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Old 2nd Dec 2021, 12:19 am   #41
Craig Sawyers
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Default Re: Quad 34 preamp volume knob clicks

Plucking more recent FET input opamps from the TI stable, the OPA1641 (single) and OPA1642 (dual) might be worth a deeper look. About twice the current per amp channel as compared with the TL072.

https://www.ti.com/product/OPA1642

Surface mount only of course, so if this is an option, they will have to go from SOIC 8 to DIL via adaptors (cheap).

Of course, the question is: why not stick with the 071/072?
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Old 2nd Dec 2021, 5:56 pm   #42
shaunpw
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Default Re: Quad 34 preamp volume knob clicks

ok, here we are with todays update.

I have now replaced IC1 & 2 (cd4066), IC24, 25, 26, 27 (all now OPA2134)
All the tests carried out with an input short plug.

I got myself a better DVM, now reading proper millivolts.

So, with radio selected, no resistors shorted, I get -41.1mv/-37.mv on IC27 op.
as above with cd selected, I get -4.5mv/-3.7mv on IC27 op.

Applying a short across R30, R29, R13 all give the same results, -0.5mv/-0.1mv on IC27 op.

I did find the track between pins 6,7 of IC27 and C30 was bringing a bit of stray voltage to the party so I removed the through pins and hard wired the connection with a section of screened cable, that helped stabilise all the testing.

The volume clicks are still present although it only happens once per input selection, if left on radio or cd, no more clicks after the first volume increment.

I've also checked the values of all the resistors on the input side and they are all within tolerance.

Thats as far as I have got.

Thanks for reading !
Shaun.
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Old 2nd Dec 2021, 7:05 pm   #43
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Default Re: Quad 34 preamp volume knob clicks

(I think I might get in the habit of swiping the volume off before changing input selection, if the act of ramping it back up used up the remaining click at a very low volume - and telling myself I had fixed it )
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Old 2nd Dec 2021, 7:52 pm   #44
shaunpw
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Default Re: Quad 34 preamp volume knob clicks

HaHa ! Yes, I had thought of that, once CD is selected, turn the volume very slowly until the caps charge up, it does work.
Unfortunately my OCD knows there is a fault and its going to bug me until I either fix it or find out it can't be fixed due the age/nature of the beast !

I'm not ready to give up yet.
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Old 2nd Dec 2021, 8:18 pm   #45
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Default Re: Quad 34 preamp volume knob clicks

Quote:
Originally Posted by shaunpw View Post
ok, here we are with todays update.

I have now replaced IC1 & 2 (cd4066), IC24, 25, 26, 27 (all now OPA2134)
All the tests carried out with an input short plug.

I got myself a better DVM, now reading proper millivolts.

So, with radio selected, no resistors shorted, I get -41.1mv/-37.mv on IC27 op.
as above with cd selected, I get -4.5mv/-3.7mv on IC27 op.
The only difference in signal path between radio and CD inputs is the internal switch selection within the 4066. Although all should be 0.00 volts you have a ten fold difference between the two here. There is however one important difference...

Look at the circuit.

If C89 and C90 are present and good then we can discount anything on the input side of those caps. The important difference I mentioned are the values of R143 and R144 (10k) and R11 and R13 (100k).

Your fault at this stage seems to suggest to me that the issue is caused by leakage somewhere. The 10k effectively pulls those nodes down better toward zero volts than 100k and so reduces the apparent DC voltage.

Lets try and be logical with this, the offset has a negative polarity at IC27 output and the opamp is configured for unity gain so that implies the leakage is from a negative rail onto the signal line.

I've drawn the signal line in red for one channel. I suspect anywhere on this line is seeing leakage from print in an area that runs close to a negative rail.

A quick off the wall calculation... -/+8 volt rails (so we work with 8 volt) means we have a 'leakage' resistance of around 18 meg ohms. That would cause the 100k to see 45mv across it. Swap that to 10k and we get 4.5mv.

I think something like this fits your fault symptoms. What to do though... I'm a great believer in washing boards but I'm worried here whether any leakage is down more to something with the board material or soakage into it.

