As the circuit is operated from a battery, and not split supplies, then R1 and R2 bias the output voltage at the op-amp output to half supply voltage. Any ac signal is bypassed by C2, whilst R3 increases the op-amp input impedance and prevents "shorting" the input signal through C2.
The input is from an electret condenser microphone (ECM), and R5 is used to power the ECM. The input signal is coupled via C1. A dynamic microphone could also be used instead of an ECM type, in this case, just omit R5.
The bridge rectifier is formed from diode D1 to D4 which are in the feedback loop of the op-amp. Without the diodes, the op-amp would be wired as a buffer or follower. The DC output voltage, at pin 6 of the op-amp is sufficient to bias the diodes, whilst at the same time, rectifies the output signal.
A 100uA FSD meter is connected to the bridge rectifier, which has a typical coil resistance of 1200 ohms. R4 is a 470 ohm preset resistor which allows for adjustment of other meters with different coil resistance. C3 decouples DC voltage while allowing the ac signal to pass.
The simulation below shows that for an input signal of 10mV pk-pk approximately 50uA current is sent through the meter. The meter was replaced with a 1200 ohm resistor for the simulation. 100uA meters can be found on Ebay or at Rapid Electronics Pinouts for the TL071 can also be found on my practical section.
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