[LAU] USB audio interfaces >= 8 channels

Fons Adriaensen fons at linuxaudio.org
Tue May 15 20:59:35 UTC 2012

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.

This gets confusing. 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 ?



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