Stuart Welwood wrote:
"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


Transformers are another alternative, but you need to get good ones,
I've got some sitting in my junkbox that simply sound awful and I have
not touched them for years. (garbled midrange, weakend bass)

These transformers were always marketed in the pro audio world so most
consumers would not even be aware of them.

Thats why I'll usually prefer a ground lift of the interconnect, or
re-binding / isolating the chassis (assuming everything is already
safety grounded).

As for the OP's issue with the cable TV shield, I would suggest first
trying a power strip that has CATV protectors on the strip. This will
put the local cable at the same potential as the safety ground. These
protectors usually have your typical power outlets as well as F
connectors and telephone connectors for grounding phones and cable
boxes from lightening damage. A nice by-product is that they will put
the cable shield down, usually these are placed at entry to the house
but there are models for branch circuit use too.

It's worth a try and he'll get some lightening protection to boot.