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Old 29th Nov 2021, 6:13 pm   #21
Mooly
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Default Re: Quad 34 preamp volume knob clicks

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Originally Posted by shaunpw View Post
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.
-0.04v or -40mv is quite high, it really should be zero.

Quick and dirty tests. It is much better if possible to try and troubleshoot in as minimally invasive way as possible.

1/ Select Radio and link out R30, the 10meg on on IC27 input. The output of IC27 must now be 0.00 volts. Check it is.

2/ Assuming that is OK remove the short on R30 and now short R29 (10meg) which is the input to the bilateral CMOS switch IC1. The voltage on IC 27 output should still be zero volts. Check it is.

3/ Assuming that to is OK now remove the short on R29 and short R13, the 100k at the input. Again the output of IC27 must be 0.00 volts.

Remove all shorts.

If it passes all those tests we have few more involved scenarios. Ideally a check with a scope should be made on all the above nodes to make sure that the problem is pure DC and not some dynamic signal that is present.

Other possibilities are leakage on the PCB which can be hard to trace and prove.

Also check that ALL opamps are FET types and original such as IC25. Use of a bjt type would create a small DC voltage at its input due to the tiny bias current flowing out of the input pin.

The Bilateral 4066 are not beyond suspicion although in my experience the opamps probably are (if original).

Leakage on a PCB can sometimes be inferred by burying the probe tip into the board near a supply line and seeing if you get a small voltage reading.
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Old 29th Nov 2021, 6:13 pm   #22
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Default Re: Quad 34 preamp volume knob clicks

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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
Thanks. I upgraded my 44 with OPA134 op amps and I could definitely hear an improvement in detail.
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Old 29th Nov 2021, 8:16 pm   #23
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Default Re: Quad 34 preamp volume knob clicks

Hi Mooly,
I can see your method here, its basically what I was trying to do but without shorting the inputs (I wasn't sure that would be a good idea and my Tannoy Eatons are irreplaceable !) I don't own a scope anymore.
I have noticed that one of the op amps is an OPA381 so someone must have replaced this before I bought the preamp. I not sure if its a good replacement for the original.
I'll try the shorting method tomorrow.
Thanks.
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Old 29th Nov 2021, 8:20 pm   #24
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Default Re: Quad 34 preamp volume knob clicks

On a positive note, I gave the 34/405-2 a good test this evening, I'm not sure if my ears were tricking me but it does seem to sound much better than it did, absolutely crystal clear with no background noise .
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Old 30th Nov 2021, 12:37 am   #25
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Default Re: Quad 34 preamp volume knob clicks

Quote:
Originally Posted by stevehertz View Post
Quote:
Originally Posted by Craig Sawyers View Post
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
Thanks. I upgraded my 44 with OPA134 op amps and I could definitely hear an improvement in detail.
Although the OPA134/2134 have higher performance (particularly in lower distortion and noise) as compared with the 071/072, they are much higher current draw, 4 to 5 mA per opamp as compared with 1.5 to 2.5mA. So the power supply needs some careful consideration.

Craig
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Old 30th Nov 2021, 7:06 am   #26
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Default Re: Quad 34 preamp volume knob clicks

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Hi Mooly,
I can see your method here, its basically what I was trying to do but without shorting the inputs (I wasn't sure that would be a good idea and my Tannoy Eatons are irreplaceable !) I don't own a scope anymore.
I have noticed that one of the op amps is an OPA381 so someone must have replaced this before I bought the preamp. I not sure if its a good replacement for the original.
I'll try the shorting method tomorrow.
Thanks.
The OPA381 is a bizarre choice of part. It is called a 'transimpedance' amp and not designed for the type of normal opamp circuitry used in the Quad.

However, it also has a maximum supply voltage rating according to data sheets of 5.5 volts.

It needs to come out now. Which opamp on the circuit has been replaced by this?

OPA381 DATA SHEET

Also Craig makes a valid point on current consumption. Given the high number of opamps a small current increase adds up. You need to make sure the 7918 regulator is not running to hot
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Old 30th Nov 2021, 7:54 am   #27
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Default Re: Quad 34 preamp volume knob clicks

Quote:
Originally Posted by Craig Sawyers View Post
Quote:
Originally Posted by stevehertz View Post
Quote:
Originally Posted by Craig Sawyers View Post
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
Thanks. I upgraded my 44 with OPA134 op amps and I could definitely hear an improvement in detail.
Although the OPA134/2134 have higher performance (particularly in lower distortion and noise) as compared with the 071/072, they are much higher current draw, 4 to 5 mA per opamp as compared with 1.5 to 2.5mA. So the power supply needs some careful consideration.

