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Phantom power, in the context of professional audio equipment, is DC
electric power Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions ...
transmitted through
microphone A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publ ...
cables to operate microphones that contain active electronic circuitry. It is best known as a convenient power source for
condenser microphone A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...
s, 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 console A mixing console or mixing desk is an electronic device for Audio mixing (recorded music), mixing audio signals, used in sound recording and reproduction and sound reinforcement systems. Inputs to the console include microphones, signals fro ...
s,
microphone preamplifier The term microphone preamplifier can either refer to the electronic circuitry within a microphone, or to a separate device or circuit that the microphone is connected to. In either instance, the purpose of the microphone preamplifier is the same. ...
s 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.


History

Phantom powering was first used (and still is used) in copper wire based landline
telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
systems since the introduction of the rotary-dial telephone in 1919. One such application in the telephone system was to provide a DC signaling path around transformer-connected amplifiers such as analogue line transmission systems. The first known commercially available phantom-powered microphone was the
Schoeps Schalltechnik Dr.-Ing. Schoeps GmbH, known as Schoeps Mikrofone, is a German manufacturer of professional studio condenser microphones for recording and broadcast. The privately owned company is based in Karlsruhe, south-west Germany, and was fo ...
model CMT 20, which came out in 1964, built to the specifications of French radio with 9–12 volt DC phantom power; the positive pole of this powering was grounded. Microphone preamplifiers of the
Nagra Nagra is a brand of portable audio recorders produced from 1951 in Switzerland. Beginning in 1997 a range of high-end equipment aimed at the audiophile community was introduced, and Nagra expanded the company’s product lines into new markets. ...
IV-series tape recorders offered this type of powering as an option for many years and Schoeps continued to support "negative phantom" until the CMT series was discontinued in the mid-1970s, but it is obsolete now. In 1966, Neumann GmbH presented a new type of transistorized microphone to the
Norwegian Broadcasting Corporation NRK, an abbreviation of the Norwegian ''Norsk Rikskringkasting AS'', generally expressed in English as the Norwegian Broadcasting Corporation, is the Norwegian government-owned radio and television public broadcasting company, and the larges ...
, NRK. Norwegian Radio had requested phantom-powered operation. Since NRK already had 48-volt power available in their studios for their emergency lighting systems, this voltage was used for powering the new microphones (model KM 84), and is the origin of 48-volt phantom power. This arrangement was later standardized in DIN 45596.


Standards

The International Electrotechnical Commission Standards Committee's "Multimedia systems – Guide to the recommended characteristics of analogue interfaces to achieve interoperability" (IEC 61938:2018) specifies parameters for microphone phantom power delivery. Three variants are defined by the document: P12, P24 and P48. In addition, two additional variants (P12L and SP48) are mentioned for specialized applications. Most microphones now use the P48 standard (maximum available power is 240 mW). Although 12 and 48-volt systems are still in use, the standard recommends 24-volt supply for new systems.


Technical information

Phantom powering consists of a phantom circuit where direct current is applied equally through the two
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
lines of a
balanced audio connector Balanced audio is a method of interconnecting audio equipment using balanced interfaces. This type of connection is very important in sound recording and production because it allows the use of long cables while reducing susceptibility to external ...
(in modern equipment, both pins 2 and 3 of an
XLR connector The XLR connector is a type of electrical connector primarily used in professional audio, video, and stage lighting equipment. XLR connectors are cylindical in design, and have three to seven connector pins, and are often employed for analog b ...
). 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 microphone A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...
s could be used for condenser microphones. This is in contrast to microphones with
vacuum-tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as a ...
circuitry, most of which require special, multi-conductor cables. 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 for 48 V, 1.2 kΩ for 24 V, and 680 Ω for 12 V), and the shield is
ground Ground may refer to: Geology * Land, the surface of the Earth not covered by water * Soil, a mixture of clay, sand and organic matter present on the surface of the Earth Electricity * Ground (electricity), the reference point in an electrical c ...
. 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 electronics, the common mode rejection ratio (CMRR) of a differential amplifier (or other device) is a metric used to quantify the ability of the device to reject common-mode signals, i.e. those that appear simultaneously and in-phase on both ...
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 Mix, mixes or mixing may refer to: Persons & places * Mix (surname) ** Tom Mix (1880-1940), American film star * nickname of Mix Diskerud (born Mikkel, 1990), Norwegian-American soccer player * Mix camp, an informal settlement in Namibia * Mix, ...
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. Phantom power can be blocked in any channel with 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 amplifier An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical ins ...
s 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.


Caveats

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. 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 Shure Incorporated is an American audio products corporation. It was founded by Sidney N. Shure in Chicago, Illinois, in 1925 as a supplier of radio parts kits. The company became a consumer and professional audio-electronics manufacturer of mi ...
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 ECHELON, originally a secret government code name, is a surveillance program (signals intelligence/SIGINT collection and analysis network) operated by the five signatory states to the UKUSA Security Agreement:Given the 5 dialects that us ...
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.


