So I had this same issue and solved it. I posted about it in another forum, so I’ll just copy and paste my response from there here:
Just wanted to chime in to say I had this same issue with my RTX 3090 HTPC. With just the AVR it was fine, but when I introduced an external amplifier into my setup via RCA preouts, there was tons of electrical hum that would only go away if I unplugged the HDMI cord from the 3090. It didn’t even have to be a direct connection - if an hdmi cable was plugged in from my computer to the tv, and my tv to the AVR via a separate cable, the noise still came through! And it definitely wasn’t something that could be ignored - it was very audible.
First, I tried a high end power conditioner/filter and it did nothing. I ran across the Hum-X product listed above that I would have had to theoretically connect my computer to in order to fix the problem. This sort of sounded like the ideal solution, since it wouldn’t directly impact the audio chain and would still provide a safety measure for the grounding of my PC. Unfortunately, my PC power supply is 1000watts, and the Hum-X explicitly states not to use it for anything over 6 amps (720watts). My computer actually pulls a lot of power, so this was a no go.
Ultimately, it looked like RCA isolators were the only things that were likely to work, but given that I am using external amplification for the purpose of getting as good sound as possible, I didn’t want to go with a cheap approach that would potentially degrade the sound quality.
After lots of research, I ended up buying Jensen RCA isolation transformers that have been proven to measure basically no audible different in sound quality.
Here is a link:
https://www.parts-express.com/Jense...O_uY52H5GoKgK5RuJ654PigzZAuL1yrAaAnjyEALw_wcB
.
I’m super happy with the results. The buzzing is completely gone, my computer is still hooked up and being supplied with normal power, and though I didn’t take personal measurements, Audio Science Review has examined Jensen transformers and found them to be the way to go in these types of unsolvable ground loop situations (see this link:
Jensen ISO-Max CI-1RR Review (Isolation Transformer)).
Might have been more expensive than necessary for most people (I had to get 3 of the dual RCA transformer to cover all my channels ). Given how many things I already have in the sound chain, like multiple MiniDSP products now, I wanted to go with as good an option as possible to not detract further from the signal quality. The system sounds great, and I have no regrets.
It’s possible that running a fully balanced AVR and amplifier setup over XLR cables would avoid these ground loop issues created by the GPU altogether, though I don’t know for certain since I never tested it. For me though, I was happy with my setup and my AVR only has RCA preouts, so I decided to go this route than trying to see if that would’ve worked better.