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

Hi Chris!

On 05/16/2012 08:00 PM, Chris Caudle wrote:
> Replying to a digest, sorry if that screws up mail threading.
>
> From: Robin Gareus <robin@email-addr-hidden>
>> 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.

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.

>> /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@email-addr-hidden>
>> 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.

that's plausible.

> From: Robin Gareus <robin@email-addr-hidden>
>> 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?

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.

> 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
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Received on Thu May 17 00:15:02 2012