ACMP-81 hum sounds

The bandwidth of the amp should be a function of the phase response of the filter, so I don't think you should see any difference in phase response well inside the bandwidth-limited frequency response.

Anyway, you can add a small-value resistor in series with the cap if you are concerned with capacitive loading. These are really small capacitances though.
 
mshilarious said:
The bandwidth of the amp should be a function of the phase response of the filter, so I don't think you should see any difference in phase response well inside the bandwidth-limited frequency response.
Click to expand...

Thanks. That's what I figured, but it's good to get confirmation of that suspicion.
 
A little testing shows that 1pF doesn't have any appreciable effect on the hum. 38pF seems to be close to the bottom as far as sizes that are useful, and in one board's case (the leftmost board that has a pot), it behaves significantly better at double that (effective frequency of the filter for the least amplified path drops to 46kHz). For the other boards, it didn't make any difference.

With those tweaks, the hum is almost but not completely gone. Rotating the transformer now makes a significant difference in the remaining hum (it did not make much difference before), and I suspect moving to shielded inductors would finish it of. That's next.

Ah. You know, I wonder if the board (second from the left) that has no pot might be similarly misbehaving.
 
crazydoc said:
Well, I tried 3 different types of low noise transistors, several of each, substituting for 1Q1, 1Q2, and 1Q3. No appreciable difference. I swapped 1C1 for several different brands - no difference in noise.

I seem to remember this individual unit as being noisier that my other two - I'll have to get them out and recheck them. If no difference, I guess this noise is just what you get when you try to amplify by 80dB.
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I checked another ACMP81 and the mic preamp noise was the same, so I guess that's just the design.

I've finished working on my first 81 (except for the switch pop - I'm going to hold off to see if there's a better fix for that.) In addition to rotating the toroid, swapping the Q4 and Q5 transistors, and shielding the toroid and the inductors, I remounted the lo freq inductor horizontally in a longer shield. There is now no audible noise in line-in mode with eq engaged, but in mic mode there is still hiss/hash at the higher input gain settings with the output gain maxed, slightly increased with eq engaged.

Here is the wired-up inductor - a five conductor shielded cable with shield to be grounded at one end (though ground/shield probably not necessary.) I had removed 3 resistors and installed them on the other side of the board, as they were too close to the inductor when it was sitting vertically, so I just left it like that.

inductor.jpg


The board with the inductor clamp installed.

inductor_clamp.jpg


The shielded toroid - two layers of Mumetal with corrugated cardboard separator.

toroid_shield.jpg


And the finished unit before putting on the cover.

ACMP81_done.jpg
 
Acmp 81

So if it was up to you, would you buy an acmp 81 for vocal use? How loud is your acmp 81 after the transistor fix? Is it a decent sounding pre without the eq enabled? Do you think you could get a good vocal sound without the eq enabled?
 
I just completed my AMCP-81 mods.

I changed the transistors on the EQ board. I googled for the correct part and ordered it online. I ordered 10 of each, cost me 20 dollars with shipping.

Took around an 2 hours to do the first amcp-81 an hour to do the second 1 (since I learned what to do with the first one)

I re-assembeled, then added a tiny bit of shielding.

Make sure your gear is set at +4 going in. I would record vocals with them no problem.

Nice, full and tight sounding pre.
EQ's are musical and warm, Highs are nice assuming the mic has good hi's.

The pre is pretty true to mic. I haven't tried it with a ribbon yet.

Do the transistor mod, and you will have a good-great pre. Think John Hardy, API.
 
I finally gave up trying to coax the existing transistors into submission. They're just too unstable. Even with a solid ground reference, there was more hum than I was happy with, so I swapped out the transistors. With that change, the hum dropped from probably 1% of the original hum level to maybe .01%. I can still hear just a tiny bit, but not much.

After that, I built a can for the transformer out of MagnetShield and wrapped the nearest inductor with MetGlas shielding film (cobalt alloy). When that still didn't make much difference, I wrapped the MetGlas with copper foil tape, soldered a wire to the tape, and grounded the other end. That made a noticeable difference. At this point, I'm struggling to hear any real hum.

Now to work on the other one.
 
I tried a high saturation alloy around the toroid - then I tried an inner layer of high saturation and outer layer of high permeability (separated by a layer of corrugated cardboard) - then I tried two layers of high permeability, which seemed to work best. (The high permeability is 0.025" http://www.magnetic-shield.com/products/conetic.html , the high saturation was 0.030" thickness http://www.magnetic-shield.com/products/netic.html .)

For the inductors I've generally used two layers of 0.010 high permeability material. I can barely hear hum at max output in line mode with eq inductor boards engaged, listening through a power amp at max gain. There is certainly no hum audible using a mic.

However, with Steve Hogan's entry into this with a new toroid and other mods, the problem may be resolved at its source, and needing to shield the inductors or replace them will hopefully become moot.

http://www.prodigy-pro.com/diy/index.php?topic=27791.480
 
Just wanted to let everyone know that I successfully modded my 81 with the new transistors, and it seems to have done the trick eliminating the EQ hum! Now it's just as quiet as my 73 (if not quieter), which is awesome. Thanks a ton to Antichef for posting the pictures and instructions.

It took around 1 hour to finish, not including testing. I consider myself to have a moderate level of electronics experience, and probably wouldn't recommend the job to a newbie because it would be easy to lift the traces and brick the unit.

[commercial content removed]

Ryan
 
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Been following Steve Hogans work over at prodigy.
Fun reading and really impressive.
Kind of appalling the sloppy and cheap construction he's uncovered.
I wonder what other less visable problems are lurking.

Just crunching some numbers again this morning;

I'm not feeling too confident about fixing these myself.
So If I send my 2 acmp 73's off to Steve and have him do the mods,
i might be out another $500 with shipping RT. ( just a guess)
Add that to the $400 i've spent so far, i'd be almost $500 into each one.

considering the bad press that comes up if a potential buyer were to google
these units, would i be able to even get my money back if i were to sell them later?

I couldn't sell them locally at my cost recently.

If i spent a couple of hours stripping them of components, could i sell the boxes for $50 each?
 
I think you're right, and $200 per unit including shipping might be low. The power trafo alone is $75 target price I believe. Then all his power supply mods, replacing all the damaged resistors, fixing the gain switch (which will eventually kill that emitter resistor if not fixed) . . . isn't he going to look at the inductors too?

Since the long-term reliability of an unrepaired box is probably not very good, clearly only the repaired boxes are going to have long-term value. How much is anybody's guess. I think when he's done it will be a nice box. It probably depends on how many people get his mod vs. less than comprehensive repair. If most of the boxes floating around have various stages of repair, it could negatively impact the perceived value of all of them.
 
I think if I were having mods done by somebody else, assuming that person owned at least one of that unit, I'd probably ship just the pieces that need modification.

With cases, those units weigh 15 pounds each and the boxes measure 21"x18"x6". If I had to ship one to southern California from northern California, it would be over $13 each way. To New York from northern Cali would be $23 each way.

The boards weigh maybe a pound total an would fit in a small box. Thus, they would probably cost about $7.50 each way.

Just my $0.02.
 
interesting idea, perhaps i can send just the board.
Or maybe i can get out the soldering iron.

MS: I agree the different levels of mods on these could overall bring down the value.

For me, i would say that these are kind of a dud.

Just wondering if i should throw good money after bad.

and for the people who are happy with theirs, i'm happy for them, but
from what i've seen, don't be surprised to have some problems down the line.
 
zorf said:
interesting idea, perhaps i can send just the board.
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You've got a minimum of two boards to mod on the 73 or the 84 (assuming the new power transformer means the inductor problem is fixed - if not, there's two or three more.) On the 81 there's a minimum of 5 boards that need fixed. I'd assume if you sent just the boards in, there'd be no warranty on a working finished box after you put it back together.

I think you're better off either sending in the whole box to be fixed, or getting the mod kit (which will have instructions) and having a local tech or knowledgeable friend do the work, if you can't do it yourself. Of course, no warranty again, but warranty considerations don't seem to be a priority with these anyway. :)
 
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I just made some real progress that surprised the heck out of me. I've gone through and done the transistor swaps, added shielding around the inductors and the transformer, added extra caps and grounding. The grounding and capacitors made a big difference. The transistor swaps nearly took care of the ground-modded one, but the other one still hummed a lot. I applied the ground mods and that helped, but didn't fix it.

In desperation, I added a metal box over top of the entire supply section. It helped slightly. I then shielded the other inductor, which made little difference.

Well, the real progress happened when I swapped power supply boards. No change, but for some reason, I decided to move some wires around. Most of the cables made no difference. One made a huge difference---the 6-pin power cable that feeds the right end of the EQ boards. When I moved this DC supply cable as far away from the transformer as I could, the hum basically disappeared.

So not only is almost all the hum I'm seeing caused by induction, a sizable percentage is induced into the ribbon cable that goes from the power supply over to the right end of the EQ boards. This also explains in large part why the 81 hums and the 73 doesn't. Besides having a bigger transformer, that power cable never gets closer than about four inches from the transformer in the 73, but runs almost right under the transformer in the 81.

So it seems to me that the key to reducing the hum may well be getting that power cable as far away from the transformer as possible. Since I don't think I can realistically keep the power cable routed so far away from everything, I'm considering soldering a right angle Molex connector onto one of the jumper cards near the left end and feeding power in from the opposite end of the device. (The power rails run straight across without resistors, diodes, or voltage regulators, and there are decoupling caps between each stage, so it shouldn't really matter which end of the bus provides power.) Thoughts?
 
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