Quad 33 restoration (Pt I)

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.



One thought on “Quad 33 restoration (Pt I)

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

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