Are they fibreglass boards or paxolin?

Given that modern signal sources (not old DIN standard) can happily drive loads down to about 600 ohm you could reduce the 100k's (and the 10k's).If you remove (link out) C89 and C90 then with FET opamps fitted the output of IC 24 should force a zero volt condition... assuming there is no similar leakage at that opamp input.

If IC 24 has 0.00 volts at its output now then you could do that trick. If not the problem is essentially confirmed as leakage across the whole board (imo), however R135 and R136 could be taken much lower if that input was used with a CD player and that would again force a near zero volt condition.
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Old 2nd Dec 2021, 9:34 pm   #46
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Default Re: Quad 34 preamp volume knob clicks

The pcb is fibreglass but the component side has a pale blue laquer on it that over time causes a type of black wire condition.
Either shorting or removing C89 does nothing to effect the -0.41mv on IC27.
I see what you mean about the 10k and 100k resistors, that has probably been leading me up the garden path !
I have a few more things to try yet, I'm replacing the rest of the caps on the inputs and the 4 zeners on the radio input.
Is there a way to "block" the signal from the Disc input at IC22 perhaps, I want to eliminate that part of the circuit.

Thanks
Shaun.
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Old 3rd Dec 2021, 6:51 am   #47
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Default Re: Quad 34 preamp volume knob clicks

Devil is in the detail here...

C89 affects only the CD input when that input is selected. -0.41 millivolts is getting down to acceptable levels and within what the 'input voltage offset' of a TL072 or OPA2134 might be.

Earlier we had:

Quote:
as above with cd selected, I get -4.5mv/-3.7mv on IC27 op.
which is to high.

Work with one input at a time and if needs be isolate everything on that line until you find what it pulling the line down.

You can use braid to unsolder pins 9 and 10 of IC22 (so they are free floating) and that will isolate them from that run of print.

This is for an OPA2134, the TL072 will be in the same ballpark.
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Old 3rd Dec 2021, 8:30 am   #48
Ted Kendall
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Default Re: Quad 34 preamp volume knob clicks

If you are getting tracking through the board, you could try leaving it above a radiator for a few weeks to dry things out - I know of one 34 where this procedure worked.
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Old 3rd Dec 2021, 8:41 am   #49
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Default Re: Quad 34 preamp volume knob clicks

Sorry that was a mistype, it was meant to be -41.0mv not -0.41mv.
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Old 3rd Dec 2021, 9:09 am   #50
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Default Re: Quad 34 preamp volume knob clicks

Quote:
Originally Posted by shaunpw View Post
Sorry that was a mistype, it was meant to be -41.0mv not -0.41mv.
So work with that value of -41mv. By elimination you have to isolate any possible feed in points to that line.

Heating the board as Ted suggests is actually not a bad idea. Try a hair dryer, any improvement? Try breathing on it. Any worse?

Randomly changing parts like caps does apply a lot of heat locally and that could confuse results. You might think replacing something seems to fix or improve it when in fact the heat was the reason.
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Old 3rd Dec 2021, 12:47 pm   #51
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Default Re: Quad 34 preamp volume knob clicks

So i've spent a few more hours chasing my tail today !

I have found where this -41mv originates, it does indeed come from the Radio input section. There are so few components for that input too and replacing them has not helped.
I have also removed all the through pins associated with that input and linked the points with wire effectively removing the pcb from the equation (this pcb is double sided but relies on pins to link the connection to the 2 sides)
I can change the -41mv to a lower voltage by altering the value of R13 but its not a fix.
I have more or less given up trying to fix it.

I have connected the 34 back to the amp and even with the "stray" voltage, the original fault has almost gone. The clicks are worst on the Radio input but you can hardly hear them now unless you crank the volume all the way up and then it only happens once (before, its was constant and even at low volume)
When CD is selected, the clicks are even less and almost unnoticeable.

To me this is as good a fix as I'm going to get and for the sound quality, its a small price to pay.

Thankyou for everyone's time and effort in getting me this far (especially Mooly).

