Thanks guys, but I've allready sovled the problem with floating DAC ($6). I just wanted to understand what's happening behind the scenes, but I guess it's complicated and that I'll have to read on it in tech books (most info I have, deals with noise in industrial automation systems and guitar tube amps).
That's whole another story, let me share... I recently bought (and returned) Tascam 16 channel USB audio interface with mic preamps. It was rich in features and price was great. Software was bad - but I could've live with that. What I could not live with is the noise, it's like there was DC present on almost all channels and there were resonances at multiples of 4kHz, screenshots:
noise profile: [MEDIA=googledrive]1E-nuuhvVQN5_spgUNzNs1B6XA5SwrZVD[/MEDIA]
normalized inputs recording silence: [MEDIA=googledrive]1euK8TV1N4vaz-204s9i4eVbp-YqraqT3[/MEDIA]
Tech support said that that's normal?!? so I went back to my old PC with 2 internal PCI Sek'd Sienas (unbalanced line inputs). Now, that system has multiple PE ground loops (PC, mixing console, powered monitors/speakers), and it's quiet. It wasn't quiet in the beginning (same issues I'm having now with the new PC) when I tried to avoid ground loops between the console and the PC, but as soon as I connected the multicore shields on both ends it went quiet. No "hum eliminators".
