[LAU] loud noise out of external soundcard in the bandroom when AC-powered

[Matthias Schönborn]

Hello list,

When playing with my band in the bandroom, I'm recording the whole session 
with ardour and playing the phasex resp. zynaddsubfx via a midi keyboard. To 
do this, the outputs of my external soundcard (m-audio fast track pro) are 
plugged into the PA. Because the sound is very much amplified, an incredible 
noise can be heard when no one's playing, approximately as loud as two people 
talking (yes, it's very annoying!) - however, yesterday I noticed that this 
only happens when my laptop is AC-powered! If it runs on batteries, it's 
completely silent and there's no noise in the background.

Now, before I start fiddling around, is this because of a poorly shielded 
powersupply? Or is this problem related to the shielding of the laptop? How 
can I build an effective shielding?

Greetings
Matthias