"Brian" wrote in message
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Apologies to the group if this has been discussed prior:

No problem, if you keep an open mind. Grounding problems with subwoofers are
very common, and have been discussed many times. Try searching using words
like subwoofer and hum at www.google.com .

When I plug my subwoofer into my receiver and turn on the receiver, I
hear a hum. The sound is similar to putting a finger across the
subwoofer cable lead.

I am trying to understand if there is an obvious cause for this hum,
other than a problem with the receiver's subwoofer output jack. I do
not believe this is a ground loop issue.

That makes about as much sense as having a short-term cough and sniffles and
not suspecting that you might have a cold.

It's probably a ground loop issue or another kind of grounding issue called
a "grounding potential difference".

Probably, the first thing to do is to eliminate the fact that you have the
sub and the receiver grounded at separate points as a potential source of
problem. IOW plug the receiver and the subwoofer into the same outlet or
plug strip, and see what happens.

Also, eliminate the rest of your system as a potential source of problems.
Remove all connections from your receiver but power, speakers, and the
connection to the subwoofer.

Once you get reasonably hum-free operation, add things back one at a time
and see if any of them are causing grounding problems.