On 05/15/2012 10:59 PM, Fons Adriaensen wrote: > On Tue, May 15, 2012 at 08:44:37PM +0200, Robin Gareus wrote: > >> Here it mainly concerns the outputs: In my case there's a ground loop >> between my Screen (Asus VE278) and the active speakers when connected >> via computer and an USB UA-25 (not UA25-ex which features a ground-lift >> switch) -> 1/4inch TRS -> B2031A speakers. >> >> Disconnecting either the screen or the speakers (and using headphones) >> works, otherwise there's a hum on the system; luckily no sparks, no >> x-runs, either :) >> >> A multimeter shows a constant 1mA current and ~3-4mV AC potential. >> (measured between the USB-shielding of the PC and the disconnected >> USB-cable of the UA25) when the screen is connected. >> >> Both the screen as well as the speakers (and computer) are connected to >> the same power-stripe and share the ground. >> >> Fun-fact: When disconnecting the screen the potential between USB and PC >> goes up to ~150mV (IFF the computer is not battery powered otherwise >> it's 0) but the current drops to nil. > > Very few multimeters are capable of measureing AC current with any > level of accuracy. If you have 150 mV between two points, and zero > current when you short-circuit them (as a current meter is supposed > to do) then at least one of the two measurements is bogus. They were not measure simultaneously. I first measured the voltage and _then_ the current. In both cases the device was not directly connected: I measured between USB-plug and socket shield. I expect that the voltage potential drops to near zero when I measure the current, but I have no way of verifying this (I don't have a 2nd multimeter at hand). > This gets confusing. Thanks for getting back to me about this nevertheless. > You have at least four items, laptop, UA-25, screen, > and active speakers. How are they connected ? Is there an audio connection > between the screen and PC or UA25 ? Screen <-VGA-> Laptop (TP X60s) <-USB-> UA-25 <-TRS/jack-> Speaker(s) All devices are connected to one power-strip (except the UA25 which takes power from USB). To take one more thing out of the equation, I run the computer on battery. So it's Screen vs. Speaker. The screen has a switching power supply, but the speaker has a hardwired one (230V fixed). Any suggestions before I add a switch to the ground of the screen? bzzzzzzt, robin _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
