Quad Spot

Pricing for Amplifier revision by Dada Electronics

2011-12-31T21:03:00.003+01:00

We use the best components and we work together with the best specialists.
After a revision there is a 1 year full warrantee (excluding transportation costs).
This is the pricing (in Euro) for the Quad-revision/upgrades:

FM3: 85,-
FM4 including battery: 95,-
33: 155,-
303: 195,-
303 including High-end board-set: 395,-
306: 195,-
34: 185,-
44: 195,-
405: 215,-
405 including High-end amplifier-boards: 455,-
405 including all High-end options (boardset, Dual mono PSU and LS Protection/delay): 610,-
303, 306 or 405 conversion to monoblock: add 25,- to the revision-price
510: 215,-
520 (f): 245,-
66Pre: 195,-
606 MKI: 215,-
606 MK II, 707 or 909: 255,-

Cyrus revision:

Cyrus One or Cyrus Two: 195,-

Bose 9-series Equaliser revision:

901 series III or series IV: 170,-

KEF Kube revision:

KEF Kube (including BB Opamps): 120,-

Other technical work: 40 Euro per working-hour.

The Do-It-Yourself kits with support remain @ the same price:

33-Kit: 22,-
34-kit MK1: 50,-
34-Kit MK2: 58,-
44-kit: between 30,- and 58,-
303-Kit: 74,-
306-Kit: 72,-
405-Kit: 80,-
405-Kit without Power-supply capacitors: 35,00,-
405-Kit with OPA604 High-end boards: 346,-
405-Kit with OPA627 High-end boards: 396,-
606 Basic kit: 65,-

Other Kits: Ask for a quotation

Dada Electronics has all Quad (33 to 909) parts & replacement transistors in stock.

Documentation & 7/7 support by experts are free.

Upgrade/Revision manuals, Service manuals and User manuals can be downloaded for free on http://www.dadaelectronics.eu/.

You can click on the pictures to enlarge them. When you click on the products or services you will go directly to the Dada Electronics Webshop.

Stefaan, Esmond, Stew and Joost

Chinese Quad 405 boards

2011-07-20T10:23:00.003+02:00

On Ebay Chinese boards for the Quad 405 are for sale.

We ordered some to try them out.

They are based on the first version of the 405 board with some modifications in the in- and output connectors.

They will not fit the original aluminium heatsink-profile and they are 10mm longer and 5mm larger than the original board so they won't fit in a 405 but they may be interesting for DIY-projects in another chassis.

They come without schematic diagram or documentation.

Stefaan

Fixing Quad 303 boards in the chassis

2011-05-06T14:30:00.005+02:00

We often get the question if we can supply the plastic clips for mounting the Quad 303 driver-boards and power-supply boards in the chassis, as they can break off while removing one of the boards.

In fact a simple and cheap alternative solution is using 4 mm plastic cable-ties, like in the pictures.

They are sold in most hardware-stores or in the Velleman cable-ties set we sell in our Webshop.

The 303 in the pictures belongs to Ad from the Netherlands and has been fitted with Dada High-end boards, new MJ15003 and MJ802 transistors and better internal cabling.

(Click on the pictures to enlarge.)

Stefaan

BHC Aerovox ALC10C instead of ALP22A in Quad 606-Mk2, 707 and 909

2011-05-05T12:15:00.006+02:00

BHC recommends using the ALC10C capacitors instead of the older ALP22A in new developments. It is less expensive, smaller and it has better specifications. The ALP22A is still produced for replacements in older circuits (but is out of stock for the moment, new stock is announced for July 2011).

In the Quad 606-Mk2, 707 and 909 the ALC10C 15000µF 63V can be used but the pins won't fit the printed circuit board. This can be solved by cutting the two unconnected pins and drilling an extra hole 1,5mm) in the PC-board (see the picture).

Make sure to double-check the polarity, when the polarity is wrong the capacitors will be damaged. The - pin is beside the gold line on the side and it has a "-" sign on it. The + pin is the middle one of the 3 pins on the other side. The two pins beside it have to be cut.

Dada is developing an new Printed-circuit board with a 500 Watt Amplimo transformer and 4x BHC ALC10C 15000µF for replacing the transformer and the capacitors in a 606 Mk1, making it an Mk2.

The 15000µF 63V ALC10C can be bought in the Webshop here.

The (general) BHC Aerovox datasheet can be downloaded from our Website.

Stefaan

405-clone needs service after 30 years...

2011-03-14T16:07:00.004+01:00

Holger (from Germany but now living in Australia) has been using his 405-clone for 30 years but then there was this 18th Birthday-party of his daughter...

Back in the old student days 30 years ago some friends were obsessed with the idea of building audio equipment and one project amongst many others was to build a replica of the Quad 405.

The benchmark was to get better measuring results than the ones published by Quad and following some hard work and applying what was taught at Uni, the result was impressive.

Thanks to choosing better components, using custom built inductors, redesign of the power supply etc. we achieved what was then shared around a circle of mind-liked people.

The unit never failed in 30 years time, but has recently given up following a wild 18's party of my daughter.

Thanks to the team of DaDa Electronics it will hopefully spring back to life and will last another 30 years...

Holger

Ivor's Quad 33 revision

2011-02-05T11:57:00.005+01:00

Quad 33 Revision. A beginners tale.

I am a professional violinist who also has a small recording studio, and In the past couple of years I have built a small collection of Quad amps. This currently includes a 33/303, 405-2 and grey 44/606mk2. I realised after years of trying different equipment, both for domestic listening and in my studio that the Quad power amps are perfect for my needs. They are fairly cheap, built fantastically well, and sound great. I use the 303 with Quad Esl 57's , 405-2 with Harbeth P3ESR (In the studio) and the 44/606 await Harbeth Super Hl5's. The big problem I have experienced is with the Pre-amps. The 33 just sounded dull to my ears, and I could not enjoy using it even after a Quad service. I still have a large record collection so need a phono stage, and I like the look of the Quad preamps, but needed to try to improve the sound to the point where I felt I could use them whilst keeping as much originality as possible.

Dada kits seemed like a sensible proposition without costing too much. The down side for me was I have had absolutely no previous electronics experience except for a couple of poor attempts at soldering broken mic leads. Here are some perceptions I experienced whilst doing the work.
I had to do some basic web searches to work out the values of resistors, and youtube provided me with tuition on soldering and the use of solder pumps. Initially I just wanted to do the power supply modifications, and although there was a picture provided of the modified board, I did manage to put a couple of capacitors in the wrong way round. (Negative to positive) . I experienced some issues then when trying the preamp as my mistake had damaged the two new capacitors. A couple of emails to Stefaan in Dada support sorted that confusion out, and when I refitted the new Capacitors (which I re-ordered from Dada) the preamp worked like a dream and that “dull” sound was replaced with much more transparency.

Success lead me to want to do all the 33 modifications in the kit (except the reduction in input sensitivity since I use an Fm3 Tuner). The biggest problem I then experienced was coming across slight variations in board layout of the Disc Pre Amplifier Board M12019 (issue 4) and Tape Adapter Board M12027. Since I cannot read a circuit diagram yet or don't actually really know what the components even do yet, I was stuck.

The instructions from Dada are very good, but a few colour photographs of finished boards would have helped me. I found a site online where someone had completed the Dada mods on his 33/303, and using his pictures of finished boards that had the same layout as mine, I was able to work out the placement of a couple of components. I then took photos of my completed boards and sent them to Stefaan at Dada to make sure I had not made any silly mistakes. (I enclose my pictures of those two boards after modification)

The end result is that I now have an extremely good sounding preamp that cost relatively little, and looks perfectly in keeping with the rest of my equipment. It took the 33 from not really usable to very good, and I do trust my ears! At the same time I am learning more about electronics, and feeling more confident that I can service my own Quad amps in future. That saves me money, gets me a better sound, and gives me a feeling of personal contentment. I will do more Dada mods to my other amps, why not, it was rewarding, not too expensive and sounds great.

Ivor

Quad 50E and D Upgrade /Revision manual

2011-01-07T14:23:00.003+01:00

DaDa Electronics has published a Upgrade/Revision manual for the Quad 50E and D professional mono amplifiers. Together with the user manual you will find all the information you need to succesfully revise these bomb proof amplifiers. Both documents are in the download section of our website. We will not produce a kit, but all the components will be available in our webshop.

Joost Plugge and Stefaan Verdonckt

DaDa Electronics

The best CD4066 switch for a Quad 44 is.........

2010-12-26T13:49:00.010+01:00

Instead of looking for the right 4066, I build a relay solution. The two relays (4 contacts) are placed on a small Pcb, piggybacked on the copper side of the Psu board.

The only awkward thing to do, is soldering a 14 pin Ic socket to this side of the board, but Quad made large soldering "islands", so with a steady hand and a small soldering tip it went ok. The relays are driven by transistors in so called “open collector mode”. The Pcb’s are degraded to prototypes because of a small non critical (repairable) layout error. How can you make an error in such a simple Pcb? Don’t ask!

In a 44 there are two switches in series with each signal path, lucky for me all the CD4066 are configured the same, so the layout of the Pcb is standard for all positions. The relays are high quality ones with gold plated contacts.

I measured THD (non weighted) and IM (Din) distortion with the Virtins analyzer (left and right channel). This is a 44 MK I with single OPA’s. All the OPA’s are BB OPA 627.

With all the original CD4066 in place:

THD 0.0055 and 0.0062 IM 0.013 and 0.015

The selector switch replaced by a relay:

THD 0.0032 and 0.0039 IM 0.009 and 0.010

The output switch replaced by a relay:

THD 0.0008 and 0.0007 IM 0.0017 and 0.0012

So a reduction of 17 dB or so of THD and IM, the relays operate click free in the signal path, but the mechanical clicks reminds me of the good old past (I am a sentimental old man!).

Altough the results of the relays are very good, the original performance of a 44 with new Op Amp's is still outstanding, considering the age of the design!

Joost Plugge

Update 8 march:

New PCB's have arived, also a connector sytem instead of a DIL socket, watch this space for the anoucement and pricing.

DaDa Electronics

What are the best capacitors for Audio?

2010-12-18T15:50:00.017+01:00

The quality of the capacitors, specially when they are in the signal-line, have a very big impact on the sound-quality of an audio-circuit.

There are several factors that determine the quality of a capacitor, some of them are very important for audio-applications:

The tolerance and the real capacitance (this is important for usage in filters)
The dependence of the capacitance on the frequency (1µF at 1000 Hz does not mean 1µF at 20 KHz!)
The internal resistance (ESR)
The leakage-current
The ageing (how will those factors evoluate over time)

The best capacitor-choice depends on the application within the circuit and the capacitance we need.

1pF to 1nF range: HF management and feedback circuits

This range is mainly used for HF-elimination in audio-circuits or for feedback purposes like in the bridge of a Quad 606 amplifier.

The best choice in this range are the Silvered Mica capacitors. They have a very good tolerance (up to 1%) and very low distortion- and noise-figures but they are rather expensive.

MKS or MKP are a good alternative. Ceramic capacitors should be avoided in the signal-line as they can cause an extra non-linear distortion of up to 1%.

1nF to 1µF: Coupling, decoupling and supression of oscillation

These are most used in audio-circuits, ao. between stages when there is a DC-level difference, to eliminate oscillation or HF-parasites and in feedback-circuits.

Typically film-capacitors will be used in this range, and even up to 4,7µF. The best choice is Polystyrol/Styrene (MKS) like Wima or Polypropylene (MKP) like Vishay. Polyethylene (MKT) is an alternative at a lower cost (but we are talking cents here).

1µF and above: Power supplies, output capacitors, filters, decoupling

In this range Electrolytic capacitors will be used. The advantage is the very high capacitance (up to 1 Farad). But there are several disadvantages:

Electrolytic capacitors are subject to ageing and drying out. After 10 years or more the oil is drying and the important factors like the ESR are changeing. They should be replaced every 10 years otherwise they will have a negative impact on the sound.
They are polarised. There should be at least 1 volt DC over an electrolytic capacitor otherwise non-linear distortion will increase to 1% or higher.

When designing a coupling-circuit electrolytes in the signal-line can often be avoided by recalculating the time-constant (RxC) for a lower capacitance below 1µF so we can use film-capacitors. If this is not possible it is important that there is at least 1 Volt DC over the electrolyte and that a good quality capacitor is used (BHC Aerovox, Nichicon, Epcos, Panasonic ...).

Bipolar capacitors should be avoided, they are in fact two electrolytes in series with opposite polarity and twice the capacitance. This is against the "Less is more" High-end philosophy. In many cases the polarisation can be calculated or measured.

This is often a problem in Opamp-circuits with a symmetric power-supply where only the (low) offset-voltage is available for polarisation. In this case a lower capacitance is often possible because of the high input-resistance of the opamp and we can use film-capacitors. Or we can replace the capacitor with a wire-link.

By choosing the best solution for every application the best sound-quality can be reached. And investing in quality capacitors will have a more positive effect on the sound quality than with any other component.

Comment by Joost

Within the company we had some discussion about Tantalum and other polar capacitors.

I found two very good documents on this subject. One by C. Bateman and one by W. Jung. Both documents are in the download section of our website (Theoretical and practical Electronics papers).

Although the documents are “old”, I did not find any recent quality documents (comments welcome!). Most opinions on this subject are based on those two studies.

In a recent article in the Dutch magazine Elektor, they measured an Electrolyte capacitor intended for the input stage of their Op Amp based power amplifier (yes they did! 32 NE5532 per channel). They rejected it because of the distortion compared to a film capacitor. The distortion can be reduced by connecting two capacitors back to back, as Jung and Bateman also found.

Bateman find high levels of distortion in Tantalum capacitors and rejected them for audio usage. Jung also did notice the high distortion, but with some special techniques they can perform reasonable. Will be continued.

Stefaan

Interesting links about capacitors:

http://sound.westhost.com/articles/capacitors.htm

http://stephan.win31.de/capdist.htm

http://www.mennovanderveen.nl/nl/download/download_4.pdf

Quad 66 Revision Kit Design considerations

2010-12-18T12:47:00.005+01:00

We get more and more questions about how the sound quality of the Quad 66 can be improved.

The 66 is often used together with the 606 which may be one of the best Quad amplifiers ever made. But there certainly is room for improving the 66 by replacing some of the components with better quality.

The bipolar capacitors can be replaced with Nichicon polar electrolytes
Some TL072 opamps can be replaced by Burr Brown OPA2604
The switch-IC's van be replaced by better quality with lower THD

It is not clear why Quad used cheap bipolar 100µF electrolytes between the stages. We switched on the 66 and measured the voltage over each electrolyte to determine the polarity. This way we could replace them all with high-quality polarised Nichicons which are a lot better for audio. Bipolar caps are in fact two capacitors of twice the capacitance in series and this doesn't correspont with the "less is more" High-end Hifi philosophy.

The TL072 can be easily replaced with the pin-compatible Burr Brown OPA2604 which offers a much better sound. On the Disc-board we will use OPA2134 because of it's good signal/noise figures and excellent behaviour at low signal levels.

The TL074 quadruple opamps are still a problem as there is no Burr Brown alternative in a DIP-package. We are looking into the possibility to replace them with OPA4134 with a SOIC to DIP adaptor but maybe we will leave them in place.

Finally we are testing Fairchild and Maxim alternatives for the switching IC's

The first 66 we revised had a much more transparent and dynamic sound after replacing the critical components. We are still making decisions about some other components and writing the revision manual and we will offer the kit in the Webshop in the beginning of 2011. We can already do the revision for you in our workshop.

See also the forum-discussion Quad 66 Hybrid Elements on this subject.

Stefaan

New power supply for the Quad 405 1 and 2

2010-10-29T18:31:00.004+02:00

A new double power supply for the Quad 405 1 or 2 and DIY projects.
We developed a new supply with an integrated DC protection for your speakers. It is based on the circuit in the Quad 306, 606, 707, 909 and 520f range. It uses the so called virtual earth technique.

Some advantages:

Integrated DC protection for your loudspeakers.
Only one fuse in the AC circuit, integrated in the power supply.
You can remove the clamp circuit from the 405 modules or the chassis.
Also you can remove and short circuit those non linear fuses in the DC circuits of the 405 modules. These two measures are strongly advised!
Symetrical clipping due to PSU voltage offset, like in the 606 range.
No transformer needed with a central tap, simple cable layout.

Price information will follow soon. The production unit may differ in detailing, also there will be a choice in capacitors, but the minium will be 10.000uF.

Joost Plugge

Update 18 december: The Dual Mono power supply with 4x15000µF BHC Aerovox and Loudspeaker-protection is available in the Webshop now. Click here.

One step further: Piotr's 405-project

2010-10-25T11:36:00.006+02:00

Piotr, from Poland, revised his 405 with the Dada-kit ... and some other components:

He wrote:

Specs:

-Dada Electronics basic revision kit on OPA604,
-mains voltage selector removed,
-double power supply with common zero (chasis),
-8x10000uf/63V BHC power supply capacitors,
-WBT speaker terminals,
-Cardas Rhodium RCA sockets,
-Furutech pure copper ICE socket,
-3N silver, gold plated wire for most power supply connections (plait),
-new rear panel, moved backwards to fit in with lots of capacitors,
-Cardas 3%Ag solder used,
-all PCB pins shortened and resoldered,
-Ultralink Challenger 2 signal cable (6N OFC),
-C1 caps removed (short),
-abandoned PCB fuses .
-1,2V input sensitivity

General philosophies:

-Minimum connections - zero connections except primary fuse, PS capacities, and ground to case screws.

-Power supply wiring made of silver wire is constant, uncut, unsoldered, no connectors, handmade plait - from "source" to "target" (rectifiers to PCBs passing capacitors, case to speaker's zeros, all zeros passing one crossing)

I didn't meet any serious problems. Almost didn't test the unit before run. Started from the first time. Results are stunning. Believe me. No partial distortions. Perfect, stereophonic, focused sound. Pleasant and reach at the same time. Pure black background with natural, creamy recording's noise. Much more agressive, faster, precise but still natural, rich, transparent sound to live with. Bass not foggy. Similar bass shape and depth like before but this is not coffee with milk anymore. This is more like.. dark brown, thick espresso that you can feel, but you cannot touch.

Cheers!
Piotr
prezydent@o2.pl

Upgrading and Monoblocking a pair of 520f amplifiers.

2010-09-03T12:11:00.010+02:00

By Stefan Vertongen, Belgium.

Description of the amplifiers, as bought on the second hand market:

-Serial number: 005 095; one amp board with power transistors MJ15003, the other with MJ15024; furthermore capacitor C9 and C11 were not foreseen by factory , drivers “40872 – no brand”
-Serial number: 005 097, original power transistors RCA 17556, drivers RCA40872
Both of them had power amp boards 12828/7 (not the later -8), the original power-supply elco’s, XLR inputs and LS outputs; there were no output limiters or input transformers; XLR pins 1 and 3 were bridged. In fairly mint state, with scratches; used in a tennis club during 20 years, but apparently no heavy use (no blackened components).

