Phantom Power
Phantom Power is a means of powering condenser microphones remotely, using balanced microphone cable. The AES (and IEC 61938) endorses two voltage levels, 48 V and 12 V, and they are referred to as P48 and P12 (distinguishing the latter from T12 – Tonader Power). An intermediate P24 was introduced but never caught on and is now defunct. P12-48 refers to microphones which can accept both 12 V and 48 V.
There is a tolerance of +/- 4 V on P48 and 3 V on P12 so it is common to meet microphones which claim “9 – 52 V” powering. Absolute voltage levels for phantom are not too important but their symmetry is, and sagging voltage rails are often indicative of inadequate supplies.
The required voltage is applied equally, usually via twin buffer resistors, to both legs of the cable and the current is returned via the ground wire or shield. The resistor values should be 680 ohms for P12 and 6800 ohms for P48 and should be matched to <0.5%. If the lines are shorted to ground <15 mA will flow.
Most audio mixers supply 48 V phantom via their microphone input sockets. If an unbalanced microphone is used, or if a fault develops in a cable, it is important to be able to switch off the phantom supply. Some mixers provide this facility on a per channel basis, while others may offer a less versatile “global” phantom power switch.
It used to be common for mixers with an input Jackfield to use a separate phantom supply unit on the “studio” side. The aim was to avoid a DC path through shorted jacks causing magnetisation of the input transformer cores. This was particularly relevant when the centre-tap was used as a common phantom feed point (with just one resistor). This arrangement is virtually never used nowadays and if input transformers are employed they are often DC isolated. Thus in-desk phantom supplies are the norm and microphone jacks frequently carry 48 V DC. It should be noted that 48 V represents a very loud audio signal. The transients caused by inserting or removing a jack plug or even just moving a dirty jack can easily damage equipment down stream, particularly loud speakers.
A simple but effective tester for Phantom Power, nicknamed a Bright Eyes, is easy to construct.
For more information on Microphone Powering see the Library section on the Microphone Data website.