Phantom power - AbsoluteAstronomy.com

Phantom voltage redirects here. For unintentional voltages in power distribution systems, see Stray voltage
Stray voltage
Stray voltage describes the occurrence of electrical potential between two objects that ideally should not have any voltage difference between them. Small voltages are often measured between two grounded objects in distant locations, due to normal current flow in the power system...

Phantom power, in the context of professional audio equipment

Professional audio

Professional audio, also 'pro audio', refers to both an activity and a type of audio equipment. Typically it encompasses the production or reproduction of sound for an audience, by individuals who do such work as an occupation like live event support, using sound reinforcement systems designed for...

, is a method for transmitting DC

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

electric power

Electric power

Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

through microphone

Microphone

A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...

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 box

DI unit

A DI unit, DI box, Direct Box, or simply DI , is a device typically used in recording studios to connect a high-impedance, line level, unbalanced output signal to a low-impedance microphone level balanced input, usually via XLR connector...

es 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 desks

Mixing console

In professional audio, a mixing console, or audio mixer, also called a sound board, mixing desk, or mixer is an electronic device for combining , routing, and changing the level, timbre and/or dynamics of audio signals. A mixer can mix analog or digital signals, depending on the type of mixer...

, 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.
Three variants of phantom power, called P12, P24 and P48, are defined in the international standard IEC 61938.

Technical information

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

XLR connector

The XLR connector is a style of electrical connector, primarily found on professional audio, video, and stage lighting equipment. The connectors are circular in design and have between 3 and 7 pins...

). 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 in contrast to microphones with vacuum-tube

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

circuitry, most of which require special, multi-conductor cables.There are phantom-powered vacuum tube microphones, such as the Microtech Gefell UM900 and Audio-Technica AT3060
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.

Many mixer boards have a switch for turning phantom power on or off; in most high-end equipment this can be done individually by channel, or on smaller mixers all mic channels can be either on or off, also this can be done in groups in most mid-range boards and some smaller ones. 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. 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

Instrument amplifier

An instrument amplifier is an electronic amplifier that converts the often barely audible or purely electronic signal from musical instruments such as an electric guitar, an electric bass, or an electric keyboard into an electronic signal capable of driving a loudspeaker that can be heard by the...

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.

History

Phantom powering was used in telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

systems since the introduction of the rotary-dial telephone in 1919 before it was applied to condenser microphones. One such application in the telephone system was to provide a DC

Direct current

signaling path around transformer connected amplifiers in analogue line transmission systems. The first known commercially available phantom-powered microphone was the Schoeps

Schoeps

Schalltechnik Dr.-Ing. Schoeps GmbH, known as Schoeps, 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 founded in 1948....

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

Nagra is the trademark referring to any of the series of mostly battery-operated portable professional audio recorders produced by Kudelski SA, based in Cheseaux-sur-Lausanne, Switzerland....

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 of Berlin, Germany, presented a new type of transistorized microphone to the Norwegian Broadcasting Corporation, NRK. Norwegian Radio had requested phantom-powered operation. Since NRK already had 48V available in their studios for their emergency lighting systems, this voltage was therefore used for powering the new microphones (model KM 84), and is the origin of 48-V phantom power. This arrangement was later standardized in DIN 45596.
The prevailing international standard, IEC 61938, defines 48-volt, 24-volt and 12-volt phantom powering. The signal conductors are positive, both fed through resistors of equal value (for 48-volt phantom powering, the standard value is 6.8 kΩ), and the shield is ground

Ground (electricity)

In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

. The 6.8 kΩ value is not critical, but the resistors must be matched to within 0.4% 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.

Implementation variations

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

Shure

Shure Incorporated is an American corporation originally 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 microphones, wireless microphone systems, phonograph cartridges,...

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

Ground lift

In sound recording and reproduction, ground lift or earth lift is a technique used to reduce or eliminate ground-related noise when connecting signal lines between two or more pieces of equipment. It interrupts the ground line at some point. It is particularly effective at eliminating ground loops,...

switches have the unwanted effect of disconnecting phantom power; there must still be a path for pin 1 of the microphone to reach the negative side of the 48 volt supply if current is to flow.

Digital phantom power

Digital microphones complying with the AES 42

Audio Engineering Society

Established in 1948, the Audio Engineering Society draws its membership from amongst engineers, scientists, other individuals with an interest or involvement in the professional audio industry. The membership largely comprises engineers developing devices or products for audio, and persons working...

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

XLR connector

, the XLD connector, may be used to prevent accidentally interchange of analog and digital devices.

Other microphone powering techniques

T-power, also known as A-B powering or 12T, described in DIN 45595, is an alternative to phantom powering that is still widely used in the world of production film sound. Most mixers and recorders 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. Adaptor barrels, and dedicated power supplies are made to accommodate T powered microphones. Sonically there is no difference between microphones featuring this method, and microphones with P48 powering. In this scheme, 12 volts is applied through 180 Ohm 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.

Line powering, or plug-in-power (PiP), is the low-current 3 V to 5 V supply provided at the microphone jack of some consumer equipment, such as portable recorders and computer sound card

Sound card

A sound card is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs. The term sound card is also applied to external audio interfaces that use software to generate sound, as opposed to using hardware...

s, is sometimes incorrectly called "phantom power". It is often used for powering electret microphone

Electret microphone

An electret microphone is a type of condenser microphone, which eliminates the need for a polarizing power supply by using a permanently charged material....

s, which will not function without power. 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 (48 V) phantom power. 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 a not-yet-released version of IEC 61938.

Some condenser microphones can be powered with a 1.5 volt battery contained in a small compartment in the microphone or in an external housing.

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.

Phantom power is also used by workers in avionics

Avionics

Avionics are electronic systems used on aircraft, artificial satellites and spacecraft.Avionic systems include communications, navigation, the display and management of multiple systems and the hundreds of systems that are fitted to aircraft to meet individual roles...

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 to 16 volts DC in series with a 470 ohm (nominal) resistor as specified in RTCA Inc.

Radio Technical Commission for Aeronautics

RTCA, Inc. is a volunteer organization that develops technical guidance for use by government regulatory authorities and by industry. Requirements for membership include a fee that is based on information in the application for membership, and an interest in aviation...

standard DO-214. . These microphones evolved from the carbon microphone

Carbon microphone

The carbon microphone, also known as a carbon button microphone or a carbon transmitter, is a sound-to-electrical signal transducer consisting of two metal plates separated by granules of carbon. One plate faces outward and acts as a diaphragm...

s used in the early days of aviation and the telephone which relied on a DC bias voltage across the carbon microphone element.

External links

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