General (my) philosophy:

- Some people prefer to keep everything in ‘original’ state (in order to keep a decent market price for the private museum lovers), and/or only repair what is faulty; to the contrary, I have a tendency to upgrade components/circuit whenever it will have a positive impact on overall sound quality, and will change even the ‘look’ if it improves technical functionalities (also, it gives a personal exclusive touch!).

- The conversion to mono-blocking was in my configuration mandatory: I use one power amplifier for one loudspeaker; the challenge was to see if those Quads would be competitive with the legendary Studer A68’s or ACCUPHASE M100…

- I am a music/audio-lover, not a qualified technician (with some basic technical knowledge acquired during the years); so, many thanks to the prompt, no nonsense advices of Stefaan (by the way, he also sells the components !) and Joost from DaDa Electronics, and especially Louis (audio technician of Microware Solutions workshop in Dendermonde, disposing of a scope and frequency generator); I am sure, especially for the latter, that it wasn’t always easy to satisfy all my inquiries, indeed “paradise” –not even the technical- isn’t to be easily found on earth !!

- Never underestimate the complexity of things; Mister Peter Walker’s principle ‘Keep it simple’ was not always followed up by his production people !(different boards, omissions, many versions/changes in the 500 Series); by the way, this project was a piece of cake compared to the revision of the integrated ‘MARANTZ PM94’…don’t even think buying this nightmare !!

The upgrading of the amplifiers

First of all: I applied strictly the content of Joost’s blog (just go to the blog ‘520f’, nice pictures). Very clear on ‘what to do’ issues, except details for mono conversion. This was our main reference.

Additional:

- I removed the existing XLR LS connections – what an unpractical connection!- and replaced by WBT looking terminals. Of course, the old oxidized input XLR is replaced by a new Neutrik.

- A SOWTER 3575 input transformer was installed (in order to match the incoming balanced line from my preamplifier to an asymmetrical signal; although Joost didn’t like the idea, I installed after the transformer a cinch (RCA) connector (flexible use in other audio installations); a switch to isolate the transformer in the circuit was necessary: if not, a square wave of 30 Hz input signal looks awful because the RCA input line is somewhat influenced by the transformer…NB: this transformer was optionally installed by Quad in the eighties and is still produced.

- Substitution of the power transistors MJ15024 by MJ15003 for conformity.

- Replacement of internal signal cabling by a multi-wired core, shield, both Teflon insulated; they are not very flexible, nor easily stripped.

- Replacement of all small 0.1 ohm resistors R32 (one of them was faulty; the rest didn’t ‘look’ in good shape, metal film 1% on all power-amp boards.

- Original opamp: Stefaan is right, although recommended by several discussions on websites, a replacement of the original isn’t necessary (because NOT in the signal path)

- Mono conversion: look at the components that Stefaan added to the blog of Joost; as the potmeter in the Quads aren’t sufficiently accurate for ‘trimming’ the output voltage between the two channels, a dedicated 5Kohm was installed and one of the original was put on retirement. Now, this resulted in an acceptable imbalance of about 20 mV between the power-outputs. Placement of the 2 loudspeaker connections, with Dale resistors.

Some measurements: (input of 230 Volt mains with a variac):

- Peak to peak voltage of about 90 Volt, resulting in a 32 Volt RMS or about 130 Watt (before clipping) power on a LS load of 8 ohm. The old RCA transistors gave even a couple of volts more!
- On a power supply board, an imbalance between H+ and H- of some volts: 51/54 Volt with no load (don’t worry says Stefaan…); the other unit approached the total 110 Volt of supply voltage.
- Very nice scope analysis of 15 kHz and 30 Hz sinus signals.

- The volume-potmeter indicator was somewhat calibrated to the notional ‘0db inscription’ with a signal of 1 kHz / 0,775 V and ‘just’ before clipping.

- On one of the mono-amplifiers I had an audible (ear against loudspeaker; nothing at 0,5 meter distance) hum…Spent a lot of time on efforts to cure it… a headache (not the elco’s but: mains transformer, input transformer, cabling, earth loop, ??!). Finally, back to the basics: I found the bad guy, the R40 was burnt!! Replaced with 1 watt dissipation type.
Nb you’ll find on diy websites many complaints about ‘noise’ from these amplifiers; they refer mostly to the standard xlr connector, where you have to connect the pins ‘earth’ and ‘cold’ signal ,with an asymmetrical signal input.

-Some further possibilities:

*This is a Professional amplifier, but surprisingly without a ‘standard’ overload or ‘clipping’ indicator. An omission, Mister Walker? I found a reliable circuit description – look at http://sound.westhost.com/project23.htm - based on a drop in the supply voltage when reaching high output levels. I think that Joost won’t like this ‘rape’ of the 520 front… I still hesitate, but I use to run high output levels and my preamplifier can deliver much more than 1 volt on dynamic cd’s! Beware: “clipping” is a complex subject!!

* Driver replacement by MJE 15031(don’t expect huge improvements)

Now, how does it sound?? "The proof of the pudding is in the eating"

After ten hours of listening: no disappointment. YES,.. they sound very dynamic, neutral, punch, very good lower bass, monitor like…’richer in details’ than my modified 405 II, perhaps lacking a bit the ‘velvet’ sound on voices/piano.
I keep them, for sure….together with Studer and Accuphase! Worthwhile!!!
Nb listening on loudspeakers Tannoy Kensington, alternate : Sonus Faber Amati (both of them very ‘musical’).

Stefan Vertongen

Comments by Joost: DaDa Electronics

First of all: we are very pleased with customers like Stefan. They try something different and are willing to share there experience with us and other Quad-fans.

The 520 has no voltage output limiter or indicator, that is correct but it is safegaurded by a strict current limiter and a thermal protection. Which simply swithes of the amplifier when it gets too hot.

Stefan mentioned "Pro" use and the omission of a clipping indicator. In most cases the power amp in pro use is out of sight, on stage with the loadspeakers, while the mixing desk is in the public area. So: set the levels right and use the clipping indicator on the mixing desk.

I will make a separate article about the monoblocking issues and the extra resistors needed.

I don't like input transformers, if you can avoid them: do it. But in the setting of Stefan they are needed. Quad used the Sowter's but replaced them by there own opamp-based input modules in later versions.

I keep the equipment as orignal as possible, but I understand the pragmatism behind Stefan's conversions.

Joost

Measuring distortion with software

2010-08-25T11:49:00.018+02:00

The case with the “Chinese” Quad 909 was the first occasion we worked with a software based distortion analyzer.

I used to work with a Sound Technology 1710A. The unit needs calibration and some repair; I have sent mails to ST, but no response until now. Just a calibration is a staggering 995 dollar or so, ex P&P, if they accept the work. Then I realized that even if they were willing to repair the unit, the performance will not be good enough to measure modern or modified vintage equipment. So we moved on.

Distortion analyzers like the ST are based on wideband AC RMS meters. First you measure the output signal of the amplifier under test, then you remove the test tone and measure the residue. Eventually you can calculate the distortion. This is off course distortion including noise.

The software distortion analyzers are based on a spectrum analyzer program and analog-to-digital and digital-to-analog converters. This type of analyzers measures the individual frequency components, the rest is “only” calculation!

Spectrum analyzers existed in the pre digital days, but they where very expensive.

The theory behind this is the fact that every repeating signal can be decomposed in so called harmonics and the basic frequency component. To measure amplifiers you also need a signal generator and an oscilloscope in (preferable) one software package. I started with a freeware solution from Sillanum Soft and the standard sound card in my PC. The results were very promising. The next phase was a high quality, 24 bit, 192 KHz sampling rate, low distortion, soundcard with balanced outputs (ESI Juli@). The results were more than ok. Because the Sillanum Soft package only has the basic functionality, I bought a package from Virtins Technology and built a proper brake out box with volume controls, a variety of connections and a possibility to connect load resistors and other lab equipment. To get more speed (calculation power) from the PC, I also installed a 64 bit operating system (dual boot option).

From the website of Audio Precision, a high end distortion analyzer manufacturer, I downloaded two very useful documents. The Audio Measurement Handbook and a large document titled "How to write and read audio specifications". By reading and digesting the last document you will find that most specifications of commercial audio equipment are not complete and meaningless in most cases! Even our Quad heroes wrote those kind of specs!

A practical case: measuring a Dada Electronics revised Quad 303

See the specifications in the picture. First I checked the distortion. Quad measured at a strange frequency of 700Hz. Also the term “unrestricted bandwidth” is undefined. What does ”up to 45 watt” mean in real terms of voltage and watts? My own settings: 1 KHz test signal, bandwidth: 20Hz to 20 KHz and 45 watt in 8 ohm loads, both channels driven and loaded. As you can see the specification of 0.03% is easily met. If you remove or lower the load the result is spectacular. Very low distortion indeed.

The next measurement was the crosstalk. Quad did not specify the output parameters. As you can see, with the channels loaded with 8 ohm and the power at 45 watts, the channel separation is below modern standards. Only good enough for vinyl replay or FM broadcasts. If you remove or decrease the load, it is ok.
So there are people who say the 303 is the best Quad amplifier ever made. We don’t agree if you use the 303 with modern digital source material combined with high volume settings. This is one of the reasons we advise to monoblock a 303 or use it in a bi-amp application per channel. The load dependent cross talk can influence the stereo soundstage.

The next myth: high bias current

In the Quad service manual it is stated that the DC bias current of the power transistors must be between 10mA and 5mA. A popular “mod” is increasing the current to 35mA and in some cases even higher. Well, in real life the distortion is only 0.05% at a bias current of 0.5mA. At 5mA it is 0.01% and will not get lower at higher bias currents. Also the spectrum analyzer will not give a better (cleaner) picture at high levels of bias current. So Quad was (is) right! For practical reasons, stability of the circuit, 10mA is a good value for the bias current.

Joost Plugge

Virtins Technology
Sillanumsoft
ESI
Audio Precision

Improving a new Quad 909

2010-08-08T12:49:00.030+02:00

One of our French customers, who is working in professional audio, bought a new Quad 909 to drive his Tannoy studio-monitors.

He has always used Quad 303 monoblocks with Dada High-end boards and was very pleased with the result so, as he needed more power for another project he bought a 909.

The Tannoys are very sensitive and transparent (as studio monitors should be). When he connected the 909 he heard a hiss and distortion in the mid-tones.He brought back the amplifier to the reseller and it was tested by Quad. They said everything was working within specifications so there was nothing they could do.

So he sent the amplifier to Dada Antwerp and asked us to find the reason of the problem and to see what we can do about it.

Joost and me took an afternoon to do some tests with Virtins Multi-instrument Audio analyser and the other lab-equipment.

First we did a listening-test. The sound was rather "harsh" and "metalic" compared to other (revised) Quads.

We measured a Total Harmonic Distortion of 0,8854% (!) and a Signal-to-Noise Ratio of 81,4dB at full power with 8 Ohm load. The total power before clipping was 120W RMS. Very bad figures for a high-end amplifier... And much worse than the "classical" 606 the 909 is based on.

We decided to take the amplifier apart and to concentrate on 3 possible sources of distortion:

The power-supply

The amplifier-boards

The input-board for the Quadlink

The power-supply board is the "classical" board known from the 606-II and the 707 with a Toroïdal transformer and 4x 15000µF capacitors in a dual-mono design. Only, the original BHC Aerovox capacitors have been replaced by Chinese CapXon capacitors.

We took them out to test them with the Hameg LCR meter.

The BHC Aerovox was the ALP22A 63Volt, the CapXon has no type-indication, it is a plug-in two pin 80Volt 105° type.

We measured the capacitance at 100Hz, 1KHz, 10KHz and 25KHz with a 1V DC bias voltage. Both were within specifications (15mF or more) up to 1KHz. At higher frequencies the BHC scores better with 3x higher capacitance, for example 1100µF at 10KHz instead of 300µF for the CapXon.

The measured internal resistance is about the same and around 10mR at 1KHz.

This leads us to the conclusion that both capacitors are very comparable with a slight advantage for the BHC specifically in the higher frequencies. Of course this test doesn't say anything about the ageing of the capacitors after several hours of usage. The BHC has an excellent reputation for ageing. We decide to replace the 4 CapXon in the power-supply with BHC Aerovox.

Then we take a look at the amplifier boards. We replace the low-cost capacitors as usual, the electrolytes with Nichicon, the range between 1nF and 1µF with Wima MKS and the range below 1nF with Silvered Mica.

We also recalculated the bridge and used the exact values for the bridge components. Joost will comment on this later.

We connect the driver-boards to the power-supply. Now we measure a THD of 0,025% and a S/N Ratio of 87dB which is a lot better yet. A listening test gives the rich and transparent sound we are used to with Quad amplifiers.

Finally we take a look at the input-board that also contains the switch-on delay and the symmetric -> asymmetric conversion for the Quadlink.

The asymmetric signal from the RCA-inputs also passes through the output-stages of the preamplifier and again we find some low-cost capacitors on board. At a low input voltage the THD of the board is 0,003% but increasing the input-voltage increases the THD of the board. Around 10V we have 0,4% THD.

As the customer won't use the Quadlink we decide to remove the board from the signal-line and to connect the RCA-inputs directly to the amplifier-boards like in the 606.

The conclusion is that every detail has it's importance for the final sound-quality, specially the quality of the components and that it is best to avoid the Quadlink if you don't need it.

Stefaan

Practical: Component list

The 909 PCB is a thhrough-metalised PCB. You should use a good desoldering-pump or a desoldering-station to remove the old components.

For soldering on the PCB's only use lead-free solder. Tin/lead solder won't mix with the solder on the board.

You can click on the components below to go directly to the Webshop for ordering or for seeing more technical information or price and availability.

The 4 Power-supply capacitors on the central board are replaced with BHC Aerovox 15000µF 63V ALP22A or ALC10C.

(2x) R29 (in the bridge) is replaced with 10R 1/2W 1% Metalfilm

(2x) R30 (in the bridge) is replaced with 47R 1/2W 1% Metalfilm

(2x) C1, C4 and C6 (6 in total) are replaced with 330pF Silvered Mica

(2x) C7 is replaced with 100µF 63V Nichicon

(2x) C9 and C11 (4 in total) are replaced with 470µF 63V Nichicon

This way we have replaced all electrolyte capacitors with the best quality and we have adapted the bridge to the optimum value.

The specifications of the 909 transformer are: 2x 80,5 Volt 500VA

Measurement of a 606 MKII (my own) as a reference:

Vout is 32.2 V, no load, left and right channel:

THD 0.0009 and 0.0010

IM Din 0.0045 and 0.0041

Vout is 32.2 V, load is 8 Ohm:

THD 0.0129 and 0.0059

IM Din 0.0342 and 0.04

So the left channel is out of spec, THD should be lower than 0.01 some work ahead!

Joost Plugge

Revision of a Quad 520f

2010-04-28T11:40:00.029+02:00

The Quad 510 and 520 were the first professional current dumping amplifiers. Produced from 1984 until 1995.

Production numbers:

Quad 510 Mono 19" Rack Amplifier(Pro), produced from 1984-1989 numbers: 3200
Quad 510 Mono 19" Rack Amplifier(Neutrik)Pro), produced from 1989-1995 numbers: 600
Quad 520 Stereo 19 " Rack Amplifier(Pro), produced from 1984-1986 numbers: 4000
Quad 520f Stereo 19 " Rack Amplifier(Pro), produced from 1986-1989 numbers: 7500
Quad 520f Stereo 19 " Rack Amplifier(Neutrik)Pro), produced from 1989-1995 numbers: 3000

Introduction:

The 520f with Pcb 12828-8 was based on the final circuit of the 606 and 306 which were introduced in 1986. Prior to the Pcb 12828-8 there was also a 12282-7 version with a slightly different circuit diagram (mayby there are more variants?). It shows how Quad solved the problems around the current limitter circuit and the H+ rail for the class A amplifier. Still not the best solution, but a fare and good working compromise. The 520 differ on two major aspects from a 520f, a double conventional power supply instead of the power supply with the regulated earth to prevent any DC current through the loudspeaker, and discrete build current sources in the input circuit. The 606 design was based on single integrated current sources. The 510 never got the “f” suffix but was after 1986 or so also based on the new circuit (and modules).
The 520f had standard unbalanced inputs, balanced high quality input modules could be ordered separately or factory installed. The model type suffix was then changed in “M”. There was also an “S” model where no input volume controls where installed. In the first series of 520f the inrush current limiter was an option, from serial 6500 this was implemented in the factory. When buying second hand; be alert, the suffix on the front for all models is the same;”f”. There was also a special version for the BBC; 521f. Here the type number on the front was changed. The 521 had all the options, including balanced input volume controls and a provision to separate the mains earth from the chassis. To make it simple; Quad also built 520’s to special order. Recently I saw units with the number 522 on the front for sale on eBay, as far as I know, these were also build for the BBC.
Because of the pro use, it is very difficult to buy an unmolested 510 or 520(f). Only a few found their way to domestic applications or received TLC in pro environments. But from a technical (electronic) viewpoint most units can be restored to pristine order.
The 510 and 520 families have double output transistors in stead of the triples in a 606. This limits the output power, but in practice you will not notice the difference when playing music. The 520 ranges of amplifiers are thermal protected, so if you wish, the current limiters can be changed to allow a higher peak output current with low impedance loads. The 510 is not thermal protected, althoug it is mentioned in the manuals.
If you can chose: buy a 520f. The 510 is also very good, but is single channel and primarily designed for driving special loads.

Revision challenges.

Except for the power supply capacitors, all the other components which are needed are standard and easy available. In the original amplifier a 6800uF 63V capacitor is used in the PSU, a BHC capacitor from the ALP’s series. If you reed the BHC datasheet’s carefully, BHC do not advise to use the ALP series for new projects, only for replacement. Also most suppliers only have a limited stock of ALP series. I chose a 15.000uF capacitor from the ALC series to replace the ALP, it has the same can size and plus and minus pin spacing, but has only four pins instead of five and uses a different pin layout. The two or three (ALP) extra pins are added for stability and so a rigid PCB and capacitor coupling as a result. But there is a design problem. The extra pins are normally not connected to the plus or the minus of the capacitor, but in some circumstances the extra pins will be internally connected to the minus due to electrolyte leakage. This will not degrade the performance of the capacitor. But, in the Quad Psu printed circuit board design some of the extra pins are connected to the plus or even worse H+. Because of this layout issue, I assume a lot of 520’s ended up with short circuit power supplies and are re ”capped” for the wrong reason!
If you can get hold of some original ALP capacitors, isolate the extra pins by removing the copper track around the pins, don’t remove the solder eyelets. Or do as I did, remove (clip) the extra pins and use heavy duty two sided sticky tape to firmly connect the capacitor mechanically to the Pcb. To accommodate the pin layout of the ALC, some new holes have to be drilled, see the picture. Remove the laquer around the new holes for proper soldering. Check the position of the holes before you drill the holes, make a template. Also the two sets of resistors R104 and R105 have to be placed at the copper side of the Pcb.
Replace C7, C9 and C11 by capacitors with the same value. That’s all to restore it to an improved electronic state. If the inrush limiter is fitted, also replace C300 with a 2.2uF bipolar capacitor. The 520 has a different Psu Pcb layout, but I think the ALC series will fit after some work.

