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The classic QUAD 33 although very good for its vintage is now behind the times as far as sound quality and technology are concerned. The sound quality of the 33 can now be brought up to very high performance levels with  new plug in circuit boards (PCBs) which have been specifically designed to fit inside the chassis just as the originals did, but with much improved performance.

The Disc Amplifier Board with its partnering Disc Adapter Board

This board is the main input amplifier for vinyl users with the input sensitivity and RIAA response selectable with the standard Quad Disc Adapter Board. The Disc Amplifier Board can also be configured so that the disc input can accept higher-level signal sources such as a CD player. Input cartridge loading is 68K as defined on the original Quad Disc Adapter Board in parallel with 47pF on the new Disc Amplifier Board. This capacitive loading was not previously available on the standard Quad 33.

Features of the circuit are:-

  • Very high quality OPA604AP opamps are used for low noise and ultimate fidelity.
  • Virtually no electrolytic capacitors are used in the signal path or biasing stages.
  • Individual regulators on each PCB giving improved bass, cleaner sound, better channel separation and reduced cross talk.
  • PCB is a Ground Plane design improving the signal integrity.
  • Surface mount resistors (underneath the PCB) are used providing shorter signal paths and improved sound quality.
  • Long life components are used in the power supply providing improved reliability.

The treble is smoother and more incisive. Bass is better formed and easier to follow. Sound staging is bigger and wider with improved separation between individual instruments and voices with less smearing. Overall the sound quality is vastly improved over the original Quad Boards with it being more "out of the speakers" and realistic.

A Quad 33 Pre-Amp with the modified PCBs fitted has been driven from a DAC with a digital volume control so that it can be compared with the DAC feeding a power amp directly. The Disc Adapter Board was set to provide the correct sensitivity on the disc input for CD replay. With the Pre-Amp in circuit with high quality cables the new PCBs are virtually totally transparent compared to the DAC feeding the Power Amp directly with no Pre-Amp in the signal chain. This confirms the minimal loss encountered when signals are passed through an upgraded 33.

For the PCBs to function correctly a small modification has to made to the Quad 33 PSU PCB (Please see photo). The original 120 Ohm resistors need replacing by 47 Ohm resistors. These are also supplied preformed and ready to fit and could probably be done for a small price by your local TV repair shop.

All signals from either the disc input (configured for either disc or CD), radio 1 and radio 2 passes through the buffer on the tape adapter PCB and then through the two output PCBs and then on to the output connector. A 33 with the full upgrade kit will surely be a vinyl enthusiast's dream.

Please Note:- With the new resistors fitted to the original Quad Power Supply only one Disc Amplifier or two Output Amplifier boards can be fitted as the original PSU cannot supply enough current for both. For more info on upgrading without any internal modifications being required please see the  Full Upgrade Page.


Disc Amplifier Board


Preformed Resistors


Resistors Fitted to Quad Power Supply


Disc Amplifier Board Fitted



Quad 33 Disc Amplifier Upgrade MK2

Disc Amplifier upgrade includes:-

  • One Disc Amplifier Board.
  • Two preformed resistors.
  • Full instructions on how to fit the upgrade modules (PDF Format)
Total Price excluding P+P = £65.00

OPA627 Option

OPA627 opamps can be fitted to produce the best performance. These opamps are generally known to be of the ultimate sonic quality but are expensive compared to other audio grade devises.

Total Price of OPA627 Option = £26