Shaun.
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Old 3rd Dec 2021, 2:33 pm   #52
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Default Re: Quad 34 preamp volume knob clicks

Thanks for the kind words

I think we can say 'leakage' for this one. Whether washing and drying the boards would be a cure is impossible to say.

As to R13/11 (the 100k's) then if you are using a modern tuner or other source then you can go right down to 1k here. That should virtually remove the dc offset problem. It may not be a 'fix' but it is not an outright bodge either and the circuit operation is unchanged. It is just a change in input impedance.

If you try that then remember to upscale the two 330nF coupling caps to maintain the low frequency response. Small electrolytics are fine such as 33uF16v.

Good luck and happy listening.
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Old 3rd Dec 2021, 4:22 pm   #53
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Default Re: Quad 34 preamp volume knob clicks

You can't just scale capacitors here. the 330nF are film capacitors. Replacing these with electrolytics you need an extra factor of at least 5, and preferably 10 to prevent distortion. Douglas Self (a complete objectivist) presents a whole bunch of measurement evidence about that. Small Signal Audio Design, 2nd Ed, p71-74.

He measures the distortion of a 47uF electrolytic looking into 1k (nominally a 3.5Hz break point - so looks OK). But he distortion starts to rise from a base level of 0.0002% at about 80Hz, and by 10Hz is 0.04%.

The solution is simple, he says - increase the value. At 10Hz into 1k: 100uF, 0.008%. 220u 0.0015%. By 470uF it is close to the 0.0002% baseline.

So 33uF electrolytic into 1k (instead of 330n into 100k) will guarantee increased distortion. You need at least 220uF.

Craig
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Old 3rd Dec 2021, 8:14 pm   #54
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Default Re: Quad 34 preamp volume knob clicks

I think the distortion figures we are talking about there would be swamped by many orders of magnitude with either FM or DAB radio as a source.

As you rightly say, Doug is purely 'objectivist' based in his methodology.

33uF is small enough to be very practical as a fitment. It gives the op choices and ideas if he wishes to pursue reducing the DC offset.

The 0.33uF/100k or 33uF/1k is only 250mDB down at 20Hz.
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Old 4th Dec 2021, 9:29 am   #55
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Default Re: Quad 34 preamp volume knob clicks

Another downside of reducing the input resistors to 1K on radio (and presumably aux too) is that you will get a reduced signal level. Most sources have a series resistor to ensure stability into cable capacitance, of between ~200 ohms and ~1k. Giving between -1.6 and -6dB.

When I had a 34, I can recall absolutely no clicks from the volume control at all.

C30/31 100uF should take out any small DC offset going into the volume control. In principle of course this should be non-polar since the offset polarity is not known.

Likewise your suggested 33uF input capacitor.

Craig
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Old 4th Dec 2021, 12:41 pm   #56
Ted Kendall
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Default Re: Quad 34 preamp volume knob clicks

The other point here is that the FM4's output capacitor is specified on the assumption of standard input impedance on the 34/44. If this is lowered, the bass rolloff frequency rises, as some have found when running an FM4 into other amplifiers.
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Old 4th Dec 2021, 1:20 pm   #57
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Default Re: Quad 34 preamp volume knob clicks

You can get ceramic multilayer capacitors with remarkably high capacitance, a 100uF 2.5V job is less than a quid https://uk.rs-online.com/web/p/mlccs...citors/1692788
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Old 4th Dec 2021, 2:04 pm   #58
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Default Re: Quad 34 preamp volume knob clicks

Quote:
Originally Posted by merlinmaxwell View Post
You can get ceramic multilayer capacitors with remarkably high capacitance, a 100uF 2.5V job is less than a quid https://uk.rs-online.com/web/p/mlccs...citors/1692788
You can indeed.

But look carefully at the full data sheets on any you intend using. The capacitance value is strongly affected by the applied voltage. Often not mentioned in data, these things are also highly microphonic.

These parts are useful as decouplers on power supplies and offer high density with freedom from the drying out lifetime limitations of electrolytics, but they re highly non-linear and are effective vibration transducers.

David
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