balanced help

[Magnus Danielson]

From: Martin Czech <czech at Micronas.Com>
Subject: balanced help
Date: Fri, 9 Jun 2000 10:19:30 +0200 (MET DST)

> 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!

OK. My first attempt would be:

Trafos in the reciever ends, no earth connection from primary to
secondary side. The cable shield should be connected in the source
end. If you have a shield in the trafo, hook it to the shield. If you
have a double shield, hook the one closest to the primary side to the
shield and hook the other one (closest to the secondary) to the earth
end at the secondary side.

If you have the time, throw together a balanced drive using
buffers/inverters using standard op-amps. You could veroboard them
quickly and they would most probably be worth the effort I think.
Just don't forget the caps ;)

Just running unbalanced from one end into a transformer is not as
efficient as running balanced since electrostatically the shield is
roughtly at the same level as one of the signals in the unbalanced
case where as in the balanced case it is rougthly inbetween.

If you spend the money on the trafos, do go ahead and spend some small
amount more on the balanced drivers.

Cheers,
Magnus