Further improvements:

Double the capacity of C7, C9, and C11. Decouple these capacitors with 0.1uF film type capacitors on the copper side of the circuit board. Also decouple D1 and D12 which these type of capacitors. Replace C2, C3, and R5 by 0.33uF, 1uF and 62kOhm, this will bring the input and DC feedback circuit to Quad 909 level. Because of excessive heat produced by R16, R17 and R24, place C7 at the copper side of channel two (2), and reposition R16 , R17 and R24 at the copper side of channel one (1).
The channel 1 board is on the left hand side of the case, if you look at the front of the case.

Current limiters.

Quad struggled with the design of the limiters from the first 405 schematic. The key problem is the position of the upper dumper driver. Ideal this driver, which is a part of the class A stage, should be connected to H+ (like the rest of the class A stage). In most schematics, including some of the the 606, the voltage of the emitter of the driver is modulated by the voltage across the current sensing resistor in the dumper circuit. The final solution from Quad is to connect the class A stage to this point. And in this way trusted on the very good Psu ripple rejection ratio of the class A stage. But in this case the ripple is not only mains borne, but also ”musical”. Why not implemented it the “right” way? Any comments are welcome on this subject. See also the post I made on 19 september 2008.

Mono blocks :

To build a parallel monoblock some extra actions are needed. Connect the two power supplies together on the Psu Pcb, use low gauge insulated wire (min 0.75mm2). Connect the earth, H+ and H- from one power supply to the other. Remove from one power supply T100, T101, R100, R101, R102, and R103. Remove from one channel R40 and C14. Because the 502f already has input potmeters, we don’t need an extra potmeter. Connect the two inputs together, use a nominal input signal of 500mV 1Khz, measure with an AC Milli volt meter the difference between the two outputs, adjust one potmeter for minimum reading. Lock the potmeter in the usual way. Then connect the loudspeaker via two (one in each channel) series resistors of 0.3 Ohm to both channels and earth. Check the 405 documentation on this point in the download section also for an alternative way to adjust the potmeter by using a loudspeaker.
A 520 has a normal double Psu, so skip the part “Remove from one power supply T100, T101, R100, R101, R102, and R103”.

Dada Electronics will not launch a 520f revision-kit but all the components you need are available in the Webshop. The Quad documentation and the BHC data sheet with all the information can be downloaded for free in the Download section of the Website.

Note:

In practice the use of the standard potmeters will give problems, so use a multi turn trimpot. See also the post of Stefan Vertongen.

Joost Plugge, may 2010

To simplify things I have made a list of the components you need for the above revision / upgrade with direct links to our Webshop:

4x BHC Aerovox ALC10C 15000µF 63V for the power-supply
2x MKT 330nF 100V 10mm for C2
2x MKT 1µF 100V 10mm for C3
2x Nichicon 100µF 63V for C7
4x Nichicon 470µF 63V for C9 & C11
1x Bipolar 2µ2 100V for C300 (inrush limitter)
10x MKT 100nF 63V over C7, C9, C11, D1 & D2
2x Metalfilm 62K 1/2W 1% for R5

And for Monoblock-conversion:

2x Dale 0R3 5W 1%

Stefaan

A fully restored 33 set

2010-03-21T15:27:00.005+01:00

A beautiful 33 set, consisting of a 33, FM2, FM3, AM3, 303 and two 50 E amplifiers. Take a look at the Thorens SME combination and the B&W loudspeakers.

All this equipment belongs to an enthusiastic Dutch Quad collector.

(Click on the picture to enlarge.)

Fixing Worn or Cracking Potentiometers, Switches and Connectors

2010-03-08T12:03:00.004+01:00

Kontakt Chemie explains on their Website how to repair old and worn out Potentiometers, Trimmers, Switches, Contacts or Connectors.

As Vintage Hifi specialists we found this very interesting for ourselves and our customers. We made sure to provide all the necessary chemicals in our Webshop and we outline the Kontakt article below:

There are 3 steps:

Remove all corrosion with Kontakt 60
Clean the contacts with Kontakt WL
Put an anti-oxydation film on the contacts with Kontakt 61

From the Kontakt-website:

"The cleaning and oxide-dissolving action can be improved by mechanical support. If switching or connector contacts are readily accessible, they can be carefully wiped, for example, using cellulose, leather or cotton swabs.

If it is impossible to provide mechanical support, allow Kontakt 60 some 10 to 15 minutes to penetrate.

Optimum results are obtained, if dissolved dirt and corrosion products are then rinsed off with Kontakt WL.

After that, Kontakt 61 is applied as a protective film. Kontakt 60 works in many cases even without after-treatment. However this increases the risk that corrosion damage will appear again after a short time."

Stefaan

Revised Quad FM3 Service Notes, nov 2010

2010-02-21T12:06:00.006+01:00

Quad produced some 50,000 FM3 tuners between 1971 and 1982. So there are plenty of units out there, so it is well worth while developing a service approach. This article is the result of having three customer FM3s on the bench at the same time as well as my own unit, which I’ve owned for 20 years but am only just renovating now.
The FM3 should not be confused with the similar-looking FM2, which is a valve unit based on the original FM tuner that goes with the 22/II valve equipment (of which 7,500 were built between 1968 and 1971: they are now much sought after). The FM2 and FM3 are clearly marked as such on the back, and the FM2 has the stereo indicator on the left and the tuning lights on the RHS of the scale: the FM3 has them the other way around. The FM3 is a solid-state tuner with excellent performance and a very sweet sound, well suited for areas with good signal strength and not too many stations in the band.
There are three principal variations of the FM3:

1: s/n to 5884: This series uses a two-IC front end and a Motorola MC1305 stereo decoder.

2: s/n 5885-9,9999: This series uses the same front end and an MC1310 stereo decoder.

3: s/n 10,000 up: This series uses a single-IC front end and the MC1310 stereo decoder.

Within each series there are also minor variations detailed in the Service Manual, the most interesting being that for a short period in 5885-9999 a different tuning gang and tuning scale were used, with ‘MHz’ appearing in the centre of the scale instead of at the right-hand end. I’ve never seen one of these.
Servicing these units should be confined to the following unless you have RF and IF alignment equipment (sweep generator, marker generator, stereo modulator, and oscilloscope).

1. Check that +-14VDC is available at the red lead and the long black lead on the underside. Pre s/n 5885 adjust the PSU RVs as necessary; otherwise maybe replace the Zeners and resistors. You can update pre s/n 5885 to the later power supply, by removing R106/7, shorting out RV102/3, and changing MR101/2 to 15V zeners. I have seen FM3s fitted with 7812/7912 voltage regulators but the MC1410 really needs more than 12V for lowest distortion: do not imitate; undo if found.

2. Replace the two PSU caps with 1000uF/25V. Quad used 680uF occasionally in one or other of these positions: this was strictly a stock or economy measure; do not imitate.

3. Replace the audio PSU decoupling cap C121 with 470uF/16V.

4. The audio coupling capacitors C124/125 (C116/118 pre s/n 5885) are rather small at 100nF, especially if you’re not using the FM3 into a Quad 33 or another preamp with 100k input impedance. Change them to 470nF high-quality poly capacitors. This will improve bass response and phase into lower impedances.

5. From s/n 5885+ only, you can consider replacing the resistors in the final multi-pole Sallen-Key filter chain: R115-120 with 10K 1% metal; R121/1 with 1K 1% metal, and R123/4 with 4k7 1% metal. If you can, check the values of C115-120 and C122/3, for tolerance within 5%, and replace with quality poly as necessary. Whatever you replace, replace in both channels.

6. There have been suggestions to replace resistors in the earlier RF and IF stages ‘to reduce noise’. This is complete nonsense of course, as the signal is FM-modulated at this point, and resistors produce noise in the amplitude domain only. However if you find a burnt or out-of-tolerance resistor there (you won’t), replace it.

7. Replace Tr107/8 with BC549C.

8. You can also consider replacing the composite-audio coupling capacitors C100 and C104, both located next to the MC1310, with 0.68uF/16V, but note that Quad used high-quality ‘orange drop’ capacitors in this position so personally I would avoid it unless you know they are faulty. On pre-5885 models these capacitors do not exist.

9. Adjust the output level at RV3 to be no more than 250mV on normal reception, or on 75% modulation if you have an FM RF generator. Too high a level can cause distortion inside the decoder chip.

10. Replace the stereo lamp assembly with a 5mm yellow LED in series with 1k (or raise R101 to 1k2). The green wire should be connected to the negative (short) lead of the LED, the black wire to the positive (long) lead.

11. Replace the tuning lamps and assembly with two 5mm yellow LEDs in series with a 1k2 resistor (0.25W will do). The left-hand lamp looking from the front is connected as (4) above; the other one is connected the other way round (because the black wire here is supplying a negative voltage). In all three cases the green wire is ground. The old lamps can be removed complete with their mounting flanges with a pair of pliers: just squeeze the ‘U’ shape together and it lifts out to the rear of the tuner. If you can figure out a way to mount an LED in the panel lamp position, you can also fit an LED there, on the same principle, i.e. also in series with 1k2. Otherwise, if you have to replace this lamp, make sure that the swinging lever is free and doesn’t contact the chassis and earth its supply voltage anywhere.

12. Using either a digital counter (preferred) or a DVM with a kHz range, adjust RV100 so that exactly 19kHz appears at pin 10 of the MC1310 with power on and no signal at the aerial input. Adjust to within +-2Hz if your meter is accurate enough. If you have s/n 5885-9999 and you can’t quite get to 19kHz, change R111 to 15k and if you can’t get accurate enough change RV100 to 10k linear as per 10,000+. This step aligns the stereo decoder, reducing distortion, and lights up the stereo lamp if it wasn’t on before. Pre s/n 5885 you need a stereo modulator to adjust all those coils for the MC1305 decoder: just forget it.

Do not adjust anything on the left-hand RF/IF board (including the part to the right of the metal divider), or anything on a pre-5885, unless you have FM alignment gear. Note that the factory instructions for s/n 10000+ assume you have an Sound Technology ST1000A and don’t tell you how to do it with an ordinary sweep generator.

Dada Electronics Australia, Esmond Pitt, may 2010

Quad 33 with s/n 1067

2010-02-10T15:23:00.006+01:00

Andreas from Vienna has one of the very first 33's on his working-table for a revision with the Dada Electronics 33-kit.

The first 33's that were actually sold had serial numbers from 1000 on. So this really is an early one.

As you can see on the pictures of the Disc-amplifier board Quad made some last-moment modifications when the PCB version 1 was already in production.

Andreas writes:

"Attached two pictures for your records. Yes, the components on the solder side of the board are indeed a correction of the PCB layout. The tracks beneath the 1k resistors are cut and the caps are 2200p. Comparing the attached component side picture with the ones from your manual one can clearly see that the components C309, C310, R317 and R318 (mistakenly named R308 on the Fig. 5 picture from QUAD) are missing on the component side.

So if you ever stumble over an disc board that old you know....... ;-)"

Stefaan

Revox Manuals can be downloaded now

2010-01-10T19:17:00.005+01:00

A collection of the "Most Wanted" Revox User Manuals and Service Instructions are available in pdf-format in the Download section of the Dada Electronics Website now.

You will find documentation for the Revox A720 - A740 - A77 - B285 - B77 - B780.

These are the original manuals from Studer. They are large, so you will need a High-speed Internet connection, but the resolution is excellent.

Stefaan

Extra repair/mod information concerning the Quad 44

2010-01-08T13:52:00.014+01:00

Correction for the 7815 pin layout!!!

In the first series of the 44 (before serial 12.000) the output mute relay was fitted on the tone control board. There is an official Quad mod to decrease the switch off time of the relay: a capacitor of 4u7 (35V minimum) between the +HT rail and the base of Tr501. To prevent switch off noise, always apply this mod. On the tone control board M 12512 iss 8 (somewhere between serial 2401 and 12000) the cap was fitted in the factory.

In some cases the relay will rattle, Quad refers simple to: HT problems. Modern OPA’s draw substantial more current than the very efficient TL071’s. Also they pull more current from the negative supply in respect to the positive supply.

If you look at the relay circuit diagram, it is a critical design. With a total supply voltage of 30V the relay “on” voltage is exact 24V minus the Vce of Tr501. But the voltage is not 30V, there is only 28.8V max at hand.

To cure this, I placed one diode in series with each of the 15V zeners (D400 and D401). The same way Quad builds the 8.2V supply for the Switch board. The next step is reduce R400 and R405 to 5 ohm’s and increase the zener and transistor circuit current by reducing R402 and R403 to 1k5 Ohm’s. In most cases the supply will now deliver enough current to feed the hungry OPA’s and the relay. It is good practice to replace all the zeners (also the 7.5V and the diode D403) by 1,3 Watt versions.

If the rattling still occurs: increase the relay voltage by decreasing R550 to 22k ohm.

The last (final) step to get the relay silent is the following: replace the negative supply by installing a 7915 Voltage regulator. Until now I had to do this once (a time consuming warrantee case for Stefaan).
Remove R402, R404, R405, Tr402, Tr403 and D401. Replace R405 by a wire link; replace Tr402 with a tripod wire link. Place the 7915 in the position of Tr403 with “legs” 2 and 3 (input and output) crossed. Don’t use the metal nut and bolt to fasten the regulator.

Topview 7915

The same kind of operation can be performed to rebuild the positive supply with a 7815 voltage regulator.

Topview 7815

Remove Tr 400, Tr401, R400, R401, R403 and D400. Replace R400 by a wire link; replace Tr401 by a wire tripod. Place the 7815 in the same position as Tr400 on the copper side of the Pcb. With ‘legs’ 2 and 3 (output and ground) crossed. Place the 7815 in an upright position.

Modern 24V relays are functionally compatible, not pin compatible, see the drawing for correct placing. The relay is in an upright position on the component side of the pcb. Use always a relay with high impedance, 1K Ohm or more.

Other useful mods.

In Quad 44’s below serial 23.000 the volume can not be reduced to zero, like in a 34 or 44’s above serial 23.000. Remove R502 and R503, replace R500 and R501 by a wire link, and replace R552 and R553 by 10 k Ohm. In this way the series resistor is reduced, this brings the gain of this stage to the original value.

In the Radio module and the first series of Aux modules two extra resistors are placed in the OPA circuit. R202, 204 and R203, 205. Why they did this? In this way the circuit is unity gain, but the OPA runs at a higher gain. In those days OPA’s weren’t always stable at unity gain. Remove R204 and R205 and replace R202 and R203 by a wire link.
In the Aux module: remove R606, 607 and replace R604, 605 by a wire link.

Joost Plugge

Dual Mono PSU & Velleman K4700 in a Quad 405

2009-12-28T15:45:00.005+01:00

Joost built a Velleman K4700 Loudspeaker-protection kit in a Quad 405 with a Dada Electronics Dual Mono Power-supply.

As there is no space above the PSU he fixed it to the Back-panel behind the transformer. This may be the best solution with a classical PSU as well...

The "-" of the Speakers goes to the Power-supply and from there to the central Mass-point. Another wire from the central Mass-point goes to the Velleman.

This is not only a practical solution, it works very well!

Click on the picture to enlarge.

The Velleman K4700 kit and the Assembled version are available in the Webshop.

Stefaan

Quad 306 - 606 Monoblock & Virtual Earth

2009-12-03T16:30:00.007+01:00

Corected version 20-02-2010

A Quad 303 or 405 power-amplifier can be easily converted to a parallel monoblock that will deliver more power to speakers with a lower impedance, will be more stable at higher volumes and can handle difficult loads like ESL speakers in a better way.

All you need are a trimmer potentiometer to divide the signal between both channels and (for the 405) some 0,3R 5W resistors to connect the outputs together. The Quad monoblock instructions are available in the Download-section.

In the 606, 707, 909 it is more complicated.

These amplifiers have a non-conventional power supply with a “virtual mass”. The advantage is that no DC power can go to the output so the clamp-circuit is no longer needed and that you will only need one fuse to protect the power-supply.

Both channels in are completely separated, the power-supply as well as the earth. Normally this technique is only used in professional audio. In the 606 the masses of both speaker-outputs (the black binding-posts) are connected… via the preamplifier, we may consider this a serious design-error!

Fortunately the virtual mass circuits in the separated power-supplies in both channels will keep the power-supply voltage at the correct level.

But when converting to monoblock we don’t want separate voltage regulation in both channels.

Another issue is that both safety-circuits (R32 and C12) are now in parallel between the signal-earth and the chassis, resulting in an impedance that is too low. When converting a 606 to monoblock those components will have to be removed in one of the channels. In the 306 they are only mounted once so they don't have to be removed.

Because of the separate power supplies both channels in the monoblock will not always react the same way. We will have to remove one of the regulation circuits and connect both power-supplies together. Also we have to remove one safety coupling. Look at the specific diagram's.

The 306 has a slightly differant layout, the mass of the two channels is allready conected together and there is only one safety coupling to mass.
To modify the amplifier to a monoblock , this is what we have to do:

Connect C10 (positive tag) in both channels in parallel by soldering an isolated copper wire between them on the copper-side of the Printed Circuit-board.
Do the same with C11 (negative tag).
In one channel remove the regulator circuit by removing R28, R29, R30, R31, T11 and T12.

Now the amplifier has one power-supply (with twice the power), and one regulator-circuit. We can now replace the RCA or Din input(s) by one new input, add a trimmer to divide the signal between both channels like in the 303 or the 405 monoblocks (tip: Glue the multiturn trimmer to the inside of the back-panel beside the RCA input) and the resistors between the outputs of both channels and the red binding-post. If you want to retain the input connector (Din or RCA) which is directly connected to the PCB, than connect the output wires of the pot to R1 in both channels and solder the input of the pot to the connector.

Stefaan & Joost

The secret project

2009-11-27T14:33:00.007+01:00

Some pictures of my own project, a stereo Bi- Amping amplifier based on the DaDa JPE modules. You see four 405 modules, two double power supplies and four delay units. The delay units are stacked in pairs. The transformer is a custom build Amplimo. The units run on 60V supplies, the aim was to make the amplifiers less sensitive to load changes, not increasing the output power. The units are still 100W at 8 Ohm at nominal input. There is only one switch at the back and no indicators or whatsoever on the front. There are some led’s on the inside for control purposes. We are not planning to make these amps on a commercial basis. But............

Joost Plugge

Changing the Online - Shop

2009-09-30T00:00:00.000+02:00

Dada Electronics has replaced the Ebay-shop by their own Webshop.

The Quad Dada Electronics Upgrade-kits and High-end boards will still be offered in the small Ebay-shop.

The fact Ebay increased the pricing for Shop-publishing without prior announcement and does not seem to negociate with their professional customers is not the only reason.

On www.dadaelectronics.eu we will offer hundreds of technical articles and papers, free downloads of schematics, user-manuals and service-manuals and the Webshop is open!

