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Old 6th September 2013, 09:20 PM   #1
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Default Another Subwoofer Hum problem

I know this comes up a lot so please bear with me. I have a powered subwoofer from Madisound . . . 12" with a KG 5230 amp. It developed a hum a couple of months ago that I haven't been able to eliminate. I've checked for a ground loop and that doesn't seem to be the problem. . . the hum persists without any other component plugged into the sub. I checked the polarity on the plugs and tried plugging the sub into various outlets in the house. Nothing effected the hum.
Since it appeared to be a problem with the amp someone suggested that the most likely problem was the power source capacitors. I replaced both capacitors (
United Chemi-Con had them in their discontinued line and sent me 4 free of charge! A fantastic company). When I first powered up the sub the hum was gone and I thought I had fixed the problem but now the hum is back. The tonal quality of the sub is much better with the new caps but the hum is very annoying.
I'm hoping someone out there can walk me through troubleshooting this, checking the resisters, diodes, rails, etc. I'd like to nail this problem down before I simply order a new amp.
One last thing I noticed which might help - the sub is connected to the main amp via the line in connections. When I power up the sub (connect or not to the amp) if I touch the positive lugs for the right or left speaker level connections (in or out) the humming gets much worse and sounds a bit like feedback. If I touch the negative lugs nothing happens.
Sorry about the length of this post. If anyone can offer any help I'd really appreciate it. Thanks, Jim
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Old 6th September 2013, 10:06 PM   #2
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Does it hum with nothing plugged into the input? If it doesn't check the socket ground line. If it does, use better quality smoothing capacitors, maybe that is why they were old stock??
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Old 6th September 2013, 10:16 PM   #3
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You've run it without a ground, with a cheater plug? Just checking.
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Old 6th September 2013, 10:27 PM   #4
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If the new capacitors solved the problem and then the problem returned, perhaps the replacement capacitors are bad. You should measure the voltage across the capacitors and make sure that it's not higher than the voltage rating of the capacitors. The capacitors should have a voltage rating higher (perhaps 25% higher) than the voltage that they will see during normal operation.
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Old 7th September 2013, 12:53 AM   #5
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Thanks for the replies, I really appreciate it.

The hum persists with nothing attached to it. A "cheater plug"? Do you mean one of those adapters that plugs a 3 prong plug into a 2 prong wall socket? If so, yes I did that, ie. I plugged it in without a ground. I even plugged it in at a shop and it hummed.

It's possible that the new caps could be faulty but I think the odds for that are low. They are good quality Nippon Chemi-con audio caps (@ 1%), not the best but certainly better than the OEM's. I think they are 2000hrs @ 85 or 100c. I had a very hard time finding 10000uf, 100v, 2 pin leads that were the correct size. No one seems to carry them any more. I couldn't find them at Mouser, Digikey or any other dealer I could find. Panasonic didn't have them. If I remember right there was a lot of low volt stuff but no 100v around. I called Chemi-con because they used to carry the caps I needed but stopped. I was hoping they would have something close but it so happened they had some old stock left.

That being said, I will test them just to be sure. What is the correct procedure for testing the caps? Do I have to remove the caps from the ps board to measure them?
If they are bad I think it is more likely that there is something going on in the amp that ruins the caps than that I got 4 bad caps from Chemi-con. I'd hate to put 2 more caps in until I'm sure. I mean, if I had a lamp that kept blowing light bulbs I'd check out the socket before I kept putting new bulbs in.
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