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Old 26th Nov 2021, 6:43 pm   #1
shaunpw
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Default Quad 34 preamp volume knob clicks

Hello everyone,
I've searched every where to try and resolve this one.
I have recently bought a Quad 34/405-2 combo, I did plenty of research on the problems these things have.
I replaced all the leaky/old capacitors in the preamp and power amp.
There were a few corroded tracks on the preamp which I have repaired, used Servisol on all the pots but I can't seem to get rid of the audible "click" when I adjust the volume.
The 100mv Radio input is the worst, it really makes my speakers jump !
Has anyone else come across this problem ? and if so, how did you fix it ?

Regards
Shaun.
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Old 27th Nov 2021, 9:12 am   #2
Mooly
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Default Re: Quad 34 preamp volume knob clicks

Can you clarify please...

The volume control should of course be silent in operation. Are you saying the control is more noisy when you have 'Radio' selected?

Look at the circuit diagram. You need to measure and confirm you have zero (0.00) volts on pin 6 of IC 5 and 6 when you have 'Radio' selected. In fact you should see zero volts for any input selection.

It is possible the volume control is just noisy and the circuit configuration (the active volume control) relies on 100% continuous contact of the pot wiper to the track in order to fix the opamp DC conditions.

Check that voltage first though.
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Old 27th Nov 2021, 11:42 am   #3
shaunpw
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Default Re: Quad 34 preamp volume knob clicks

Hi, thanks for the reponse.
To clarify, each click of the volume knob causes an audible click through the output which gets louder the higher the volume knob is turned.
My CD input is set at 300mv so I guess is less sensitive than the factory set Radio input which i believe to be 100mv.
Therefore, as I turn the volume up with nothing playing the clicks get louder to the point of a thump ! at higher levels.
I have partially dismantled the volume control and the steps are quite evident in the design, it could be a faulty wiper in the pot but it affects both channels equally. Maybe the pot is worn out, which would be a real pain to replace.
I was looking at maybe replacing the 4 op amps in the volume circuit.
As a temporary fix, I have increased the output of the 34 by swapping R118, 119, 121,122, and fitted 500mv jumpers in the CD input flags, it has definately helped but the Radio input is still the same.
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Old 27th Nov 2021, 2:37 pm   #4
Ted Kendall
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Default Re: Quad 34 preamp volume knob clicks

Sounds like you have DC on the volume control somewhere...
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Old 27th Nov 2021, 2:49 pm   #5
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Default Re: Quad 34 preamp volume knob clicks

You have to measure the voltage I mentioned in post #2. The opamps are FET input devices and so the DC voltage on the output pins (with them configured as they are) has got to be 0.00 volts DC.

That has to be proved or otherwise before going further.

If you remove or isolate C30 and C31 you remove everything to the left of the volume control.

The volume control under those conditions has to be silent in operation. If it is not then either the pot is faulty or there is some contamination causing conduction on the board.

Pins 6 (and also pins 2 and 3) must all be at 0.00volts DC.
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Old 27th Nov 2021, 3:56 pm   #6
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Default Re: Quad 34 preamp volume knob clicks

If it is a noisy pot a hundred k or so across the 15p capacitors will keep it stable during glitches.
 
Old 28th Nov 2021, 11:34 am   #7
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Default

ok, so i've been searching for IC5 & 6, then i realised the diagram you sent is an early version. My 34 is a later M12730-555, the circuit is quite different as to having input buffers. I am going to remove the caps C30 & C31 (which are linked out) and test again.

Update : with c30 & c31 removed, the amp is silent so the problem must be the input section I guess.
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Old 28th Nov 2021, 1:12 pm   #8
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Default Re: Quad 34 preamp volume knob clicks

Quote:
Originally Posted by shaunpw View Post
caps C30 & C31 (which are linked out)
Has someone been at this thinking the removal of those 'horrible' electrolytics from the signal path would make everything sound 'better'? - i.e. do the links look original?

Cheers
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Old 28th Nov 2021, 1:54 pm   #9
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Default Re: Quad 34 preamp volume knob clicks

The links are certainly not "original" but quite a few of the electrolytics are shorted and i've read that its normal, in fact a company that specialises in quad upgrades carries out this mod.
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Old 28th Nov 2021, 2:09 pm   #10
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Default Re: Quad 34 preamp volume knob clicks

The mist clears, I think Trigon has it, good spot.
 
Old 28th Nov 2021, 3:20 pm   #11
Ted Kendall
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Default Re: Quad 34 preamp volume knob clicks

Quote:
Originally Posted by shaunpw View Post
The links are certainly not "original" but quite a few of the electrolytics are shorted and i've read that its normal, in fact a company that specialises in quad upgrades carries out this mod.
Anything but normal, in my view. Get the thing the way Quad say it should be and you won't go far wrong.
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Old 28th Nov 2021, 4:15 pm   #12
shaunpw
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Default Re: Quad 34 preamp volume knob clicks

ok, so this is where I am so far.
I removed the opa2604 op amps on all the inputs - no clicks
I refitted opa2604 ic27 - clicks came back
so as Trigon suggested,
I removed the links from all the shorted 100uf caps and fitted new caps
except the tantalums on the balance pot - clicks still there but faded away after turning the volume up and down, so we are getting somewhere.
Once the clicks have gone, they come back again if I switch inputs.
I have now removed all the mods I can see on the board and put the 34 back to standard.
I think I'm going to replace the opa2604 op amps and then ic's 1&2 which are 4066.
Thanks for everyones input so far.
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Old 28th Nov 2021, 8:09 pm   #13
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Default Re: Quad 34 preamp volume knob clicks

I don't think 4016 vs 4066 is of much significance - the 4066 has lower distortion over a wider voltage range, but them Quad went to some lengths to operate it within its safe zone. General trend is in the right direction, though.
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Old 28th Nov 2021, 8:54 pm   #14
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Default Re: Quad 34 preamp volume knob clicks

The linked out caps do sound like a typical but misguided upgrade attempt... that's all another story.