In the coming days we will add hundreds of products to the Webshop. You can already take a look and inscribe for the newsletter.

Stefaan

Update 18/9: We added all Dada Electronics Quad Upgrade Kits and a lot of Velleman Kits and Connectors. The Webshop is fully operational now.

We have made it very simple and straightforward for the Electronician or the Audio-lover to find his way. We added 300 products up to now. 1700 to go ;-)

Update 28/9: We started the Forum where you can ask all Quad & other High-end technical questions. New technical papers and hundreds of new products have been added.

Thierry Martin's Vintage collection

2009-09-26T16:15:00.003+02:00

One of the nicest collections of Vintage Hifi in mint condition I have ever seen belongs to Thierry Martin in France.

This is a must see!

Click here for Thierry's website.

Impedance of Quad ESL's and Monoblocks

2009-06-26T09:30:00.009+02:00

The original ESL was a notorious hard to drive loudspeaker. See the graph from Sheldon Stokes. A low impedance at around 10 Khz and of highly capacitive nature. The ESL 63 has a more friendly impedance curve, but the same behaviour at around 10 Khz. The Martin Logan approach, hybrid systems, will not help in this area. The problems are in the HF section, the LF section of a full range electrostatic is easy to drive. On the Sheldon Stokes site you can find a diagram of a dummy load electrostatic, very useful to test an amp without the risk of damaging your fragile ESL.

To improve the current output, and prevent high distorsion due to current clipping, the two channels of an amplifier can be connected in parallel (offcourse with some extra measures). This is useful when you use a 405 (2) to drive an ESL63. The improvement is also there when you use a 303 and ESL57 combination. In both cases the channel separation will be on CD level, which is not so if the individual amplifiers are used as a two channel combination. The 303 and 405 where built in a era when 25dB channel separation from your high end pick-up cartridge was top!

Joost Plugge

Dada Electronics & Elektor

2009-05-27T16:12:00.004+02:00

Elektor is the leading international editor for the professional electronics engineer and the hobby-electronician.

Dada Electronics is now an official reseller of all Elektor books, CD's, projects and kits.

4 pole capacitors

2009-04-16T17:27:00.007+02:00

The idea came from a request from a customer to mount Jensen 4 pole capacitors on our 405 dual mono power-supply board. This was not possible on Dada's standard board. But I was triggered by the idea. Is this a way to make extra money by selling snake oil? No!

BHC and Jensen manufacture this kind of capacitors. What is the theory behind the four pole capacitor? An electrolyt capacitor can be modelled as a resistance, an inductance and a pure capacitor in series. In a 4 pole capacitor these resistors and inductance are used as a HF filter. This is done by placing the input and output wires at separated places along the metal foil. In real life you can’t separate the individual components, but the filter effect is there. On the Jensen website the history of the 4 pole capacitor is explained, as in most cases, the 4 pole is an old idea.

In normal setups the capacitor in a power supply delivers a parallel supply current to the load. That’s why most people consider the Psu capacitor “out of scope” considering the signal path. In my view every component which has an influence on the signal path is a part of the signal path! In the case of the four pole capacitor, the foil is in series with the load. So the filtering capabilities also increase the internal impedance of the power supply. There is no difference in behaviour when you put an extra filter in the Psu, the plus is that you use the natural L and R part of the capacitor. The load current flows through the capacitor, so it will get warm at maximum power output of the amplifier! In a normal Psu setup the internal L and R are responsible for the increasing impedance of a capacitor at high frequencies, now they are used to filter the HF components!

The downside:

Only two manufacturers: BHC distrubited by DNM and Jensen.
Different pin layout, Jensen even has two different layouts!
A bit more expensive, compared to normal capacitors.
Complex setup especially with dual mono power supplies.

The plus side:

High quality capacitor's
The HF filter works!

We developed a new PCB for these capacitors; the layout is in accordance with the BHC T pin configuration.

Joost Plugge

Quad 606 Transformer Swap

2009-03-14T11:27:00.017+01:00

François' (Besancon, France) Quad 606-1 started making a rumbling sound....

Some QUAD 606 MK1's start to make a rumbling sound after some years - from the amplifier itself, not through the speakers.

This means that the transformer is worn out and needs replacement (we know the same problem from the Quad 405 where we can replace the old transformer with a Toroïdal version).

The problem with the 606-1 is that a 400VA 2x 80V Toroïdal transformer won't fit as the diameter is much too large. Quad uses a Toroidal transformer in the 606 MK2, the 707 and the 909 but they developed a special PCB for it that wont fit the 606-1 chassis.

The only alternative seemed to order a "New Old Stock" "classic" replacement transformer for the 606-1 but this appeared to be very expensive and we don't like the quality of this transformer neither.

So we thought about using 2x 250VA 2x 40V Toroïdal transformers. This offers the advantages of extra power headroom (500 instead of 400VA), (even) better channel-separation as we use two different transformers and the possibility to use larger PSU capacitors.

We decided to use 2 Noratel 250VA 2x 40 Volt transformers. They are 50mm high and 120mm diameter.

But how to build them into the Chassis? Chris Devrieze has been puzzling with the question during several days and came up with a very good solution.

We remove a part of the old transformer mounting-frame on the bottom of the (upside-down) chassis so the new transformers will fit in between

As we can't mount the capacitors and the diode's on top of the transformers anymore - there is no space - we glue them with 2-component Epoxy on both sides of the transformers

Both 115V primary windings are soldered in series (for Europe) or in parallel (for the USA & Canada) and the secondary windings are soldered in series to obtain 80V

We drill a hole in the top cover-plate and we use a M6 x 120mm screw to fix both transformers to it after connecting all the wires. We put 4 rubber feet on the bottom to avoid all movement of the transformers once they are fixed

We mount the cover-plate with both transformers to the chassis with M5 x 40mm screws in the four corners

Of course we also did a full revision of the amplifier-boards to bring the 606 to a Quad 909 level and we replaced the connectors.

Click on the pictures to enlarge.

Dada Electronics will offer the 606-1 transformer swap as an option to the 606 revision service and will sell the Noratel 250VA transformers in the Ebay-shop.

Any questions? Info@dadaelectronics.com

Stefaan, Chris

Hum

2009-03-04T18:40:00.015+01:00

Most of the hum problems are related to “earth” loops in external cabling systems.
Input "hum" can arise from small ground potential differences between signal inputs and outputs.
The grounds between amplifiers are established by the AC outlets, and through signal cables. Various factors can cause small ground potential differences between the sound input (power amp) and output (pre-amplifier).
A one-volt difference, while seemingly insignificant, can cause large loop currents to flow through the ground conductor of the signal cable. Depending on the circuit configuration of the audio hardware, the audio signal impedance and the impedance of the signal cable ground, the currents may cause induced voltages in the audio circuitry.

Source: Channel D

This is a source of the familiar 'hum' problem.
In the “old” days, like the Quad 33 and 303 combination, the 303 has no separated ground connection. Grounding was done trough the signal cable between the 33 and 303.
The advantage of such a “star” system: no hum loops. With modern safety regulations such a star system is highly illegal in most countries, and more important, not safe.
In the multi channel (Stereo) world the problems are even more pregnant. Also in most cases the aerial connection of the tuner is also connected to “ground”. In professional installations signal transport is isolated from ground connections (balanced connections); in normal audio systems engineers mix signal ground and chassis ground.

How to avoid hum loops?

Use only one wall socket for the mains connection, if possible. Use the preamp and the associated cabling in the “star” configuration. Separate the safety ground and the signal ground in the power amp, so the chassis is connected to the safety ground of the wall socket, but not connected to the signal ground of the preamp. The same recipe for other equipment connected to the preamp (not always possible!).
General advice; use cables as short as possible. Use shielded cables for the power cables. In some cases, a so called “isolator” is unavoidable; first test one in the aerial connection of the tuner. The tuner signal is FM modulated, so an isolator does less (no) harm to the signal quality. In RCA cables a small signal transformer play this role. Don’t spend a lot of money on special cables and end up with a cheap isolator in between.
In some cases increasing the resistance of the shielding of the signal cable has a positive effect, apply this only if the amplifiers have a proper installed safety earth connection!

How to detect and solve hum loops?

In general; a hum loop generates hum independent of the volume settings. If the hum is dependent on the volume setting; the hum is equipment borne in most cases. But the hum could also be a combination of both situations! The elimination of a hum loop is a painstaking process. Trial and error and applying logical thinking is the best solution.
Start with only the power amp and the loudspeakers, no signal cables connected. Then rebuild the configuration item for item. A cable is also an item! Once the source of the hum is known, a measure can be determined.

Measures

In a Quad 405 and 303 the chassis ground can be separated from the signal ground. In both amplifiers they are bonded together at the DIN input chassis part. In a 405 with factory fitted RCA inputs the two grounds are already separated. The same applies for a 606 and the higher numbers. In Quad tuners it is difficult to perform this trick, but in most cases the hum loop is caused by the aerial connection. A coaxial cable isolator will fix this problem. For testing and analysis it is acceptable to use a non earthed power extension cable for detecting the source of the hum, but always restore the safety earth connection to the chassis afterwards for your own safety! So you should *either* have 3-core mains leads from every piece of equipment *and* signal cables with the shielding disconnected at *one* end, if separation from the signal ground and chassis ground is not possible, or else a single mains connection and everything earthed via the signal leads, as in the case of a 33. Read also the comments before you change anything to your equipment!

A Jensen coax cable isolator

A simple DIY HF isolator!

Joost Plugge

New release of the 34 - 306 - 606 Upgrade manuals

2009-02-23T14:07:00.006+01:00

After several hundreds of upgrade-projects by our customers who used the Dada kits we released the next step, based on the feedback and the questions we got from you.

The new manuals have more pictures and schematics so it is even easier to upgrade your Quad yourself.

You can download de manuals for free from www.dadaelectronics.eu/downloads.

Stefaan & Joost

Dada Loudspeaker Protection & Delay pre-announcement

2009-01-01T17:16:00.000+01:00

The Loudspeaker protection / Delay is available in the Dada Electronics Webshop here.

Dada Electronics and JP Engineering are proud to pre-announce:

The Dada High End Loudspeaker DC protection with delayed switch-on, fast switch-off. (this is the first pre-production version, after the first ten units, real production starts. Size 24 x 89 mm. Product specifications might be changing. )

No switch-on/off noises like “Plop” or “Pmp” anymore.

Protects your valuable speakers against DC voltage from the output of your amplifier when something is wrong. Outperforms the original 405 'Clamp' circuit. The clamp board or components on the modules can be removed. A led will be used to indcate the activation of one of the protection circuits.

Designed for Quad transistor amplifiers, but can be used with almost any other transistor amplifier up to 500W RMS.

Fixed delay: 4 a 5 seconds. Other values on request. The switch on delay in a Quad 34 or 44 is not needed anymore.

AC detection: Immediate switch-off when mains power is removed from the amplifier. The AC detection is done via an optic coupler so no ground loops or other related problems.

Runs on the DC power supply of the amplifier 35 - 90V DC. Can be used with the Quad 303 as well, as with most other power-amplifiers. No separate mains power needed.
Less mains-power leads inside the amplifier means less induced hum.

Audio Grade loudspeaker relay (the only one we know of) - very low contact resistance, double contact, one heavy duty and one gold plated silver contact, the gold contact closes after the heavy duty contact and opens before the heavy duty contact.

DC sensing and protection must be used when you have an amplifier with symmetric power-supply. It can be switched off by removing a jumper, when you have an amplifier with output-transformers and a single power supply or output-capacitors. With this option the relay contact can switch the loudspeaker without any galvanic connection with the rest of the amplifier.

Mono design. The unit can be used with amplifiers which are built as four pole devices, like the Quad 520 and 606 range. Or when a 405 is fitted with our double PSU in full floating mode. For use in a Stereo amplifier you should use 2 modules.

Placement of the unit in a 405, it will fit beside the modules. Firmly connected to the bottom cover with self adhesive feet.

High-end Audio-grade Opamps available again...

2008-12-25T12:18:00.006+01:00

Dada Electronics has the hard-to-find High-end Audio-grade Opamps in stock again.

Click on the links for pricing - quantity - datasheet from our Webshop. We ship within 24 hours.

These opamps are the ideal choice for Quad (pre-) amplifiers from the 34 on. Because of the power-supply rails you would rather use the OPA134 in a Quad 134 and the OPA604 in a 44.

Stefaan

Burr Brown OPA134 (for Quad 34, 44 ao.)

Burr Brown OPA2134 (dual opamp for Quad 34, 44 ao.)

Burr Brown OPA604 (for Quad 44, 405 ao.)

Burr Brown OPA2604 (dual opamp for Quad 44 ao.)

Dada Electronics is specialised in Vintage & High-end audio.

Quad Power-supply design generations: pro & contra.

2008-12-07T18:02:00.013+01:00

Second edited version 3 june 2010, thanks Peter!

Some thoughts about the design of the power supply in the 606 till 909 series.

Quad introduced a new power-supply in the 606 with an active earth circuit.

This basically means they removed the central earth wire from the transformer. As both power supply voltages now only depend on the value of the capacitors, an additional transistor-circuit was added to adjust the voltage across the two power supply capacitors.

If you used ‘matched’ capacitors, the extra circuit would not be necessary.

Looking at the simplified circuit diagram you will notice that it is impossible to have any DC current trough the loudspeaker. Quad introduced an elegant DC protection. The DC protection itself has nothing to do with the active earth, and without the transistor circuits, still there will be no DC current.

If one of the dumpers would fail the voltage over the capacitor will be doubled, hopefully the fuse will be blown before the capacitor does, as it is only rated 63V. Anyway, this is a far better solution than the Clamp circuit in the 405. From the DC point of view…

But is there a “Free lunch”?

If you compare the return current from the loudspeaker in a conventional setup, like the 405 (see the diagram), and the 606 supply, it looks like there is an extra power supply capacitor, a diode and half the secondary transformer winding in series with the loudspeaker in the case of the 606. This is worse than the output circuit of a 303!
Remember, the capacitors are of large capacity and the diode and the transformer winding are “behind” the capacitor. In the real world the loudspeaker current is delivered by the capacitors, they act as power supplies. Even in the situation were the centre connection of the transformer is used the current is supplied by the capacitors. There is only a small current through this wire, this is due to the tolerance in capacitors value. If the capacitors were matched, there is zero current in that wire. Check this with two power supplies in series and two resistors, the principle is the same. So in both setups the speaker current goes ‘’ through’’ the capacitors. In both setups the quality of the capacitors and the value are important. In the case of the Quad virtual earth the stability of the centre voltage is determent by the value (capacity) of the capacitors and the “speed” of the regulating transistor circuits.

Why this post?

In this paper I described two situations, the conventional earth wire and the 606 situation where the wire has an infinite resistance. Based on a idea of Keith Snook, what will happen when the wire is replaced by a resistor? If the value is low enough, the voltage on the power supply capacitors will stabilise, without the need of the extra transistor circuits. If the value is high enough the DC current will be limited.

The idea was to build a 405 PSU with the resistor and omit the DC protection. Until now I did not found a value that will match both the former criteria. The main reason for this is the fact that there is only very little time (5mS for the 50Hz situation, 60Hz is even worse in this aspect) for the capacitors to charge, the value of the resistor must be very low, but a low value wont protect your valuable speakers.

So until someone comes with a better idea, our own DC protection or the Quad implementations will still be needed.

Joost Plugge

DaDa Electronics

405 in the air

2008-09-25T19:05:00.005+02:00

Justin Wood (UK) used the Dada 44 and 405 kits to upgrade his Quads. He wrote:

Thought you might like the photo below...

Perhaps the first time a Dada Electronics Quad 405-2 was given its own upgrade to 'Club Class'!?

As you know I did the upgrade work while flying S61 helicopters up in the Shetland Islands. At the end of the contract I flew the helicopter with the 405-2, back to our home base in Penzance, Cornwall, UK.

The 405 loved the trip!

Justin

Today Justin added:

Just back to Cornwall from Stornoway in west Scotland.

Plugged in the 44 ....and I'm very very pleased to say it works like a dream; perfect in every way. There is certainly a most worthwhile improvement in the sound -- much greater detail, transparency and channel separation. I am looking forward to plenty of great music as the winter nights draw in.

I am absolutely delighted with the 'Dada improved' 44/405-II combination as it sounds fantastic. I hope lots of other Quad 44 owners will take the trouble to do the Dada up-grade and re-discover how good this pre-amp can sound. We all know how superb your 405 mods sound!

The photo shows the freshly upgraded 44 ready to ride home to Cornwall on the instrument dash-board! ...actually it had to go under the seat for safety....

How about a challenge for the weirdest places to find a Quad?

Justin Wood [justinnawood@hotmail.com]

Trying out the 1-day Quad revision...

2008-09-25T17:23:00.002+02:00

The eternal problem with a Quad-revision is the transport. A 405 has a weight of around 10 kg, a 606 even more... So it is expensive to send it by mail. By the way, I had bad experiences sending amplifiers by mail ;-(

Pierre (from Calais, France - 250 kilometers from Antwerp) wanted to have his 405 upgraded and Burr Brown chips installed but he did not feel like driving 4 x 250 km.

So we invented the "Visit Antwerp & Upgrade your Quad" roundtrip.

If the Quad is delivered before 11 am it can be picked up around 6 pm IF all eventual problems are described via e-mail and a rendez-vous is made in advance (we have to make sure we have all the components ready).

Antwerp is an excellent city-trip - see http://www.visitantwerp.be/.

Stefaan

OPA134 pin 8 in Quad revisions / upgrades

2008-09-24T15:55:00.002+02:00

When using a Burr Brown OPA134 opamp for revising / upgrading Quad amplifiers like the 34 and the 306 make sure Pin 8 is connected to nothing.

Pin 8 is the offset-input. If it is connected to pin 6 (like in some versions of the 306) this wil cause problems with the DC-feedback and result in a DC Voltage on the output.

You have to cut off pin 8 or cut the track on the PCB-board.

In general, when using the OPA134, OPA604 or OPA627 in a Quad revision project pins 1, 5 and 8 are not used and should be disconnected.

Stefaan

Marcos Di Paolo uses a 303 with Dada's High-end boards

2008-09-20T10:55:00.004+02:00

Marcos is one more professional musician who has chosen Dada's High-end boards in his studio for their excellent transparency and natural sound.

Marcos was born in Argentina and lives in Rotterdam today. He studies the guitar for more than 25 years and is now working as a full-time professional musician in several projects. See his website http://www.marcosdipaolo.nl.

He writes about his new High-end boards:

"And the most important, ...it's really a pleasure listening music with this amp!!, I've never heard a high freq range so clear like I'm listening at this very moment. I'm really happy with the sound of my new Quad, thanks a lot!!!."

Stefaan

Current Limiter in the Quad 405 and other limiters.

2008-09-19T15:42:00.008+02:00

We decided to implement the 405-II current-limiters on request only. We have built the limiters with normal components. If you insist on having the limiters on your boards, ask for a quotation.