Craig
Oh... Have you any experience of what needs to done in practical terms?
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Old 30th Nov 2021, 10:49 am   #28
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Default Re: Quad 34 preamp volume knob clicks

Hi Mooly,

The OPA381 had been fitted in IC26 slot. The new op amps are on their way so I will have a matched set, i'll check the regulator when they are all in.

Back to the clicking problem.
I've carried out the shorting sequence and can confrim :

R30,32 short - 0.0v all input selections.
R11,13 short - radio 0.0v, cd 0.007v.
R29,31 short - 0.0v all input selections.

All these test are with the OPA381 installed but with R145,146 shorted to isolate its output.
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Old 30th Nov 2021, 11:29 am   #29
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Default Re: Quad 34 preamp volume knob clicks

Something doesn't add up here...

IC26 is shown as a dual opamp which would be a TL072. The OPA381 is a single opamp but there is an OPA2381 which is a dual.

As to the voltage checks... if we concentrate on the radio input then all seems good as far as those tests go. Now the big question is whether there is leakage anywhere.

I'm assuming you removed the shorts before moving on to the next i.e. R30 shorted and zero volts... all good. Then remove that short and move on to shorting R29 before finally testing with R13 shorted and all other previous shorts removed. It is a sequence working forward toward the selected input to see where the voltage is appearing.

Anyhow, was it silent operating the volume control when Radio was selected and you had 0.00 volts on IC27?

R11/13 and the 7 millivolts. Hmmm. Are C89 and C90 linked out? Is the 7 millivolts occurring at IC24 output pins and carrying through.

It's not easy working from multiple screens trying to keep track of it all
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Old 30th Nov 2021, 11:39 am   #30
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Default Re: Quad 34 preamp volume knob clicks

Just adding a bit more to this to hopefully make it easier to follow as its getting a bit hard to navigate (for me ).

Concentrate on one input at a time. With the correct opamps fitted then zero volts DC at any point in the selected signal path must be achieved.

The input impedances of any floating inputs are quite high and could be picking up noise which might register on a DVM as the pickup would be asymmetric. To avoid that it would be worth shorting each input that is under test at the input socket itself. A Din shorting plug would be ideal.
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Old 1st Dec 2021, 9:28 am   #31
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Default Re: Quad 34 preamp volume knob clicks

I have the Quad service data manual which shows how to make up a test din plug with 1k resistors, is that the one you mean ?

When the voltage on the outputs of ic27 is 0.0v, the volume knob is silent, and yes I was removing the shorting links before moving on.
C89 & 90 are not linked, I removed the links from all shorted caps.
I still need to trace the voltage back.
I'm kind of understanding how this thing works now and i'm sure i'll find the problem (especially with all the help you have given me !)

I'll post again when i've fitted the new op amps.

Thanks.
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Old 1st Dec 2021, 9:44 am   #32
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Default Re: Quad 34 preamp volume knob clicks

That OPA381 wouldn't be a LM381 perchance?

That's a dual low noise preamplifier though it's bipolar rather than FET input. It would at least work as unity gain buffers in the circuit.
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Old 1st Dec 2021, 11:24 am   #33
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Default Re: Quad 34 preamp volume knob clicks

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That OPA381 wouldn't be a LM381 perchance?

That's a dual low noise preamplifier though it's bipolar rather than FET input. It would at least work as unity gain buffers in the circuit.
No not the same but the LM381 is a good chip never the less.
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Old 1st Dec 2021, 12:09 pm   #34
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Default Re: Quad 34 preamp volume knob clicks

The orgingianation of the linked out capacitors is from DADA audios "upgrades" where they replace caps. with links, fine in a static condition for the '34 but as we have heard not to good in reality. I would put it back to QUAD specs..


Handy hint, when replacing easily "socketable" ICs do use a socket, save wrecking the board on the third attempt.
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Old 1st Dec 2021, 12:33 pm   #35
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Default Re: Quad 34 preamp volume knob clicks

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Quote:
Originally Posted by Herald1360 View Post
That OPA381 wouldn't be a LM381 perchance?

That's a dual low noise preamplifier though it's bipolar rather than FET input. It would at least work as unity gain buffers in the circuit.
No not the same but the LM381 is a good chip never the less.
Agreed, but the bias currents are higher than the 071/72, which might give rise to clicks.
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Old 1st Dec 2021, 1:54 pm   #36
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Default Re: Quad 34 preamp volume knob clicks

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I have the Quad service data manual which shows how to make up a test din plug with 1k resistors, is that the one you mean ?

When the voltage on the outputs of ic27 is 0.0v, the volume knob is silent, and yes I was removing the shorting links before moving on.
C89 & 90 are not linked, I removed the links from all shorted caps.
I still need to trace the voltage back.
I'm kind of understanding how this thing works now and i'm sure i'll find the problem (especially with all the help you have given me !)

