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Old 3rd July 2002, 12:27 AM   #11
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Possible the problem is somewhere else besides the amp. What's the signal source? Is there a separate pre-amp involved?
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Old 3rd July 2002, 03:36 AM   #12
fender4 is offline fender4  United States
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Hi Bill,
I am not using a separate pre-amp...just a signal fed through a pot to the amp. I have tried the amp with a Toshiba SD-3750 DVD player (my main source), a Sony Discman, and an old Fisher 25 disc changer. The problem presentation was identical with all three sources, so I ruled that out.

I just did a refresher course on Zobel networks, and I think it might solve my problems. If I am understanding this correctly, high frequency oscillations can occur when there is an absence of load at high frequencies. This is consistent with my "purge the system" volume burst that eliminates the symptoms temporarily. The distortion seems to appear the most when the signal level is at its lowest. An RC Zobel network can reduce these oscillations. So does adding an inductor in series with a resistor at the output just cut off the high frequencies at a level lower than the oscillation? Which one of these fixes would most adequately address my problem? (I would ask which solution would degrade the sound quality the least, but at this point I'll take any clean sound I can get! )

Sorry to bother everyone with elementary electronics, and thanks again for the help. I should have the parts in hand tommorrow to try the Zobel.

-f4
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Old 3rd July 2002, 03:57 AM   #13
Duo is offline Duo  Canada
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Hmm, just read up on this thread again. You can use an inductor and resistor as well, some people even put it in with a zobel at the same time. The zobel however, will deliver higher sound quality and is used by many of the better amp designers. Though, you can design an inductance network that won't really reduce quality, but it's a bit harder and I wouldnt consider it worth it.
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Old 3rd July 2002, 09:00 AM   #14
fender4 is offline fender4  United States
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Default Some progress!

I haven't put in the Zobel yet, but I did a little snooping with a jumper wire. Here's what I found:

Each channel has its own metal chassis, but both are fed by the same power supply. Also, both metal chassis's are housed in a single wooden enclosure that contains a dual-ganged pot (I think that is what it's called) and the PS. When I ran a length of wire between the star ground of each amp, I was able to listen distortion free...1 hour and counting!

I'm not sure what was going on. I assumed that the grounds on the dual pot were isolated from each other. If they are not, that may have been a source for a ground loop, I think. Other than the PS, that was the only possible common connection.

I will still try the Zobel network for stability's sake. There is a slight hum at full volume/shorted-input, but it is 1000x better than before. And it is consistent...no changes during a listening session. It sounds great! Thanks again for everyone's help.

-f4
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Old 3rd July 2002, 09:18 AM   #15
Duo is offline Duo  Canada
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Weird, there must have been some potential between the grounds causing some offset in the chips or oscillation somewhere... Either one, I'm happy you fixed the initial problem...
Things like that can get extremely tricky after a while!
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