Why don’t we implement the limiter in our High-end boards?

It affects the sound quality, not only because of the topology of the limiter itself but also the way Quad implement it. In a 405, but still in the 909, the current trough the sensing resistor modulates the voltage of the positive power rail of the class A stage and the Opamp.

The Opamp is the least of our worries, because it has a high power supply ripple rejection ratio.

The modulation is done by a dirty half wave class C signal from the upper dumper transistor. In the original 1975 days this was not such a problem, noise, crosstalk and other distortions masked this problem. But in search for the highest quality, the limiter has to go. Also the current sensing resistors can now be short circuited.

The right way to implement a limiter with this topology: (This is a 405-2 example, the 405-1 is less complex)
The limiter in the negative part of the output circuit can stay as it is, it is part of the dumper section, the class A stage will compensate for errors via the current dumping principle!

Input limit is the role of the Opamp.

In the first production version of the 405 the power supply voltage of the Opamp was plus and minus 12 V. Also due to the DC stabilisation there was a DC offset at the output of the Opamp of 1.7 V. This means the theoretical input voltage of the Opamp stage is 486mV max, conclusion; with 500mV input no 100W in 8 ohm undistorted output power was possible!

The PSU voltage went up to 15V and the DC offset was lowered to 1.2V, still the headroom is only 2.3dB for undistorted output. So a popular en reasonable update/mod is raising the PSU voltage and selecting the proper Opamp. An OPA604 running on 24V gives a healthy 6dB plus headroom in the OpAmp stage.

The rest? Calculation shows that the maximum theoretical input voltage needed to let the dumper stage clip is only 1,9 dB above the nominal input, so the figure of 2,3dB implemented by Quad looks sufficient. So be careful with the input level, there is only a small overload margin.

Output limit.

The output can be limited to 20V by inserting a link in the OpAmp circuit; this was done to protect the ESL57 from overloading. We do not advice this option, this is not a soft clipping device, and feeding 20V square waves in to a fragile ESL57 is not a good thing to do. Buy a refurbished 303 or Quad II to drive the ESL57. Clipping of a 303 and an II is still possible, but only with high input levels.

DC detection and protection.

A low pass filter is placed in parallel with the output, when the DC voltage is about 6 volt, the Diac opens and the Triac will short circuit the output. Due to the characteristic of the Diac the output stays in short circuit mode, hence the name ”Clamp circuit”. It was the intention that the fuses would be blown, in practice also the output transistors will be thermically killed! But there was a problem anyway with the circuit. Low voltage Diac’s are hard to get in these days.
Dada Electronics' own DC protection circuit is relay based and has a switch on delay function built in to it. See Ebay-object 200243187533.

Joost

Dada's 405 High-end boards

2008-09-17T14:09:00.007+02:00

We had our new High-end boards photographed by Photology.net.

Click on the pictures to enlarge.

Stefaan

Dada Electronics wants to buy your old Quad boards.

2008-08-27T16:41:00.003+02:00

When you buy one of the new Dada 405 High-end boards with OPA604 or with OPA627-chips we will buy your old 405 PCB-board, even if it's broken.

We offer 30 Euro for a 405-MKI board and 40 Euro for a 405-MKII board.

When you buy a Quad 303 High-end Driver-board or Power-supply board or the 303 High-end board & capacitor kit we'll offer 20 Euro for the old driver-board and 10 Euro for the old power-supply board.

This way you can easily upgrade your Quad yourself by simply swapping the boards.

Stefaan

Elias from Cyprus upgraded his 303 with the Dada kit.

2008-08-22T17:20:00.004+02:00

Elias writes:

"Some time ago I realised that I had to, at least replace, all the electrolytic capacitors in my Quad 303 power amplifier. With that in mind I started searching the internet for possible suppliers, when I came across DADA electronics of Belgium. After studying in deptth their site I decided to purchase their Quad 303 DIY Upgrade kit Deluxe. In addition I opted for a second Nichicon 4700μF capacitor for the power supply,so that I could increase the power reserve of the amplifier and also for new power transistors for both the amplifier and the power supply output stages.

The parts were received in no time after placing my order and straight away I got down to work. I came across some difficulties but these were very quickly resolved by emailing DADA electronics. I must here thank these very courteous people for their so quick service.

After completing the work I calibrated the voltage and quiescent currents and connected the QUAD 303 to my hi-fi system. And what a surprise! It now sounds a much better amplifier than before. I can imagine how much better it would have been had I replaced the amplifier and power supply boards with their high end ones. One day I will do it. But before that I will upgrade my Quad 405, hopefully with DADA's 405 high end boards, BHC Aerovox dual mono power supply unit and loudspeaker protection units."

Stefaan

Quad 44 upgrade kit(s)

2008-08-15T15:08:00.006+02:00

Dada announces the Quad 44 upgrade-kits.

There are 2 versions:

Up to serial number 23000

Above serial number 23000

If you have a special configuration of input-modules or if you want OPA627 output-chips send an e-mail to info@dadaelectronics.eu.

Dada now offers a full range of upgrade-kits, High-end boards and revision services for all transistor Quad amplifiers from the 33 to the 606.

If you have a Quad 44 and you want to have an idea about the upgrade / revision, send an e-mail and we'll reply with the Quad 44 Upgrade-manual.

You can download the upgrade/revision manual from our website for free, see www.dadaelectronics.eu/downloads. The kit is available in the Webshop on our site.

Stefaan

Dada announces the 405 High-End boards.

2008-07-30T16:43:00.011+02:00

After a period of testing, improving, listening and testing again we can proudly announce the 405 High-End boards today.

They were designed by Dada Electronics and JP Engineering and we are convinced we will set new standards in High-End audio.

We applied the same philosophy as last year when we launched the 303 High-End boards. Except for some practical or obvious improvements we started with the latest original Quad 405-MK-2 schematic (M12333.10), but with today's very best Audio-grade components only.

We used:

Burr Brown OPA627 opamps, the highest standard in precision operational amplifiers (an alternative board with OPA604 is available).
Silvered Mica capacitors for HF-handling
Wima MKS4 for decoupling
Nichicon Low ESR electrolyts
1% Metalfilm low-noise resistors
Epcos inductors
MJ15003 Power-transistors and BD242C drivers

In this version we left out the current-limiters, you won't need them if you use the Dada LS Protection/Delay and they would have a negative impact on the dynamics of the sound.

You can now easliy upgrade every Quad 405 yourself by replacing the 2 amplifier boards, the power-supply and the LS protection circuit. As the concept is modular you can use one of the 3 options separately as well.

The new High-End boards are available in our Ebay-shop in a Burr Brown OPA604 and a Burr Brown OPA627 version.

Stefaan & Joost

Willem Tetteroo from NL was our first customer. Here is what he tells about the board (original NL text + my translation):

"Beste Stefaan en Joost,

Ik ben laaiend enthousiast! Super!

Na een heel weekend intensief geluisterd te hebben kan ik niets anders zeggen!

Sprankelend geluid tot in detail. Mooi strak en diep!

In vergelijking met het origineel vind ik dit een enorme verbetering, en ik was al gek van het origineel. Heb veelvuldig vergeleken en vind de Dada echt de fraaiste!"

"Dear Stefaan & Joost,

I am extremely enthousiastic! Super!

After a whole weekend of intensive listening I can't say anything else!

Sprankling sound upto every detail. Nicely neat and deep!

In comparison to the original I find this an enourmous improvement and I was already crazy about the original (450-2). I compared many times and I find Dada really the nicest one."

All Quad replacement Capacitors available now!

2008-07-14T14:53:00.009+02:00

All Replacement capacitors for the Quad amplifiers between the Quad 33 and the 909 are available in the Dada Electronics Webshop now.

From the 3p3 you need in the Quad 303 driver to the BHC Aerovox 15.000 µF 63 Volt for the Quad 606 MK-II.

But that's not all.

We have tested several alternatives with our lab equipment and most of all by listening to them. You will only find the best choice from our selection in the Ebay-shop.

We are convinced "cheap" doesn't do the job in high-end equipment like Quad. So you will only find the best alternatives like Wima, Nichicon, BHC Aerovox, Silvered Mica caps, Vishay and other top-quality alternatives.

We give 7/7 e-mail support and a 1 year full warrantee (excluding shipment costs).

Our central stock is in Antwerp (Europe) but in most cases we can ship from Dada Electronics Australia or USA, with reduced shipment costs.

We can deliver worldwide and we ship the same day if the order is placed before noon on working days.

Stefaan

Mike Cofferon from Dublin used the 34 & 405 kits

2008-07-14T12:41:00.002+02:00

Mike did a revision of his 34 and his 405 with the Dada Upgrade kits.

Here is what he says about it:

"I have completed both upgrades now and spent the weekend listening to the results. I sounds fantastic!

Can't recommend this upgrade strongly enough. Obviously I'm comparing the old tired, dried out sound of the Quads before the upgrade, but even so…

Stereo Image, Timing and Timbre are all spot on, but the increase in information still doesn't overload the listener, it's still a supremely MUSICAL amp combo. One unexpected bonus is the hightened effect of the "Tilt" control. I never got the change in soundstage that Quad described in their user manual before, but now it's clearly evident. I've been able to tune the frequency curve of the amp to compensate for my less than ideal listening room. All this for less than the price of a few metres of speaker cable you guys should really be charging more for this upgrade :)"

Stefaan

Quad 405 replacement transistors

2008-05-14T14:46:00.007+02:00

This is the complete list. The original Quad transistor is in Italics our preferred alternative(s) are in Bold.

Click on the hyperlink to see the Datasheet or to buy the transistor in the Dada Electronics Webshop.

TR1: BC214C - BC559C BC560C BC415C
TR2: ZTX304 - BCX32 BC682 BC546B MPSA06 2N5551
TR3/TR4: ZTX504 - BC556B
TR5/TR6: BC214C - BC560C BC415C BC559C
TR7/TR8: 40872 - 2SA740 BD242C 2N6134 TIP42C BDX78
TR9/TR10: 17556 - 2SD424 2SD676 2N3773 MJ15003
The Bridge-diode can be replaced by KBPC3504

All other Quad 405 replacement components are available in the Dada Electronics Webshop as well, or send an e-mail to info@dadaelectronics.eu.

Quad 303 replacement transistors

2008-05-14T14:22:00.015+02:00

This is the complete list. The original Quad transistor is in Italics our preferred alternative is in Bold.

Click on the hyperlink to view the datasheet or to buy the transistor in the Dada Electronics Webshop.

TR1: 38494 - 2N3055 MJ15003
TR2: 38494 - 2N3055 MJ15003 (or Push-pull alternative* MJ15004)
TR3: 40411 - MJ802
TR100: BC154 - BC560 BC214C
TR101/TR107: BC109 - BC109 BC237C BC184K
TR102/TR103: U17219 - BC546B ZTX304
TR104: U17229 - BC556B ZTX504
TR105: 38496 - BC461 2N5322
TR106: 38495 - BC441 2N5320
TR200: U17229 - BC556
TR210: 38495 - BC441
MR 100/101/105/106 can be replaced by 1N4148

* The 303 can be modified to a PNP/NPN Push-pull amplifier using the Dada Electronics or standard boards.

All other Quad 303 replacement parts are available from the Dada Electronics Webshop or send an e-mail to info@dadaelectronics.eu.

Eduardo Niebla chooses Dada's 405 revision

2008-05-10T16:04:00.006+02:00

We revised Eduardo's 405.

He uses it in his studio to monitor his recordings.

We used the Dada dual-mono power supply and Burr Brown OPA627 opamps.

Edouardo wrote:

"Received my Quad 405 with your upgrade plus the Velleman K4700 loudspeaker protection.

The packing was very secure, well done!

The sound now is much more open and extended; bass to trebles through the whole spectrum are very clear and rich. A very wonderful sound. I'm very very happy . I now can hear very clear my mixing and recording work in much more detail.

Thank you very much for all you wonderful and professional work - I have no hesitation in highly recommending your upgrade services and give you 10/10.

Best wishes to you all (I'm definitely a happy customer)."

Click on the picture to visit Eduardo's website.

Stefaan

Quad 50D & 50E Connectors available now

2008-05-10T14:08:00.004+02:00

We found the original manufacturer of the 50E and 50D connectors.

The company was willing to make a small batch for us from NOS (new old stock) parts.

So these are new connectors.

The input connector for the 50E and D are the same, the output connectors are different.

Now you can restore these magnificent amplifiers to their original glory.

Joost

Installation instructions for the 405 Dual Mono Psu

2008-04-09T13:26:00.010+02:00

This post explains the installation of the Dada / JPE dual mono power supply for the Quad 405 and 405-2, all models. The PSU and the tools are available in the Ebay Webshop.

Joost & Stefaan

Second introduction and important extra information

This dual mono power supply is a fully functional replacement of the original power supply, but it has a lower crosstalk figure as a result and the elco capacity is being doubled, giving a more stable power supply voltage. In a standard 405 the secondary windings of the transformer are combined to form a single supply. In post 405 amplifiers, like the 606 each channel has its own supply. There are two reasons for this new design. First it is necessary to support the more elaborate protection circuitry in the successors of the 405, second, it lowers the mutual interference of the two channels. By building the amplifier as a four pole device, the channel interference is at its minimum, because there is no common ground plane or wiring, it can/will also reduce earth loops in the feeding cabling systems.

Preparation and beware!

Start with a clean bench and the necessary tools. Although the Psu board is not very sensitive for static electricity, discharge yourself by placing your hands on the bare metal of the chassis before you touch the electronics. Or wear a special bracelet connected to the chassis. Read this instruction before you start working on the amplifier. If you power up the PSU without the amplifier modules, the voltage on the capacitors will be there for a long time! Discharge the elco’s with a bleeder resistance with a value of 1K. In the full floating setup the chassis is not connected to the signal earth anymore. For safety reasons the chassis must be connected to the wall socket or the pre-amp via a power chord with a proper earth connection!

Tools

· Philips (Posidriv) no 2 screwdriver
· Soldering iron, 25W, a soldering station with temperature control is the best
· Wire cutters and clippers
· A DC multimeter

Removing parts

For easy access: remove the top, bottom and side plates of the amplifier.
Remove the wiring from the capacitors and the rectifier bridge. Cut the wires where they are entering the cable beam. Remove the capacitors and the clips. Remove the Rectifier bridge. Remove all wires from the secondary windings of the transformer, leave the blank connecting wires in place on each secondary winding. Also remove the wires to the negative (black) speaker terminals.

Mounting the Psu in the chassis

Place the plastic self adhesive feet in the corresponding holes. Clean the area where the feet are to be placed with a solvent. Mount the board after the wires are soldered to the connecting pins, by removing the protective tape. and push the board firmly in position. You can always remove the board from the pins. Before final installation, test the placement procedure with the protective tape still in place.

Basic wiring

In this setup we only implement the power supply as a dual supply. The Din connector and the earth connection at that point will stay in place. For Quad 405-2’s with factory fitted cinch connectors: read the following paragraph. If you use additional cinch connectors, connect the signal ground to the earth point of the Din chassis part. Also reuse the earth connecting point at the location of the old rectifier bridge. Connect the wires as shown in the diagram. Start with the wires on the Psu, place the Psu in the chassis and connect the other wires. Also connect the two wires leading to the led on the two led pins on the Psu. Because there is already a series resistor on the Psu for the led, short circuit the resistor on the small Pcb underneath the amplifier. Or replace the resistor on the PSU by a wire link, and leave the resistor on the small Pcb underneath the amplifier in its original state. Both PSU’s have there own led connections, so if you wish you can connect a second led.

Building a floating amplifier, for the advanced DIY user!

To gain full benefit of the double supply, a more complicated route has to be followed. The chassis to earth connection on the Din connector should be removed. Also a new central "earth" point on each amplifier module is necessary. This is created by removing the centre screw on the heat sink, use a special transistor isolation washer (on the heatsink side) and a new bolt and nut, M3x15 (all from a TO220 transistor mounting kit) between the head or the nut (depending of the way the bolt is inserted) and the heat sink. Use a copper or steel washer on the PCB side to ensure a good contact. In this way we can connect wires to this point without making any contact with the heat sink and the chassis. Or solder the wires to the copper side of the module. For best results we only use the isolated cinch connectors and remove the wiring from the Din connector. If you have to use the facilities of the Din connector use the spare pin for a second signal earth connection.

In all Pcb's prior to M12565 issue 3 (serial number 1 to 50000) there is a serious layout problem, the signal (input) ground must be connected to the output ground, in this case our new created "earth" point, or R2 replaced by a wire bridge! See also the "Hum" post and comments.

Quad 405-2 with standard factory fitted cinch connectors. Serial 85000 and higher

Keith Snook mentioned high values of the DC offset on the speaker output terminals with late versions of the 405-2 with the standard isolated Cinch connectors. This problem is the result of the mixed use of signal earth and chassis earth to connect components on the Pcb. Between those two "earth" connections a resistor R2 with a value of 10 Ohm is placed. In the Quads with a Din connector those resistors are effectively short circuited by connecting the signal ground to the chassis. In the Quads with the cinch connectors R2 is not short circuited anymore, in real life, the problem Keith mentioned is smaller when you use a preamp with a common signal ground (as most of us do), the two R2 resistors of both channels are now in parallel, so the value is halved. But the two channels now have shared components, bad news for channel separation! The second action to be taken to minimize the problem: use a power cord to the 405 with an earth connection from the preamp. The voltage difference across R2 should be very low, only depending on the resistance of the wires of the cables. But if you want top performance, every detail counts! Why Quad did change the layout? In an all Quad system (34 or 44 connected to a 405), a hum loop can be introduced, depending on length and routing of the cables, because of the use of cinch cables and an earth connection, via the power cord from the preamp to the chassis of the 405. In the early DIN days the Power Amp was earthed via the signal cable, see also the connection between a 33 and a 303.

How does this relate to our double PSU? Basic or floating?

If you don’t experience any hum loops in your current system, in the basic wiring setup: connect the signal ground of the Cinch chassis parts to the chassis, as was the case with the Din connector. If there is a hum loop, solve this one in the power cord of the 405 by disconecting the earth wire at the 405 side. Is illegal in some countries! See the "Hum" post.

In floating mode, there are two solutions, first connect extra wires from the signal ground connection of the cinch chassis part of each channel to the new central earth point on the corresponding modules, in effect short circuit R2, or replace R2 by a wire link. The latter is the best and more elegant solution in my view. These solutions can also be aplied to the basic wiring setup, but it is not so basic anymore.

If a connection between the amplifier ground and the chassis is necessary, build it like Quad did in the 606. Use a separate 10 Ohm resistor with a small capacitor (4n7) in parallel to connect the Psu ground to the chassis.

Checks before and after switching on the amplifier

Connect the amplifier to the mains socket. On the red wire there should be 50V referring to the chassis or the module earth point. On the black wire there should be -50V. If you installed the Psu as a fully floating device, before switching on, check whit an ohm meter that there is no connection between one of the green wires and the chassis. Also check that there is no connection between the green wires of the two channels.

