Compressor For Electret Microphone

  
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The FM Remote Control Receiver` (available on this website in Infra-red circuits section) has a connector where an analogue output is made available. To make a simple intercom or P. A. system the associated transmitter needs a microphone pre-amplifier that outputs a signal at the correct level. And that is exactly the function of this circuit. Actu
Compressor For Electret Microphone - schematic

ally, this design is adapted from a circuit published last year (AM Modulator for Intercom`). A few things have been changed so that it can work with the 5 V supply from the transmitter module. The OTA (IC1) used here is the single version (CA3080), which has slightly different characteristics from the dual CA3280. The quad opamp is the same rail-to-rail TS924IN, made by ST. The turnover frequency of the ¬lter (3rd order 1 dB Chebyshev) has been increased slightly to improve the intelligibility of speech and is now about 5. 5 kHz. The ¬lter now ampli ¬es the signal by a factor of 10. In practice it is possible that due to various tolerances and the fact that the opamp is not perfect, the ¬lter characteristic shows some deviation from that required. In our prototype it was necessary to change R15 into 2k7 to straighten the response curve. The DC current variation at the output of the OTA and the resulting offset variation at the output of current/voltage converter IC2d is such that the gain of IC2d has to be substantially smaller than in the old` design. Otherwise the output could easily rise to the supply voltage at low signal levels. The value of R6 has therefore been made smaller by a factor of 10. This has reduced the gain of the circuit by 20 dB, which is compensated for in the ¬lter. The amplitude of the signal from IC2d is fed back as a control current to the OTA by peak recti ¬er D1/C3 and inverting ampli ¬er...



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