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My Logitech Z313 speakers constantly emit a low audible hum. It's constant and not relative to the volume level. There is no noise if I connect headphones to the computer and it doesn't go away in the speakers when I disconnect them from the computer, so it's not coming from the audio line out, nor does it seem to be a ground-loop.

It's not cell-phone RF interference since that shows up very noticeably when my cell communicates with the tower and is very distinct.

My router and wireless internet modem are a few feet away (above) the speakers and about a foot more above the amp/subwoofer.

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For troubleshooting purposes, try putting different speakers on your computer (if you have some laying around) that are non headphones. Try hooking your headphones up to another computer. Also, just for general information, which OS is it? – David Jan 18 '11 at 21:47
O/S is Windows 7. – Lawrence Dol Jan 18 '11 at 22:00
up vote 2 down vote accepted

Figured it out - it's RF interference from my DEC 6.0 telephone base station which is on my desk not far from the speakers; it gets slightly louder when a call is active, and moving the base-station changes the noise. I am not sure why I have not made the connection before.

Putting the base station on the shelf above my desk has eliminated the interference.

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+1, Nice detective work, you can mark your own answer as the solution. – Moab Jan 18 '11 at 22:44
@Moab: The system won't let me for 2 days. Then I will. – Lawrence Dol Jan 18 '11 at 23:54
I just realized, also, that the sub was humming/buzzing too - it is also (not surprisingly) now silent. It's amazing to me to realize how subtly annoying the noise was, now that it's gone. – Lawrence Dol Jan 18 '11 at 23:55

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