Appendix 1 Basic wiring
Appendix 2 Fully isolated wiring

Quad FM1 - FM2 revision notes

2008-04-02T01:35:00.006+02:00

Despite their external appearances being so different, the Quad FM1 and FM2 tuners are internally almost identical.

They are both very well-regarded FM tuners and are still capable of delivering a quality FM sound when in good working order. The guidelines for renovating these are basically the same as for any piece of valve equipment.

The notes below also include the results of a lot of experience with these two tuners in particular.

But first a WARNING: valve equipment contains lethal exposed voltages: in the case of the Quad valve tuners, there are a couple of hundred volts DC.

Don't attempt any internal measurements or repairs unless you are aware of all the appropriate safety precautions. Dada Electronics accepts no responsibility.

1. Replace all the resistors attached to the HT line with 1 watt carbon.
These will typically have drifted higher in value, and the larger the value the larger the drift.

2. Replace all the red Hunts capacitors in the (larger) IF/audio section.
These will typically have increased in value by 200% or more, or decreased in value by 50% or more. Replace with ceramic.

3. Replace all the electrolytics that are easy to replace. Replace the main PSU capacitor if you get funny noises, or low or unstable HT voltage, or it shows up badly on an ESR meter: otherwise leave it alone.

4. Don't disturb anything inside the (smaller) RF section if you can help it, regardless of the above. Otherwise you are up for an RF realignment at least.

5. Check the anode and cathode voltages of each valve against the voltages shown on the circuit diagram. If the anode voltage and cathode voltages are both out, replace the valve. If just the anode voltage is low, check the grid voltage of the next stage: if it is high, you have a leaky coupling capacitor between the two, so replace it.

6. Always use the same values, or as close as possible, when replacing components, except that you can double the values of the electrolytics apart from the main PSU electro. Always observe the correct voltage ratings for capacitors.

7. Don't attempt any kind of alignment unless: you really know what you're doing; you have the necessary equipment, i.e. an oscilloscope and an RF sweep generator; and you're aware of, and prepared for, the consequences of damaging a tuning slug, which are fatal for the tuner. The alignment instructions in the owner's manual are barely adequate even as a last resort, and won't give you minimum distortion, or decent stereo.

Dada Electronics Australia specializes in valve and solid-state tuner
alignment: mailto:dada.australia@bigpond.com. We also refurbish and align the sterep decoders that go with the FM1 and FM2.

Esmond Pitt
Dada Electronics Australia
Copyright (c) Esmond Pitt, 2008. All rights reserved.

Dada Electronics lab "Speakers"

2008-03-19T16:01:00.005+01:00

We wanted to show you the "speakers" we test the amplifiers with at full power for several hours.

Instead of noise they produce heath, 2x 400 W RMS.

Joost developed them, inspired by modern architecture...

Stefaan

About the Gazember 2x 160W 303

2008-02-18T22:40:00.010+01:00

The Quad 303 160W RMS per channel. Part two.

Last Thursday, on Valentine’s Day, I was in Antwerp, as Stefaan sent me an interesting e-mail (nothing to do with Valentine). A customer sent us a Quad 303/33/FM3 set for a check up, all bought from the man whose name should not be mentioned on this blog, (only this time).

Yes, the "Gazember" 160 W 303 was on our workbench. The pictures on Ebay reveal a 303 in a sorry state, in real life as well I can say now. The output capacitor banks are "glued" to the chassis, with wood glue, so they where rattling inside the cover of the 303 after transport.

The extra capacitors on top and beside the original power supply capacitors where held together with an elastic band.

The original trimmer pots where still in place after 40 years.

The solder work is below any standards.

So far for first impressions.

Technical review:

The regulated power supply has been removed, including the 40411 transistor.

The emitter resistors are halved in value by soldering a second pair over the existing ones.

The standard number of current limiter diodes is unchanged.

All the output transistors are replaced with MJ15003, which in itself is a good idea.

There are two rectifier bridges screwed to the chassis, with more extra diodes soldered on top of it.

All the wirering and the soldering looks unprofessional.

Measurements:

Instead of measuring with constant input voltage whe measured the maximum output before clipping, the way it should be done according to IEEE definitions.

If you let the amp clip until the level of square wave output, "almost" any RMS voltage is possible.

A 1 KHz test signal was applied.

The results:

16 Ohm 24W RMS both channels
8 Ohm 42W RMS both channels
4 Ohm 62W RMS both channels

Technical analysis.

By using the wrong (in our opinion) secondary windings from the transformer, combined with the voltage selector on 240V, you will get a power supply voltage of 70 V DC without load.

With two channels at full power, the voltage drops to 62 Volts DC.

This explains the lower performance with an 8 Ohm load compared to a standard Quad 303.

As a result of halving the emitter resistances the theoretical current limit doubled. This explains the result at 4 ohm load.

There may be 3 possible reasons why "Gazember" made the mistake to state the 303 had 2x 160 Watt RMS:

He was confused between Ac Peak-to-Peak Voltage and RMS Voltage. Power into 4 Ohm = output voltage under load (both channels) Square / 4.

Output-power is defined as power before "clipping" or before 1% distortion

The power-supply has to be able to deliver the total power. In the case of a Push-Pull like the 303 this is 200% to be on the safe side. A 2x 160W would need a 640W PSU.

Advice.

It could be possible that whe are looking at a "Monday-morning" product. So check those units before you buy them, if you like what you see, buy it.

Until it is clear under which conditions the amplifier should be measured in order to produce the promised results, and it has been checked by an independent lab, I have very strong reservations.

And there is still the issue of the "misinterpreted graph" (see my comments in the first article).

Joost Plugge

The original Joost Plugge text - not for publication - can be obtained via info@dadaelectronics.eu only. You will need Joost's agreement to publish parts of the text.

Stefaan

Gazember - continued

2008-02-17T13:26:00.000+01:00

Andy Williams added the following information:

I am also involved in the upgrading of QUAD amplifiers and during the course of this I have seen a Gazember. The owner of this had owned it for around 12 months and said it had been returned for repair two or three times and had eventually lost faith in the product.

Opening the case revealed the original 30 year old power and output caps had several low value caps soldered in parallel and the original caps had finally ruptured and leaked onto the circuit boards causing the amplifier to fail yet again. The original large capacitors should have been replaced when serviced, to not do so is just penny pinching and is not in the customers interest as the amplifier would be no more reliable than a non serviced 30 year old 303 which is not what the customer would expect after spending money on his Quad.

The old trimpots were still in place as was the original rectifier bridge however the power supply board had been removed and the current limit increased by removing half the turns on the wire wound emitter resistors and putting them back in place. New resistors of the correct value would have been far more professional and actually easier to fit - again penny pinching at the customers expense. The resistors windings are no longer in contact with the ceramic former and would be far more prone to failure than a new resistor.

I looked at his ebay shop recently and notice he is now selling QUAD's valve powered equipment - I would like to express my concerns to prospective customers of the high voltages involved in valve equipment and the required safety measures that should be followed when upgrading or repairing such equipment. My advice would be to ensure that whatever they purchase, if it has been modified or repaired that it has been done so by experienced qualified technically competent people and not someone who's past record shows a distinct lack of electronic knowledge and know how. These amplifiers can be very dangerous if the work is not competently carried out and could pose a serious electric shock hazard to children as well as the owner.

Regards Andy Williams

Quad 306 Upgrade-service & DIY Kit launch

2008-02-16T14:40:00.014+01:00

Dada Electronics launches the Quad 306 Revision-service and the Do-It-Yourself kit. They will be available in the European, USA and Australian Ebay-shop.

The input-filter and the DC Feedback circuit are upgraded to the Quad 909 quality-level.

Low-noise 1% Metalfilm resistors give less noise on the base of the input-transistor. The TL071 opamp in this amplifier is not replaced as it is not in the signal-line and so it has no impact on the sound quality.

All 4 zeners in both channels are decoupled with 100 nF MKT, thus eliminating HF ripple and further improving signal/noise level of the zener's output voltage.

The old 4700 µF Power-supply caps are replaced with new 6800 µF caps with low ESR. The higher capacitance reduces ripple and increases stability of the PSU voltage.

Finally the input-sensitivity is adapted to modern standards. The original value was 0,375 Volt which is much to sensitive for modern preamplifiers or input-sources. As this is done by increasing the local feedback of the input stage, as Quad did also in the later 707 and 909 models, this also improves overall signal to noise ratio.

The kit and the service are available in Dada's Ebay-shop, just click on the links in the article above.

If you want a (free) copy of the upgrade/revision manual just send an e-mail to info@dadaelectronics.com.

Stefaan & Joost

Listening session

2008-02-07T14:24:00.001+01:00

Hans Aussems (NL) upgraded his 2 303's to monoblocks with Dada Electronics' High-end boards. He wrote:

Hi, Stefaan,

Yesterday I visited an old friend after a long time. I know him since we studied Hobo and Fagot at the conservatorium. What we also have in common is our love for listening to music and audio equipment.

I took my 2 Quad 303 monoblocks to listen to them with with his vintage 7-ties Tannoy Dual-concentric speakers which are completely revised by a Tannoy-guru from the UK.

I have to say I was very impressed by those speakers! Despite of my Electrostat-ears this was a very pleasant experience. Many modern loudspeaker-builders could learn a lot from this.

But the best is yet to come: the Quads.

After listening a while via my friend's Rega amplifier we connected the Quads. What a great sound!

The sound is tranquil at all times, without sharp edges or turbulence but with a lot of power and authority. What a dynamic can be realised with those speakers! We couldn't do anything else than listening to our music, stopping was not an option.

The Rega clearly was the lesser one, it did not have the fineness or the natural sound of the Quads.

At the end of the session we also compared with a tube-amplifier but this one had too much unrest, sharpness and distortion.

My friend is completely convinced now of what I knew already: the great capacities of the 303. You can soon expect a new order for a set of 303 Highend-boards.

Hans

A 160W per channel Quad 303?

2008-01-30T11:30:00.001+01:00

On Ebay such a beast is for sale, but what is it?

We get a lot of questions about this offering. Can you do that? Yes we can (maybe), but we won’t. See comments.

Can you also glue new capacitors on old ones, and leave corroded trimmerpot's in to place to maximise the profit? Do you also falsify graphs to proof your point? Yes we can, but we don’t.

The claimed 80W RMS in 8 Ohm is easy done, remove the regulated supply, ignore the fact why Quad did design this in the first place, and run the amp on an unregulated 90V supply voltage.

The second part, 160W RMS in 4 ohm is a lot more difficult, if possible. Our advice: check before you buy it, if the unit really deliver this kind of power. RMS, two channels, not peak-to-peak.

Why don’t we do this kind of work?

We work within the Quad design philosophy. We only upgrade and improve things if they really matter and are necessary because of modern speakers and sources.

If you need a 160W in 4 Ohm, buy a 520 series or a 606, 707 and 909. But remember, you need ten times the power to hear a sound at twice the original volume.

Joost Plugge

Who is behind Dada Electronics & the Blog?

2008-01-18T21:47:00.002+01:00

We are guys in our 4-ties who have been working in informatics. Strange, isn't it? All 3 of us? Gregg joined us, he wasn't in informatics but in photography.

Stefaan Verdonckt (BE)

Joost Plugge (NL)

Esmond Pitt (AU)

Gregg Deking (US)

Quad 405-2 AC voltage considerations

2008-01-12T14:00:00.000+01:00

From serial number 83000 and onwards Quad omitted the AC mains voltage selector on the Quad 405-2.
I recently checked such a late model Quad. It did not perform well. The power delivery was not on spec. On the outside of the unit there was a marking: 220V. But in real life the unit was wired for 240V. The Quad 405-1 had a selector with 6 positions, in the service documentation of the 405-2, only for four voltages is explained how to connect the wire links to the primary windings of the transformer. In the Netherlands the AC voltage is 230V, this is one of the missing voltages in the documentation, and the other is 130V. Because of the design, the DC power supply voltage of the amplifier is proportional to the AC mains voltage. So 130 V or 110 V does matter! To complete the documentation I drawn the missing link positions. You can also use these diagrams to rewire any Quad 405 and omit the voltage selector with those tiny wires. If you can hear a difference? I can’t tell you, judge for yourself. Before you rewire the amp, check the AC voltage during some weeks, check what is really coming out of the mains socket. Click on the diagram to get a fully detailed version, I also added some colour.

Joost Plugge

Gold - New DACT components available from Dada

2008-01-09T18:11:00.000+01:00

Dada now has the DACT CT 2-50K-1, CT 3-5-4, CT 3-5-8 and CT 4-250K-2 in our stock.

Check out Dada's Ebay-shop.

Stefaan

Quad 405 DIY illustrated guidelines - Step 1 to 3

2008-01-01T14:00:00.000+01:00

This is the original 405-revision article published on this blog in 2006. We republish it on general demand.

Stefaan

I thought it was time to make an online step-by-step guide for upgrading a 405.

If you are in the US and you are looking for a DIY upgrade-kit, check out www.quad405.com. The guidelines remain the same but you won't have to pay more for low Dollar-rates and high shipment-costs ;-)

As I recently bought a very nice (but very old) 405-2 (thank you Leigh, I hope you had a good time in Antwerp) I will use this one as a photo-model. I will take pictures of everything I do and explain why I do it. All comments and tips are welcome of course.

The revision/upgrade will be a basic one without fancy or expensive things like the BB627, but I'll mention those options in the PS.

Click on the pictures to enlarge.

So here we go...

Step 1 - Getting the components together

This may be the hardest part... Aspecially if you are looking for audio-grade components at an affordable price. The components I will use in this revision are:

BHC Aerovox 10.000 MF 63 Volt for the power-supply. These are quite expensive (+/- 16 Euro each) but in the power-rails we need the best quality. A less expensive alternative may be BC-components or Philips.

100MF and 47MF 63V Audiograde capacitors for C5 & C10. In the 405-2 we will need 3 47MF per circuit-board.
Bipolar capacitors of 10MF (for the clamp-circuit) and 47MF (for the DC-feedback) x2
Metalfilm 1% 0,5W low-noise resistors for R2 and R6, and probably for R1, R3 and R4 as well (we'll come back to that).
Burr Brown OPA604AP-chips for the input-channel. We'll need 100NF caps to stabilise the power-supply of the chips and 16V zener-diodes to make the chip feel happy.
We'll also need some supplies like new fuses, IC-sockets, thermal compound, cables to replace the old cable-beam, LS & RCA connectors, PCB-connectors adl...
Last but not least: the schematic that corresponds with the exact serial-number. That won't be a broblem: send me an e-mail.

What equipment will we need?

The minimum is of course screwdrivers, pliers, soldering-equipment and a multimeter
Better would be a 2x50V lab power supply, a LF tone-generator and a 2 channel scope (but it is possible to do without them)
Ideal would be a low-distortion sinus-generator and a distortion-meter as well. I don't have those myself (yet) so don't worry;-)

Step 2 - Taking the 405 apart

The first step is to remove the top- bottom- and sidepanels. Sometimes, after 30 years, the screws are hard to remove. The best screwdriver to use is a Philips n°2 with a length of at least 20 cm.

We now understand why Quad made 4 hole's on the left- and the rightside of the backpanel: they were intended to facilitate manipulation of the screws of the circuit-boards.

I don't need to mention it's usefull to keep the screws apart. They can be replaced (they are M4 - which is a standard-size) but of course there is not much use in replacing the screws.

The best way to proceed is to remove the top-panel, then the bottom-panel and then the 2 side-panels.

The first impression of a newly opened 405 is usually a dusty one. This doesn't matter (in this stage ;-).

On the 405 on my table the voltage-selector is at 240 Volt. Switch it to 230 Volt. The connectors will have to be replaced (except the DIN-connector).

The LS-connectors were not only lousy in the first place, they are oxydated and will give a bad contact. The RCA-connectors are clearly worn. It's surprising there are any RCA-connectors, this is not standard. This amp has been modified.

The PCB-connectors have to be replaced anyway, they are oxydated and we will use new cabling.

I didn't spend time desoldering the cables, I cut them off. They will have to be replaced anyway.

The cable-beam of the 230V-side of the transformer should stay in place, there is no improvement in replacing it as there is only +/1 Amp max going through it. On the other side of the transformer we'll deal with up to 10 Amp.

Something is very strange with this amplifier. The Serial number is 42472 and the amplifier boards are M12565:6. The earlier version of the M12565:5 was used from serial number 59001 on. It says "Quad 405-2" on the front panel (launched in 1983 and) it says "1977" on the transformer. One can always find the year of production stamped on the transformer.

This is a recomposed 405! It has 405-2 circuit boards in a 405-1 case. It also has 2 clamp-circuits (one on the 405-2 board and one on the LS-outputs like in the 405-1).

That's no problem, we'll throw away the 405-1 clamp-circuit and make this a great-sounding 405-2.

Step 3 - Fitting in the Power Supply capacitors and the connectors

We'll do this step first and we'll fit in the capacitors with transparent Silicone, so it can dry for 24 hours before we continue tomorrow.

As modern capacitors are a lot smaller (and better ;-) than the ones used 30 years ago we can either buy new fixing-rings to fit them in or use Silicone to fix them in the original rings. Another alternative is to wind some self-adhesive tape around the caps. This will work if the caps are not nuch smaller than the rings.
We'll mount the LS-outputs and the RCA-inputs before we fit in the capacitors. This makes working on the back-panel easier.

The mass LS-outputs do not have to be isolated from the chassis but the "hot" LS-outputs have to be isolated. The new RCA-inputs have to be isolated as well as in the 405 there is a difference between the input-mass and the output-mass.

The best order to mount everything is:

The mass (black) LS-connectors with a (green) wire to the central mass-point on the frontpanel (this wire will be connected to the central mass-point later). We already put this wire in because it is easier to solder it when the capacitors are not in place. The mass LS-connectors don't need to be isolated from the chassis.
The hot LS-connectors. Those need to be isolated from the chassis.
The RCA-inputs. They are soldered to the L/R connectors of the 4-pin DIN-plug with normal wire.
The capacitors. The wiring of the power-supply will be done later, after the Silicone has dried. Make sure to fit the first capacitor with the + on the left and the second one with the - on the left. The left-side will be our central 0Volt mass of the power supply.

Quad 405 DIY illustrated guidelines - Step 4 to 6

2008-01-01T13:00:00.001+01:00

Step 4 - Upgrading the circuit-boards.

The number of this circuit-board is M12565-6 (this is about the first 405-2 circuit-board). This means that the clamp-circuit is integrated on the board and that some improvements have been made about the current-limiting and the input circuit.

The first thing we'll do is connect it to a lab + & - 50 Volt power-supply to see what amount of power it eats. The - power-line should eat about 110 mA and the + power-line about 120 mA.

It passes the test well. This means the transistors are OK. (Zero Killed can be taken literally in this case). Both boards are OK.

The 0 Volt power-line has to be connected to the transistor-chassis. There is a 10R resistor between the input-circuit mass and the power-circuit mass. It will smoke if you are confused between masses.

OK, we'll remove all components that have to be replaced. We won't touch the other board for the time being. This way we can always compare.

