Bias Stability in a Quad 303
The new v2 High-end boards are now officially launched and available in the Webshop.
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...
Quad 33 Revision. A beginners tale.
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
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.
The best capacitor-choice depends on the application within the circuit and the capacitance we need.
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%.
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
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.
Interesting links about capacitors:http://www.mennovanderveen.nl/nl/download/download_4.pdf
We get more and more questions about how the sound quality of the Quad 66 can be improved.
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.
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.
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.
Update 18 december: The Dual Mono power supply with 4x15000µF BHC Aerovox and Loudspeaker-protection is available in the Webshop now. Click here.
Piotr, from Poland, revised his 405 with the Dada-kit ... and some other components:
By Stefan Vertongen, Belgium.
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.
One of our French customers, who is working in professional audio, bought a new Quad 909 to drive his Tannoy studio-monitors.
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.
This way we have replaced all electrolyte capacitors with the best quality and we have adapted the bridge to the optimum value.