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How can I eliminate the hum in my speakers ?

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Hello folks ! I have the tuffest one :|

I have 2 studio monitors (speakers). They are powered and they are plugged with balanced cables to an audio interface (FireWire) that has balanced output. I know the problem is not the speakers or the interface (I tried many different cable configuration, grounded no ground chassis ground etc)- just to say it's not from there.

The problem comes from when I connect the FireWire cable to the computer. It picks the noise of the PSU, even when it's not turned on (but is plugged in the wall).

I read that I could attach some ferrite core to the FireWire cable to filter out that nasty subtile noise. But I cannot find any in the town that I live in and whenever I go to a store and ask them about ferrite core they don't know anything about it.

I do have a spare FireWire cable, is there a way to mod it ?

Btw, everything is plugged on the same powerbar and have the same ground.
 
Take the ferrite core off a keyboard or other devices and try that.

Is it 60hz humm? If it is it's likely not the PSU the frequency would be much higher. Sounds more like a ground loop issue, never had any experience in fixing them because I've never run across one.
 
Hey man, thanks for the reply.

It sounds something like 1kHz I'd say.

As for the ground loop, it's almost impossible... My audio interface's chassis is grounded to my computer's chassis, I tried balanced cables and unbalanced cables between the audio interface soundcard and the speakers. I even tried powering the monitors with the ground pong cut and still there. As soon as I unplug the firewire cable, the monitors go back at their normal state. Everything share the same ground point. I might try grounding my audio interface on my desktop chassis or directly at the powerbar's ground instead of going by the computer, but I believe it won't change a thing (not supposed to have a loop there).

As for the ferrite cores on a keyboards, that's a good ideas but I think there are differant ferrite cores for different frequencies. I'm clueless.

I sent 2 emails to Edirol (Roland music.. Boss gears, etc) never had a reply yet. I really don't want to look into another product because that interface and any Roland gear I had so far always worked great. They were supposed to include the snap-on ferrites with the interface but I might have lost them years ago, and it was never a problem with passive monitors...
 
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