AC adapter making my speakers buzz -- and it's not a ground loop.

March 23, 2009 7:13 PM

New, generic AC adapter is causing my computer speaker system to hum. I thought it was a ground loop, but a cheater plug (fitted on the jack of the AC adapter) didn't help. (The speaker system itself is not grounded.) I know it's the AC adapter, because if I run my laptop on battery power, the hum disappears. Anything I can do, short of forking out $70 for an original adapter? This adapter has been working fine otherwise.

• I'm using an external Sound Blaster Live! 24bit as my sound card, but the problem persists when I switch to the on-board sound card.
• The speaker system works perfectly well with other laptops.

In short -- it's the AC adapter. But what should I do?

posted by ori to computers & internet (8 comments total)

Is there a ferrite bead on the cord?
posted by fake at 7:16 PM on March 23

It could be a ground loop, but it could also be EMI. Rearranging wires so that cables carrying AC are not near cables carrying low-amplitude sound (pre-amplifier) may help. Or, as the above says, strategic use of ferrite cores.
posted by Chocolate Pickle at 7:25 PM on March 23

Yes, but it's somewhat smaller than the one I had on my previous cord.
posted by ori at 7:25 PM on March 23

It's probably due to poor regulation or filtering of the AC adapter's output, causing some 60 hz ripple on the output voltage. Short of opening the thing up and trying to add some more filtering yourself, about the only other thing you could try is another brand of generic AC adapter in the hope that it'll be better.
posted by FishBike at 7:26 PM on March 23

Yeah, I've tried re-arranging the cables and it didn't help. I'll go buy some clip-on ferrite cores, and if that doesn't help, another AC adapter.
posted by ori at 7:42 PM on March 23

Remember that the wire needs to loop around the core at least once. I.e. it needs to pass through the middle of the core at least twice.
posted by Chocolate Pickle at 8:20 PM on March 23

Anything I can do, short of forking out $70 for an original adapter?

70 dollars? Once you know your voltage just hunt around on the net, you can easily get one for a fraction of the price. Heck, you can get a universal for under 50.
posted by damn dirty ape at 6:38 AM on March 24

I think fishbike has it- cheap adapters don't care too much about putting out a pure DC "signal". The more expensive ones made for audio components do. Similarly, if the speaker set is expecting to have a clean power source, it's apt to not have any filtering inside of it.

If it is EMI, I'd doubt it would sound 60hz. It's worth a shot, but I doubt it. When I think of EMI on lines like this, I'm thinking of impulse noise.

You could probably do some math and put a capacitor or two across the terminals to smooth out the ripples. The capacitor fills up, and as the voltage rises and drops, it fills and empties against the ripple and smooths it out.
posted by gjc at 7:05 AM on March 24