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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.

These PCBs are a direct plug and replace boards for the current items found in a Quad 33 Pre-Amp and have been designed for ultimate transparency. They buffer the volume control before the signal is fed to the tone control circuitry and second amp. This second amplifier is the output buffer and tone control amplifier.

Feature of the circuit are:-

  • Very high quality Op-Amps with 0.00008% distortion are used for ultimate fidelity.
  • FET Input buffer is fitted to the MK2 for improved sound quality and musicality.
  • 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 - power supply is improved on the MK2.
  • New De Thump Circuit added to ensure noise free switch on - delay is between 6 and 10 seconds.

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.

For vinyl enthusiasts there is a Disc Input Amplifier Board available. With the mod to the PSU as listed above only the Disc Input Amplifier PCB or Output PCBs can be fitted but not together as the internal power supply cannot power up both upgrade PCBs. To resolve this problem a tape input card is available which has been extended in length to accommodate a DC power socket. This enables an external 18V to 24V power supply to be plugged in and power up all available cards. Removing the mains from inside the pre-amp has additional benefits on the sound quality. Any upgrade which includes the tape adapter PCB will not require any internal modifications to be made to the internal PSU and are plug in upgrades.

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 with the new tape and output board circuit improvements over the original MK1 NET Audio 33 full upgrade.

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.


Output Amplifier Boards


Output Boards Fitted inside a Quad 33

The new MK2 boards ghave more circuitry and look different to the boards pictured below but improvements have been made to the electronics for even better sound quality.



NET Audio 33 MK2 Output Amplifier Upgrade

Output Amplifier upgrade includes:-

  • Two MK2 33 Output Amplifier Boards
  • Two preformed resistors.
  • Full instructions on how to fit the upgrade modules (PDF Format)
Total Price excluding P+P = £80

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 devices.  The price below is for two as one is fitted to each channel.

Total Price of OPA627 Option = £26