ACMP-81 hum sounds

dgatwood said:
Which is relatively minor
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I'm not sure about that. Recall that earlier on this thread it was demonstrated that the "pop" position caused a DC bias on the output transistor that resulted in power dissipation at the emitter resistor that exceeded its rating. There was one failure of that part observed; this could be a long-term reliability problem, not to mention a potential hazard.
 
crazydoc said:
Loosen the bolt holding the transformer down. Then rotate it clockwise or counterclockwise while listening, to find the place where noise is minimized. You have to do the transistor swap first on the ACMP81 - otherwise the oscillation noise overpowers the inductor hum.
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It may depend on your 81. That certainly doesn't jive with my experience. My experience was that fixing any single source of hum improved things noticeably.

The oscillation causes all sorts of weird behavior in certain positions of the pots, but the hum itself seemed to be primarily induced noise---mostly in the 5-line power wire to the EQ boards. The transistor swap fixed the weird swishing noises and sudden changes in hum when you turn the trip pots.

Ultimately, though, moving that power connector from board 6 to board 2 made the biggest difference in noise level for me (down to an undetectable level). If I were doing this again with a fresh unit, I'd start by moving that power connector and go from there.
 
dgatwood said:
Interesting. This at least suggests that all of these hum problems are caused by variations in the power transformer.
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As noted by Steve Hogan several months ago:
http://www.prodigy-pro.com/diy/index.php?topic=27791.msg391935#msg391935

These preamps seem to have such individual idiosyncratic problems that no single fix applies to all the units.

All the more reason to change the circuitry to with with a single-secondary transformer that can be purchased off the shelf instead of the current (junk) custom power transformers.
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I'll be very interested to see if you can get all circuitry to work correctly off a single supply, and if it fixes the noise issues. (BTW, the transformer you suggest, http://www.radioshack.com/product/index.jsp?productId=2102703&tab=techSpecs , won't fit in a 1U case, but I'm sure there are smaller ones available.)

But I still wonder why they put the output amp on a different supply from the rest of the circuits - it doesn't make sense from a cost standpoint, and I can't see an obvious reason from an electronic standpoint. Maybe you'll find out the reason for it, or that there is no reason for it.

I was hoping Steve Hogan would have some info by now on his transformer's effect on a fix for the noise issues (which after all was the main issue to be addressed in fixing these units,) but nada so far.

I have fixed all the issues in one 81 and a 73 to my satisfaction by shielding, transistor swap and gain switch mods. My only remaining problem is noise at high gain in the first preamp stage, which substituting different low noise transistors has not helped appreciably. But it's probably not a real life problem in normal recording situations.
 
mshilarious said:
I'm not sure about that. Recall that earlier on this thread it was demonstrated that the "pop" position caused a DC bias on the output transistor that resulted in power dissipation at the emitter resistor that exceeded its rating. There was one failure of that part observed; this could be a long-term reliability problem, not to mention a potential hazard.
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By "minor", I meant that it doesn't affect normal operation of the unit and can be easily avoided by just not leaving it in a nonexistent position. I'm not saying it shouldn't be fixed. I fixed mine by dropping a 5W wire-wound in there.
 
crazydoc said:
But I still wonder why they put the output amp on a different supply from the rest of the circuits - it doesn't make sense from a cost standpoint, and I can't see an obvious reason from an electronic standpoint. Maybe you'll find out the reason for it, or that there is no reason for it.
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The 81 is the only one that does it that way. Both the 73 ad 84 have only one 24V supply that feeds both the amp board and the EQ boards. I have no idea what they were thinking... with regards to any of the weirdness in this power supply design.
 
dgatwood said:
So a few minutes with the Dremel and a few minutes of painful bridge soldering later, I got the ACMP-81 hooked back up with the DC power cable plugged into the second board from the left (because there was plenty of space to drill holes on that board). Apart from a bzzzert sound for a fraction of a second at startup, the hum level is zero with the output turned all the way up and a medium input gain setting. The EQ hum that plagued me before is just plain gone.
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You and I seem to have very different iterations of the 81.

From your first posts about yours (so many months ago) IIRC it seems a lot of the problems (at least in one unit) were related to relatively large ripple in your DC power. The ripple on mine was just a few millivolts.

