Phantom power

Phantom power is a DC power supply provided on the microphone inputs of many audio mixing desks and similar equipment, and used for powering microphones, DI boxes, and similar devices.

It is provided between the shield and the two signal lines of a balanced audio connector, usually, but not always, an XLR connector. Power is required by condenser microphones including electret and back electret designs, and by active DI boxes. Some of these are battery powered, some powered by phantom power, and some by both options.

Phantom power typically is about +48V DC, but some equipment uses 12V. The original voltage on valve equipment was 48V, transistor equipment introduced the 12V standard but the voltage has been rising ever since and now 48V is again most common. The signal conductors are positive, both fed through large resistors, and the shield negative.

Many desks have a switch for turning phantom power off or on. On some top of the line equipment this can be done individually by channel, more commonly all channels are either off or on at once. If it is desired to disconnect phantom power from one channel only, this can be done by using a 1:1 isolation transformer, or two baluns can be used back to back which has the advantage that these baluns are more commonly available.

Instrument amplifiers rarely provide phantom power. To use equipment requiring it with these amplifiers, a separate power supply must be inserted into the line. These are readily available commercially, or alternatively are one of the easier projects for the amateur electronics constructor.

Cautions

Some experts advise removal of the batteries from dual-powered units when phantom power is available, as even some top quality units will incorrectly recharge the batteries under these conditions, risking battery failure, leakage or even explosion. Some units will continue to drain the battery even if good phantom power is available. Others will drain their batteries if left connected for a time (perhaps overnight) with the phantom power supply turned off. Many will distort or fail to operate at all if a flat battery is installed, or once a good battery has been drained in this way.

However, some equipment makers advise always using batteries in case of phantom power failure. A properly engineered phantom power supply is far more reliable than many other components of the input circuit, but one advantage of having batteries installed is that should the phantom power be deliberately turned off because of some other incompatibility, the unit will continue to operate. Good batteries may also reduce (but will not eliminate) the hum associated with cheaper DI boxes, or produced by inadequately filtered phantom power supplies.

Most earth lift switches have the unwanted effect of disconnecting phantom power.

The low-current 5V supply provided at the microphone jack of some computer sound cards is sometimes called phantom power but is more properly called a polarising supply, and is connected in a completely different way, using the extra conductor as a dedicated power supply conductor. It is suitable only for powering microphones specifically designed for use with this power supply, and damage may result if these microphones are connected to true phantom power.