We'll replace R3 with 1% metal-film low noise 22K.

We'll replace D1 & D2 with 15V 1,3W.

We'll replace the electrolyts C4/C5/C18/C19 with new audio-grade caps.

We'll replace C2 and C10 with bipolar caps.

Of course we'll replace the opamp with BB604AP and the PCB connectors. The PCB-connectors have to be replaced because we have to redo the cabling. After more than 20 years they usually are oxydated anyway. We'll use 1,3mm silver-coated PCB-connectors here.

Soldering them to the PC Board is quite difficult as they tend to fall out and as they should be fixed very well in a horizontal way. I us a small carton box of 1 cm height to position them one by one.

We'll add 100 nF caps in the powerline of the chips on the backside of the PCB.

We'll also reduce the sensitivity of the input to 1,5V (from 0,5V). This will also reduce noise & distortion with another 10 dB.

We'll do this by increasing local feedback and DC feedback. We replace C4 with 150 nF MKT, R4 with 6K8 and R6 with 100K 1% metalfilm.

On this subject you should take a look at Bernd Ludwig's text and schematic (see the Quad-links in the left column).

Before adapting the second circuit-board we have to test the upgraded board to make sure it's OK. This way, if there is a problem, we can compare the voltages on both boards to locate the error.

Step 5 - Testing the circuit-boards

When both boards are upgraded we'll connect them to a sinus-generator and to the scope. We'll connect + and - 50 Volt with our lab power-supply and connect a true-RMS multimeter to check the input- and output voltages.

Following measurements are OK:

- 0,01 Volt DC on the outputs
- 30 .. 32 Volt AC on the outputs before clipping. This corresponds with 110 .. 125 Watt into 8 Ohm.
- 1,7 Volt AC on the input before clipping. This corresponds with line-voltage for full power.

If you don't have all this lab-equipment, skip this step. We will make sure later that there is O VDC on the output and that the power-consumption in the power-lines is 120 .. 130 mA. In this case we'll do Step 6 first.

Step 6 - Cabling

We start by cabling the power-supply. For internal cabling counts: the shorter the better. We'll use high-quality flexible 1,25 mm in 5 colors for everything except the 230 Volt lines.

We'll use following colors:

Yellow for AC power-lines (between the transformer and the bridge)
Red for the +50 Volt line
Black for the -50 Volt line
Green for the output-mass
Blue for the LS-output lines

We do all the cabling of the Power Supply lines and test the voltages first. They should be around + and - 51 .. 54 Volt DC. Before connecting the power re-check the polarity of the caps and the rectifier-bridge.

Don't forget to connect the mass-lines of the capacitors (one + to one - side of each cap) and the ground-line of the transformer to the central mass-point on the chassis.

It is better to twist all cables that have to be soldered to the same point together before tinning them, it makes soldering them a lot easier. Foresee cables to the boards as well.

Warning If the polarity of the caps is wrong they will explode!

Once the power-supply is OK, we fix in both boards adding thermal paste to ensure maximum heat-conduction with the chassis.

We can now connect the + and - power-lines to the boards (don't switch them or you'll kill the output-transistors!) as well as the LS-output cables to the red LS output-plugs.

It is no luxury to use some Raychem (or other) Heat Shrinkable Tubing for insulating the +, - and output connections, as there are large voltages between them.

Before connecting mains-voltage we should check all cabling again with the Ohm-meter. We should alse re-check that each channel uses 120 .. 130 mA in the + and the - power-line and that there is less than 1 Volt DC on the outputs (typically 0,01 Volt).

If it passes these final tests we can now connect the speakers and the preamp and... enjoy the music ;-)

Stefaan

Growing fast...

2007-12-28T11:57:00.001+01:00

You can now get the same high-standard technical support, upgrade-kits and service from Dada Electronics without expensive shipment-costs.

4 Continents, 1 e-mail address: info@dadaelectronics.com

For orders or technical support, see our Ebay-shops:

Australia & SE-Asia: http://stores.ebay.com.au/dada-australia

Europe, Middle-East, Africa: http://www.dadaelectronics.eu

USA & Canada: http://stores.ebay.com/DaDa-Electronics-USA

Stefaan, Joost, Esmond, Gregg

First edition of Dada (1917)

2007-12-13T22:19:00.000+01:00

Click on the picture to read more about Dada.

Stefaan

Quad 405 clamp-circuit on the driver-boards

2007-10-24T17:51:00.000+02:00

When did Quad integrate the clamp-circuit on the driver-boards?

In the 405-2 (from serial number 59001 on) with board-number M1265.3 the clamp-circuit was integrated on the driver-boards.

Some 405-1 (serial number between 57301 and 57600) as well as some 405's made in OEM for the BBC also have those new boards.

If you want a copy of the 405 service-manual in pdf, including the version definitions, send an e-mail to info@dadaelectronics.eu.

Stefaan

K4700 & Toroïdal transformer in the Quad 405

2007-10-22T14:30:00.000+02:00

On general request: a picture of the Velleman K4700 and the 2x 36V 300VA Toroïdal transformer mounted in a Quad 405.

Several articles on both subjects have been published on this blog. You'll have to scroll down a long way....

Stefaan

Quad 303 & Dada's High-en boards & Tannoy 215DMT

2007-10-11T10:47:00.000+02:00

Francois Lay, Export Sales manager at Tannoy, uses a pair of 215DMT Studio-monitors, the absolute top-range in professional audio.

He has put his McIntosh aside and uses the 2 channels of one 303 with new Dada High End boards to drive the two speakers in each of the monitors in biamping mode.

The result is... well here is what Francois says himself.

Stefaan

"Everyone knows about Tannoy, like everyone knows about Quad.

I own a pair of 215DMT. These main studio monitors use a 15’ bass driver and a 15’ Dual Concentric per channel.

The design is not really for "home use" ;-) and it is really because they sound quite well that my wife hasn’t left me yet.

Nevertheless the 215DMT have been designed to be matched with powerful amplifiers - more than 300 watts - and a very high damping factor is more than welcome to control both drivers which are playing partially on the same bandwidth.

Thus, matching a simple Quad 303 with these speakers could be a challenge… 45 watts and limited performances in terms of control of the membranes, but an inimitable midrange and a exquisite atmosphere could be the final result with these 98db/W/m speakers… at least if the amplifier is working well, which was not the case with the one I bought some month ago in France.

The first step has been to find someone who could repair this amplifier. My search on internet brought me to Stefaan from Dada Electronics, who suggested to send him the unit. A few days later the verdict came: one board was broken, the other one was not reparable… But I agreed with Stefaan to give a chance to this old and respectable Quad 303, and to give it a rebirth with Dada Electronics’ High-end driverboards and PSU.

The second step has been to ask Stefaan if he could find and upgrade me a second unit, as the idea came to me to Biamp the speakers. Fortunately Stefaan had another Quad 303, which has been adjusted precisely in order to work identically with the first one.

Few weeks later I received my new toys…What about the result? Simply fantastic.

First of all the power is now almost 60 watts, which is never too much with these speakers. The “Quad sound” and its wonderful midrange are still there, but with an extended response in the low frequencies. The transients are much faster, the high frequencies are simply better, more detailed, and the soundstage is much wider and deeper than before, and gives you the feeling that you will permanently rediscover your music…I think that I will stay with this new setup for a while!

Thanks Stefaan, and congratulations for the fantastic work!"

Francois Lay

Export Sales Manager

Tannoy Limited

http://www.tannoy.com/

francois.lay@tannoy.com

Skype: francoislay

DACT CT-2 Attenuators available again

2007-10-03T16:10:00.000+02:00

Picture (c) DACT.com. The DACT on the picture is the 8-channel version.

We have a new stock of 10K and 50K DACT Stereo stepped attenuators.

The DACT CT-2's are without any doubt the best High-end volume attenuators on the market.

They can be ordered in our Ebay shop and will be shipped immediately. Dada Electronics is official DACT dealer.

Stefaan

Dada Electronics Ecological mission statement

2007-09-26T13:48:00.000+02:00

From the beginning Dada's business plan had an important ecological aspect.

Bringing high-end audio equipment back to life and saving it from the dustbin is in itself a strong concept that gives us a "negative ecological footstep", which means we give more to the environment than we take from it.

But today we want to go a step further for nature. We decided to:

Print less documents. Invoices, manuals, documentation... will be sent in pdf format.
Only use 100% recycled unbleached paper for those documents (like address-labels) that need to be printed.
Only use 100% recycled packaging-material.
Only use ROHS-compliant electronics components
Only use lead-free solder
Only use 100% "Green" electricity
Make the ecological aspect an essential negotiation-point with our suppliers
Advise our customers about ecological solutions

We are addicts of great music and great sound equipment. If we want our grandchildren to enjoy this heritage we have to preserve it.

Stefaan

Picture: Leica R3, Summicron 50, Kodak Portra N 100 ASA, f5.6 .60 sec, Warnant, Belgium, 2003

Click on the picture to enlarge

Quad 44 Erratic Input Switching

2007-09-26T11:43:00.001+02:00

Thanks to Henk Vocks (NL) we can publish Quads paper on input-switching problems in some 44 preamplifiers.

The date of the Quad paper is 23/05/90 and the ref. is M12436-5.

QUAD 44 ERRATIC INPUT SWITCHING

A small number of 44 control units appear to be subject to pulses, triggering the electronic switching causing the unit to change from the input selected either to another input or to no input at all. The required input can then be regained by pressing the appropriate input selector button.

The cure is to:

Resolder both sides of the 10 feed through pins marked D, E, +, -, B, C, H, G, F and A on the mother board. Ideally the pins should be replaced with tinned copper wire. The pins are ringed in the attached diagrams.

Connect a capacitor min 4,7 µF across the +8,5V and -7,5V rails on the back of the mother board, as shown.

You can download this paper from the download-section of our website www.dadaelectronics.eu.

Stefaan.

Quad 405 switch-on behaviour

2007-09-19T15:38:00.000+02:00

Take a look @ the voltages on the output of a Quad 405 board when it is switched on.

We are talking about 110V Peak-to-peak during 60 mS.

That's why we'd better use a loudspeaker delay like the Velleman K4700.

Dada Electronics will launch a loudspeaker-protection / switch-on delay adapted for the Quad 405 to 909 Current dumping amplifiers in the coming weeks.

Stefaan

Quad USA Seventies price list

2007-09-09T12:22:00.000+02:00

I recently bought a lot of Quad documentation, there was a price list of Quad USA, I think 1970 or so, those where the days..........

Joost Plugge

Quad 34 capacitor problems

2007-08-24T14:18:00.000+02:00

Last week I visit our shop in Antwerp. Stefaan mentioned a problem with a Quad 34. It was virtually impossible to solder a electrolytic capacitor. After scraping the track with a hobby knife, a good solder joint was possible. On the way back to the Netherlands I remembered a remark Keith Snook made on his website about the placing of some capacitors, they where incorrect biased, with leakage of the elco’s as a result. I bought a batch of 34’s with different colours and serial numbers, so all the mod levels where present. The major conclusion after looking at the diagram’s and the real thing; the incorrect placing of the elco’s is not an incident but a structural problem in the 34. In the picture you can see the result of the leakage of the elco’s and the different phases of decay of the Pcb, which causes the solder problems. Because of the asymmetrical power supply the DC voltage level of the output of all Opamps, except IC25 and 27 in the model above serial 8000, will be negative, relative to ground. So the minus connection of the elco should be facing the output terminal of the Opamp. In the diagram and in real life most of the elco’s are inverted, except C30 and 31, check your 34, the status maybe vary from model to model. There is only one solution, replace all elco’s and solder them in the correct position concerning the bias level. Check the DC voltage across the elco’s with no signal connections made to the 34. The bias voltage should be positive, a few millivolts is enough. So the plus of the elco should be positive, relative to ground level when measuring the DC voltage.

Joost Plugge

Power supply voltage in a Quad 33; 12, 15 or 16V? Modification december 2010

2007-08-10T11:52:00.003+02:00

The discussion started with the first article in the Netherlands about the revision/upgrade and modification of a Quad 33 and 303 by Ruud Janssen. See appendix 1

This upgrade, apart from the standard replacement of all the electrolytes, was based upon the increase of the power supply voltage to 16 volt, reducing the gain of the output amplifier stage, to cope with high level inputs, and increasing the gain of the PU amplifier to compensate for the lower gain of the output stage. Also the power supply voltage for the PU amplifier was kept at the same level by changing R300. Although the series resistors in the PSU are decreased when you install a 16V zener, this is still one of the disadvantages of this type of regulation, a PSU with a high internal resistance. This gives poor load regulation and higher crosstalk figures.

Considering the input signal levels, I built 33’s with 7812 and 7815 regulators, (check the remarks about the 7812/7815 below) There is more headroom with a 15V supply, correct. So with certain inputs, there is an advantage.
There is a big difference between the standard (12V) zener based supply and the regulator based supply, but the regulator based supply is difficult to sell in a DIY upgrade kit. Experienced DIY users can do this job, also it can be done in one of our shops.

The reason for keeping the PSU voltage at the same level for the PU amplifier was the following: To preserve the noise level produced by the first BC109 transistor on the PU board, Ruud Janssen stated that the DC bias current should not change. This looks like a valid argument, but increasing the gain of the PU amplifier to compensate for the lower gain of the output amplifier increases the noise level of the PU stage.

The decrease of the gain of the output amplifier is no guarantee that a high level output from a CD player or other source will not overstress the input amplifiers on the tape adapter board. All the input signals, except the tape input are routed via the input amplifier on the tape adapter board. This is even more a consideration when someone uses the modified PU input selector board for connecting a CD player, trough the PU amplifier with the normal PSU voltage setting.

My own observations:

R300 together with C313 forms a low pas filter to reduce the hum and noise in the PU stages. The -3dB point is around 3hz. By putting in a 7812 or 7815 in the PSU, this filter is no longer necessary. Keep the capacitor in place and replace R300 by a wire link. This also reduces PSU induced crosstalk. If you are consequent replace resistors R305/308 and R315/316 to maintain the DC bias current settings and the voltages as mentioned in the diagram, if you are a purist also replace all the other resistors concerning the DC bias settings of the other stages when you are applying a 7815 regulator or the 16V zener option! The 7812/7815 will not work in all situations, in some cases the input voltage of the 7812/7815 will not be sufficient. The LM2937ET-15 low voltage drop regulator is a little bit more expensive, but performs very good under all feasible conditions. So we advice to start with the LM2937ET-15.

In the diagram you will see the placement and the pin layout ( 1,2, and 3) of the regulator.

Replacing R305/308 and R315/316 is a better option than increasing R300 when you want to reduce the PSU voltage of the PU amplifier. Increasing R300 also means increasing the PSU induced crosstalk, but also a unwanted feedback effect between the two collector resistors of each channel but also between the channels. All four resistors are fed trough R300! Because the filter formed by R300 and C313 also stabilize the voltage at R305 (10,25V) and the rest, the effect will be small, but it will not be zero!
There is also a positive effect, the filter formed by R300 and C313 will have a lower -3dB point when increasing R300.
In the table the values are given for the three power supply voltages, situation one: keep R300 in place and situation two: replace R300 by a wire link.

Keep the DC PU settings:

PSU Voltage, Value in Ohms
12V, R300=560
15V, R300=2K2

16V, R300=2K7

Remove R300 (replace by wire link)!

12V, R305/308=100K, R315/316=4K4
15V, R305/308=133K, R315/316=6K5
16V, R305/308=140K, R315/316=7K2

Instead of reducing the gain of the output stages I use a resistive network to adjust the input sensitivity, like Quad themselves does concerning the tape input. Reducing the gain of the output amplifiers helps, but increasing the gain of the PU amplifier does not help. If you don’t need all the inputs, i.e. incidentally a turntable, and don’t want to connect the FM2, FM3, FM4 or AM3 tuners to the preamp then adjusting the gain of the output amplifiers and the PU amplifier is a valid option.

So to sum up: my revision advise for experienced users. Remember, there is nothing wrong with the standard upgrade, these are extra's.

If you use Quad tuners, leave the gain settings as they are. If you use other high output equipment, change the gain of the output amplifier boards. Use a LM2934ET-15 volt regulator in the PSU, remove all the unnecessary components, also the extra supply for the switching of secondary equipment. Because of the regulator, the caps in the PSU can be of relative low value, there is also the extra 100uF on the PU board. Replace R300 by a wire link in the PU amplifier and replace R305/308 and R315/316 to maintain the DC settings. Change R101/102/103/104 to 47K, this gives a more or less standard impedance of the PU input. Replace all the electrolytic capacitors. Connect a CD player to the Tape input, 1 Volt setting.

Joost Plugge

Appendix 1

The Ruud Janssen 33 upgrade and revision in a nutshell. (See the Quad-links in the left column for the original text - in Dutch). This is the basis for our standard upgrade kit.

Power Supply

Replace R500/501 (120 Ohm) by two 28 Ohm resistors. Replace MR500 by a 16V 250mW zener.
Replace C500/501 with radial capacitors of 4700uF 25V, maximum dimensions: 35 x 17mm, replace C502 with an axial type, 1000uF 25 V (30 x 16mm). Replace MR501/502 with 1N4002 diodes. Remove C503/504, R502 and MR503 (My advise).

Output amplifier boards

Reduce gain, from 5 to 2,4

Change R411/411 from 1800 Ohm to 1000 Ohm, change R412/412 from 470Ohm to 1200 Ohm

Replace:
C401/401 2,2 uF 25V
C405/405 47 uF 25V
C406/406 22 uF 25V

PU amplifier

Replace:

C300/303 47uF 25V
C301/302 100uF 25V
C307/308 47uF 25V
C311/312 47uF 25V
C313 470uF 25V

Change R300 from 560 Ohm to 1500 Ohm
(I don't agree with this value, to low!)

PU Adapter board

Replace R101/102/103/104 with 47K to change the input impedance from 68K to 47K.

Use M1 position of the adapter board to compensate for the gain reduction of the output boards. Otherwise change the values of R105/107 for the left channel, and R106/R108 for the right channel, to change the gain of the PU amp. No values are specified, only that the sum of R105/107 and R106/108 should stay the same for a proper RIAA correction. A lower value of R107 will increase the gain. A gain setting above 78x is not recommended.

Optional: replace all the resistors in the RIAA section and the first transistor stage with low noise types.

Motherboard

Replace:

C5/6 100uF 25V

PNP or NPN TR1 in the Quad FM2?

2007-08-09T16:14:00.000+02:00

We recently had an enquiry from one of our customers in Turkey, a Quad fan who was trying to make sense of TR1 in his Quad FM2 valve tuner. TR1 controls the stereo lamp which in his unit was off although he was definitely hearing FM stereo. He was confused because TR1 is shown as a PNP device in the circuit diagram but listed as a Mulllard AC176 which is listed in the datasheet as an NPN device.

The answer seems to be that the Quad FM2 circuit diagram is wrong and the device TR1 should indeed be shown as NPN to agree with the datasheet and the electronics. TR1 has a collector voltage more positive than its base voltage, and receives a positive base voltage to turn it on, so it is clearly an NPN device.