You need to understand the mechanism of the clicks now and that will help you diagnose and perhaps correct the problem.

Lets take one channel and look at C30, the DC voltage on the volume control ed of the cap must be 0.00 volts at all times. The FET opamp IC9 ensures that is so. Check it though.

The voltage on the other end of the cap should also be 0.00 volts DC. Again check.

The OPA2604 opamps I think are still unobtainable these days (there were problems that came to light with later production and the chips having issues when used at the upper end of their supply voltage range) although fakes exist.

If these are recent fitments then at least make sure that these chips also have 0.00 volts on their output pin.

There should be no clicks audible on rotating the volume control once any input has been selected and a few seconds have elapsed. If there are then confirm the voltage is 0.00 v DC on both sides of C30.

If there is a small voltage on C30 on the 'input side' that changes as you change inputs then this will cause both an initial click and also could make the volume control noisy for a few seconds until the charge current in the cap falls to zero.

So it is important to make sure the DC conditions are correct.

Changing the opamps (assuming those OPA2604's are genuine) will not achieve anything. It is vitally important to use FET opamps and not bjt types like the NE5532 in these circuits.
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Old 28th Nov 2021, 11:49 pm   #15
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Default Re: Quad 34 preamp volume knob clicks

Return it to the original Quad design. There was quite a bit of subtlety. For example, the power supply rails for the 071 opamps is +8.6V and -9.4V. That is because Quad noticed that the 071 clipped asymmetrically, and so chose asymmetrical power rails to compensate and ensure symmetrical clipping.

That is not in the spec sheet for the opamp.

Craig
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Old 29th Nov 2021, 12:06 am   #16
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Default Re: Quad 34 preamp volume knob clicks

This is a page of measured results for the unadulterated 34 https://www.kenrockwell.com/audio/qu...m#measurements

It is pretty darned good. Now audiophiles turn up their noses at the 34, for no good reason.

I'm pretty sad in hindsight that I sold mine

Craig
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Old 29th Nov 2021, 5:31 pm   #17
shaunpw
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Default Re: Quad 34 preamp volume knob clicks

Update: Chasing voltages all day !

Sticking with C30, there was -0.040 volts on the input side(+) 0.0v on -ve
This same voltage was present on ic27. I removed the through hole link to isolate the track to c30 (no interference there).
The voltage was -0.040 when "Radio" was selected. I removed C3&4 & D3,4,5,6 just leaving the 100k resistor on IC2 input. The voltage dropped to -0.025.
When I selected "CD" input, the voltage dropped to -0.003, so looking at what else might be introducing a voltage, I removed C27&29 effectivly removing the "Disc" input.
The voltage of -0.025 is still present when "Radio" is selected but drops to virtually nothing when "CD" is selected.
Unfortunately the clicks are still there.
I still think that IC1 or 2 might have something to do with introducing the voltage and for now I have run out of desolder braid !
I've ordered cd4016 & cd4066 and some tle2072 op amps (burr browns)
I'll let you know what happens.
Many thanks again.
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Old 29th Nov 2021, 5:38 pm   #18
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Default Re: Quad 34 preamp volume knob clicks

Quote:
Originally Posted by Craig Sawyers View Post
This is a page of measured results for the unadulterated 34 https://www.kenrockwell.com/audio/qu...m#measurements

It is pretty darned good. Now audiophiles turn up their noses at the 34, for no good reason.

I'm pretty sad in hindsight that I sold mine

Craig
Craig, how do the 34 and 44 compare?
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Old 29th Nov 2021, 5:57 pm   #19
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Default Re: Quad 34 preamp volume knob clicks

My take on the 44/34 only having read the service manuals over lunch today. 44 was before the 34 and did much the same thing with the added virtue of swappable input modules, you could have five turntables! They (QUAD) where the best kit using available components designed well. I reckon that if Peter Walker was doing it today there would be a DSP preamp and digital power amplifiers, much as current AV systems are.
 
Old 29th Nov 2021, 6:15 pm   #20
Craig Sawyers
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Default Re: Quad 34 preamp volume knob clicks

Yes - the 44 (1979) was introduced before the 34 (1982). The 44 used the more conventional +/-15V rails. But sometime between those two dates, Quad figured out that the 071/072 clipped asymmetrically and skewed the (lower) voltage rails to +8.6V -9.4V.

But the 44 was very flexible with input modules, all of which were pluggable in any combination. The 34 had a single selectable module for disc - either moving magnet or moving coil being the choices.

Amazingly the 34 only consumes 2.5W, which is pretty impressive.

Craig
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