Film-Tech Forum ARCHIVE: Powered subwoofer hum (home)

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Posted by Andy Muirhead (Member # 712) on 06-15-2006, 06:36 PM:

Hey all, thought I'd post in this forum although it is my own subwoofer at home! (it may have relevance to theatre subwoofers though)

My 2.1 sound system for my PC has a really annoying low frequency hum. I have traced it to the powered subwoofer itself. I know this as the hum is still there even when all inputs and outputs are disconnected.

Some things I know;

1. It is not a ground loop issue - there is no ground connection in the AC plug and like i said, there are no inputs or outputs connected.

2. The hum dissapears when you turn the bass pot right down, sounds stupid i know as nothing is connected, but i feel i must at least point it out.

3. I got so annoyed i didn't use it for a week or so. when i switched it back on, the hum was not there, even on maximum bass, it came back over a period of time.

4. It seems that the hum gets worse over time.. initially on power up it is there, but gets worse over time.. or as the unit gets hotter i should say.

Things I have tried

1. Took the unit apart and checked over everything visually just in case there were any loose connections or anything else out of the ordinary

2. Reversed the AC connections just in case - no dice

Now over to you guys! Can you think of anything I can check or what components could be causing this. There isn't really a lot to these things so swapping out a cap or whatever is not a problem, I'm just not sure what really to do now.

Any help would be appreciated

Posted by Gordon McLeod (Member # 33) on 06-15-2006, 06:52 PM:

since it started after it warmed up I would suspect a bad filter or rectifier issue or a hybrid amp chip that is flaky

Posted by Bruce Hansen (Member # 281) on 06-17-2006, 07:32 PM:

Maybe it hums, because it doesn't know the words. OK, can you tell I'm trying to get this 17 beer a day thing down. I'm almost there. [Big Grin]

It does sound like a filter cap that is going bad, when it heats up it loses a little more of it's capitance. It should be the largest cap in the unit. It could also be the bias on the output transistors changing with heat, causing the outputs to draw more current than the power supply and filter cap is designed for. Does it sound destorted after it warms up? Does it get hot?

Posted by Andy Muirhead (Member # 712) on 06-18-2006, 06:40 PM:

Ah, keep working on the beers, Bruce [beer]

Have to have a look inside the unit, from memory I think there are two caps, same size. The sound doesn't distort at all. It does get pretty hot.
The hum is pretty much there from power on even if it has not been on for 12 hours. It does get worse over time. The only time the hum is not there is if it has been of for days.

It's pretty much been like this since new, and since It probably cost about £20, I'm not going to bother sending it back to fix, the hassle and postage would be more bother than anything. But hey if I can fix it myself, or at least get an idea of whats up, I have nothing to lose, so thanks for any help!