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Pinpointing the source of hiss in a phono stage
Pinpointing the source of hiss in a phono stage
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Old 15th November 2019, 07:51 PM   #91
MarcelvdG is offline MarcelvdG  Netherlands
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Hi Chris,

Doesn't it seem odd to you that Matt sees the oscillations with inputs shorted rather than inputs open? If it were due to parasitic capacitive coupling to the oversized input capacitors, I would expect the biggest problems to occur with open inputs.

In any case, if your hypothesis is correct, then the circuit should work fine once the capacitors are replaced with physically smaller capacitors.

If my hypothesis is correct, then the circuit will still have a large resonance at some deep subsonic frequency once the capacitors are replaced with somewhat more lossy types, at least when it is driven from a very low impedance source. Besides, since you have the experience that the circuit should not require lossy capacitors, there must then still be something wrong somewhere (faulty component, loose contact, whatever). If it is something that is not common to left and right, that will probably mean that one channel will work fine and the other not.

So I'd suggest to measure the frequency response driven from a low impedance on both channels from whatever frequency it used to motorboat on up to 20 kHz. If it looks fine on both channels, then everything is well. If not, then we have to take it from there.

Best regards,
Marcel
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Old 15th November 2019, 08:11 PM   #92
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by MarcelvdG View Post
I already did when I wrote that the losses might stabilize the loop (ESR + 470 ohm causes a zero in the loop that has a stabilizing effect).

(...)

However, in this case, it all doesn't add up. 53 ohm of ESR is much less than the 470 ohm that was already there. It could only help when the loop is at the edge of instability - which it very well might be, considering that it sometimes oscillates and sometimes doesn't. But I haven't a clue why it would be at the edge in the first place.
Having another look at the schematic, I don't see how the ESR of the input capacitors could cause a zero in the loop gain, so my hypothesis goes down the drain. Nonetheless, I still think it a good idea to check the frequency response on both channels, just to be sure it is not at the edge of oscillation due to some unknown defect.
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Old 15th November 2019, 09:05 PM   #93
dbxdx5 is offline dbxdx5
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I've been following this discussion (sort of, as you both have worlds more experience and knowledge under your belts). The one thing we know is that putting the tantalums back in the circuit did stop the motorboating. If size was the issue with the Panasonic film caps, then I should be fine with smaller Nichicon UKLs. But there is an "if." So when I order the UKLs, I also want to get a few plan B caps that would be the closest possible approximation to what the tantalums do right, in addition to being small, that might be playing a role here. A much smaller stacked film than the ECQ Panasonics?
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Old 15th November 2019, 09:58 PM   #94
anatech is offline anatech  Canada
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Pinpointing the source of hiss in a phono stage
Hi Matt,
Yes. Those are what I use (stacked film). I have often said that your replacement components should not be any larger than the originals, and this is for good reason.

Hi Marcel,
I have tested the phono stages after the work is done to verify response and THD. They don't deviate that much, given their simplicity. But I don't know if Matt as a reverse RIAA filter network. I do see the value in what you are thinking about in this case. My current belief is that the larger capacitors simply picked up more signal causing a very low frequency oscillation. Shorting the inputs would generate a charging shock in the base circuit. Maybe that "bell" just wants to ring that low in frequency. There is certainly enough low frequency gain available. Nothing else seems to work as an explanation. So I'm saying that the system gets an impulse from being shorted, and the large capacitors can pick up more stray noise to continue the oscillation (=motor-boating). Beyond that, I am at a loss to explain the way it is acting.

-Chris
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Old 15th November 2019, 10:12 PM   #95
MarcelvdG is offline MarcelvdG  Netherlands
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Hi Matt, I don't think anyone has found any reason to believe that using small film capacitors would help if it doesn't work with Nichicon UKLs. Of course you could buy some film caps that are as similar as possible to those that Chris uses, just to be sure.
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Old 15th November 2019, 10:34 PM   #96
MarcelvdG is offline MarcelvdG  Netherlands
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Hi Chris and Matt,

You can also measure the response of a phono preamplifier without using a reverse RIAA network, just with some resistive attenuators. Measure one point per octave and compare the results with the theoretical values. That's the way I've always done it; I'm sure it is more tedious, but when you only measure phono amplifiers once per decade, that's OK.

In this specific case the purpose of the measurement is to check for big subsonic peaks or other gross errors, so there is no need to measure very accurately. Just very slowly sweep the frequency and see if there are any weird bumps in the subsonic region. If not, check whether the gain drops with roughly the correct slope between 50 Hz and 20 kHz.

Regards,
Marcel
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Old 16th November 2019, 04:38 AM   #97
anatech is offline anatech  Canada
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Pinpointing the source of hiss in a phono stage
Hi Marcel,
Yes, used to do it that way. It's a pain and is very critical to get the frequency accurately set. If you don't, your values will be out. Not something I would wish on others having done this myself.

-Chris
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Old 19th November 2019, 11:17 PM   #98
dbxdx5 is offline dbxdx5
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The UKLs didn't solve the issue. In fact, when I swapped them back out for the tantalums, the motorboating was present with those as well. So it would seem that something had changed when I first reinstalled the tantalums that stopped the problem, but not putting in the caps themselves.

When I installed the UKLs I was careful not to make any other changes: I unscrewed the board, tipped it out at a 90 degree angle with the chassis, installed the new caps, tipped the board back, and screwed it in. The only thing I can think of that could have changed, even slightly, was the board and it's relation to the wiring running behind it. I mention this because the proximity of the board to that wiring and the section of chassis behind it is the only way that I can stop and start the motorboating: With the board tipped out at a 90 degree angle, there's nothing, but as I tilt it back toward where it's supposed to sit, the motorboating will consistently begin when it comes within less than an inch.

I've tried prodding the wiring, pushing it away from the back of the board, while the motorboating is occurring. And it does sound as if this causes the frequency and amplitude to change, though the wires are all grouped so closely together that it's difficult to tell which might be responding.

Anyway, I'll continue to investigate.
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Old 20th November 2019, 06:12 AM   #99
MarcelvdG is offline MarcelvdG  Netherlands
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That's a pity... If I were you, I would look again for poor solder joints or tiny fractures in PCB traces. If that doesn't help, take your scope and a 1:10 probe and measure the waveforms on all nodes you can access on both the left and right channel. Look for suspicious differences between left and right and for anything that doesn't make sense. Of course you can also take pictures of the waveforms and post them, maybe they give someone on this forum a clue as to what is going on.
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Old 20th November 2019, 07:39 PM   #100
dbxdx5 is offline dbxdx5
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Thanks Marcel. It certainly is odd. The fact that the problem cleared up temporarily when the old caps were reinstalled leads me to believe that the cause is likely the solder joints for those caps and any near to them, the wiring coming to and from the board, and/or the wiring running behind it. Nothing else changed.
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