A DC (direct current) voltage, usually 48 volts, applied to pins 2 and 3 referenced to pin 1 of an XLR microphone connector that can be used to power transducers with active electronics. Condenser microphones require a pre amp close to the very high impedance (See WFTD archive “impedance“) diaphragm which requires power to operate. Back in the ’50’s and ’60’s this power was often provided by a separate power supply that came with the microphone. Later manufacturers began to provide a source for this power at the microphone input to mixers or pre amps. Since the power is carried on the same wires that carry the audio signal, and since most dynamic microphones and other passive devices are not affected by this DC voltage it was known as “phantom” power. The theory was that only devices that needed it would be wired in such a way that they would use it. Nowadays almost all condenser microphones and active direct boxes are able to use phantom power when it is present on a microphone cable. Consequently most mixing board manufacturers include this feature in their products.