17 Mar
2008
17 Mar
'08
8:26 a.m.
Quoting Matthias Schönborn <mbs1303(a)gmx.de>de>:
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.
You have a ground loop in your system.
First a warning, whatever you do: do NOT disconnect any earthing pins from
any equipment to defeat the loop. Doing such things will put you and your
equipment at danger.
You can try a few things, like connecting your laptop PSU and amp power to
adjacent terminals. This can help if your laptop PSU is earthed.
The best solution (and the one which works almost every time) is to use a
transformer to isolate the laptop and the PA system. This can be in the form
of a specialized transformer isolator (available usually with RCA-RCA
connectors) or a stereo DI box with a ground lift switch.
Sampo