[LAU] OT interference / grounding problems

pete shorthose zenadsl6252 at zen.co.uk
Fri Feb 20 22:49:39 EST 2009

On Sat, 21 Feb 2009 13:10:06 +1100
James Cameron <quozl at us.netrek.org> wrote:

> 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.

Sorry this was incorrect. I meant to say the XPS has a 3 core earthed AC
adapter. Mine has a 2 core cable and so is not earthed. Hence the problem.
I realise now that I never had problems before because I used a desktop and that was
fully earthed. 

> > 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
> manufacturers.

I just ransacked my old gear pile and found a commodore CDTV gathering
dust. This is the only audio gear I have left that is earthed. I connected the 
audio out to my amp and plugged it in but left it switched off. The power
switch does not disconnect the earth so even powered off it serves as a sink.
As expected.. no noise, no shocks (and this is with my current noisy AC adapter)
It may not be ideal to have the path to ground traveling though several devices
but in this instance it works. 
If I remove the CDTV power plug I hear noise, if I plug it back in, no noise. 
This is pretty conclusive.

Ah well Here's for design tailored to the lowest common denominator.  

Thanks for the tips all. I'm very happy to have solved this. I guess I now know
why I often got shocks from my Iriver IHP140 too. 


nb: Someone measured the voltage coming off the case of a later model dell at
around 100v. Nice huh. Good job the current is low. Apples have had problems too
or so I've read.

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