The amplifier

frostbe

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What is the purpose of R4?
 

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Increase input impedance and set gain. Input impedance at T1 base node is low due to voltage controlled current ("shunt-shunt") feedback, not suited for dynamic microphone.

I think the amplifier topology isn't first choice for microphone amplifier. Suggested load impedance for standard 200 ohm moving coil microphones is > 1k, even higher for 600 ohm type.

Looks like an elektor circuit?
 



Rough look at performance.

Note Zin levels off at ~ 1.5K @ 1 Khz
 
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