Source of ground looping? Different ground for USB and laptop's line out?


I have a Behringer UCA202 (no-external power) USB Interface (DAC):

Bus 006 Device 003: ID 08bb:2902 Texas Instruments Japan PCM2902 Audio Codec

and an Intel audio controller which is built into the laptop.

00:1b.0 Audio device: Intel Corporation 82801I (ICH9 Family) HD Audio Controller (rev 03)

When I plug the both of them into the mixer, I get loopback. It sounds like a high pitched oscillating noise. I'm wonder if anyone has any idea of what this nose could be from and how I further trouble shoot it. A simple configuration with the mixer to Behringer and audio-out on laptop to Mixer in can cause this feed back.

Is this nose perhaps because the line-out on the laptop and behringer In have two different fake grounds?

Any ideas are appreciated.r

Evan Carroll

Posted 2012-10-30T06:51:59.573

Reputation: 903

Can't you describe that "high pitched oscillating noise" a bit more precisely? If it's really a feedback issue I'd expect it to be very loud (drive one of the inputs into clipping). If it's not particularly loud, it's probably rather something like this and has indeed to do with grounding.

– leftaroundabout – 2012-10-30T20:19:20.153



This is nothing to do with ground issues, but more to do with the fact that your config is set up to allow output on one or both devices to be fed to an input.

Check your audio properties. You should be able to assign devices to input or output, or mute when recording.

Rory Alsop

Posted 2012-10-30T06:51:59.573

Reputation: 5 762

I agree, ground loops occur at 60 Hz. – ObscureRobot – 2012-10-30T14:59:32.863

1Or 50 in the UK:-) But definitely not high pitched - this is more of a feedback issue. – Rory Alsop – 2012-10-30T15:19:10.967

Both devices are fed to two different inputs on the mixer, and this is required. I want to play two sources at the same time to the mixer. – Evan Carroll – 2012-10-30T15:50:32.700

Having them sent to two inputs is fine, but it sounds like something is also being set as an output, hence the feedback – Rory Alsop – 2012-10-30T16:20:19.237