Digital phantom power

Digital microphones complying with the AES 42 standard may be provided with phantom power at 10 volts, impressed on both audio leads and ground. This supply can furnish up to 250 mA to digital microphones. A keyed variation of the usual
XLR connector The XLR connector is a type of electrical connector primarily used in professional audio, video, and stage lighting equipment. XLR connectors are cylindical in design, and have three to seven connector pins, and are often employed for analog b ...
, the XLD connector, may be used to prevent accidental interchange of analog and digital devices.


Other microphone powering techniques

T-power, also known as A-B powering or T12, described in DIN 45595, is an alternative to phantom powering that is still widely used in the world of production film sound. Many mixers and recorders intended for that market have a T-power option. Many older Sennheiser and Schoeps microphones use this powering method, although newer recorders and mixers are phasing out this option. Adapter barrels, and dedicated power supplies, are made to accommodate T powered microphones. There is often no audible difference between microphones featuring this method and microphones with P48 powering. In this scheme, 12volts is applied through 180ohm resistors between the microphone's "hot" terminal (XLR pin 2) and the microphone's "cold" terminal (XLR pin 3). This results in a 12-volt potential difference with significant current capability across pins 2 and 3, which would likely cause permanent damage if applied to a dynamic or ribbon microphone. Plug-in-power (PiP) is the low-current 3–5 V supply provided at the microphone jack of some consumer equipment, such as portable recorders and computer
sound card A sound card (also known as an audio card) is an internal expansion card that provides input and output of audio signals to and from a computer under the control of computer programs. The term ''sound card'' is also applied to external audio ...
s. It is also defined in IEC 61938. It is unlike phantom power since it is an unbalanced interface with a low voltage (around +5volts) connected to the signal conductor with return through the sleeve; the DC power is in common with the audio signal from the microphone. A capacitor is used to block the DC from subsequent audio frequency circuits. It is often used for powering
electret microphone An electret microphone is a type of electrostatic capacitor-based microphone, which eliminates the need for a polarizing power supply by using a permanently charged material. An ''electret'' is a stable dielectric material with a permanently em ...
s, which will not function without power. It 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 (48 V) phantom power through a 3.5 mm to XLR adapter that connects the XLR shield to the 3.5 mm sleeve. Plug-in-power is covered by Japanese standard CP-1203A:2007. A similar line-powering scheme is found in computer sound cards. Both plug-in-power and soundcard power are defined in the second edition of IEC 61938. These alternative powering schemes are sometimes improperly referred to as "phantom power" and should not be confused with true 48-volt phantom powering described above. Some condenser microphones can be powered with a 1.5-volt cell contained in a small compartment in the microphone or in an external housing. Phantom power is sometimes used by workers in
avionics Avionics (a blend of ''aviation'' and ''electronics'') are the electronic systems used on aircraft. Avionic systems include communications, navigation, the display and management of multiple systems, and the hundreds of systems that are fit ...
to describe the DC bias voltage used to power aviation microphones, which use a lower voltage than professional audio microphones. Phantom power used in this context is 8–16volts DC in series with a 470ohm (nominal) resistor as specified in RTCA Inc. standard DO-214.http://www.rtca.org/ RTCA DO-214 These microphones evolved from the
carbon microphone The carbon microphone, also known as carbon button microphone, button microphone, or carbon transmitter, is a type of microphone, a transducer that converts sound to an electrical audio signal. It consists of two metal plates separated by gra ...
s used in the early days of aviation and the telephone which relied on a DC bias voltage across the carbon microphone element.


Other uses

Phantom power is also used in applications other than microphones: *
Active antenna An active antenna is an antenna that contains active electronic components such as transistors, as opposed to most antennas which only consist of passive components such as metal rods, capacitors and inductors. Active antenna designs allow antenna ...
s *
Low-noise block downconverter A low-noise block downconverter (LNB) is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish and converts them to a signal which is sent through a cable to the receiver ...
*
Power over Ethernet Power over Ethernet, or PoE, describes any of several standards or ad hoc systems that pass electric power along with data on twisted-pair Ethernet cabling. This allows a single cable to provide both data connection and electrical power to ...


Notes


See also

*
Bias tee A bias tee is a three-port network used for setting the DC bias point of some electronic components without disturbing other components. The bias tee is a diplexer. The low-frequency port is used to set the bias; the high-frequency port passes the r ...
*
Power line communication Power-line communication (also known as power-line carrier or PLC) carries data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers. A wide range of power-line communica ...
, data communication over
mains electricity Mains electricity or utility power, power grid, domestic power, and wall power, or in some parts of Canada as hydro, is a general-purpose alternating-current (AC) electric power supply. It is the form of electrical power that is delivered to h ...


References

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External links


The Schoeps CMT 20 microphone of 1964 – the world's first phantom-powered microphonePhantom Powering – Balanced Lines, Phantom Powering, Grounding, and Other Arcane Mysteries
Loud Technologies Inc, 2003

* Microphone Design and Operation – contains alternative condenser microphone powering techniques including T-power/12T/A-B powering/DIN 45595
DIY tester – for the presence of phantom power and limited wiring testing
Audio engineering Microphones