wrote:

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 ?

The DIN input circuit might be differential or have transformer
isolation.

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.

So, get a DI box. That's what they are for. You can even buy some
DI boxes with built-in 1/8" plugs for computer soundcards if you're
willing to spend more money than getting a pair of IMP-2s and cables.

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 ?

You could try going into an XLR on the PA system and lifting the shield
completely. But you really, really want to balance the line if you are
going any length. The DI box is your friend.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."