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The AD820 opamp has a gain of 11 times and the TL071 opamp has a gain of 10 times for a total gain of 110 times.
The Twin-T notch filter between the opamps cuts 52Hz to 1/50th.
Then 52Hz is amplified by 2.2 times.
Maybe the electret mic has its own power source and is connected to this preamp using ordinary wires instead of shielded audio cable so the wires pickup mains hum.
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