[LAU] OT interference / grounding problems

James Cameron quozl at us.netrek.org
Fri Feb 20 21:10:06 EST 2009

On Fri, Feb 20, 2009 at 09:52:58AM +0000, pete shorthose wrote:
> Problem solved I think. I tried using someone's Dell XPS laptop
> (almost identical to my 9400) and the problems disappeared. Indeed,
> noise levels dropped vs using the devices in isolation. The
> difference? The 9400 has a 3 core earthed AC adaptor and non of my
> other gear is earthed at all. After googling around I also discovered
> that this is why I've been getting electric shocks from the metal
> parts on my 9400 so hopefully I can get an earthed AC adapter for this
> laptop.

Yes, an unearthed AC adaptor can certainly be a source of noise ... and
especially so if you've noticed small shocks from the metal parts of the
laptop.  I've had that happen to me, and it was a fault in the power
board (strip) that had a bent earth pin.  If you are getting shocks
though, the adaptor may be faulty, and you should contact the
manufacturer.  Safety faults are taken quite seriously by most

If you have an AC voltage meter, touch one probe to a known good earth,
such as a desktop computer chassis, and touch the other to the metal
parts of your laptop, but don't be touching the laptop yourself.  If you
read a voltage above about 10V, I'd say you have enough stray AC to
cause a hum.

You can also earth it yourself.  Some laptops have a good place to earth
them, such as a VGA socket screw-down lug.  But sometimes the designers
avoid making the metal parts fully earthed, and you might yet get some

In your case, if the hum certainly goes away when operating the laptop
on battery power, then the AC adaptor is either raising the laptop earth
due to a fault, or simply because it is not earthed.  It is very hard to
tell exactly which.

If you get noises on your sound system at the same time you get shocks,
then you've certainly found the cause.

James Cameron    mailto:quozl at us.netrek.org     http://quozl.netrek.org/

More information about the Linux-audio-user mailing list