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and Schoeps continued to support "negative phantom" until the CMT series was discontinued in the mid-1970s, but it is obsolete now.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 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 48 V 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. 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.
Read more about this topic: Phantom Voltage
Famous quotes containing the word history:
“This above all makes history useful and desirable: it unfolds before our eyes a glorious record of exemplary actions.”
—Titus Livius (Livy)
“The principle office of history I take to be this: to prevent virtuous actions from being forgotten, and that evil words and deeds should fear an infamous reputation with posterity.”
—Tacitus (c. 55–117)
“Indeed, the Englishman’s history of New England commences only when it ceases to be New France.”
—Henry David Thoreau (1817–1862)