ACMP81 oscillation sounds

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antichef

antichef

pornk rock
On to the next issue :D -- I've been running mixes through my ACMP-81s using the outs and ins on my audio interface. All was happy until one of them started motorboating :(

I messed around with it some -- it seems to be related to the total gain level across the system -- above a certain threshold, the oscillation starts. If it's oscillating, I can stop it with either the input gain or the output level, and then as I creep it back up (either one), it hits the threshold and the oscillation starts again. The EQ can make it worse if the EQ is acting to increase the gain a little, but other than that, doesn't seem related.

Here's an mp3


This time, the beats are much quicker, and it sounds more like a cylon than a motorboat - that changed when I opened it up. The sound you here is me turning up and down the input gain to incite, then squelch the oscillation. At the end, I have the EQ engaged, which changes the character and intensity of the zappiness, but that's it.

Aaarggh!! :mad: All the transistors on the preamp board and the output board are BC184C - I went to see if these were sub-spec, and noticed a lot of people use them in Neve-like circuits.

edit: I should have mentioned - this is one of two units - both were functioning fine this morning. The oscillation happens with or without signal going through (made the MP3 when there was no signal). This is happening on the "line" setting. On the mic setting, it also happened, but I had to turn both the input and output all the way up -- there was no mic cable attached.

Anyone got any ideas?
 
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Wow. I've never heard anything like that before. That's one weird sound. I assume you've tried swapping power supply boards to make sure you don't have some failing rectifier or something? Hmm. Now that I think about it, that shouldn't be affected by gain if it were a power supply problem. *shrugs* I've got nothing. Try swapping one of the two main preamp boards (the leftmost board at the front of the case and the board behind it) and see which board the problem follows? That might at least help narrow it down a little.
 
The first sound was a slower oscillation, more like a chainsaw, motorcycle, or motorboat. I need to be careful using pseudo-technical terms, because I haven't heard this sound or that sound, or many other common/uncommon pathological sounds.

I'll try those swaps next chance I get and report back. The one 81 is dead in the water (which means I'm effectively not using both, because I treat them as a pair) :( -- Hope it's just mine that developed this problem.
 
That sounded quite resonant, but that could be due to the fact that there is an eq. section in the circuit.

I know it sounds obvious, but make sure you aren't accidentally looping-back any audio coming back from the unit back to your soundcard output feeding the unit.
 
I can get a very similiar sound on Input gain setting 7.
Also, check to see if there any new metal touching a lead to a component.

Possibly ground interuption?

Just to through this out there, a bad capacitor, perhaps?
 
By input gain setting 7, do you mean the infamous -pop- place?

Thanks for the responses everyone -- I will follow up with better information soon.
 
Well, I took both 81s out of the rack and attempted to reproduce the problem, but couldn't - not sure what was going on. Between the software and hardware routing and my lack of experience, I can't be 100% sure it wasn't a routing problem as Rodabod suggested, so I'll go dormant on this one until it crops up again, and then hopefully I'll have enough time to go into information-gathering mode immediately.
 
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