Phantom power


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.

History

Phantom powering was used in telephone 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 signaling path around transformer connected amplifiers in analogue line transmission systems. The first known commercially available phantom-powered microphone was the Schoeps 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 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and Schoeps continued to support "negative phantom" until the CMT series was discontinued in the mid-1970s, but it is obsolete now.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. Norwegian Radio had requested phantom-powered operation. Since NRK already had 48 V available in their studios for their emergency lighting systems, this voltage was used for powering the new microphones, and is the origin of 48-V 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" specifies parameters for microphone phantom power delivery. Three variants are defined by the document: P12, P24 and P48. In addition, two additional variants are mentioned for specialized applications. Most microphones now use the P48 standard. 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 lines of a balanced audio connector. The supply voltage is referenced to the ground pin of the connector, 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.
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", 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, 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. 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 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.

Caveats

Some microphones offer a choice of internal battery powering or 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 48-volt phantom-powered condenser microphones had simple circuitry and required only small amounts of operating current, 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. 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. Some microphones will also show lower sensitivity.
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.

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 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 and the microphone's "cold" terminal. 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, is the low-current 3–5 V supply provided at the microphone jack of some consumer equipment, such as portable recorders and computer sound cards. It is also defined in IEC 61938. It is unlike phantom power since it is an unbalanced interface with a low voltage 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 microphones, 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 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 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 resistor as specified in RTCA Inc. standard DO-214. These microphones evolved from the carbon microphones 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: