Humming when I plug my laptop into the sound system

[anony]

I run the audio/visual presentation equipment for a small church. We run presentation software through our laptop to a projector and the audio from our worship leader's mics, pianos, guitars, etc to a mixer and then to power amps then to the speakers....

At times, we play music or dvds through the mixer from the laptop. The problem we are having is a low-level hum/static sound that arises when I connect the computer to the mixer in any form. I have tried the headphone output into one of the mixer's channels as well as the headphone output to an RCA adapter to the RCA input on the mixer. I have the same problem both ways. I do not have the problem, however, when I take the computer off AC power and let it run on the battery. I'm no expert, but is what I'm having a ground-loop problem, and if so, are there in any easy ways to fix it?

Thanks.

[Tomi Engdahl]

What you are having sound like a classical example of a ground-loop problem.
And good news is that there is an easy ways to fix it: audio isolation transformer.
Feed the audio signal from PC to suitable isolation transformer and from the transformer to your mixer.
The end result is that the humming noise is gone (no matter if PC is powered from mains or not) and
the sound from PC goes to mixer (pretty much) unaffected by the process.

More details on those isolators can be found on those web pages:
http://www.epanorama.net/documents/grou ... ators.html
http://www.epanorama.net/phpBB3/viewtop ... 12&t=47729
http://www.epanorama.net/phpBB3/viewtop ... 12&t=20937
http://www.xitel.com/USA/prod_gli.htm