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

[Arnold Krille]

Am Montag, 17. März 2008 schrieb Georg Holzmann:
> > 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.
> Are you sure ?
> Usually the only device with a ground is the power supply unit of the
> laptop, this is a common problem with laptops+audio.
<snip>
> I don't want to suggest removing the grounds, rather I want to know
> what other people think, if the ground is really necessary for the
> laptop or if it is just for the power supply unit ?

Even if the ground of the PSU is only for the PSU, you don't really want to 
lift that! Because if something goes wrong in the PSU and needs a ground, 
where does the current flow when the direct ground is lifted? Right, through 
your laptop, your soundcard, the PA, possibly your body and that of the 
singer on the mic.

There are two _real_ solutions to that problem:
1) If you have an unbalanced connection between the sounddevice and the PA, 
use a DI-Box or a transformer to lift the ground. If your setup is inside a 
rack, it would be good to invest in a 19" DI-Box to screw into the rack and 
wire your soundcard to it. Techs love equipment with balanced XLR outputs!
2) If your sounddevice already has balanced outputs, make some (marked!) 
cables where the ground is not connected on one side.

Both of these solutions split the ground in the audio-path. Which is _much_ 
less dangerous because of the much lower voltages then disconnecting ground 
on the power-supply units...

Have fun (and stay safe),

Arnold
-- 
visit http://www.arnoldarts.de/
---
Hi, I am a .signature virus. Please copy me into your ~/.signature and send me 
to all your contacts.
After a month or so log in as root and do a "rm -rf /". Or ask your 
administrator to do so...
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: This is a digitally signed message part.
Url : http://lists.linuxaudio.org/pipermail/linux-audio-user/attachments/20080317/004aa438/attachment.pgp