Arny Krueger wrote:

"Eeyore" wrote
Ron Hardin wrote:
Adrian wrote:

Then you'll find you get enormous AC hum when you connect to your
computer, owing to a ground loop. Put a Radio Shack ground loop
isolator in
that line (cat 270-054). I keep a bunch of them around to isolate
every
different thing that's plugged in on AC mains.

Is that true even if I utilize the USB link into my Notebook?

Yes, it's a problem in the connecting audio cable. The computer and the
audio components don't have quite the same idea of what ground potential
is.

That's the traditional reason for 'hum loops'.

In this case the problem is made worse but the use of switching power
supplies (certainly in the
computer) and in some audio gear (I'm fairly certain the Behringer model
mentioned also has one).
These swiching power supplies have filters on the ac power input that
injects a noise current
into the ground conductor. This messes up your grounding arrangements very
effectively.

I've always been under the impression that the simple presence of a hard
chassis ground was all it took to pretty well guarantee a ground loop, if
there was a second hard chassis ground anyplace in the equipment that was
attached to the PC.

For this to be the case, there has either to be some source of electromagnetic
induction ( like the stray field from a transformer) imposing itself on a
connecting wire or a current intentionally dumped to ground (as with the EMI
filters). Or, not likely in the context of equipment used in a single room, a
different earth potential at the various power sockets.


IOW, the switching power supply might have made the
ground loop more obnoxious-sounding, but it would be there regardless.

The mere presence of a loop doesn't of itself generate any hum. The loop simply
provides a path that's a perfect 'shorted turn' for a magnetic field to create a
current in.

You can in fact very easily have a 'hum loop' like that *inside* a piece of
equipment. I've measured potential differences of the order of tens of uV
between different parts of a steel chassis caused by electromagnetic induction
from the stray field of a large power transformer inside the equipment for
example.

Graham