Sunday, January 08, 2006

Quad Revision FAQ's: Part II: The Quad 33

The 33 has a very beautiful design (to be discussed ;-) but also electronics from the 6-ties. Can it be upgraded to modern high-end standards?

Personally I love the 6-ties for the design in furniture, fashion, wall-paper and for the music. The fact I was 2 years old in 1960 & 11 years old in '69 must have something to do with it ;-).

6-ties design has come back in today's fashion as an icon of the best design of the previous century, beside Art Deco.

The Quad 33 was one of the best examples of 6-ties design. It was the first hifi-design to show that the highest quality does not mean the most buttons, a large size and impressive meters, lamps & switches on the front-panel.

But can the 33 be adapted to modern high-end standards?

Sure, it can.

The 33 is completely transistor-based. Later pre-amps used Opamps. Quad used them from the 34 on.

There is nothing wrong with Opamps. Only, the first opamps made in the 7-ties (like the LM301) did not match the specs of a well-designed transistor pre-amp. In the 34 the sound passes throught (at least) 4 Opamps, each containing (at least) 24 transistors. Later it got worse...

Less is more is the design rule for the best pre-amps designed today.The 33 respects this principle in a great way.

What can be done to upgrade my 33 to high-end standards?

Ruud Janssen's website is the best advice on the 33 (and the 303) - see the links on the left. Unfortunately it is only in Dutch.

What has to be done in any way is:
  • Replace all capacitors > 1 mF with new ones. They have dried out after more than 20 years.
  • Increase the powersupply-line from 12 Volt to 16 Volt (the reason it was 12 Volt is because modern capacitors would have been too big to fit in).

These are the basic things.

How about the Volume-attunator potentiometer of 50KOhm with a switch on it?

The potentiometer is 50K Ohm Log with a switch. This one is no longer available. A company in the UK sent me an offer on my request, but I had to order a minimum of 1.000 of them to start a production-line "for a small quantity".

The best alternative is a DACT-attenuator (a switch with 24 positions, stereo, logaritmic) that is applied in most modern high-end.

Anyway, count >100 Euro for this one (see and you will have to add an external switch. That's what I did.


To be continued...


Anonymous Anonymous said...


the preamps are generally considered the weakest products made by Quad. Why spend a lot of money on a Q33 when you can buy a Chinese tube preamp like the Ming Da MC-7R for not a lot of money. Once you connect this to your Q303 power amplifiers (upgraded by Stefaan) you are in audio heaven! Net Audio also offers complete upgrades but if you just add up all the costs involved you might as well buy a new tube preamp.
My 0,02 cents to this "discussion". Cheers, Robert Kok

3:34 PM  
Blogger Stefaan said...

Hi, Robert,

Before, when Quad amplifiers were designed, a pre-amp was an amplifier.

One had to deal with Vynil & RIAA inputs, "Radio" inputs of 100 mV adl.

Line-voltage was defined as 500 mV by Quad, which was like high-current at the time.

You had "tone control" with "treble" and "bass" in order to correct bad speakers and so on.

Today less is more . What do you need in a pre-amp? A volume-control and a switch between the input-sources of +/- 1,2 Volt.

What else would you need?

I am working on the "Maurice Calier" pre-amplifier (this is a project name) that respects this principle.

An ADC/DAC, maybe to connect it to digital equipment?

It should be an insertable module.


3:11 AM  
Anonymous Anonymous said...

To me, the 3-series is a prime example of seventies design.

10:02 AM  
Anonymous Anonymous said...

Hello Stefaan, You can still buy a good replacement volume control potmeter (including a 2 amp switch)for the 33. The Benelux representative Transtec still have them. I payed 40 euro's ex tax and postage. Regards Joost Plugge

11:21 AM  
Blogger Stefaan said...

Thanks, Joost,

I have been looking for a solution for more than a year now.

I'll check out Transtec and keep you (and the group)posted.

For the 33 we need a 47K Log x 2 with switch.


9:58 PM  
Anonymous Anonymous said...


I am very interested in knowing how you are going to improve the 33 preamp.

I have made some modifications according to the instructions at this site:

Basically, I by-passed all tone-control but left the LC filter at the output alone. And I replaced all coupling caps with polyprophelene caps, upgraded power supply caps.

The improvement is quite noticeable. I have two 33 preamps, one stock and one modified. Comparing to the stock unit, the modified unit sounded less veil and harsh. In fact, it is quite musical driving an upgraded 303.

The modified 33 is essentially a selector with a volume control. I suspect that replacing the volume control may improve the transparency.

Thank you for a great site.

8:34 PM  
Anonymous Anonymous said...

