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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.

Quad 33 control amplifier


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 chance.

Quad 303 power amplifier


The Quad 303 power amp is perhaps the more well known of the two components. Not only does it have the unique "triples" circuit, but it also has a reputation for robustness and is known for sound quality even now.

I am very lucky in that this example of the Quad 303 is in perfect condition. Not a scratch on the heatsink grills, for instance, which are often quite tatty on well used 303s.



Internal view of the Quad 303 amplifier. This is also a modular design, with two identical amplifier boards on the left (for left and right channels) and a power supply regulator on the right. It is quite rare for a power amplifier to have a regulated power supply, but this one does.

The only problem with my Quad amplifiers as they came to me was their age. Electrolytic capacitors dry out with time, so I felt I couldn't entirely trust the amplifiers, and the switches didn't work perfectly every time.


Updating the 33 and 303

Unfortunately, the amplifiers languished for over a year until I got around to working on them, between Christmas and New Year 2010. I had originally planned to take the amps apart and work out for myself what parts were required to service them, and to also adjust the gain at the same time so that they'd have better compatibility with modern sources (the output of CD players etc. are a bit "hotter" than older components) but I never got around to it.

I started looking about for information and realised that there were kits of parts available which meant I didn't have to duplicate work which had already been done by others. I chose the DaDa Electronics kits as they stay true to the original design of Peter Walker. The kits provide only those parts which must be changed due to age, those which adjusting the gain to cater for CD, and a few which tighten up tolerances and widen the bandwidth of the amp just a little. These are logical changes which I'm sure Quad would have done themselves if they'd carried on producing the amplifier for longer.

As well as all the parts required, Dada also sent a link to very good online manuals. I also ordered a small quantity of lead solder from them. It's not a good idea to mix up lead free solder (which I already had) with the lead solder on the original boards and components as this can result in unreliable solder connections.



Older parts gave way to new parts. On the left is an updated 303 amplifier board, while that on the right has not yet been updated.


All the removeable boards from the Quad 33 with replacement parts fitted. While some parts in my amplifiers had a 1969 date stamp on them, my amplifiers had actually been serviced by Quad in 1986 and some parts had been replaced then. However, that's still 24 years ago, so it didn't hurt to replace some things again.


That's me, working on the amps. The switch cleaner rejuvenated the switches, some of which were a bit temperamental. After a few hours work, the Quad gear could take up its rightful place next to the speakers they were designed to match.

And the sound is... wonderful. I'm very happy with the amplifiers. The aesthetics are good too, so good that they won a design award in 1969. Read the article on the right for more information or click here.

An interesting aside about using the Quad 303 with ESL 57s

The Quad 303 power amp has an output impedence which is a little higher than most modern power amplifiers. Normally, this would be expected to have no effect with normal speakers, but the complex load of the ESLs might be expected to have an effect on the overall response. As it happens, and as pointed out by Jim Lesurf, the Quad 303 output impedence interacts with the ESL, but does so in such a way that it slightly flattens the frequency response. Whether this was by design or merely a happy accident, I don't know. However, the 303 and the ESLs really do work excellently together.

And what do I think of the Quad 33/303 now I'm using them ?

It's only been a couple of weeks as I write this, but they sound marvellous. It's much the same story as when I got the ESLs. That ended my desire to swap speakers. I think the same has now happened with amplifiers.

Problems which occurred with my Quad 33/303

Mostly I've been getting on with enjoying listening the music through the amp. However, there have been a feww problems along the way:

One channel sounds dull


Servisol to the rescue

Two years after the upgrade, I realised that one channel sounded very dull compared with the other. It was a bit difficult to identify at first because the problem was intermittent. It was not absolutely clear whether the problem was with the pre-amp, power-amp or the speakers. I eventually decided it was definitely the pre-amp by excluding other variables. Resoldering components and cleaning the connectors in the pre-amp seemed not to achieve much so I asked for assistance on the Dada Electronics forum. The reply was about cleaning the selector switches, which I did using a tin of Servisol. This resolved the problem. In retrospect, this should have been obvious. The same problem had occured two years earlier and I'd resolved it in the same way.

I would suggest that cleaning the selector, mono/stereo and filter switches should be your first action if you have a channel imbalance or dull sound in one channel with a Quad 33.

One way crosstalk in Quad 303


An easy problem to find

It can be difficult to work out where intermittent problems come from, and at one point I convinced myself that the problem with one channel sounding dull came from the power-amp, not the pre-amp. Unfortunately, while I had it apart to take a close look, I accidentally disconnected one of the wires between an amplifier board and a power transistor. On reassembly there was a strange symptom of crosstalk in one direction only (in my case the "<-mon" switch lead to sound in both speakers while "mon->" made sound only in the right speaker) and a high level of distortion in one channel. Reconnecting the wire solved this problem.

Quad resources

Various Quad 33 / 303 related web pages that I have found useful:

DaDa Electronics - supplier of the upgrade kits that I used.
Quad Spot - Dada Electronics blog.
Quad World
Quad Musikwiedergabe
Sheldon Stokes - has lots of information about the ESLs.
Richard Brice on modifying the Quad 33 in particular to be compatible with modern hifi components.
Net Audio do quite extensive upgrades of Quad equipment.
Jim Lesurf on 303 and 57 interaction

Peter Walker's articles on electrostatic speaker design

I've more hifi component reviews on my hifi page.



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