Hum in output of iRig HD2

by **dmitch** » Wed Mar 31, 2021 9:55 pm

I'm coming up to speed on the iRig HD 2. I have a problem with hum at the 1/4" output. When the audio source is USB (rather than thru), and the USB is "active" (LED is bright blue instead of dim blue), there is a fair amount of hum in the output. Not as loud as the signal, but loud enough to be a problem. I.e. louder than I think it should be. :-)

Observations:

It doesn't matter what the audio source on the host side is.
iRig is connected via USB to an iMac.
I tried different USB ports (one on the iMac, one on a USB hub). No effect on hum.
It doesn't matter what the 1/4" audio out is connected to - I tried different cables, different destinations. Hum is always the same.
The hum is *not* audible in the headphone out, so it seems like the hum is coming from the amp or buffer that drives the 1/4" out.
The hum does *not* happen when the audio source is "thru".
When sending USB audio to the iRig from the iMac, the LED on the iRig is bright blue whenever there is active audio on the USB. When the audio stops, the LED turns dark blue in 1-2 seconds. The hum stops exactly when the LED switches from bright blue to dark blue.
I'm generally pretty careful about hum in my project studio, I know about ground loops and balanced cables (which of course I am not using with the iRig). The rest of the audio gear used in the this test is pretty decent stuff (Mackie mixer, Focusrite interface, etc.). I'm pretty sure that this is a problem with the iRig.

Does this ring a bell with anyone? Are there any tips/techniques to avoid this hum?