The original volume pot is one of the main culprits of a "veil" and "grainy" sound of the 33.

I replaced the pot with a newer and cheap carbon pot. I must say that the sound is now much better. The veil is gone. The little bit of harsh in the mid is also gone.

By replacing all coupling caps with better quality ones, the 33 sounded quite transparent. It may not sound as detail as a tube preamp. But, it is cheap and has an excellent phone stage.

7:02 PM  
Blogger Stefaan said...


The best replacement for the old pot in the 33 is the DACT-attenuator... if you've got enough budget ;-)

See the "DACT" link on the left.

Otherwise use a good quality "plastic" Log pot of 50K.


6:34 PM  
Blogger Stefaan said...

I'll post an article about "Quadrevision live: the 33" on this blog soon - as soon as a 33 arrives to be revised.

A lot of people send me 405's but no 33's anymore %-)

If it takes too long I'll buy an old 33 on Ebay myself.


6:39 PM  
Anonymous Anonymous said...


To up the power supply you just change to 16V diodes, right? Any other progress on your project?

4:03 AM  
Blogger Stefaan said...

No, it's somewhat more complicated. I'll add a new post to blog to explain how to adapt the power-supply voltage.


1:38 PM  
Anonymous Anonymous said...

In the Quad 33 I own since 1977, I just replaced a worn volume pot with switch (between position 1 and 3, the volume in the left channel was at 100%...) by a low-end carbon stereo 2 x 47k log. Got a mono pot with switch from the same manufacturer (Omeg, at Cotubex, but can be found about everywhere), I detached the back and mounted it on the stereo pot.

A bit of mechanical work, but (at least in this case) the result is good, the pre-amp sounds better than before (i.e., even compared to the state after replacing the capacitors etc.). Total cost about 5 euro at my local parts seller. I have kept a spare or two as this pot may not remain healthy over the coming 25 years ...



3:41 PM  
Anonymous Anonymous said...

hi stefaan,
do you have a solution to obtain
the by-passe of all the tone-control the LC filter and the balance ? I think it would be an other good mod;
and what do you think about the polycarbonate condenser?

5:39 PM  
Anonymous Anonymous said...

Hay Stefaan;

Nice to know that poeple still listnen to a Quad 33!
I've just replaced the volume pot on my Q33, and indeed imho the sound changed a bit: it sounds a bit more 'midy'.
I will try to change the capacitors when time comes. What type of capacitors do you advice to use?
Since I'm not a specialist in analog electronics, I've got two (maybe stupid) questions:
-1 :I've got the impression that the "pan-pot" or "balance" potentiometer is getting very tired as well. Do you think that it is usefull to replace the pot by fixed foil-resitors? If yes, by what ohm value should I replace what potentiometer?
-2 I've got a Protools 192 I/O interface, which outputs audio signals at +4 dB, and so I'm driving my Quad 33 a bit to high high on the input. What resisters could I place in serie on the input of the Quad 33 préamp, so i don't get my input signal to "hot"?
Thank U somuch in advanced for your help,


Huby AT mac DOT com

3:13 PM  
Blogger Stefaan said...

Hi, Huby,

To reduce the sensitivity of the 33 it is better practice to increase the feedback than to use resistances in the input-channel.

I think you should take a look at the upgrade/revision manual. If you want a free copy send an e-mail to

For the balance-pot, if you want to eliminate it you can just desolder it. What it does is: it puts one of the channels more or less to mass.

Or you can replace it with a new 1K Linear mono pot of course.

Kind regards, Stefaan

3:24 PM  
Anonymous Anonymous said...

Has a modification to replace the 2 pin US 110 - 115 Volt AC style sockets on the QUAD 33?

In Australia the mains voltage is 230 volts and the plugs/sockets are subject to voltage overload.

11:02 AM  
Anonymous Anonymous said...

The mods to the 33 amplifier boards described at seem pretty pointless to me. For better performance, it would have been better either to completely bypass the second stage including the 2nd and third transistors, just using the first stage/transistor as an emitter follower, or else bypass the first stage and reconfigure the two-transistor 2nd stage as a complementary feedback pair, which would greatly reduce the distortion.

But I like my 33 the way it is. A regulator in the PSU does far more than any of these mods to clean up the sound.


4:27 AM  
Anonymous Anonymous said...

Re the volume control/power switch, they are still available from Quad.


4:28 AM  
Blogger yereverluvinunclebert said...

Quad does not have the repalcement switches any more. They use cheap Allen Bradley replacements which are much lower quality. In the potentiometer there is far less volume range and the sound starts way too high. The switch is acceptable but it does not have the solid-sound that the original has.

7:02 PM  

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