Phantom Voltage - History

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and Schoeps continued to support "negative phantom" until the CMT series was discontinued in the mid-1970s, but it is obsolete now.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.