Upgrading and Monoblocking a pair of 520f amplifiers.

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