Phantom power, in the context of professional audio equipment, is DC electric power transmitted through microphone cables to operate microphones that contain active electronic circuitry. It is best known as a convenient power source for condenser microphones, though many active direct boxes also use it. The technique is also used in other applications where power supply and signal communication take place over the same wires.
Phantom power supplies are often built into mixing consoles, microphone preamplifiers and similar equipment. In addition to powering the circuitry of a microphone, traditional condenser microphones also use phantom power for polarizing the microphone's transducer element.
Phantom powering consists of a phantom circuit where direct current is applied equally through the two signal lines of a balanced audio connector (in modern equipment, both pins 2 and 3 of 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 or a ground wire in the cable or both. When phantom powering was introduced, one of its advantages was that the same type of balanced, shielded microphone cable that studios were already using for dynamic microphones could be used for condenser microphones. This is in contrast to microphones with vacuum-tube circuitry, most of which require special, multi-conductor cables.
One method of supplying phantom power. A microphone or other device can obtain DC power from either signal line to ground terminal, and two capacitors block this DC from appearing at the output. R1 and R2 should be 6.81k ohms for "P48" 48 volt phantom. R3-6 and Zener diodes 1-4 protect the output to a maximum of 10v.
With phantom power, the supply voltage is effectively invisible to balanced microphones that do not use it, which includes most dynamic microphones. 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 conventional microphone to an input that had parallel powering enabled could very well damage the microphone.
The IEC 61938 Standard defines 48-volt, 24-volt, and 12-volt phantom powering. The signal conductors are positive, both fed through resistors of equal value (6.81 kΩ for 48 V, 1.2 kΩ for 24 V, and 680 Ω for 12 V), and the shield is ground. The 6.81 kΩ value is not critical, but the resistors must be matched to within 0.1% or better to maintain good common-mode rejection in the circuit. The 24-volt version of phantom powering, proposed quite a few years after the 12 and 48 V versions, was also included in the DIN standard and is in the IEC standard, but it was never widely adopted by equipment manufacturers.
Nearly all modern mixing consoles have a switch for turning phantom power on or off; in most high-end equipment this can be done individually by channel, while on smaller mixers a single master switch may control power delivery to all channels. If one desires to disconnect phantom power from one channel only, this can be done by using a 1:1 isolation transformer or blocking capacitors. Phantom powering can cause equipment malfunction or even damage if used with cables or adapters that connect one side of the input to ground, or if certain equipment other than microphones is connected to it.
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.
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 since batteries may corrode and leak chemicals. 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. The original DIN 45596 phantom-power specification called for a maximum of 2 mA. 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 that 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 and Josephson microphones, 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 that the microphone can handle without overload (distortion). Some microphones will also show lower sensitivity (output level for a given sound-pressure level).
Most ground lift switches have the unwanted effect of disconnecting phantom power. There must always be a DC current path between pin 1 of the microphone and the negative side of the 48 volt supply, if power is to reach the microphone's electronics. Lifting the ground, which is normally pin 1, breaks this path and disables the phantom power supply.
There is a common belief that connecting a dynamic or ribbon microphone to a phantom-powered input will damage it. There are three possibilities for this damage to occur. If there is a fault in the cable, phantom power may damage some mics by applying a voltage across the output of the microphone. Equipment damage is also possible if a phantom-powered input connected to an unbalanced dynamic microphone or electronic musical instruments. The transient generated when a microphone is hot-plugged into an input with active phantom power can damage the microphone and possibly the preamp circuit of the input because not all pins of the microphone connector make contact at the same time, and there is an instant when current can flow to charge the capacitance of the cable from one side of the phantom-powered input and not the other. This is particularly a problem with long microphone cables. It is considered good practice to disable phantom power to devices that don't require it.