Hum when using external souncard for external fft

[mowgli]

I'm using a Behringer XENYX Q1202USB as the hub for my av set up. I have my macbook pro outputting audio to the mixer via de built in jack. I want to use the mixer's USB audio as the channel for external FFT in Resolume. It works, but as soon as I select it in Resolume's preferences, I get a humming noise coming out of my speakers. Any clues as to what might be the cause?

[Oaktown]

You probably have introduced a ground loop in your setup so to get rid of the hum, you'll probably need to use a stereo DI box (or two mono if you have them already). I personally use the Radial ProAV2 and they work like a charm (http://www.radialeng.com/proav.php).

[deepvisual]

[Oaktown]

I took a look at the DTI unit and I think the Radial Engineering unit might have a few advantages such as:

• 1/8" TRS input
  Input through
  Pad
  Lift
  Ground Lift on output side

proav2-panels-lrg.jpg (43.62 KiB) Viewed 6806 times

[mowgli]

Thanks for the tips. I managed to resolve it by wiring everything differently. I had the USB audio going through a hub, it's now connected directly to the laptop and the hum is gone.

[Oaktown]

Was it a powered USB hub?

[mowgli]

yes it was

[Oaktown]

Any equipment without ground is likely to introduce a ground loop so that makes sense. It's not a bad idea to always carry a stereo DI in your A/V kit to interface with venues. That way you can always send audio to FOH using balanced cables.