Phantom power (labeled as +48 V on most audio equipment) is a method that sends DC voltage through microphone cables. It is best known as a power source for condenser microphones, though many active DI (direct input) boxes also use it.
Phantom power supplies are often built into mixing desks, microphone preamplifiers and similar equipment. In addition to powering the circuitry of a microphone, in traditional (DC-polarized) condenser microphones the phantom powering directly or indirectly supplies the voltage used for polarizing the microphone’s transducer element ("capsule").
Polarizing the microphone’s transducer WHAT? What’s a Capsule?
Condenser microphones are based on an electrically-charged diaphragm/backplate assembly which forms a sound-sensitive capacitor/capsule. This capsule stores an electrical charge. When the element is charged, an electrical field is created in proportional size to the distance between the backplate and the diaphram.
Take a standard grocery store tub of whip cream. Empty it and then cover it tightly in saran-wrap. Now if you press down on the saran wrap, you are decreasing the potential energy because there is less space inside. If you pull up on the top of the saran-wrap in the middle you are increasing the potential energy because there is more room.
The variation between the spacing (the increasing and decreasing pressures) produces the electrical signal which represents the sound detected by the condenser microphone.
Sound To Energy
Microphones have transducers, either dynamic or condenser types (hence dynamic or condenser microphones). A transducer changes energy from one form into another, in this case, acoustic energy into electrical energy.
Back to That Old Phantom Magic
A condenser microphone needs some means for maintaining the electrical charge / polarizing voltage. Creating the charge on the capsule of condenser microphones can be accomplished by battery, phantom powering, and by using fixed charge materials in manufacturing.
In the case of phantom powering, the voltage supplied through the microphone cable, such as by a mixer equipped with phantom power, is equal on Pin 2 and Pin 3 of a typical balanced, XLR-type connector. For example, a 48 volt phantom source, would have Pin 2 at 48 VDC and Pin 3 at 48 VDC, both with respect to Pin 1 which is ground.
Because the voltage is exactly the same on Pin 2 and Pin 3, phantom power will have no effect on balanced dynamic microphones: no current will flow since there is no voltage difference across the output. In fact, phantom power supplies have current limiting which will prevent damage to a dynamic microphone even if it is shorted or miswired.
Phantom power can be thought of as just the right about of power where you need it. In some sound board mixers, multiple phantom power buttons can be present for different channels. For example, a button for channels 1-8 and one for 9-16. Knowing what microphones require it, tells you what channels will need it.
Note: condenser microphones that are made with pre-charged metals or are battery charged may be damaged if used in conjunction with phantom power so make sure you read the instructions with the microphone.