Re: hum when biamping -- ground loop or interference?


"Brian Running" <brunning@xxxxxxxxxxxxxxxxx> wrote in message
news:2B6ak.20382$co7.12367@xxxxxxxxxxxxxxxxxxxxxxx
I totally agree that there should be an AC ground connection, but as long
as one amp is grounded and both are connected together, there's no more
danger than if both are grounded through AC plugs. It's quite common to
see PA systems with all AC grounds lifted but one, the connecting cables
insure that everything is grounded, and there is only one common ground,
eliminating ground loop problems.

This is complete insanity, please, don't do what's described above. If
you do, then only one piece of equipment's chassis is grounded, the rest
of them are not. If you rely on the instrument and mic cable shielding to
ground the chassis of the gear, then the only route for current to ground
if there's a fault is through the gear you're holding -- your mic, your
bass, whatever. You are in the circuit. If you are connected in any way
to ground yourself -- standing on dirt or concrete, holding a metal rail,
whatever -- then you are potentially part of the circuit. Unbelievably
dangerous.

Chassis are grounded through the power cord so there's a direct link to
ground, without making you a part of that circuit in any way. If there's
a fault inside an amp, for instance, and the chassis becomes "hot," then
it goes directly to ground and pops the circuit breaker. If you've lifted
the ground of any single piece of gear, then it has no direct connection
to ground, and if the chassis goes hot, it stays that way until something
gives it a path to ground. Would you want that path to be your bass
cable?

Even by Mike Rieves standards, this is so incredibly stupid, irresponsible
and dangerous, there aren't even words to describe it.

Lee has both amps rackmounted.
Each amp is dead quiet when turned on individually.
When both are turned on there's a slight hum even when nothing
is connected to either input.
This suggests imo a chassis-ground loop via the rackmounts...
He will know for sure if he removes/disconnects one amp from the
rackmounts.

This link should be interesting
http://www.epanorama.net/documents/groundloop/

Pedro





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