ACMP-81 hum sounds

iownrocknroll said:
After removing the jumpers, I realized that was how the pre-amp recieved power.


Do you know where I could get replacement inductors for the 2 EQ board that require them?
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The inductors are a total red herring. The distance to the transformer in the 81 is similar to the distance in the 73 and 84, but neither of them has hum problems. That's not saying you can't improve the sound quality by replacing them, but that's the icing, not the cake.
 
hrmmmmm........So you then think the hum is the power supply/input transformer?

I don't get that, since the pre-amp on it's own shares the same power supply and without the EQ enguaged there is no noise.

I'm assuming the noise is coming from the EQ signal path
 
iownrocknroll said:
hrmmmmm........So you then think the hum is the power supply/input transformer?

I don't get that, since the pre-amp on it's own shares the same power supply and without the EQ enguaged there is no noise.

I'm assuming the noise is coming from the EQ signal path
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I think the hum comes from three main sources:

1. The power transformer producing 60 Hz hum in the power/ground to the EQ boards
2. Insufficiently large filter capacitors on the EQ boards (they're much, much smaller than the ones on the 73)
3. Bizarre oscillation problems (caused by defective transistors with insufficient power dissipation)

I think most or all of the 81s oscillate, but I'm not 100% convinced that this is the main cause of the hum. It is clearly the source of all sorts of weird clicks, swishing noises, etc. when you adjust the controls, though, so you definitely should fix that.

There may be some interference from in the inductor closest to the power supply, but those sorts of inductors are usually pretty resistant to EMI. I did shield mine, but it made only maybe a fraction of a dB difference in hum, and that was before I made several of the other changes that made a much bigger difference. It's possible that with the other changes, it might not have made any audible difference at all. Hard to say without trying every possible combination of mods. It's definitely the last mod I'd do, either way.

Do the transistor upgrade first. It will either help or it won't, but at minimum, it will prevent the transistors from dying prematurely due to running so far outside their proper thermal/bias range.

Next bump up the size of the filter caps. 1,000 uF or 2,200 uF @35V or more for all the EQ boards except the filter board.

After that, I'd probably shift the power cable down to the other end.

After doing those three mods, if you still have hum problems, then upgrade the inductors.

Just my $0.02.

Regarding the preamp by itself and supply ripple, by the time the power reaches the preamp stage, it has gone past a filter cap on each of five EQ boards.... :) Remove a couple of those EQ boards with a jumper cable and you'll get hum even with the EQ disabled, assuming your power cable is still hooked up at the rightmost end.

If you're seeing this supply hum, you'll know it because you can move the five-pin power cable around and the hum will drop significantly. You may have to make a number of other mods before that problem will be obvious, though, as that noise may well be masked by the other problems.
 
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iownrocknroll said:
Which cap, is the filter cap, and what do you recommend replacing it with?
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The power filtering caps are the big ones in the center at the top of each board between the +24V power rail and ground. Like I said in another post, 1,000uF or 2,200uF. Either way, use parts rated at 35V (at least) because of the size of the thermal envelope. Also, use shrink tubing to coat the top half inch of the leads and leave them sticking up a bit above the board. Otherwise there isn't enough horizontal clearance for such large parts at that spot on the 81's boards.
 
I have no opinion on capacitor brands. Given how little these caps have to deal with, you could probably put even the crappiest stolen-electrolyte-with-missing-ingredient caps and they'd hold up, so I wouldn't worry too much about it. I bought whatever they had at Fry's.
 
Tried shielding the power transformer. That got me another dB or so. Didn't come close to fixing it, though. That's just way too much EMI to block with shielding alone.

I'm at a conference all week. When I get back home, I'll shoot some photos of my hacked-up 81s.
 
Quick update:

After reassembling everything, one of my two 81s seems to be behaving. The other one gets weird dropouts in the output when I adjust the output level rapidly while EQ is enabled. If I change the output level slowly, it doesn't misbehave even at the same spots on the pot. At one point, I slammed it up to the top and got it to stick off for a couple of seconds before I moved the pot.

Anybody with a strong background in analog electronics have any theories on what could cause this behavior? No hum, just randomly shutting down at random output gain settings.

It appears to be confined to the second EQ board from the right (board #5). If I jumper across that board, the problem goes away; jumpering across board 4 doesn't make any difference. I didn't try jumpering across board 6, so it's possible that the problem is some weird interaction between boards 5 & 6, but my gut says it is probably just board 5. Either way, the fact that I can jumper across a board and fix the problem means it isn't just a dirty pot or something similarly trivial.

I'm going to try subbing in pulled transistors from the other boards in place of the remaining transistors on board 5 (one at a time) to see if one of the transistors is marginal. If that doesn't work... I guess I'll pull the new transistors on board 5 and replace them with more new transistors and see if that helps.... Beyond that, I'm about out of ideas. There's no bad behavior I can measure on a scope or anything because it is so transient that by the time I know it is happening, it isn't happening anymore.... :o
 
Measured hFE on the removed transistors ranges from 450 to 490, which either means my meter is a little off or several of those parts have higher hFE than the specs say they should. Not sure if this is good or bad, but my gut says I'm probably needing to lower the hFE. The one remaining BC557B on board 5 measured 492. I'm going to sub in the one that read in the 450s and see if the problem goes away. I've soldered it in place, but I can't test it until I figure out where I lost one of the jumper boards. Their color matches my throw rug....

My BC461 parts seem to be in the 110-120 range, which is well within spec for BF461-6 parts in spite of them being uncategorized BC461 parts. Not a surprise. These days, I'd expect a lot less variation in manufacturing than when those spec sheets were written up. :)

Edit: looks like I should have done the test jumpering across board 6. I swapped that transistor and there's no difference, but skipping board 6 fixes the problem just like skipping board 5 does. So the issue could easily be either of them....

Any ideas (besides tearing down the working '81 and swapping one board at a time)?
 
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I'm not the sharpest pencil in the box when it comes to circuit design, but I'd say variations in transistor gain is not your problem. Circuits are designed to minimize these effects. (Of course, this is a Chinese copycat, so...)

If you can't reproduce the problem consistently enough to check a signal at the output of each board, I'd think the easiest thing to do (easier than switching a bunch of transistors, particularly as this possibly isn't the problem) would be to swap out boards 5 and 6, one at a time, between the good and bad units, to see if it fixes the bad one and breaks the good one. Then you've probably localized it to one board, and you can proceed from there.
 
crazydoc said:
I'm not the sharpest pencil in the box when it comes to circuit design, but I'd say variations in transistor gain is not your problem. Circuits are designed to minimize these effects. (Of course, this is a Chinese copycat, so...)
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The thing is, this is almost identical to the previous behavior I saw when adjusting the EQ pots that each effectively were changing the output level of each EQ board within a certain frequency range. That problem was solved by a transistor swap (to a different kind of transistor with more current handling capacity, but much lower hFE), which is why I suspect that there is at least one other transistor that is out of spec in some way.

Interestingly, two of the three additional transistors from board 5 are of a different type than the pulled transistors, as are all four transistors on board 6. The one thing that really jumps out at me is that transistor 1 (6Q1) on board 6 is a BC214KC. On every other board, it is a BC557. This is the only consistent difference between 6 and the other boards. Does anybody have any theories about why they used a different part here on this one board?
 
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I think there must be some very serious sensitivity to differences in capacitance or something. I swapped board 6 from the working 81 into the non-working 81 and the problem went away, so clearly the problem is board 6. Then, I stuck the problem board 6 into the previously working unit and it still works. So now I have two fully working 81s. :D

I still suspect that I should probably replace those BC214KC parts with BC557s to match the other boards, but....
 
dgatwood said:
The inductors are a total red herring. The distance to the transformer in the 81 is similar to the distance in the 73 and 84, but neither of them has hum problems. That's not saying you can't improve the sound quality by replacing them, but that's the icing, not the cake.
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There is an easy way to get another piece of evidence to support this. Namely, connect any handy coil to your spectrum analyzer. I used a guitar pickup.

In the 81s I have here, the hum radiated by the power transformer is mainly 60Hz but the buzz problem is mainly 120Hz -- and the two spectra are quite different. (If anybody gets a different result, I'd be interested.)

Since the 60Hz hum is not that bad, it seems to suggest that neither the amount of power transformer leakage flux nor the ability of the eq inductors to pick it up is the main problem in the 81.

A guy in Sydney (alexc on prodigy-pro) claimed to get a hum reduction from replacing and shielding his inductors, but this was in a unit where the main buzz had already been eliminated.
 
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wscrane said:
Since the 60Hz hum is not that bad, it seems to suggest that neither the amount of power transformer leakage flux nor the ability of the eq inductors to pick it up is the main problem in the 81.

