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. > /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. 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? 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. 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 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. 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. -- Chris Caudle _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
