"Lots of people talking, but few of them know..."
The real story behind the most common source of hum in unbalanced audio
systems (common impedance coupling and inter-chassis current) and how to
properly treat it is well covered in Bill Whitlock's paper for Jensen
transformers:
http://www.jensentransformers.com/an/an004.pdf

The "ground loop" explanation may lead one to eliminate the hum, but it is
not usually the real culprit.

Stuart Welwood
6 sigma


"johnelstad" wrote in message
oups.com...
Well, I'm at my wit's end, but I'm hoping this will be obvious to
someone with more experience. Please help if you can.

I recently noticed a mild-moderate hum coming from my speakers that is
constant regardless of volume or input. Here are the results of my
attempts to isolate it.

Turn off and unplug pre-amp = hum (no change)
Disconnect pre-amp output cable = no hum
Replace cable connecting pre-amp to amp = hum

Not sure if this is helpful, but my pre-amp has a headphone amp that
produces no hum.

Equipment:
Sonic Frontiers Line 1 Tube Preamp w/ 5-year-old tubes
McCormack DNA-1 Solid State Amplifier
NHT 1.5 speakers.

Thanks a bunch!

John