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Phantom Power

Phantom Power is a standardised professional method of providing power to the electronics of some types of microphones via a balanced XLR connection. The relevant standard was first conceived in the early 1970s, and is now recognised as IEC 61938:2018. Several variations of phantom power are detailed in the document. 

Most professional systems operate with a nominal supply voltage of +48V DC, although there is an acceptable tolerance range of +/-4V (ie. 44 to 52V). This arrangement is described as the 'P48' format, and the phantom power supply is connected with the positive side going to each of the two balanced audio lines via individual 6k8 current-limiting resistors. The negative side is returned via the cable screen. In this configuration the maximum current available to the microphone is 10mA, providing up to 170mW of power. Most microphones draw around 4mA (or less) of current, but some models require more and a few need the full 10mA .

Battery-powered equipment often uses the P12 format, which uses a 12V power supply with 680 Ohm feed resistors allowing up to 15mA and 100mW of power. A newer addition to the specification, called P12L (low-power), uses 3k3 resistors to give 4mA and just 22mW of power.

At the other end of the scale is the new SP48 variation which uses the 48V supply voltage but connects via 2.2K feed resistors allowing up to 22mA and 520mW of power. 

Although rare, some manufacturers choose to provide non-standard supply voltages, such as 15 or 24V — something which seems prevalent on budget, compact, active PA loudspeakers. Some microphones are very tolerant of the supply voltage (many AKG mics can accept anything from 9-52V, for example), while others won't function correctly if the supply voltage falls too low.

Other microphone powering schemes are also available, including 'Plug-in Power' for consumer electret and lavalier mics, and Tonader (or A-B) power for battery-powered legacy professional equipment. 

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