I’d be curious to know what consoles you’ve encountered that have more than one phantom power supply and even more curious to know which consoles have a separate power transformer for each channel. Does this include separate rectifiers, capacitors and regulators for each channel? I can remember some old consoles the had local regulators for the power supply rails but not complete local phantom supplies.
Common practice is to use only one phantom PS rail per console, but channels in premium consoles can have individual on/off switches and local RC filtering. Budget consoles routinely use global switch for all channels, and this created problems in the early days of wireless mics when their receivers couldn't handle 48V (modern receivers can).
I’ve got an old Mackie Onyx 2 channel interface that gets its phantom power from a home brew DC-DC circuit, running a fair bit above the audible range. Apparently earlier versions of the circuit ran *in* the audible range!
I couldn’t use that phantom power with my MacBook because something in the FireWire connection also had a little bit of very high frequency tone on it, which mixed together with the slight tone on the phantom supply produced an audible difference tone.
Doesn’t take much to be audible in a mic preamp.
Mic preamps are not all created equal.... more likely a preamp with poor CMRR at HF...
Many preamps will include HF filters in their front end, if those filters on the + and - inputs are not identical, common mode HF noise will not be presented equally to both inputs so won't cancel completely.
I repeat there are multiple factors at play. A good preamp should reject common mode noise on the phantom supply whether it's 120Hz or 100kHz, while 100kHz will be harder to scrub.
JR