A guy in Sydney (alexc on prodigy-pro) claimed to get a hum reduction from replacing and shielding his inductors, but this was in a unit where the main buzz had already been eliminated.
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It definitely makes a small difference (like a couple of dB). Moving the power wire away from the transformer and routing power the opposite way through the boards, by contrast, was... by my ear, probably a 30 dB decrease in hum. The transistor swap made probably a 20 dB-ish difference. Note that these are guesses---I didn't do any measurement during this process.

What's interesting about that, though, is that the two main hum sources seemed to be the output transistors and induction on the power line, neither of which should have generated a 120 Hz hum.... Very odd.
 
dgatwood said:
...neither of which should have generated a 120 Hz hum.... Very odd.
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Yeah, how a transistor produces supply hum is a real mind bender.

Quick summary of my take on the zmix transistor upgrade:
- The four BC547B (Q5 on the EQ boards) in the ACMP-81s here were apparently manufactured backwards, so they were operating in reverse mode.
- They need to be flipped or replaced but do not require upgrading.
- They were drawing some extra current but were not overheating or oscillating.
- The BC557B are working ok and don’t need to be replaced.
- One way to see if you have this problem is check that the voltage across R13 on the EQ boards is not in excess of about 1.2V. If you measure something larger it is very likely that Q5 is installed backwards.
- Although the buzz is clearly supply hum, it is not a problem with the supply or power transformer. Replacing the four BC547B fixes it.
- I’m not exactly sure why this fix works but my guess is that since Q5 serves as the load for Q4, with Q5 operating in reverse there may be a loss of open loop gain which would affect the ability of the circuit to subtract out supply hum via feedback. This is just a guess since the Neve design is new to me. Maybe somebody else has a comment on this?
 
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Hey ws - great detective work. So maybe all this noise was just from a bad batch of transistors! (But still causing oscillation?) You might want to post this over at Group DIY (prodigy pro) also.

So now that you have this out of the way, you can start on the inductor hum and the gain switch blast. :)
 
antichef said:
OK here's my instructional picture:

ACMP81Guts.jpg


I should have said that the big knob actually has two screws (the other one is under the visible portion of the knob in the picture), and sometimes you have to loosen both - you probably want to tighten both when you put it back on.

Some variations are: one or two boards have an additional ribbon cable or two, but it should be obvious, and one board has a big brass post (for the lid screw) instead of a regular screw in one of the positions (it's visible in the picture to the right of the subject board, just under the inductor with the comment about my sloppy mu metal fix-attempt), but you just unscrew that, too.

after you do the stuff in the picture, you can pull the board out, then replace those two transistors with the new ones. The picture is post-mod. The little dots on the tops of the new transistors are there to show the orientation of the pins.

I repeated this for each of the 4 eq boards that have those transistors in Q4/Q5. The first EQ board has a different configuration, and a bunch of BC557 transistors (like 7), but no BC547. I left those alone for now, because the hum on my EQs is gone.

I have ACMP-73s, and they don't use the same transistors.

Another edit: Hopefully my posts don't give anyone the impression that I know what I'm doing :D -- the transistor swap was prescribed by zmix on another board, and anything else I've done has been suggested by someone else who knew better than I did. Zero original thought here. I just thought it would be convenient and maybe encouraging to see this stuff.
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thanks for the post, those transistors did the trick!
 
Thanks crazydoc, but unfortunately some mystery remains. I thought the manufactured-backwards Q5 was the end of the story, but it was not. In some cases (as Zwicky said) the Q4s had to go too, in spite of testing ok and having the correct pinout. One interesting fact is that the TNC BC557Bs only caused the buzz when their hfe measured over about 350. I tried other transistors with the same results.

So here’s a theory … Suppose a miniscule amount of supply hum gets into the Q4 of an EQ board and the board feedback is unable to completely keep it off the collector. This hum will pass through any subsequent EQ boards unaltered. It then goes to the output pot and finally to 7Q1. Since the supply has some resistance, if the caps on the board are unable to keep it perfectly clean, the hum in 7Q1 (and 7Q3) will return through the supply to Q4 as positive feedback. While there are multiple measures to prevent it, apparently if Q4 has enough gain some kind of loop like this can come into play.

Its kind of far-fetched, but it does explain all the key features of the buzz. Namely why it resembles what you get from a poorly filtered supply and why its depends so critically on the gain and environment of Q4, the output pot setting, and the position of the wire from board 7 to the output pot.

It would seem that Neve knew about this problem since there would have been no reason to spec a transistor like the BC461 in this application except specifically for its gain ceiling of 250.

By the way a cheap low noise transistor that works fine for Q4 is the MSPA56. I ended up using Fairchilds from Mouser which consistently have a gain around 200.
 
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