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Phantom power - a great breakthrough in microphone technology

Capacitor microphones used to come each with their own power supply. Then phantom power was invented so that any number of mics can be powered from the mixing console. So why are some manufacturers returning to the old ways?

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If you have a very long memory, you will remember microphones such as the AKG C12 and Neumann U67 - tube microphones that came with their own dedicated power supplies. You plugged the mic into the power supply, the power supply into the mains, and took a feed of the mic signal via the power supply to the mixing console.

Doing this with one or two mics isn't any big deal. But what if you wanted to have twenty mics on an orchestral session? You would soon get tired of all that extra plugging.

But somewhere in Norway, way back in 1966, plans were afoot to change all of this.

Norwegian State Television at the time was already using a 48 V DV powering system for much of its equipment. They wanted to take advantage of this ready supply of voltage to power their capacitor microphones, rather than use a separate power supply for each one.

The Neumann company responded to this with their model KM 84. This is a small FET (field-effect transistor) microphone and was the first to be capable of being phantom powered.

Neumann's phantom power system uses a 48 V DC source and sends this equally to the hot and cold conductors of the audio signal cable, and thence to the microphone. The word 'phantom' is appropriate because you don't see the power supply - it can be contained within the mixing console and requires no additional wiring beyond the signal cable.

The word phantom is also appropriate because unless a microphone is designed to receive such power, there is no voltage difference between the two signal conductors. A microphone that is not designed to use phantom power simply will not notice it is there (as long as it has the usual output transformer).

Phantom power is supplied to the conductors through two well-matched 6800 ohm resistors. In fact one power source can be used, and voltage supplied to many microphones, each through a pair of resistors. This provides protection against short circuits - even if one mic is shorted to ground, only 14 milliamps can flow, which is peanuts to any properly designed power supply.

Once phantom power was accepted by the microphone manufacturing industry, the limitation on the number of microphones employed caused by all those individual power supplies was removed.

The strange fact is that currently there is an increasing number of microphones that are returning to the old ways. In the case of tube microphones, this can be justified - tubes require higher voltages. Also, microphone that are capable of handling very high sound pressure levels can benefit from a higher power supply voltage.

However in the vast majority of cases, phantom power works just fine. It is in fact a brilliant invention - brilliant in its simplicity.

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By David Mellor Sunday April 11, 2010
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