Unveiling the Mystery of Phantom Power
+48v Phantom power, in the context of professional audio equipment, is DC electric power transmitted through microphone cables to operate microphones that contain active electronic circuitry. It is best known as a convenient power source for condenser microphones, though many active direct boxes also use it. The technique is also used in other applications where power supply and signal communication take place over the same wires.
Phantom power supplies are often built into mixing consoles, microphone preamplifiers and similar equipment. In addition to powering the circuitry of a microphone, traditional condenser microphones also use phantom power for polarizing the microphone's transducer element.
To know what phantom power is, you need to understand how condenser microphones work. Condensers work on the principle of variable capacitance. Sound waves vibrate a diaphragm (usually gold-sputtered mylar) that is stretched in front of a metal plate (called the backplate). As the diaphragm vibrates, the distance between the diaphragm and the backplate changes, which changes the capacitance.
Those tiny electrical variations which must be amplified before they leave the mic so there’s an internal preamp inside the mic. The phantom power provides the voltage to charge the diaphragm and also powers the preamp inside the mic.
Phantom powering consists of a phantom circuit where direct current is applied equally through the two signal lines of a balanced audio connector (in modern equipment, both pins 2 and 3 of an XLR connector. The supply voltage is referenced to the ground pin of the connector (pin 1 of an XLR), which normally is connected to the cable shield or a ground wire in the cable or both. When phantom powering was introduced, one of its advantages was that the same type of balanced, shielded microphone cable that studios were already using for dynamic microphones could be used for condenser microphones. This is in contrast to microphones with vacuum tube circuitry, most of which require special, multi-conductor cables.
With phantom power, the supply voltage is effectively invisible to balanced microphones that do not use it, which includes most dynamic microphones. A balanced signal consists only of the differences in voltage between two signal lines; phantom powering places the same DC voltage on both signal lines of a balanced connection. This is in marked contrast to another, slightly earlier method of powering known as "parallel powering" or "T-powering" (from the German term Tonaderspeisung), in which DC was overlaid directly onto the signal in differential mode. Connecting a conventional microphone to an input that had parallel powering enabled could very well damage the microphone.
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