Just to clarify a few things about microphone power supplies:
Tubes work kinda like a valve on a garden hose. They shoot a stream of electrons from the filament (the little heater part that glows red) to the "plate" (the big flat black part near the outside of the tube). In between those two pieces is another element called a "grid" which can be used like a valve on a water hose to control that stream of electrons. With a very small signal applied to the grid, you can make a very big version of the signal appear at the plate.
The filament needs a small voltage (usually around 6 to 12 volts) to heat up so that it will start shooting out electrons, and the plate usually needs high voltage (usually around 200 to 300 volts) since the signal is going to be magnified - a lot.
So why do you need a tube in a microphone?. For one thing, you can steal some of that plate voltage to charge the diaphragm. Secondly, tubes don't mind seeing very high source impedances as their input, which is what a condenser capsule is - a VERY high impedance source.
One of the problems with tubes is that once you have this higher level signal on the plate, you can't use it with out one more step.
Since the signal you want is riding on the DC plate voltage, you gotta get rid of the DC to get the AC music signal you want.
Enter the output transformer. Transformers won't pass a DC signal, so they block the DC. letting the AC music signal come thru. And the transformer can convert this signal to a low impedance balanced signal, so that you can run it fairly noise free to your mixer.
So, in addition to the 3 wires that you need to carry the balanced signal out to the mixer, you also need some wires to carry the heater and plate voltages to the tube in the mic.
Why not put the tube in the power supply? The high impedance of the capsule won't let you run more than a couple of inches of wire before you've lost most of the signal.
Also, if you have two active diaphragms back to back, you can change the patterns remotely from the power supply, by just changing the polarizing voltages, but you'll need to run a couple more wires from the power supply to the mic as well.
When transistors appeared, companies switched to Field Effect Transistors (FETs) which could change very high impedances to lower impedances, so the tube was no longer necessary for that fuction, which meant no heater or plate voltages were needed any more. But what about the DC voltage needed to charge the capsule?
Neumann solved that problem very cleverly - run the DC on the same wires carrying the signal, and then block it from the rest of the stuff inside the mic, by using the transformer in reverse. Since the mic wouldn't really see the DC voltage on the signal wires, it would seem like it was getting its power from a ghost power supply - a "phantom" power supply.
In fact, you could also pull off a small portion of the DC voltage to power any active electronics in the mic. Later, they figured out how to eliminate the transformer entirely, by using transistors to create a balanced output circuit.
There are electrical laws in Germany which limits the amount of DC voltage you can run thru wires in commercial facilities without special inspections and permits. It happens to be 48 volts.
Hopefully, this helps a little bit in explaining where we are, and how we got here.
