Condenser microphones have internal electronics that need power to operate. Batteries, or separate power supplies using multi-conductor cables powered early condenser microphones. 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%. More.
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