Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Easy way to filter 50hz buzz?
References: 
Torrance Bell  writes:

> Here on base we have an ampitheatre that we use to show movies and
> stuff.  My current issue is that we have a piece of crap cable coming
> from the stage to the booth, about 12.5 meters, which is just an RCA
> cable at both ends.  One end has been cut off and another cheep-o RCA
> connector has been mickey moused onto the end.  Whenever we setup the
> projector on stage, we get the buzz through our system.
> 
> I plan on laying new cables in a conduit very soon, but in the
> meantime, is there a dirty (and safe) way to get rid of the buzz?  I
> KNOW the buzz is coming mostly from the projector.  If I unplug the
> end from the projector, the buzz goes away.  Our DVD player is in the
> booth.
> It doesn't help that there is a fridge and two voltage
> converter/transformers in the booth.  (120 volt stuff)
> 
> Torrance
> 
> PS: we have a mackie 1604 with the infamous ribbon cable problem.
> this board is dusty and dirty and a general POS IMHO.  I can't use the
> mains out because of the ribbon cable problem, so I have to use a
> subgroup out.  I'll post pictures of stuff sometime in the near future.

It sounds you could have a ground loop problem. 
Your RCA cables get ground connection on the projector end. 
And at the DVD player end the RCA connector gounds get connected 
to mains gets ground connection at the booth (maybe DVD player 
has groudned power connector, or ground connnection gets to 
it through other wires connected to it). 
What you hear as noise is the differencer between the grounding 
potentials of those two locations and the end of the RCA cable. 
That's the most common reason for the problem in this kind 
of application. 
I have written a lot of information on this kind of problems 
and how to solve them to 
http://www.epanorama.net/documents/groundloop/index.html

The easiest solution to get rid of the noise (or most of it) 
with the existing wiring is to go to a car hifui supply and 
buy to a RCA audio cable isolation transformer. 
Something like the one seen in the pictrure at
http://www.biltema.fi/osteri/data/Kuvat/31_441iso.jpg
I have used this device many times to solve lots of noise problems:

Some links to other products like this 
(I have not tested those, but look pretty similar):
http://www.xitel.com/product_gli.htm
http://www.crutchfield.com/S-nUSyyx0oqIp/cgi-bin/prodview.asp?I=127SNI1
http://www.jensen-transformers.com/iso_aud.html
http://www.lundahl.se/catframe.html



-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/