15 May
2012
15 May
'12
11:43 p.m.
On 05/15/2012 10:59 PM, Fons Adriaensen wrote:
On Tue, May 15, 2012 at 08:44:37PM +0200, Robin Gareus
wrote:
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).
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.
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