This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
Condenser microphones have internal electronics that need power
to operate. Early condenser microphones were powered by
batteries, or separate power supplies using multi-conductor
cables. In the late 1960's, German microphone manufacturers
developed 2 methods of sending power on the same wires that carry
the signal from the microphone.
The more common of these methods is called "phantom power" and is
covered by DIN spec 45596. The positive terminal of a power supply
is connected through resistors to both signal leads of a balanced
microphone, and the negative terminal is connected to ground. 48
volts is the preferred value, with 6800 ohm resistors in each leg of
the circuit, but lower voltages and lower resistor values are also
used. The precise value of the resistors is not too critical, but
the two resistors must be matched within 0.4%.
Phantom power has the advantage that a dynamic or ribbon mic may
be plugged in to a phantom powered microphone input and operate
without damage, and a phantom powered mic can be plugged in to
the same input and receive power. The only hazard is that in case
of a shorted microphone cable, or certain old microphones having
a grounded center tap output, current can flow through the
microphone, damaging it. It's a good idea anyway to check cables
regularly to see that there are no shorts between any of the
pins, and the few ribbon or dynamic microphones with any circuit
connection to ground can be identified and not used with phantom
T-power (short for Tonaderspeisung, also called AB or parallel
power, and covered by DIN spec 45595) was developed for portable
applications, and is still common in film sound equipment.
T-power is usually 12 volts, and the power is connected across
the balanced pair through 180 ohm resistors. Only T-power mics
may be connected to T-power inputs; dynamic or ribbon mics may be
damaged and phantom powered mics will not operate properly. [David]