Denna förförstärkare tillsammans med slutsteget Quad 303 och tuner  Quad FM 3, har blivit klassiker i HI-FI världen. Apparater i bra skick spelar fortfarande underbart.

Quad 33 control amplifier, Quad 303 Power amplifier

The Quad 33 / 303 hifi amplifiers are classic designs. Due to being a relatively early transistor power amplifier design, from the 1960s, when high power complimentary PNP transistors were not so easy to find, they were designed to use a “triplet” circuit instead, with all NPN transistors.

This pair of amplifiers came to me through family. They were last serviced in 1986, so I’m planning to test them and probably change at least the major electrolytic capacitors before using them. They are, of course, a classic combination with my Quad ESL 57 speakers, so I am looking forward to trying them out.

The Quad 33 is a modular pre-amp which offers a range of different facilities.

In contrast to many modern hifi components, it has a useful range of tone controls. Not only treble and bass boost and cut but also high frequency filters controllable for frequency and slope which are intended to improve the sound of low quality sources.

The rear of the unit. From left to right, the two radio inputs, tape in/out above the preamp output, tape replay input and earth socket, two mains outputs (for matching tuner and the power amp), mains input, fuse and phono input. Note also the flaps which cover the tape and disc adaptor boards.

The preamp doesn’t really have enough inputs for modern usage, and the expected signal level is rather low. However, it is very configurable in comparison with modern devices.

The tape adaptor board and the phono adaptor board. Both these boards can be accessed without opening the case. They are pulled out through openings on the back of the preamp.

The screw positions on the tape board allow setting the gain of this input to match other sources.

The disc (phono) adaptor board can be fitted four different ways around. These give two different gains for moving magnet inputs, a ceramic pickup input (of little use these days) and a straight through input with holes left in the PCB for adding your own components. This can be used to convert the disc input for compact disc.

The insides of the Quad 33. All very logical and designed to be easy to service. On the left you can see the two buffer boards which pull upwards out of the motherboard. On the right is the phono board which is also removeable. The black box in the middle is the transformer, mounted on a board which includes voltage regulation for the rest of the preamp.

The buffer boards and the phono board. High quality plastic capacitors in this 1960s design (and some electrolytics which perhaps are past their best before date).
I plan to add more photos, including of the internals of the Quad 33, circuit diagrams, suggested alterations etc. when I have had a chanche.