Condenser microphones require a power source to operate. Sometimes, this power can be provided internally with a battery. More commonly, however, an external supply is used. Most consoles today have a phantom power source built in to the microphone input channel.
Phantom power, also called simplex power, is typically fed at 48VDC, but can actually range from 9V to 52V. Microphones requiring phantom power have an internal voltage regulator to accept the possible range of voltages. The name phantom power is just as it implies, being supplied apparently from nowhere. Phantom power uses the same conductors as the audio path, so no additional wiring is required.
Standard balanced low-impedance microphone wiring can support phantom power. The advantages of phantom powering are clear. Without the need for special wiring, the cost of installation or use is reduced. The power supply voltage already present within a console can be adapted for phantom use. While the phantom voltage is present, microphones that do not require it can also be used on the same inputs without any adverse effect.
The figure above shows how phantom power is put onto the audio lines from the console input or mic pre-amp to feed the condensor mic. The main difference between the two methods shown is the use of a transformer input. If the console's input transformer does not have a center tap, a hybrid resistor design can be used to create one. The value of the dropping resistor will limit the current draw of the mic to 10mA or less. The power supply return is connected through the cable shield connection.
The simplicity of phantom powering is its greatest asset. The built-in fool-proof design also allows non-phantom powered microphones, like dynamics and ribbons, to be used interchangeably.
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