I have been experiencing audio hum + hiss + screeching when both sound devices in my workstation are connected to the same stereo line mixer and the levels on the mixer are not equal. Unplugging either sound device or equalizing the levels makes this completely disappear. As of now I am currently using this setup by having the two mixer channels my workstation uses equalized to eliminate the background noise. This is not ideal though because it defeats the purpose of using the mixer in the first place. All other audio devices I plug into the mixer do not cause any noise, it only happens when both sources from the workstation are connected.
My audio setup is as follows:
Workstation:
Realtek ALC1150 onboard audio chipset (Debian 9.4 with 4.14 realtime kernel as hypervisor)
NVIDIA GTX 1070 HDMI audio to monitor with built in DAC. (Windows 10 VM)
Laptop:
Thinkpad T420s
All 3 sound devices are connected to a stereo mixer which then goes out to my speakers and/or headphones. Everything is powered by a single power strip. I tried separating the monitor, mixer, and workstation by running extension cables to other rooms, as I expected it amplified the effect.
Since it does not sound like your average ground loop hum, (its more screeching and hissing patterns that correspond with the load of the workstation than a constant background hum) would adding inline audio isolation transformers between the monitor and mixer be able to eliminate the noise? I would like to just salvage transformers from something else but am not sure what type is suitable for this application. If I don’t have anything laying around that I could hack up would someone also point me to one that will not distort the audio and is also not overpriced snake oil?