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@xxxxxxxxxx> >> Subject: Re: 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@xxxxxxxxxxxxxx> >> 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@xxxxxxxxxx> >> 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 _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
