WaveTheory
100+ Head-Fier
This problem is slowly driving me nuts haha
For sure! That noise gets irritating quickly.
USB isolators I've used like ifi defender/silencer only reduce the noise by a little bit
I had the same problem.
I'm curious about what the fixes are
Disclaimer: USB ground loops can be super hard to get rid of. Cheaper fixes may only have limited effectiveness, or they could knock it out. It all depends. Let's go cheap to expensive:
1) Run optical instead of USB - if you can. You mentioned using the optical input in a previous post, IIRC. If your PC has an optical out, just use that and disconnect USB entirely. You'll still need to use a wall wart and USB power cable for the Modi, tho.
2) Get an in-line ground loop isolator and put it between the dac and the amp. This type of thing: https://www.amazon.com/PAC-SNI-1-Noise-Isolator/dp/B000K50HJE/ref=sr_1_5?dchild=1&keywords=rca+ground+loop+isolator&qid=1625512384&sr=8-5. This worked well for me. The only time I heard the ground loop noise was when the pots on my amps were near maxed-out.
3) Get a DDC (digital-to-digital converter) with good USB noise isolation. These get pricey. The Matrix Audio X-SPDIF 2 and the Singxer SU-2 are gonna be the starting points an they're both in the neighborhood of 400USD.
4) Go completely balanced between your DAC and amp...which would of course require new dac and amp. Using balanced connections between your dac and amp will naturally reject that ground loop noise. But, it ain't cheap, and balanced tube amps are quite rare.
I'd start with 1 or 2 or both and see what it does for you. Good luck!