Phantom power
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- For the albums named Phantom Power, see Phantom Power (album).
Phantom power is a widely-used method for supplying current to devices over signalling cables, especially audio. This is most often used for condenser microphones, and occasionally microphones of other types.
Phantom power supplies are often built into mixing desks, microphone preamplifiers and similar equipment. Many active DI boxes also use it. In addition to powering the circuitry of a microphone, in traditional (DC-polarized) condenser microphones the phantom powering directly or indirectly supplies the voltage used for polarizing the microphone's transducer element ("capsule").
Phantom powering consists of direct current applied equally through the two signal lines of a balanced audio connector (in modern equipment, usually an XLR connector). The supply voltage is referenced to the ground pin of the connector (= pin 1 of an XLR), which normally is connected to the cable shield and/or a ground wire in the cable. Thus the same type of balanced, shielded microphone cable which studios already used for dynamic microphones could also be used for condenser microphones--in contrast to vacuum-tube microphones, which had required special, multi-conductor cables of various kinds.
It is called "phantom" powering because the supply voltage is effectively invisible to any balanced microphones which do not happen to use it. (A balanced signal consists only of the differences in voltage between two signal lines; phantom powering places the same DC voltage on both signal lines of a balanced connection.) This is in marked contrast to another, slightly earlier method of powering known as "parallel powering" or "T-powering" (from the German term Tonaderspeisung), in which DC was overlaid directly onto the signal in differential mode. Connecting a dynamic microphone (especially a ribbon microphone) to an input that had parallel powering enabled could very well damage the microphone severely, but this is not normally so with phantom powering unless the cables are defective or wired incorrectly.
The prevailing IEC standard defines 48 volt and 12 volt phantom powering; a 24 volt version of phantom powering was included in the DIN standard for several years, but was never widely adopted by equipment manufacturers. The signal conductors are positive, both fed through resistors of equal value (for 48 volt phantom powering the standard value is 6.81 kΩ), and the shield is ground. (In an earlier system used mainly in France, the positive side of the 9 - 12 volt supply was grounded and the phantom supply voltage was 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 blocking capacitors.
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 microphones offer a choice of internal battery powering or (external) phantom powering. In some such microphones it is advisable to remove the internal batteries when phantom power is being used (i.e. beyond the normal concern about possible leakage of corrosive chemicals from batteries). Other microphones are specifically designed to switch over to the internal batteries if an external supply fails, which may be useful.
Phantom powering is not always implemented correctly or adequately even in professional quality preamps, mixers and recorders. In part this is because first-generation (late-1960s through mid-1970s) 48 volt phantom-powered condenser microphones had simple circuitry and required only small amounts of operating current (typically less than 1 mA per microphone), so the phantom supply circuits typically built into recorders, mixers and preamps of that time were designed on the assumption that this current would be adequate. This practice has carried forward to the present; many 48 volt phantom power supply circuits, especially in low-cost and portable equipment, simply cannot supply more than 1 or 2 mA total without breaking down. Some circuits also have significant additional resistance in series with the standard pair of supply resistors for each microphone input; this may not affect low-current microphones much, but it can disable microphones which need more current.
Mid-1970s and later condenser microphones designed for 48 volt phantom powering often require much more current (e.g. 2 - 4 mA for Neumann transformerless microphones, 4 - 5 mA for the Schoeps CMC ("Colette") series, 5 - 6 mA for most Shure KSM-series microphones, 8 mA for CAD Equiteks and 10 mA for Earthworks). The IEC standard gives 10 mA as the maximum allowed current per microphone. If its required current is not available a microphone may still put out a signal, but it cannot deliver its intended level of performance. The specific symptoms vary somewhat, but the most common result will be reduction of the maximum sound pressure level which the microphone can handle without overload (distortion). Some microphones will also show lower sensitivity (output level for a given sound pressure level).
Most earth lift switches have the unwanted effect of disconnecting phantom power.
The low-current 5 V supply provided at the microphone jack of some consumer equipment, such as portable recorders and computer sound cards, is sometimes called "phantom power." However, it is connected in a completely different way and is suitable only for powering microphones specifically designed for use with this type of power supply. Damage may result if these microphones are connected to true phantom power.