Quad 33 Pre-amplifier Upgraded to Modern Audio Standards
This Quad 33 has been thoroughly overhauled using audio-grade components.
A Quad 33 in fully restored, re-sprayed condition
The Quad 33 transistor pre-amplifier was launched by Quad in the late nineteen-sixties to partner the Quad 303 Power Amplifier. Paired with its 303 power amp, the Quad 33 was the cornerstone of many a connoisseur hi-fi outfit throughout the 1970s and well into the eighties. The Quad 33 Pre Amplifier is a very high quality design and although nearly 40 years old, it still performs well if updated using modern up-rated audio-grade components. The exterior fascia is well designed providing a very modern minimalist approach whilst the interior layout is beautifully laid out in a modular fashion with separate boards fulfilling discrete functions. Two of these boards can be easily removed and the disc input modified to alter input sensitivity corresponding to the sensitivity of the input source cartridge. Interior boards can also be simply unplugged and removed for upgrading or easy swap-out repair.
The Quad 33 Pre-Amplifier uses high quality push switches and carbon potentiometers to select input sources and volume. Modification of the tone is achieved through the incorporation of filters allowing modification of the extreme harmonic ranges. The filter slope control used in conjunction with the 5k, 7k and 10k filters removes noise and high frequency distortion.
The chassis and covers of the Quad equipment are in die cast aluminium and sheet steel finished in stove-enamelled metallic beige and brown, with off-white and marigold push buttons and control panels. The variable controls are arranged in logical sequence suiting the importance of each function. As well as providing phono, tape and two radio inputs a CD source can be fed through the Quad 33 using a modified tape input board or as we did, using the radio 2 DIN input with a suitable cable.
The Quad 33 is fitted with a 4 way phono module. This module is known as the disc adaptor. The Disc adaptor has 4 positions:
M1 Low Input Magnetic Cartridge
M2 High Output Magnetic Cartridge
C1 Ceramic Pick-up
S1 Spare
The spare is basically a straight through position allowing external phono amplifiers such as moving coil type cartridges to be utilised.
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Quad 33 with top removed showing overall layout
of boards | PSU board with upgraded caps and diodes |
The upgrade using modern day components allow the unit to provide a level of performance that will certainly equal or surpass modern equipment costing much more.
o The two internal Amplifier boards have had all the electrolytic capacitors replaced using uprated high quality audio-grade capacitors, all the critical resistors replaced by metal film modern equivalents, reducing the sensitivity from 100mv to 250mv and improving the signal to noise ratio to 6dB. With this upgrade in place you may connect a CD player to the radio 1 & 2 inputs.
o The Phono board has all the electrolytic capacitors and resistors replaced adapting it to 16v.
o The tape input board has been modified to improve bass response.
o The disc input board has been modified to increase the sensitivity compensating for the overall reduction in amplification.
o The old and dried out electrolytic balance capacitors on the motherboard have been replaced by modern audio grade versions with better tolerance
o Electrolytic capacitors, resitors, diodes and Zeners on the PSU board have been replaced by modern audio grade versions. More stability has been achieved by increasing the voltage to 16v.
o Where components have expired due to old age or abnormal usage they have been replaced by modern uprated versions, the LES bulb is replaced as a matter of course.
o All soldered connections checked and re-made where necessary.
o Exterior case cleaned and degreased
o The internal chassis and components have been cleaned.
o All printed circuit boards have been dusted and cleaned with a foam cleaner to remove grime.
o New components on the PCB potted to hold them fast to the board ensuring no movement.
o Potentiometers (controls) have been internally sprayed with switch cleaner to remove crackle during operation.
o Any potentiometers that are failing will be replaced, the volume pots are prone to failure on Quad 33s and a no-functioning or 'graunching' pot will be replaced with a Modern Alps equivalent without a switch or a new (NOS) Allen Bradley dual 50k log pot with a switch.
(We use the following audio grade electrolytic capacitors:
Nichicon Muse, Panasonic FJ, Elna Starget, RO2 and Silmic)
The unit has been connected to a Quad 303 amplifier, Quad 67 CD player via the
radio 2 input, and a Yamaha cassette deck and has been soak-tested for eight
hours continuously.
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Modified Amplifier board | Modified Tape Input board |
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Modified Pre-Amplifier board | |
To sum up, the Quad 33 is an ideal device to upgrade and using modern day components allows the unit to provide a level of performance that will certainly equal modern equipment costing much more. As we have performed the upgrade for you all you have to do is sit back and enjoy the sound.
Specification:
The Quad 33. The unit will function either at 100-125 or 200-250V, 50-60Hz mains voltages. The Quad 33 is designed in combination with the Quad 303 amplifier for which it is the perfect companion.
Quad 33 specification document here
Dimensions are: 260mm wide, 84mm high and 138mm deep or 10.25" wide, 3.25" high, 5.5" deep.
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Diodes, Polypropylene capacitors, zener diodes and Evox suppressor | metal fim resistors |
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Panasonic FJ, Nichicon Muse and Elna Starget, Silmic Ro2 audio grade capacitors | |
The unit will be supplied with a mains lead with a UK plug at one end and an original Bulgin power plug at the other. The unit will also be supplied with the full manual and circuit diagrams on CD.
Due to demand - ALL OUR QUAD 33s ARE NOW RESPRAYED PROFESSIONALLY PRIOR TO SHIPPING. We remove all the paint and respray with automotive metallic colours and overspray with a matt silk lacquer to retain the semi-matt appearance of the original items.
The unit will be supplied with a mains lead with a UK plug at one end and an original Bulgin power plug at the other. The unit will be supplied with the full manual and circuit diagrams on CD.
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The unit from the front | and the rear showing input |
| sockets (from left to right) | Upper Radio1 socket
Tape Record socket
Tape Replay socket
Mains out
Mains in
Fuse socket
Disc input | Lower Radio2 socket
Output socket
Earth connector
Tape input card port
Disc input card port |
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4 way disc adaptor card | |
We hope you will enjoy using this classic piece of British electronic engineering as much as we have. I personally have an upgraded Quad 33 in combination with a similarly upgraded Quad 303 amplifier and I think the combination is superb, certainly superior to any integrated amplifier I have ever owned. I use my Quad 33/303 combination to play a range of musical styles from Show of Hands English folk through classical Vaughan Williams, to Depeche Mode 80s synthesizer and Irish punk rock. The music the Quad 33/303 combination produces has a lovely valve-feel and in my opinion it copes admirably with any music genre you choose to play through it.
Quad 33 Pre-Amplifier 