I'll post again when i've fitted the new op amps.

Thanks.
I hadn't actually seen the 1k's thing in the manual but yes, that would work as 1k is low enough. Usually we just specify a 'short' as that removes 100% of any stray pickup. Just short the pins together, job done.

As you are finding out, it is the very small DC voltages on the signal lines that are the cause of the clicks. Any DC is fed to the output as a 'step change' in level as the gain (volume control) is altered and you hear that as a click. That applies even with AC coupling as it is the sudden change that is passed along the signal path.

Quote:
Originally Posted by Herald1360 View Post
That OPA381 wouldn't be a LM381 perchance?

That's a dual low noise preamplifier though it's bipolar rather than FET input. It would at least work as unity gain buffers in the circuit.
The LM381, the 381N and the rare 381AN (ultra low noise version) are not really opamps in the sense we know today. These were really for single rail operation and would not directly slot into standard opamp configured circuitry.
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Old 1st Dec 2021, 3:49 pm   #37
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Default Re: Quad 34 preamp volume knob clicks

In general, the DC bias currents on the inputs of FET input opamps (JFET and CMOS) are orders of magnitude lower than those of bipolar opamps. Similarly, their input impedances at low (audio) frequencies are also much higher.

So the design techniques for the two families of opamps are rather different. It isn't only a matter of scaling of component values, sometimes which circuit topology around them proves to be optimum can be quite different. There are also different countermeasures to be taken to dodge the different failings of the two families.

So hopping from bipolar to FET or vice-versa needs all the work of a full redesign doing, just to check things anyway. Someone or another may say xxxxx amplifiers are good, but did they do all the sums? Did they impartially and quantitatively verify results... can you actually trust them?

Those low bias currents in the TL072 family let Quad take a few short cuts without paying penalties in other areas. Some of these issues affect things like switch clicks and noisy pots.

The OPA prefix amplifiers originally came out of Precision Monolithics inc. who were eventually gobbled up by Analog Devices, who kept the OP- prefix for that line of devices and added some to it. The issuing of numbers has no relationship to the numbers used by National Semiconductor (since gobbled up by Texas instruments). You have to be familiar with the family trees or else spend a lot of time comparing datasheets.

The opamp comparison tables in The Art of Electronics were once the best cross-brand comparisons, and they're still pretty useful.

So just swapping amplifiers alone is not a good guide to which is best, a lot is missed if circuit changes aren't made to optimise for each part in turn. Otherwise, it's as likely to be misleading as helpful.

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Old 1st Dec 2021, 7:36 pm   #38
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Default Re: Quad 34 preamp volume knob clicks

Thats a good point there about using a different device that is said to be a good replacement for the original.
My Quad 34 has OPA2604 op amps (apart from the one OPA381) which look to be factory fitted, the schematic diagram and parts list show them as TLE072.
I don't know enough about electronics to know if the circuit works better or worse with either.
Also, trying to find the correct replacement for that op amp is a mine field.
There is a fair choice of chips that would work but would some of the essence of what Quad created in their original sound be changed ?
Luckily, the prices are quite low so its not too much of a loss if they have to be changed. (I have mounted ic sockets).
Its not just DADA Audios that do the capacitor linkout, Keith Snook also carries out the same mod.
The way I see it, the circuit designer would not have put them there just to fill up space on the pcb and even though my particular fault hasn't been fixed yet, it did highlight the fact that the capacitors are doing something.
I have removed all the links that shouldn't be there.

Shaun.
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Old 1st Dec 2021, 7:54 pm   #39
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Default Re: Quad 34 preamp volume knob clicks

The OPA2604 was always was of my favourite devices but I don't think it was around when the Quad was produced. Apart from its higher power consumption the OPA2604 should be OK replacing the TL072. In fact the OPA2604 was always one of my favourite devices and it is a shame it was dropped in DIP8 form.

The TLE2072 is a later variation on the original TL072 and is a valid swap (again these were not around when the Quad was first produced).

The only real limitation of the original TL0xx series is a limited drive ability although the Quad comes nowhere near that point with its circuitry. They were and still are capable of excellent results.

That OPA381 is a true mystery. Please read the data sheet yourself to see how unsuitable it really is. The maximum supply voltage is just 5.5v, it should not even survive running on the rails the Quad uses. Truly bizarre that someone should fit that.

There are lots of misguided attempts and ideas to 'improve' equipment and while some have a genuine basis to improve certain aspects, many more do not. Correctly used electrolytics have no detrimental effect in passing audio signals.
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Old 1st Dec 2021, 8:45 pm   #40
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Default Re: Quad 34 preamp volume knob clicks

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Correctly used electrolytics have no detrimental effect in passing audio signals.
Audiophools take note!
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