Most of my noise was fixed by the Q4-5 transistor substitutions (I couldn't even evaluate the inductor hum before that was fixed, it was so loud.) Then the inductor noise was eliminated by shielding the toroid and the inductors.

There is still substantial "electron noise" both from the EQ circuits and the first stage preamp (in mic mode) at high gain.

I just now tried moving the DC power cable with toroid and one inductor shield removed - no audible difference. I then inserted the cable into a braided mumetal shield along its entire length - again no change, grounded or ungrounded. The inductor hum was unchanged. Only re-applying the shields eliminated the hum (which can still be barely heard, however, at max gain out of the preamp and max gain into the power amp, with EQ settings on the inductor boards at mid position - no boost or cut - and frequencies set at the low end.)

I don't think there is a one-size-fits-all answer for these - that's why I'm hoping Steve Hogan's new transformer will fix most of the problem. Maybe your new power supply will also, at a significantly lower cost. However, the new toroid would be essentially a drop-in replacement, allowing most folks to make the switch themselves without much electronics experience.
 
crazydoc said:
You and I seem to have very different iterations of the 81.

From your first posts about yours (so many months ago) IIRC it seems a lot of the problems (at least in one unit) were related to relatively large ripple in your DC power. The ripple on mine was just a few millivolts.

Most of my noise was fixed by the Q4-5 transistor substitutions (I couldn't even evaluate the inductor hum before that was fixed, it was so loud.) Then the inductor noise was eliminated by shielding the toroid and the inductors.
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Those aren't mutually exclusive. The reason that transistors amp circuits oscillate usually has to do with ripple on the DC power line, usually involving insufficient filter caps between amp stages. It is entirely possible that the reason my transistor mods didn't make as much difference is because the other changes I made (bigger filter caps, better grounds on the ground side of those caps) made the mods unnecessary. It's really hard to know. :)


crazydoc said:
There is still substantial "electron noise" both from the EQ circuits and the first stage preamp (in mic mode) at high gain.
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You mean the white noise hiss? Yeah, these things are anything but quiet.


crazydoc said:
I don't think there is a one-size-fits-all answer for these - that's why I'm hoping Steve Hogan's new transformer will fix most of the problem. Maybe your new power supply will also, at a significantly lower cost. However, the new toroid would be essentially a drop-in replacement, allowing most folks to make the switch themselves without much electronics experience.
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You're assuming I'm actually going to spend the time redesigning the supply. Mine are already working cleanly at this point with just shielding changes, transistor changes, cap size increases, and moving the power cable to the opposite end. I'm not sure there's a pressing need for me to build one. It's easy enough to sketch out the schematic, though, for somebody interested in building one.
 
PSU schematic sketch. Have a look and tell me all the things wrong with it.

New_PSU.png
 
dgatwood said:
PSU schematic sketch. Have a look and tell me all the things wrong with it.

New_PSU.png
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Capacitances are generally too small, try 10x or like 20x on the regulator outputs. Break up C3 into a few caps to lower ESR. Some small series resistance at the regulator outputs will help their smoothing caps. Use a full-wave rectifier on the 24V/12V rail since that is high-current, otherwise you have to fully charge C3 on the + cycle every time, which limits current/increases ripple. What is the load on the 12V rail? Watch the power dissipation on the 12V rail, you are chucking out >50% of power there.

Here is a full-wave sketch; I think this works but I will try to build it later:
 
I did not notice any drop in volume when moving the ribbon cable away from the transformer.

I did however notice that moving the transformer did reduce the hiss/hum a bit. However when I rotated it back to the original position the reduction stayed.
Perhaps there is/ was a loose wire?
 
iownrocknroll said:
I did not notice any drop in volume when moving the ribbon cable away from the transformer.

I did however notice that moving the transformer did reduce the hiss/hum a bit. However when I rotated it back to the original position the reduction stayed.
Perhaps there is/ was a loose wire?
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Heisenberg had a theory about these preamps, I believe.
 
Can you explain how you moved the ribbon cable to a jumper? I am considering doing this mod on one of my 81s.

What is the order of the wires?

