Quad 33/303

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JLH phono preamp

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Quad ESL 57 Electrostatic speakers



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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 had to service them before use. Read further down this page to find out about the restoration. There's also a section about solving problems and one about a modification to improve performance with a separate phono amplifier.

Quad's amplifiers were, of course, designed to be used with their own ESL 57 electrostatic speakers, and that's the combination which I use.

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 disc 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.

Phono modification

My record player is a few metres away from the preamp and this means I have to use long cables to reach the Quad. The capacitance of the long cables resulted in rolled off treble. I own a good quality external phono preamp which fell out of use a few years ago when I got the Quad amp, but it now makes sense to use it again so that I can shorten the low level cables taking the signal from the cartridge to the preamp and send more robust line level signals to the Quad amp.

I looked at the circuit diagram for the Quad 33 and it was instantly obvious that it is possible to completely avoid the Quad's phono preamp circuitry simply by rewiring the plug in disc adaptor. I also decided to add phono plugs at the back of the amplifier to allow normal cables to be used, but as I didn't want to drill holes in the back of the amplifier I made a 3D printed replacement disc adaptor cover so that the original could be preserved.

See the photos below for more details of this modification.

As first constructed

Modified with attenuator to prevent clipping on the tape board with the output of my rather "hot" phono preamp

The disc adaptor has four sides. The S1 side is used with this modification and the signal is connected to pin 3 (left) and pin 14 (right). Earth connections are found on pins 5 and 12. Pins 1 and 2 are covered with a piece of tape in order to disable the now unneccesary power supply connection to the internal phono preamp. Note that when connected like this, none of the components on the phono adaptor are in circuit. However, everything is retained so that original performance can be restored at any time.

After experimentation I installed a few resistors to attenuate the high output of my phono preamp. I found that 22 kOhm in series with the signal and 33 kOhm to ground attenuated the signal enough to prevent clipping in the buffer amplifier within the tape adaptor (all sources are routed through this amplifier).

I made this modification in order to use an external phono preamp with the Quad 33. However, after making this modification, any line level source can be used with the "disc" input, for example a CD player.

Surprising gain in sound quality

Apart from getting back the treble, I didn't expect much of a change from using an external phono preamp rather than the internal one in the Quad 33. However, there is actually another quite dramatic gain: The Quad 33 preamp rolls off the bass from phono sources while my external preamp does not. This actually has a dramatic effect on sound quality.

Some people express opinions that the Quad 33 has a pedestrian sound quality. I've never found this to be the case with digital and other line level sources. It now seems to me that this problem is perhaps real, but that it lies entirely within the phono preamp. Use this modification instead of the internal Quad preamp and the problem is solved.

Fault finding: Low level high frequency rustling noise

In mid 2017 I started to hear a strange quiet high frequency "rustling" noise from the hifi. At times it sounded like mistracking when playing a vinyl LP, and of course if I was listening to that source at the time this is precisely what I'd put it down to. It could also sound quite similar to what you might expect from a loose connection, dirty volume control potentiometer or selector switch. For a while it seemed to go away if I turned the pre-amp volume up and down a few times or if I selected each of the inputs in turn.

After a while, the noise suddenly became more prevalent so that it was almost always present. At that point it became possible to try more things and it became obvious that the sound continued even if the 303 power amplifier was powered up without its line level connection back to the 33 control amp. i.e. the 33 was innocent, vinyl LPs were innocent and the problem lay in the power amp. All the things I'd been doing to treat the symptoms were mere placebos. There was a fault and I knew in which component the fault lay !

The old transistor is discoloured underneath

I plugged in the 303 on its own with the bottom off so that I could compare oscilloscope traces with the rustling sound on a speaker. The rustling was very obvious on a scope trace of the power amplifier output. It also turned out to be easy to see on the regulated output of the 303's power supply. With the amplifier on its side with the PSU board folded open I could probe parts of the PSU and it turned out that TR201, a 38495, worked faultlessly at power up but became less reliable as it warmed up slightly. Tapping on it with a plastic pen caused the scope trace of the power supply line to jump up and down. The voltage was varying between the 67 V which the regulator is supposed to have been set to and a voltage much closer to the input unregulated voltage. It's possible that when people report that they can't achieve the correct PSU output voltage with any adjustment on other Quad 303 amps that the fault is this transistor.

After a bit of searching on the internet I found that the BC441 is a good substitute for the original transistor, but my local electronics shop in Assen didn't have one of those so I bought a 2N3553 instead as from their stock it had the most important features of a similar C-E voltage range and could handle a few watts. In both of these regards it's a less good choice than the BC441, but it would be adequate. In practice the C-E voltage across TR201 is usually only about 12 V anyway. The high frequency performance of the 2N3553 is irrelevent in this application.

Apart from the text on the top of the transistor it looks identical to the old

While the package of the new transistor is not the same as the old and doesn't form such a good connection with the heat sink, the heatsink does serve to hold the transistor quite firmly. I found this transistor not to become very warm anyway so I am leaving it as it is for now. If the transistor fails then I'll replace it with a new one with the correct heatsink for that package.

Make a donation

Several people have asked me if they can make a donation as a way of saying thank-you for help with repairs. I very much appreciate this. If you have repaired your Quad amplifier with help from this page and you you wish to make a donation of €5 then you can do so by clicking on the button below:

Other ways to think us are simply to link to this page, which will help others to find this information should they be looking for it, or to link to our webshop.

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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