balanced help

[Martin Czech]

I have a friend who is preparing an electronic music performance (16. July
2000 - ) in Friedrichhafen, Germany. (btw. this is where Graf Zeppelin
build his famous "silver cigars"). He wrote me that he installed four
active speakers in the corners of the room and that he put the audio
cables in "cable channels". Now, all unbalanced, you might know what's
happening: hum and radio interference all the way.

The hum may be due to magnetic coupling (no galvanic ground loop here,
no protective earth plugs), and the radio should be capacitive coupling
on the shield which is unfortunately the audio "return" path. The word
cable channel makes me sick!  I think the cure would be to use one
transformer per line, thus balancing using balanced shielded cable.

Or may be the transformer is not neccessary (no ground loop), just twisted pair
shielded cable.

The "real thing" would be certainly two transformers, one at each side, fully 
balanced, but there is short money ...

Now there are two possible positions the trafo may have: at the signal
source or the drain. I'd say that it should be at the drain. I.e. a
balanced cable comes from the unbalanced source all the way to the trafo,
there the secondary winding is fed into the unbalanced speaker input. Now , 
where do we connect the cable shield. If the enclosures are plastic, I guess I 
have to propose protective earth.

I have only one attempt to make it work, a couple of hours before the event
starts. I'm 170 km away till then. 

So any suggestions for achieving a "first hit" solution would be very much 
appreciated!


m.c.