Transformerless_microphone_preamplifier

Published:2009/7/23 21:12:00 Author:Jessie | From:SeekIC

This circuit uses two LM833s to amplify the input signal before the common-mode noise is cancelled in the differential amplifier. The equivalent input noise is about 760 nV over a 20-Hz to 20-kHz frequency band (-122 dB referred to 1V), which is over 26 dB lower than a typical microphone output from the 30-dB SPL ambient noise level in a quiet room. THD is under 0.01% at maximum gain, and 0.002% at minimum gain. R4 adjusts the circuit gain from about 4 to 270.Common-mode noise is about 60 dB (or 44 dB, worst case) when R1, R2, and R3 are of 0.1 % tolerance. If better common-mode rejection is needed, one or the R2s can be replaced with an 18-kΩ resistor and a 5-kΩ, pot to trim for a better CMRR.If necessary, use 470-pF capacitors between inputs and ground to prevent RF interference from entering the preamp inputs.



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