Can I just soilder the ribbon straight onto a jumper?
 
iownrocknroll said:
Can you explain how you moved the ribbon cable to a jumper? I am considering doing this mod on one of my 81s.

What is the order of the wires?

Can I just soilder the ribbon straight onto a jumper?
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I actually didn't move it to one of the jumper boards. That was the original plan, but I ended up moving it to the second board from the left because the traces on the jumper boards were on the wrong side and I didn't want to try to figure out how to build plated vias myself.

The order of the pins on the cable is the same as the order of the middle five pins on the jumper boards and all the parallel traces connected to those five pins on each of the EQ boards. You just have to figure out which side of the cable is the 24V side and which side is the 12V side (read the markings on the board) and make sure you get the orientation right at the other end.

Basically, I did the following:

1. Took a 6-pin male Molex header and cut off one pin with a pair of scissors.

2. Bought a Dremel Stylus, a chuck, and a set of micro drill bits. Used them to drill a fairly even row of five holes through the traces in the second board from the left (the wide board with two big knobs on it).

3. Used the Dremel to sand off the coating on the traces I just drilled through.

4. Very carefully and painstakingly created solder bridges from each pin to the exposed copper traces on each side.

5. Checked to ensure all the traces were connected correctly and were not shorted.

6. Redid steps 4 and 5 a couple of times. :)

Note that this is pretty tricky to get right. It definitely isn't the sort of thing that I'd recommend to people unless they're really comfortable with tools. Either way, use a multimeter to make darn sure that each trace still connects across and properly connects to the pins and that no adjacent traces are shorted together. :D

Ideally, you should also:

7. Apply an enamel coating over the exposed copper traces.

I haven't done that yet, though.
 
ny thought was to snip the ribbon cable and soilder to the ends of the jumpers.

i'm out of town now, however my thought still is that there is an issue with hpf and lpf section.

have you tried removing the jumper for the hpf lpf board first? will the other jumpers are connected

i don't understand why there is a jumper from hpf/lpf board that goes back to the pre amp board

i don't think that jumper is needed either.

i will start investigating my hpf/lpf and disconnect this jumper first. and also look into taking out the jumper from the hpf/lpf board that goes to the pre amp board, since the current could flow bi directional in this circuit for power, i'm wondering if this is inducing hum back into the input gain circuit, since the hum/hiss is present regardless of input gain


these conditions may be specific to my 2 81s
 
iownrocknroll said:
My thought was to snip the ribbon cable and soilder to the ends of the jumpers.
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That should work fine.


iownrocknroll said:
i don't understand why there is a jumper from hpf/lpf board that goes back to the pre amp board

i don't think that jumper is needed either.
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Yes, it is. That's how the signal gets passed to/from the EQ stages and how the preamp board gets power.

Basically, the way the device looks is this:

Main amp
board

Preamp Board EQ1 EQ2 EQ3 EQ4 EQ5

Note that the preamp board is officially called board #1, EQ1 is board #2, EQ2 is board #3, etc.

You have the main amplifier board up above the row of six cards. The leftmost board is the preamp gain stage. Its output goes two places, depending on the position of the EQ switch:

A. to the main amp board.
B. to EQ5. Actually, it goes out of the preamp stage through what is IIRC the top pin on that jumper card (as seen from above and in front of the unit), and into EQ1. From there, it passes unaltered via a straight trace across that EQ board to the next jumper, to the next EQ board, etc., until the preamp's unfiltered output reaches the input of EQ5.

After EQ5 processes the signal, it sends the signal out via what is IIRC the bottom pin on the jumper card. The signal then passes across to EQ4 for further processing, then to EQ3, then to EQ2, then to EQ1, and is finally returned to the preamp card. From there, the EQ switch decides which signal to send to the preamp stage---the output of EQ1 or the unfiltered output of the preamp stage.

You're probably wondering why the main amp board also has a cable across to EQ5. This drives the clip light for the main gain stage and is not part of the audio path (the slight potential for noise injection notwithstanding).

Also, the +/-12V rails and the +24V rail are connected to the preamp stage exclusively via that jumper card. Without those rails, the preamp stage would be completely dead. The power cable from the power supply to the preamp board provides +48VDC for the phantom power, and that's it. Only the main amp board is powered by the other +24V rail.
 
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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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