Laurence Payne wrote:
On Thu, 15 Jun 2006 07:53:13 -0700, "Richard Crowley"
wrote:

From reading the above I think the problem is a ground earth loop.
What we usually do is open the unit and remove the earth linking the
mains earth to the audio earth. This should clean the noise up.

You are only bandaging the symptom, not discovering and
remedying the cause. It is a potentially leathal "solution". It
would not be acceptable to me.

Surely a ground loop IS the classic cause of hum? There may indeed be
better ways of curing it.

Gound lifts on signal cables is a very common to-do in pro audio, less
so in home audio, it poses no danger, (as long as the connected chassis
are all on 3 prong power).

Other posters were correct in that there is no reason to open the
chassis and mess with the power ground, even if the references between
2 chassis are off a little (due to separate outlets, ground binding
corrosion, the fit of the plugs in the power strip, currents induced
into one chassis by its own circuit, etc).

If all else fails, just find out what signal interconnect has the
current flow in it's shield, and break it, and reference it to one
ground (side) or the other by trial.

Safety grounds should never depend on the audio interconnects (by using
a 2 prong adapter on one of the chassis), nor should there ever be a
reason to eliminate a safety ground. A pre-amp connected to a power
amp is easy. Whereas finding a hum loop in a pro rack with 10
processing devices, a 32 channel board, with all the aux buses and
inserts going is much harder. But flipping the ground lift switch at
one end or the other, or using a ground lift adaptor on an interconnect
is often the cure. Sometimes re-binding the chassis togeter, by wire,
in the rack is the cure. Sometimes isolating a chassis from the rack
with nylon shoulder washers is the cure.

Ground current in the audio path is pure voodoo and defies logic and
require seemingly unlogical cures.

But I think adhering to not breaking a power ground is always a good
rule, as well as adding a chassis ground to older equipment along with
removal of the death cap from same.