Quad 33 restoration (Pt 1)

The Quad 33 preamplifier is 50 years old, now.

It was introduced by Quad Electroacoustics in 1967 (to partner the 303 power amp) and 120,000 units were reportedly manufactured over 15 years.

The Quad 33/303 was Quad’s first Solid State consumer preamp/power-amp set.

My Quad 33 is serial no.6184, which means probably late 60s-early 70s manufacture.


Cosmetic condition of this example is less than 100%.


Rear view, showing 2 hatches for removable Tape Adapter & the Disc (phono) Adapter PCBs.


Quad 33 preamp block diagram (amended).


Case removed, showing compact, modular PCB design. (Note also, early ‘ribbon cable’ in 1967.)


Case removed, underside view.


Easily accessible Tape Adapter & Disc Adapter boards.


The Quad 33 has 3 inputs + a Disc (phono) input – all DIN sockets.

Original input sensitivities are:

  • RADIO 1 – 100mV
  • RADIO 2 – 100mV
  • TAPE REPLAY – 1V or 100mV or 400mV (user-selected by screws on that Tape Adapter board (see photos below).
  • DISC (phono) – 2mV or 5.6mV or 100mV (ceramic pickup) sensitivities (user-selected using plug-in Disc Adapter board (see photo below).

However, modern input ‘line level’ inputs should around 300mV. And source devices now (cellphones, CD players) have quite high output of 1V-2V, or more.

This preamp will be used for the household sound system, so I want to accommodate also a FM Tuner and maybe run my old Nakamichi Cassette Deck.

So, we need to decrease the sensitivity of the Disc (phono) and Radio inputs and set the Tape Adapter board sensitivity down to say 400mV.

Also, a number of other modifications recommended by other Quad 33 restorers.


The new parts – total cost UK£45 (~NZ$95).

I decided to replace all semi-conductor parts – the transistors with modern equivalents (BC550C) together with the diodes.  I used some bits from my own parts collection (2 x Wima caps and some diodes).

Resistors will be Takman REY metal film.  Electrolytic caps, Elna Silmic II.  Film caps, Panasonic radial polypropylene.

Good audio quality parts used, but not extremely costly – this preamp is for regular household use.


All sub-PCBs removed.


Underside of Phono Adapter board showing 4-sided plug-in contacts design.


Close-up of main Preamp/tone-filter ‘motherboard’ showing the PCB sockets. (The 2 Balance control caps (100uF 6.3v) have been replaced with Elna Silmic II 100uF 16v.)


The Power Supply board has been removed after de-soldering the ribbon cable and the rear AC Mains socket wire connections.


PSU board with new caps (Panasonic FC) ready to install.


Finished PSU board.

PSU mods:

  • PSU output voltage increased from 12vdc to 16vdc for improved stability – pi-filter resistances adjusted and original filter caps (400uF & 640uF) were up-sized to 1500uF 25v.  Zener diode changed from 12v to 16v.
  • Rectifier diodes replaced.
  • The supplementary 5v supply originally designed for power-amp switching, is removed.

(All as recommended by other restorers, see references.)


Upgraded PSU re-installed and tested – 17.55vdc out with no boards connected, looking OK.

(References – Quad 33 Service Data, Dada Electronics, QuadRevisie.)

Part II – upgrades to Amplifier, Disc, Disc Adapter and Tape Adapter boards, replace volume control.



10 thoughts on “Quad 33 restoration (Pt 1)

  1. Pingback: Quad 33 restoration (Pt II) | D a r k L a n t e r n

  2. Hi there
    Many thanks for your write up.
    I have no power on the 33 all of a sudden and having changed all fuses I can only imagine it’s the psu. I see you changed yours, was it fairly straight forward to do?

    • Hi Alex – thanks for visiting.
      Before removing the PSU, if you have a DMM meter, I would first check any volts showing at the top + lead of the 1st PSU cap (as in the last pic above) – there should be 12+vdc between this pin & the green Gnd wire. Additionally, there should be 12vdc at the red pin 4 nearby on this bd. This will tell you if the PSU is working or not. And whether to investigate further upstream (within the PSU bd) or downstream (within the preamp).
      Let me know how you get on, Owen

  3. Hi there ,

    Good job ,

    I would like to do the same job , where can I buy the “The new parts – total cost UK£45 ” ?



  4. Lovely write up.
    I have two 33s and a 303. I had one upgraded along with the 303 by a specialist who installed the lovely Net-Audio upgrades. Sadly Richard who owned the business passed away a year ago, and the kits and parts are no longer available, though his website is. Worth a peek. I also carried out the Dada upgrade on the remaining elements that were not covered, though I have subsequently discovered, I needn’t have as the mains input upgrade is not required with the Net-Audio as it’s all driven off the tape card somehow. The mains is not required – woops.

    The second 33 is still awaiting “surgery” for which I’ll use the Dada upgrade and a couple of other recommend items to increase the low end – remove the 30Hz cut off Quad put in to reduce rumble. I think its C100 or C400 off the top of my head….

    Always enjoy your work, and shall be popping back for tips in the 2nd 33 upgrade.

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