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 manufacturers. 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 hum. 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@xxxxxxxxxxxxx http://quozl.netrek.org/ _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
