Plug In Power Microphones - How to Fix Problems
Sony Popularised this Powering System, But Compatibility is Limited
Apr 28, 2009
MiniDisc had a much lower noise than typical cassette recorders, and this favoured using electret capacitor microphones, which deliver a higher output than dynamic microphones, but do require a power supply, typically provided by a small battery in the microphone.
Before the 1990s many domestic audio recorders were designed to be used with dynamic microphones, which derive their signal is derived from the energy in the sound source. This is good in that no battery is needed, but it means that the output of the microphone is low, which leads to problems with noise from the microphone amplifiers.
The battery in an electret microphone is a problem, however, in that it needs to be switched on and off, and it is all too easy to leave a battery microphone switched on after recording, which means that the battery will have run down when the microphone is taken out for use next; the battery typically lasts only a few days when switched on.
Sony's innovation was to observe that the typical electret microphone could be powered from the recorder. By putting the microphone load resistor in the recorder and terminating this on a moderate voltage source, the microphone would only be powered when the machine is recording.
Plug In Power (PiP) is Poorly Defined
This was all very well when the microphone and recorder were designed by the same manufacturer, and Sony provided a range of MiniDisc recorders and microphones that worked well together. However, they failed to provide a complete specification, and as a result it is not possible to guarantee compatibility.
Early designs (1990s) such as the Kenwood DMC-F5R tended to source plug-in-power from a 5V source via a resistor of between 2.2 kilohms to 6.8 kilohms. Later designs such as Sony's MZ-NH700 and MZ-RH1 used a volatge of 2.3V and a resistor of 6.8 kilohms. The powering capability of these is very limited, and as a result some microphones lack sensitivity and clip easily on loud signals.
Microphones such as the Soundman OKM binaural head-worn microphones draw too much current to work properly with the later plug-in-power inputs. As a result the microphones lack sensitivity, and clip easily with loud signals, resulting in harshness and a lack of clarity with high sound pressure levels.
Solve Plug In Power Issues with a Battery Box
A battery box often uses a 9V PP3 battery or similar to power the microphone via a resistor similar to that of the plug in power circuit. The higher voltage means that a battery box is compatible with nearly all plug-in-power microphones, and the higher voltage available means that the microphone can handle higher sound levels before clipping.
It is even possible to connect the output of the battery box to the line level input of the recorder - though this does not have PiP the battery box solves this problem, and the line-level input handles the high signal levels from the microphone is high sound levels are beihng recorded, such as those associated with live music. Battery boxes often also provide a low-frequency roll-off function, useful for outdoors recording.
Battery boxes are produced by manufacturers that make plug in power microphones, such as Soundman, Sound Professionals and Core Sound, among others. They solve the compatibility and performance issues, albeit at the expense of re-introducing the separate battery plug-in-power was designed to eliminate and an extra in-line box.
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