New subject: loud noise out of external soundcard in the bandroom when AC-powered

17 Mar 2008

Quoting Matthias Schönborn &lt;mbs1303(a)gmx.de&gt;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

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