Microphone
Impedance bridging

When choosing a pre-amplifier for a certain microphone, the microphone's impedance must be known. Impedance is a frequency-dependent electrical characteristic, measured in ohms (Ω), that relates voltage to current. When not concerned with power transfer, signals are generally transferred as varying voltages and this is also the case for microphones. To obtain the highest signal amplitude one uses a method called impedance bridging. In this configuration the output impedance of the microphone should be negligible in comparison with the input impedance of the pre-amplifier (in practice a pre-amp impedance at least 10 times greater than the microphone impedance is recommended). By doing so, the signal is attenuated minimally and almost no power is used in the process.

The main alternative to impedance bridging is impedance matching which maximizes power transfer for a given source impedance. However, this has not been relevant since the early 20th century when amplifiers were very expensive and produced a lot of heat. To reduce the number of amplifiers in telephone lines, power loss needed to be minimal so source and load impedances were matched. A downside to impedance matching is the 6 dB loss in signal that occurs as only half the voltage level appears at the pre-amplifier's input. Certain ribbon and dynamic microphones however are exceptions, due to the designers' assumption of a certain load impedance being part of the internal electro-acoustical damping circuit of the microphone.

Different microphones can have vastly different impedances and this depends on the design. In passive microphones, this value relates closely to the impedance of the coil (or similar mechanism). In active microphones, this value describes the output impedance of its internal amplifier circuitry.

Low impedance is considered under 600 Ω. Medium impedance is considered between 600 Ω and 10 kΩ. High impedance is above 10 kΩ. Owing to their built-in amplifier, condenser microphones typically have an output impedance between 50 and 200 Ω.
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