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Quad 33 Upgrades
Full Upgrade
Disc Amplifier Upgrade
Tape Board Upgrade
Output Board Upgrade
Minimalist Output Upgrade

Quad 303 Upgrades
Quad 303 Upgrade
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Quad 405 Upgrades
405 MK1/2 Upgrade
405 Full Upgrade
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A review of the 33 and 303 upgrades is available here.

Second review which is of the 33 MK2 boards is available here.

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 33 has 4 main sections:-

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 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.

The Tape Adapter Board - Now MK2 Version using discrete FET Circuitry

This board buffers the signals coming from the disc input board as well as signals from the Radio 1 and Radio 2 input sockets before these are passed onto the volume control. It also used to match the Tape input and output levels to suit the tape recorder you may be using - can be set for use with a standard output CD player. This is done with user selectable jumpers on the Tape Adaptor Board.

The Output Boards - Now MK2 Version using discrete FET Circuitry

The signals from the volume control are fed into these two mono boards (one for the left and one for the right channel). Their function is to drive the tone control circuitry and buffer the output signal before it is fed to the power amplifier. These boards now have higher quality components such as Rubycon ZLH capacitors on the main power supply and Wima polypropylene coupling capacitors.

Alternative Minimalist Output Buffer Boards

An alternative board can be used instead of the standard Output Boards. The Minimalist Output Buffer Boards buffer the volume control output directly and bypass the tone controls completely. The output voltage is then fed to the output of the pre amp. Please see here for further details. Now in MK2 discrete format.

The Power Supply

As with all audio equipment the power supply plays a crucial role in overall performance. A weakness in the power supply will always produce a deterioration in sound quality. Important aspects of the power supply is how the PSU voltages are produced and also their return path from the audio circuitry back to the power supply (the ground). A PSU which radiates a lot of electromagnetic noise in a sensitive piece of equipment such as a pre-amp can only degrade performance. Ideally this should be outside the preamp itself in either a screened casing or kept a few feet away.

In this case raw semi-smoothed DC at 18-22V or regulated 18V to 24V is fed into the newly designed Tape Adapter Board MK2 where it is further smoothed and then regulated to provide an ideal power source for the rest of the pre-amp. There is then also further smoothing and additional regulation for the local circuitry on each individual plug in card. This provides the best possible Power Supply for the active circuitry. As for the ground return, this is done using very low impedance ground planes which have been specifically 'split' to direct the ground currents back to the regulator ground with minimal interference to the sensitive signal grounds. This provides the best solution for the power supply design.

The Tape Adapter Board must be used if more than one other option is purchased as the internal power supply in the Quad 33 is not capable of supplying the required current for the additional boards.

Upgrade Design Features:-

  • The discrete FET Buffers are used on the Tape Adaptor and Output Boards for improved sound 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.
  • The boards are direct plug in replacements and no tools are needed to complete the upgrade except a screwdriver to open the 33 case.


Output Level

2V RMS - This is sufficient to drive a 405 MK3 to full power 

0.5V RMS at lower volume setting - This is sufficient to drive a 303 to full power

Output level is determined by the volume control

Distortion Measurements

Radio Inputs - Input Level 300mV - Output Level 1.5V RMS

0.001% at 100Hz
0.008% at 1KHz
0.045% at 20KHz

Radio Inputs - Input Level 300mV - Output Level 0.5V RMS

0.001% at 100Hz
0.008% at 1KHz
0.02% at 20KHz

Disc Input

0.004% at 100Hz
0.015 at 1KHz
0.016 at 20KHz

Distortion drops with decreasing output level - at normal to loud listening levels the distortion is proportionally less than the maximum levels.

Fitting Instructions for the Quad 33 Full Upgrade.


Disc Input Board


Tape Adapter Board - MK2 Photograph comming soon.

Tape Jumper Locations (pdf)


Output Amplifier Boards - MK2 Photographs comming soon.


Alternative Output Buffer Boards - Discrete Buffer board photographs comming soon.


Disc Amplifier Board Fitted inside a Quad 33


Output Boards Fitted inside a Quad 33 - MK1 Boards.


Fitted Tape Adapter Board Showing DC Power Connector on the rear of the 33 - MK1 Board.


Internal Power Supply

 A new internal PSU has been designed so that the 33 can be used as the original. The NET Audio Quad 33 PSU is a low noise, linear regulated PSU which fits inside the Quad 33 case in place of the original Quad 33 PSU. The original PSU cannot supply the required current for the NET Audio 33 boards.

The internal PSU solders in directly to the mains input wires and can be set for either European 230V or USA 115V AC using on board links and a single or two jumpers. The output and Quad logo illuminator bulb wiring is soldered into the 5 board holes on the other side of the PCB. Mounting screws, washers and spacers are required for correct fitment of the PSU. All screws, washers, jumpers, new bulb and spacers are supplied with the NET Audio 33 PSU for easy fitment.

The NET Audio 33 PSU adds refinement, body, detail and balance to the sound and is a significant upgrade over the standard external PSU previously supplied. MK2 Tape Adaptor boards with the external PSU socket can be modified so that your upgraded Quad 33 can be fitted with the new NET Audio PSU.

Fitting Instructions for the NET Audio Quad 33 PSU

Quad 33 Full Upgrade

Full upgrade includes:-

  • One Disc Amplifier Board.
  • One Tape Adapter Board
  • Two Output Amplifier Boards
  • One Internal DC Power Supply
  • Full instructions on how to fit the upgrade modules (PDF Format)
  • Board overlay diagram for Tape Adapter Board Jumpers (PDF Format)
Total Price PSU excluding P+P = £270

NET Audio Internal Regulated Quad 33 PSU for UK, Euro and USA customers

This PSU can be added to existing Quad 33 owners with the NET Audio upgrade and external PSU for a performance increase.

This includes:
  • One NET Audio 33 Regulated PSU
  • Three screws, washers and spacers
  • One new bulb to suit new PSU board
  • Two jumpers for voltage selection
Total Price PSU excluding P+P = £65

If you require maximum disc performance then OPA627AP opamps can be fitted instead of the standard OPA604 opamps in the disc board. Quad 33 full upgrade with OPA627s fitted to the disc board is £25 These opamps are now extremely rare and may not be available - please check at time of order.

If you require maximum performance OPA627AP opamps can be fitted instead of the standard OPA604 and OPA134 opamps in the disc and output boards. Quad 33 full upgrade with OPA627s fitted to all boards is £50 These opamps are now extremely rare and may not be available - please check at time of order.

If the Minimalist Output Buffer Board is required instead of the standard Output Boards, then please add £15 to the final price (No longer stocked).

International Postal Insurance is £7.00 on top of the UK Postage for fully insured International Air Mail delivery.