Indeed so.'Most sound desks' use op-amps as their active components which run most successfully on a dual supply rail of 15V ...
Again, indeed. Having a centre-tapped transformer secondary is probably the simplest/easiest way to get +15V and -15V rails.... the easiest way to derive this from the mains is a 2 phase ac secondary and full wave rectification....
However, what I was talking/asking about was this concept (which I think is a 'new one' on me!) of having a power supply which is 'half in and half out' of the equipment it supplies - i.e. having the transformer external to the equipment but everything else internal.
What I would normally expect is EITHER to have the equipment supplied with mains voltage, with the PSU entirely within the equipment OR (if one wanted everything in the equipment to be ELV and/or their were space issues) to have the power supply entirely external to the equipment, supplying +15V and -15V DC to the equipment (still only '3 wires').
The need for a 48V rail might possibly be one of the reasons, but it seems to me to be a pretty lame 'excuse'/explanation, in either of those two ('common') scenarios I've just mentioned. If the equipment is supplied with mains, then there is obviously no issue, and if the PSU is entirely external, then generation of the 48V is again exactly the same, the only difference beeing the need for a 'fourth wire' between PSU and equipment.Using the voltage doubler/tripler for the 48V phantom is also easy.
Kind Regards, John