16 May
2012
16 May
'12
8:57 p.m.
Hi Chris!
On 05/16/2012 08:00 PM, Chris Caudle wrote:
that's plausible.
Replying to a digest, sorry if that screws up mail
threading.
From: Robin Gareus <robin(a)gareus.org>
Well, neither the UA25, nor the Behringer speakers nor the ASUS screen
are high quality devices. They're OK for home-use and actually pretty
good for their price, but I judging by the ground-loop noise, at least
one of them is improperly designed :)
Note that the UA25-EX (successor of the UA25) features a ground-lift
switch which disconnects the sleeve pin of the master output from the
ground.
Subject: Re: [LAU] USB audio interfaces >= 8
channels
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.
You have found that there are ways to design balanced inputs improperly.
The Audio Engineering Society devoted an entire issue of the journal (June
2005) to such issues.
A multimeter shows a constant 1mA current and
~3-4mV AC potential.
Should not be a problem for properly designed balanced inputs and outputs,
can be for improperly designed equipment. /me is
pondering to cut the ground-wire from the screen..
but I have so far refrained from doing that.
Could cause other problems. Probably your choices are either to modify
the equipment so it is not quite so improperly constructed, use a
transformer to isolate the equipment from the shield current, or possibly
to construct a cable which works around the offending equipment.
From: Fons Adriaensen <fons(a)linuxaudio.org>
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.
Probably what happens is that when the screen is not connected, there is a
high impedance voltage difference between the devices, and when the
shields are connected together (through the ammeter) current actually
flows, but is so low that the ammeter does not measure it correctly.
When the VGA monitor is connected, the monitor has low enough impedance
leakage path that it can source a couple of milliamps through the shield,
and the USB interface or speaker (or both) has a common impedance path
for that current to flow on the reference potential node of a high gain
stage and amplifies the noise current. From: Robin Gareus <robin(a)gareus.org>
They were TRS<->TRS. I've just replaced them with balanced TRS<->XLR
cables and the problem went away.
Actually I previously tried that but I did not pay attention: I was
using an unbalanced (mono TRS jack - bridged the shield and ground)
TRS->XLS cable - which of course did not help.
Any suggestions before I add a switch to the
ground of the screen?
Verify the wiring, make sure that there are no broken conductors or broken
solder joints in the shield connections of the cables between the audio
interface and the speakers.
Are you using TRS-TRS cables, or TRS-XLR cables for the connection from
audio interface to speakers? Which of the equipment (video monitor, computer,
active speakers) have
three wire power connections with safety ground connected, and which (if
any) have only two wire connections.
All of them are grounded (I live in Europe). The power-supply is built
into the screen and the speakers. Only the laptop has an external PS
(also grounded) and a 2 pin low-voltage connection to the laptop, but
that does not make a difference - the noise was there when running it
from battery, too.
Equipment in the US is typically
double insulated and so does not require a safety earth connection, but I
don't know if that also applies on equipment shipped for use on European
240V power distribution.
I don't know about regulations, but non-grounded equipment is becoming
extremely rare here. Basically only some external PS units for phones,
etc are not-grounded, everything else is.
I ask because you need to give the ground current
someplace to flow to
complete the circuit which does not flow across the reference potential
node (sometimes called the "ground connection") of a high gain stage.
Could be in the output of the audio interface, or the input of the
speaker, or both, where that current is being converted to audible noise.
One way to do that is to make sure the shield of the cable connects well
to the shield of the connector. You generally have to connect the cable
that way in a TRS connector, but XLR connectors do not by default have a
connection between pin1 and the connector shell. The equipment should
connect pin 1 to a low impedance equipment shield connection internally,
but many designs do not. In that case connecting pin 1 to the connector
shell inside the connector can sometimes help.
If some of the equipment has an earthed chassis and some does not,
sometime making an external connection (using wire or copper braid)
between the different chassis can reduce the potential difference enough
that the current flowing on the audio cable becomes low enough to be
inaudible.
That's a neat tip. I remember that from old record-player days, but
would not have thought of that in the current context.
Something like one of these would probably help:
http://jensentransformers.com/dm2xx.html
http://jensentransformers.com/pi2xx.html
But good transformers are not inexpensive, it might be the same cost to
get an audio interface which did not inject so much noise current into the
output. If the speakers are causing the problem and not the audio
interface, then you may be able to make some modifications to the input
connections of the speaker interface to solve the issue. Depends on how
the amplifier assembly is physically constructed.
Thanks for all the information and feedback. Might come in handy one day.
For now I took the easy way out and just got myself two new cables :)
Cheers!
robin