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Phantom Power for Condenser Microphones
While the electret condenser microphone doesn't need a power supply to provide polarizing voltage, an FET impedance matching circuit inside the microphone does require some power. This may be supplied by a small low-voltage internal battery or by an external "phantom" supply.

Phantom powering is a technique which delivers a DC voltage to the microphone through the same shielded two-conductor cable that carries the audio from the mic. The phantom power may be supplied either by the mic mixer or from an external supply that is "inserted" into the line between the microphone and mixer input. For phantom power to function, the line between the power supply and the microphone must be balanced to ground, and uninterrupted by such devices as filters or transformers which might pass the audio signal but block DC. Phantom power also requires a continuous ground connection (Pin 1 in the XLR-type connector) from the power supply to the microphone. The supply delivers positive DC voltage equally to both signal-conducting leads, and uses the shield as a return path, or negative. Balanced-output dynamic microphones are not affected by the presence of phantom power, since there is no connection between the shield and either signal lead and, therefore, no circuit for the DC voltage.

Phantom power supplies are available in various output voltages ranging from as low as 9 volts up to 48 volts. They may be designed to operate from AC line voltages or from internal batteries.

Externally polarized or "discrete" condenser microphones seldom have internal battery power. Instead, a phantom power source is used to provide both the polarizing voltage for the element and to power the impedance converter. This type is sometimes called a "pure condenser."