[LAU] ground-loops -- was Re: USB audio interfaces >= 8 channels

Robin Gareus robin at gareus.org
Tue May 15 21:43:17 UTC 2012

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?


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