ACMP-81 hum sounds
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dgatwood
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One of the problems is likely that the decoupling caps here really aren't between the stages.... 
They look like they are in the schematic, but in the actual layout, it doesn't look that way at all.... This might reduce their effectiveness at absorbing noise on the supply rail and preventing it from leaking into the adjacent board. Dunno.
They look like they are in the schematic, but in the actual layout, it doesn't look that way at all.... This might reduce their effectiveness at absorbing noise on the supply rail and preventing it from leaking into the adjacent board. Dunno.
dgatwood
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It's an interesting theory, but I don't think 7Q* can be implicated here. The power supply to 7Q* is electrically isolated from the power supply to the other boards in the 81. So the supply resistance from 7Q* back to Q4 on the other boards should be infinite, or nearly so.
That's one the larger differences between the 81 and, for example, the 73---the 73 has only one 24V regulator, while the 81 has two---one for the main output board (7) and a second one for the EQ, filter, and amp boards (1-6).
Truly, I'm at a loss to explain why the output gain affects it other than that it is likely changing the capacitive load on the output, and oscillating circuits are frequently sensitive to such things....
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Ouch, but I'm still fond of my theory. There may be a more plausible loop with
right phase. I'll no doubt be looking -- I find the problem compelling.
crazydoc
Master Baiter
I don't know if this will help figure it out, but just now going back and looking at my hacked 81, I remember that I didn't replace the transistors on board 6. When I replaced them on boards 4 and 5 (the inductor boards) and tried it out, I got the EQ buzz for 10 or 15 seconds after I engaged the EQ switch, and then it disappeared. When I replaced the transistors on board 3, the noise completely disappeared, so I never did replace them on board 6.
dgatwood
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Zmix was saying that the bias current is set too high for the transistors, which (I think) makes sense if the original design was for parts with a much lower Hfe. That said, I'm not really clear on how Hfe, bias current, etc. all interact, so I could be completely off there.
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The currents are all determined by the supply voltage and the dividers on the bases of Q1 and Q5. The transistor specs don’t come into it as long as they are all Si types. When you do the math, the currents come to Q1:200uA, Q2:400uA, Q3:1mA, Q4:12mA, Q5:8mA.
One wrinkle I already pointed out is that the Q5 transistors TNC used were manufactured with the emitter and collector leads reversed. They are marked as Fairchilds but I would guess they are fakes. I have no idea what the backwards specs are, but in the stock ACMP-81s you end up with about 17mA in Q5. Q4 and Q5 are still well within their current and temperature specs, but they do run 5-10C warmer.
After you change Q5, all the dc currents and voltages on the board will have their spec values. The problem of Q4 and Q5 running slightly warm is also fixed. But the buzz may still be present.
It should be apparent from these facts that while zmix’s fix works, it was just a happy accident based on flawed deduction. The buzz is clearly not related in any way to overheating, excessive current, or transistors with insufficient current or temperature specs.
crazydoc
Master Baiter
There still remains the question of the 70MHz RF signal/oscillation present with the original transistors, which disappeared along with the broadband EQ noise with the transistor swap. This could be coincidental, but they seem suspiciously related to me. I don't know whether anyone but myself and Steve Hogan has documented the presence of this in their unmodified 81's.
dgatwood
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I always thought that seemed pretty odd. Still, I'm curious why it does seem to significantly diminish the buzz in most cases---maybe the shielding effect of the metal cans on the transistors?
dgatwood
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FWIW, I don't remember noticing any oscillation on my scope on either of mine, but the transistor swap still reduced my hum. So the oscillation may be a red herring even if the transistors are legitimately bad.
dg, The buzz has to do with the gain of Q4 on the eq boards. Q5 only plays a role to the extent that it changes the operating environment of Q4.
It turns out you can get the buzz with only one board powered up and all its interconnects pulled except for the power. I did this with board 6, measuring across 6R32 and swapping Q4 from a BC560C (buzz) to an MPSA56(no buzz). The gain of these transistors was 600 and 200 respectively.
Since you can get the buzz with one board in isolation, the options for whats happening are pretty limited. I think the most plausible loop is thru the supply from Q4 to Q2. A minus voltage at the emitter of Q2 makes a small current in Q2. This makes a big reduction in the current in Q4. Supply resistance will make this positive feedback back at Q2. It is apparently not at a sufficient level to cause oscillation but it does subvert the ability of the main loop to remove supply hum. Thats my best guess anyway.
crazydoc, We live in a sea of RF and any piece of metal is an antenna. It is present in every piece of audio gear. You cannot tell if something is oscillating simply by putting a probe on it.
crazydoc
Master Baiter
I live in the boonies with no TV reception and one radio station that doesn't come in well. And the signal disappeared with the transistor swap. Maybe coincidental, but I suspect they are related. Maybe I'll have to get out my virgin 81 and check it out.
Maybe you should. If you were able to prove you had measured an oscillation or more importantly demonstrate that there is a mechanism by which low level rf can increase supply hum, it would be remarkable.
My experience tells me the probablility of success is zero. I would list more reasons, but I'm sure you could come up with a clever counter for each without leaving your keyboard and it would get us nowhere.
crazydoc
Master Baiter
No need to get snippy.
Here's my original post on the 70Mhz signal present with the EQ switch engaged, and its resolution (along with the EQ noise resolution) after progressive transistor substitutions.
https://homerecording.com/bbs/showpost.php?p=3084168&postcount=7286
dgatwood
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Doesn't mean the high frequency oscillation caused the hum, though. It is much more likely, IMHO, that they both have a single common cause---probably fake Chinese transistors.
The hum on the power rail causes hum in the output that then feeds back onto the V+ rail, which is amplified again, etc.
In addition, the resistors and capacitors in the feedback path are causing the transistor to act as some sort of RC feedback oscillator, causing the 70 MHz oscillation.
Swapping the transistors likely fixed both the instability and the hum amplification by removing the feedback loop. My guess is that some of these transistors have abnormally high leakage or something.
The hum on the power rail causes hum in the output that then feeds back onto the V+ rail, which is amplified again, etc.
In addition, the resistors and capacitors in the feedback path are causing the transistor to act as some sort of RC feedback oscillator, causing the 70 MHz oscillation.
Swapping the transistors likely fixed both the instability and the hum amplification by removing the feedback loop. My guess is that some of these transistors have abnormally high leakage or something.
crazydoc
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That certainly could be - they do seem to be related - but the causal factor is still unclear.
The important thing, practically, is that swapping the transistors fixes the problem. But it would be nice to have an explanation.