> An on-line UPS like those used in datacenters or music studios is probably enough.
Only if you don’t have mains power anywhere near the recording device. You don’t need to be plugged into mains power to have it be present in a recording.
True! And that applies for every case. Even if you have 100% isolated mains with solar or battery, you'll get 50hz from neighbours (especially apartments) and power lines close by.
I remember having this issue when working in a recording studio during college, even when we switched to battery and turn the mains off, we'd still get hum in some guitars.
Funny enough I was recently reading an interview with producer Michael Beinhorn and he mentioned having some "mystery EMF/RFI event" happening in New York around 1997, coming from a specific block, and he had to relocate a recording session to Los Angeles because of how strong it was interfering with the guitar amps and other equipment [1].
Only if you don’t have mains power anywhere near the recording device. You don’t need to be plugged into mains power to have it be present in a recording.