The amplifier is intended for a supply voltage of 5 V. As supply voltages in microprocessor systems are "noisy", the bias for the Electret Condenser Microphone as well as the Opamp have been extra filtered. As it turned out, that the LM321 has an output voltage range of DC ... 3 V when powered with 5 V, we adjusted the bias for the opamp to around 1.5 V in order to have a maximum dynamic range. The output is dc-coupled and of low impedance in order to be connected directly to the A/D converter input. The gain is set by the ratio of R7 / R6. It is set to approx. 100 which is a useful value for the used microphone (ABM-713-RC). For mamimum resolution, the reference voltage should be set to 3 V - or you may use a microcontroller with 3.3 V supply.

As the used Opamp has a Gain-Bandwidth product of 1 MHz, the max. Bandwidth is 10 kHz, when using a gain of 100. Therefore the capacitor across R7 (10pF) is not necessary. It may be of interest, when using other Opamps. Especially, when using a rail-to-rail type, the bias of the positive input mayst be readjusted to half of the supply. The lower cutoff frequency is set by the values of C3, R3, R5. Increase the capacitor to move it into the sub-Hz range. With the given values, a simulation shows the following amplitude response :



Amplitude Response (Simulation), Gain vs. Frequency.



Real World : Voltage at the Output.

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