Hi,

I have an earth loop problem (or at least a hum problem and I'm
assuing it's an earth loop issue). I have a PC that I use at home and
want to play live with (using midi keyboards to play virtual
instruments). I have the PC connected to my old Sony amp at home with
a 3.5mm jack to 5 pin DIN cable and experience no hum at all.

When I tried connecting the PC to the amp using a 3.5mm jack to phono
(cinch) lead, I had terrible mains hum. No big deal, I just carry on
using the DIN connection instead. Why do I get hum using phono
connectors and none using the DIN connector ? (all wiring is sound,
all connections good). Do DIN input circuits have resistors or
something that kills the earth loop ?

However, now I'm testing the live setup and try connecting the PC to
our PA system. In short... ARRGHH !! Mains hum in abundance !!
I've tried connecting it to the Tape-In (phono) inputs on the PA and
into a Hi-Z 1/4" jack on one of the channels - same thing - lots of
mains hum. Incidentally, the hum volume is unaffected by the gain
pots on the tape-in / channel - only the master volume makes a
difference.

I tried connecting the PC and the PA to the same electrical outlet, no
change. I've obviously got an earth loop through the shielding of the
connection between the PC and the PA.

What can I do about this (that doesn't involve buying anything !). Is
it possible to put a resistor in series with the shield ? ...or will
this degrade the sound quality / level ?

Any (cheap) ideas would be very welcome ! :-)

Many thanks,

Kev.