These are very cheap 5.1 surround system for PC. Inside is pretty basic, as in it has 1 big chip doing most everything. The STA540 4-channel, class AB audio amplifier
https://www.sparkfun.com/datasheets/Components/General/STA540.pdfHad this problem for years, on may system setups, it's the Logitech for 100% for sure.
My problem is at certain volumes on it's external knob, I get a pulsating/woofing/tremolo type volume effect @ ~2 Hertz. It's super noticeable when white noise, or a rain storm, waves, certain things in music like holding big notes. I can't notice it if just speech is playing.
It's on all all 5 speakers separately, when doing speaker static test. Not sure about the subW.
I just finally looked at the right-out on a scope, and I can't get clean triggering at all on anything I tried, a 1kHz sinewave on utube looks like a dirty mess at only 10's of mV. So the scope is rolling. With it on the white noise, I think the base signal was about 20-40mVpp and @2Hz I'm seeing narrow peaks jumping to 60mVpp (at this random volume).
When it's doing it, I usually just tweak it's volume knob back/forth a bit and with luck it stops somewhere and doesn't do it. I usually listen from 1/4-1/2 volume, I haven't checked upper volumes. The knob is a potentiometer.
Another thing I just noticed when playing a 1kHz sine wave was that changing the volume back/forth, on the external knob AND/OR the PC, made brief popping/static type sound that seems to drown
out the sine wave as it happens.
I never worked on audio before, any ideas ?