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A Low-Noise High-Output Capacitor Microphone System

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The dynamic range of a capacitor microphone has been considerably extended by means of an advanced FET feedback amplifier. This microphone delivers +20 dBm into 600 ohms at maximum sound pressure levels of 110, 125, or 140 dB. Its A-weighted noise level of 15 dB is contributed mostly by the capsule instead of the amplifier.

JAES Volume 25 Issue 5 pp. 278-283; May 1977
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Scott Dorsey
Scott Dorsey

Comment posted June 9, 2020 @ 23:48:56 UTC (Comment permalink)

What is interesting about this new microphone design is the servo circuit used to keep the DC bias point of the input FETs constant.

The parallel fets and very high front end impedances are the big takeaways from this design, and neither one was really very innovative by 1977.  The low noise amplifier is a fine design but any fine points about it are not explained in the text.  One thing that would be really interesting if one tried to build a copy of this is keeping the noise from the DC-DC converter out of the audio path, which is going to be a huge challenge and isn't explained by the author at all.

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