Schematics Revision of a Quad 520f PCB | Electronic Circuits

Schematics Revision of a Quad 520f PCB

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


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.
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:
And for Monoblock-conversion:

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