The term phantom power is commonly used to describe a form of +48 volt DC power that is superimposed on a balanced audio microphone input to supply power to condenser microphones. Condenser microphones typically have a preamplifier built into the microphone housing which need some form of power supply. In addition; the microphone’s diaphragm is a form of capacitor (or “condenser”) which requires an electrical charge. In some cases, the power supply can either be a battery, or the phantom power supplied by the microphone preamplifier.
When condenser microphones were first developed, they contained vacuum tube amplifiers; and thus required high voltage DC power supplies. These typically were metal “lunch box” sized enclosures with AC power cords and XLR output connectors for the audio. A multi-pin connector and multi-conductor cable was used to carry the audio and high voltage DC power to the microphone. This type of microphone is commonly referred to as a “vintage condenser microphone.” When solid-state amplifiers became available; the internal amplifier in condenser microphones could operate on much lower voltages and it became possible to safely use a form of “phantom” power that is carried by the same cable that carries the audio- an XLR cable.
By connecting the same +48 DC voltage to both of the audio conductors of the balanced audio cable, the audio input does not “see” this DC voltage because it only amplifies the voltage difference between the two conductors- thus the “phantom” characterization. The shield of the cable is used as the “0 volt return” conductor for the phantom power. This also means that a dynamic microphone can be connected to phantom power with no damage to the microphone, or change to its audio output. The +48 DC voltage is normally connected through current limiting resistors for safety. This both limits the amount of current available to each microphone and prevents damage if one or both to the signal conductors is shorted to ground or connected to a circuit that is not a microphone input.