Possibly the mistake arose in the drawing stage because the AC187 is a germanium transistor, which are usually PNP.

However we advised our customer that the fault was most probably in the lamp itself rather than the transistor. The lamp is a standard type still available today, or can be replaced with a yellow LED in series with a small resistor.

Dada Electronics Australia specializes in the Quad FM tuners, for repairs, renovations, and RF/IF/MPX alignments. See the Dada Australia web-shop http://stores.ebay.com.au/dada-australia.

The complete FM2-schematic can be obtained for free by sending an e-mail to info@dadaelectronics.com.

Esmond Pitt
Dada Electronics Australia

"Quad 405" ?

2007-06-16T17:06:00.000+02:00

Piet (NL) brought me a "405" he bought second-hand for revision.

I have seen a lot of things inside the > 250 Quads I had on my working-table but this beats everything.

The circuitsboards were replaced with Sanyo STK070 solid-state 70W amplifiers. It is clear that this way the 405 specs are far from being met.

Furthermore the internal wiring is a total mess.

One should always ask to see the inside when buying an amplifier second-hand.

We will build this 405 again, bottom-up, with 405 driver-boards.

It looks a lot better yet, doesn't it?

To be continued...

Stefaan

Quad II-40 Service Manual

2007-05-30T15:24:00.000+02:00

The Quad II-40 Service Manual is available from Dada Electronics in Pdf for free now.

Just send an e-mail to info@dadaelectronics.eu

We have an extended library of free domain Quad (and other) schematics and service-manuals. If you are looking for something just let us know.

Quad 606 MK-1 DIY Upgrade-kit available now

2007-05-03T18:02:00.000+02:00

Dada Electronics announces the Quad 606-1 upgrade kit.

There is 1 basic version & 2 options:

Basic upgrade-kit (includes Nichicon, Wima & Silvered-mica capacitors for the driver-boards, internal cabling, gold-plated RCA & LS-connectors, silver-plated PCB-connectors). 65 Euro

Recommended option: 4x BHC Aerovox 10000µF 63V capacitors for the power-supply. 85 Euro

Option: 12x 2N3773 high slew-rate power-transistors. 45 Euro.

The upgrade-manual and 7/7 e-mail support are included.

The kit and the options are available in the Dada Ebay-shop.

Quad 33 Mono/Stereo facilities. Connect a CD player the right way

2007-04-06T14:36:00.000+02:00

Quad 33 Control Unit Mono /Stereo switching facilities. And the optimum way to connect a CD player.

Mono Stereo facilities

There are a lot of questions about these facilities, in most cases these questions arise after the upgrade of a 33 Control Unit. As pointed out below, the Disc, Radio1, Radio2 and Tape replay have different mono/stereo functionalities. The only common thing is an input combined with the Stereo button. This document is based on the circuit diagram above serial number 11.500. Below serial number 7500 some functionality is missing, in the last part of this paragraph you will find the functional differences.

No pushbuttons activated

If no pushbutton is activated, all the input signals are shortened to pin 2, earth. Quad assumed in those days that all the sources could cope with this situation. Check if all your source equipment can deal with short circuit outputs!

Disc

Disc combined with «M : L+R to L output
Disc combined with M» : L+R to R output
Disc combined with «M, M» : L+R to L and R output, full mono
Disc combined with Stereo and «M : L to both outputs
Disc combined with Stereo and M» : R to both outputs
Disc combined with Stereo and «M, M» : L+R to both outputs, full mono

Radio1

This is a special situation because the Quad FM2 and FM3 tuners carry the mono signal on pin 1 of the Din connector. So if a device is connected, without a signal on pin 1, there will be no output in the first three situations described below!

Radio1 combined with «M : Signal pin 1 to L output
Radio1 combined with M» : Signal pin 1 to R output
Radio1 combined with «M, M» : Signal pin 1 to L and R output, full mono
Radio1 combined with Stereo and «M : L to both outputs
Radio1 combined with Stereo and M» : R to both outputs
Radio1 combined with Stereo and «M, M» : L+R to both outputs, full mono of pin 3 and 5

Radio2 and Tape replay

Radio2 combined with «M : L to L output
Radio2 combined with M» : R to R output
Radio2 combined with «M, M» : L to L output, R to R output
Radio2 combined with Stereo and «M : L to both outputs
Radio2 combined with Stereo and M» : R to both outputs
Radio2 combined with Stereo and «M, M» : L+R to both outputs, full mono

On units below serial number 7500 there was no possibility to fed one channel, Left or Right, to both outputs.

Connecting a CD player to a 33 Control Unit.

Quad advised in a previous factory document three ways to connect a CD player

A modified Disc Adapter board
Via Radio 1 or 2
Via the Tape input

From a technical point of view the Tape input is the best option, the signal from that input is fed through a resistive network on the tape adapter board and than connected via the switch to the volume pot meter. Also the input sensitivity can be set to 1V.

The second best solution is the usage of one of the two radio inputs. The sensitivity of those inputs is 100mV and the signal is fed trough the tape adapter board, one transistor stage extra.

The worst option is the usage of the modified Disc Adapter board. The signal is fed through an extra two transistors, compared with the first option a total of three transistors extra, and associated components. The only advantage is the sensitivity setting can be adjusted by changing some components.

If you want to use one of the radio inputs for a CD player, buy a special cable with at least a 10dB attenuation, otherwise, in most cases the CD player will be too loud with normal volume settings.

Joost Plugge

The Quad 50E revision, including 50D info

2007-03-23T00:16:00.000+01:00

(click on the picture to enlarge)

The 50E has been produced from 1966 to 1983. The quantity that was sold counts up to 12.000. Although it shares the looks of a 303 it is a different amplifier.

The 303 was built from 1967 to 1985, numbers produced: 94.000. I own a early and a late version of the 50E, the only difference is the fuse holder! Internally there are more things that were changed during the production run. The early type has its own heat sink, the later version shares its heat sink with the 303.

Of course components differ during their lifetime, but as far as I known there are no modifications or what's so ever. Quad added a heat sink to the voltage regulator transistor, and that's it.

So the 50E is not a better version of the 303, in fact it's a pity Quad didn't develop a 50E with the famous Triplet output configuration.

The 50E has a regulated power supply for the driver stages and a well filtered supply for the output stages. A choke is very good in deleting HF rubbish. It is a technique from the tube era. The 50E will give 50W into almost any load, but not at the same time! In fact the power bandwidth concerning loads is relative small, it uses an output transformer instead of an output capacitor like the 303.

It is idiot proof though, it is funny, for professionals you design an idiot proof amplifier, for normal people.........well. There is an optional 600 ohm balanced input transformer. There also is an input potentiometer to cope with large voltage, 1966, input signals.

The first thing you should do when you buy vintage audio equipment, is to collect information about it. Never connect it to the mains supply in the first place, in most cases there will be an expensive noise! Start by cleaning the unit, 40 years of dust-collection will cause problems. As Quad used standard components, replacing the electrolytics and trimmer pots is easy. Check for suspect looking solder joints as well in the first place.

It's a pity the replacement capacitor for the power supply looks a little lost in the engine bay, although it is supposed to be twice the capacity. That should be a progress! I used a NOS, New Old Stock, capacitor from BC, with a projected life of 200000 hours. It appears to fit between the original brackets. I use a Hameg component tester to check the components within the circuit. Once everything looks ok I connect the unit with a Variac to the mains supply. If this gives a good result, I connect the unit directly and I can start testing & adjusting.

First we set the Emitter voltage of Tr3 to 5.5 volts with trimmer Rv1, then the bias currents of Tr8 and Tr9. Instead of breaking the transformer links, we measure the voltage in R30 and R31. The bias current should be 30-40 mA. Some people think this is also the correct value for a 303, well it is not. This means the voltage across R30 or R31 is .5 x 30-40mA = 15-20mV. You will set this voltage with pot Rv2 and Rv3. Purists would say that I made a mistake. I forgot the base current of Tr8 and Tr9 flowing trough R30 and R31 indeed, but that does not really matter.

The output of a 50E consists of eight (4 x 17V and 4 x 8.5V)independend transformer outputs, you can combine these outputs in any way, as long as you keep Ohm's law in mind. Most people will use the 50E with the 17V output configuration. This means in fact six 17V windings in parallel. The 50D has a different output combination, it has only the four 25,5V windings (17V and 8,5v in series)! So compared to a 50E you will miss the important 17V (6 Ohm) option, you can rewire the 50D to this specification. In the 50D there is no option for placing an input transformer and the netsupply voltage must start at a minimum of 200V AC. Also the netconnector is an odd Cannon connector. The output connector is a 11 pin version of the 19 pin 50E connector.

An interesting feature occurs: biwiring!

You can connect four windings in parallel for the woofer and two in parallel for the tweeter section.

Because all the electrolytics are used for coupling or decoupling we can change (increase) the value of the caps without worrying about frequency problems. Because capacitors are a lot smaller, you can easily tenfold the value with the same outer dimensions. Sometimes I almost did.

Well, almost all the capacitors, C6 should stay 50uF, because it forms a high pass filter in combination with R10. By changing the value of R10, the gain can be adjusted. Increasing the value of R10 means reducing the overall gain of the amplifier.

This is far more preferable (lower noise) than reducing the input signal by the input pot meter on the front panel of the amplifier. Remember that the product of C6 and R10 should stay constant, when you change the value of R10, otherwise the low frequency behaviour of the amplifier will be changed.

So with Tr1 the gain is set, feedback is applied and the high pass filter is implemented, an combination we also find in later Quad designs.

The protection of the 50E (and 50D) is based on a current limiter circuit formed around Tr10. This circuit is triggered by two parameters: the current flowing trough the emitter resistors of Tr8 and Tr9 and the temperature of the heat sink sensed by R18.

The peak output can be increased by changing (increase) the value of R26 and R27. Remember that this amp was designed in the days of constant RMS output.

With modern speakers, peak power (and coping with low impedances) is very useful. The two output circuits are identical, that’s the advantage of using an output transformer with two primary windings.

Compatible Transistors, check pin layout!

BC109 BC184 BC451
BC154 BC560 BC214
BC142 2N3020
BC125 BC301-6
40411 MJ802
40408 BC142
IS922 1N4148
IS920 1N4003
AA129 none! 200mV reference diode

Joost

If you need a copy of the schematics or the 50e-manual send an e-mail to info@dadaelectronics.com

All Quad 50E and D electronic parts are available from Dada Electronics, send an e-mail to info@dadaelectronics.eu.

Quad 405-2 State of the Art

2007-03-10T18:55:00.000+01:00

After several weeks of work I presented my latest revised Quad 405-2 on Ebay today.

The dual-mono power supply by Dada/JPE gives a dramatical improvement of channel-separation and stability (see the other article about the dual power supply below).

Of course it has OPA 604's on board and the internal cabling is High-end. In used 4x MJ15003 power-transistors in this one.

Click on the pictures for High-res.

I love soldering but I hate taking digital pictures.

Stefaan

All Velleman Components, Kits & Tools available

2007-03-10T11:19:00.000+01:00

Dada Electronics is reseller for all the Velleman Components, Kits & Tools.

Check the Velleman website for thousants of products at interesting prices.

We ship worldwide by Postal service or UPS.

Kits can be mounted & tested by Dada Electronics (ask for a quotation).

The delivery-time is:

1 day for items in stock @ Dada
1 week for items in stock @ Velleman
ask for the delivery-time for items out of stock @ Velleman

Dada Electronics is official Velleman dealer.

Send an e-mail to info@dadaelectronics.eu for a quotation, shipment-cost or leadtimes.

Stefaan

Quad 303 High-end boards: the Figures

2007-02-16T15:04:00.000+01:00

Esmond Pitt (Dada Elecronics Australia) measured the Dada High-end boards.

Output-transistors: 4x (new) MJ15003

Equipment used: Tektronix SG505 oscillator & AA501/01 distortion analyser.

Bias-setting: 7,5mA (10 mV between pins 4 and 6)

Bandwidth: -3dB at 13,5 Hz and 60KHz. Ruler-flat in the passband.

Distortion (THD): less than 0,015% at 4V RMS 1 KHz, less than 0,04% at 10 KHz
Crosstalk: less than 65 dB

Hum & Noise: less than 0,002% ref 4V output, so add 14dB ref full output

Output-power before clipping: 59 W RMS into 8 Ohm (can be limited to 45 W for use with ESL57 speakers)

It is clear that these figures are a lot better than the original 303-specs, specially the THD.

The output-power was measured by Dada Belgium with a Hameg HM8012 multimeter, Hameg HM8037 generator and a Tektronix TDS2012B scope.

The high-end boards and the MJ15003 transistors are for sale in the Dada Electronics Ebay-shop.

Stefaan

Quad 44 & 34 upgrade-service & upgrade-kit

2007-01-22T01:55:00.001+01:00

Dada electronics is proud to anounce the 44 upgrade-kit and revision-service.

The problem with the 44 is the fact there are different versions. Some versions used dual (TL072) opamps, other version used single-opamps.

After several tests we decided to use Burr-Brown OPA134 and OPA2134 to replace the TL's. See the data-sheet for more info.

The opamps are very stable, they reject HF feedback and they are designed for audio. At the voltage-level and the amplification-factor in the 34 or 44 they perform better than teh (more expensive) OPA 604 and 2604.

They are also less critical for the power-rails.

The following products will be available in the Dada Webshop next week:

Quad 44 upgrade
Quad 44 upgrade-kit

Stefaan

The following components are replaced in the 44 (depending on the version and the configuration):

On the PSU-board:

2x 47mF 63V capacitors
2x 1000mF 25V axial capacitors
1x 100mF 16V capacitor

On the Tone-control board:

5x TL072 replaced by BB OPA2134 with feet
6x 100mF 25V (instead of 68mF 6V3)

On the Radio-input board:

2x TL071 by OPA134 with feet
2x 100mF 25V

On the CD/AUX board:

1x TL072 by OPA2134 with foot

On the Disc-board:

2x TL071 with OPA134 with feet
2x Wima MKS4 2,2mF 50V

On the Tape-replay board:

2x TL071 by OPA134 with feet
2x 100mF 25V

Bernd Ludwig's complete latest 405 update text!

2007-01-20T14:32:00.001+01:00

Dada Electronics is proud to present: the latest version of Bernd's Quad 405 modification text.

You can download the text from www.dadaelectronics.eu - in the Download section.

Stefaan

PS. Bernd, thank you for this update.

Bias current settings in Quad 303 and 50E

2007-01-17T11:40:00.000+01:00

Quad produced two vintage transistor power amplifiers with a bias current, the 50E and D variant, and the 303. In the service manuals the following currents are mentioned: 30 to 40mA for the 50 and 5 to 10mA for the 303. Maybe this is the reason for a lot of discussion on this subject; the 50E is a professional amplifier, so 30 to 40mA is better than the 5 to 10mA specification for the 303.

Why a bias current?

Most analogue power amplifiers operate in class A, B or AB.

In class A the complete input signal is used to drive the output stage. In its basic form the output stage consist of a single transistor, FET or Tube. To generate the full output signal the bias current should be high enough to let the output voltage swing from the positive supply rail to ground when a signal is applied. In practice the bias current is more or less half the maximum output current.

In class B only half the signal is used for the drive of the output stage with no bias current applied. For the amplification of the positive and negative halve of the sinus you need two output devices. This led to the popular push pull design.
The problem with class B is the non linearity of the output devices at low voltage. A transistor starts conducting at 0.6V base to emitter voltage. The non linearity at higher voltages (currents) is a problem in both class A or B amps. The advantage of a class A amp is the smaller signals involved, but with the lower output associated with this.

In class AB the class B push pull circuit is combined with a low bias current. For small signals the amplifier operates as a class A amplifier. The bias current, solves the low voltage problems of the output devices.

Class A is still used in pre amplifiers and driver stages of power amplifiers; efficiency is not a problem and quality the goal. There are still class A power amps, but mostly in the High End (Cost) territory.

How much bias current?

Here the mystification and Voodoo starts.

Some people think: the higher the better. It depends; most of the class AB amplifiers start their life in the design stage as a Class B circuit, including local and overall feedback. You cannot convert such a amplifier to semi class A by increasing the bias current. You have to redesign the driver stages and the feedback regime to make a proper class A amplifier. In a well designed class AB amp, like the 303, the output transistor is part of a circuit with local feedback, in this case the famous triplet circuit. So the linearity of the output transistor is not a big problem anymore, only the cutoff and start behavior of the output transistor.

So to prevent the transistor operating in this non linear fashion a small current is applied. In most datasheets the cutoff current (this is the current where the transistor stops conducting) of the transistor is specified. In the case of the 2N3055 output transistor of a 303 this cutoff current is 5mA. So the bias current specification for a 303 of 5 to 10mA seams reasonable. The 50E’s 40411 transistors have probably a higher cutoff current, unfortunally I could not find a proper datasheet anymore of this transistor to proof this. The often used replacement transistor, MJ802 has a lower cutoff current than a 2N3055, 2 mA. In both the designs a minimum DC voltage is applied to the base emitter junction to force the transistor in conducting mode. The result of all this is a certain collector current. This is design dependent! Because it is easier to measure the collector current than the very tiny voltage difference at the base emitter junction, this is still the way to do it. So only if you have access to very sophisticated distortion measurement equipment you can play with the values of the bias current. But you will still face the problem: which bias current will sound the best? So we stick to the manufacturers specifications.

Actual settings in a 303 and 50E

In practice you can still measure the DC bias current indirectly by measuring the DC voltage across the emitter resistance of the output transistor.

According to Quad 5-10mA current trough Tr1 of a 303 is enough. But....through R124 flows also the DC current from Tr109 and Tr105. You can measure the DC voltage across R120 and R123 and calculate the current. These currents are 0.8mA and 8.6mA. So the total DC current should be between 14.4 - 19.4mA, giving a DC voltage across R124 of 4.3 - 5.8 mV to meet the Quad specification. More accurate is to measure the voltage across R124 and R125 (leads 4 and 6 on the driver boards). It doubles the value of the voltage (8.6 -11.6mV), but with some voltmeters this give better results. So a good value in practice is 10mV across R124 and R125 (pin 4 and 6 on the driver boards).

Instead of breaking the transformer links in a 50E, we measure the voltage across R30 and R31. The bias current should be 30-40mA. This means the voltage across R30 or R31 is .5 x (30-40mA ) = 15-20mV. You can set this voltage with pot Rv2 and Rv3. I made a mistake, correct, I forgot the base current of Tr8 and Tr9 flowing trough R30 and R31, but that is only a small error. The same fault applies to the measurement of the 303, but also a small error. I advise to set the bias in a 303 at 10mV ( pin 4 and 6) and in a 50E at 18mV (across R30 and R31), after the amplifier has reached his operational temperature, without any signal drive.

Joost Plugge

Dada is now reseller for DACT & Flashbacksales

2006-11-25T00:58:00.000+01:00

only wants the best suppliers.