The following illustration is the circuit schematics of single transistor microphone pre-amplifier for both dynamic and condenser type mic. The two kinds of microphone have different characteristics. Dynamic microphone does not need biasing current because the transducer generate the electrical signal. On the other hand, the condenser microphone needs a bias current to compensate the leakage in the microphone and also because of the pre-amplifier loading discharging the condenser mic’s electrical charges. Here is the circuit diagram of the microphone pre-amp which provides input terminal for both types using only a single transistor as the active components.
9V battery is used on this circuit, however, 6-15V battery can also be used without modification. The input for dynamic microphone is decoupled with C3, and thus DC-isolated from any DC voltage coming from R2 and R1. When only the condenser mic that you need, then C3 can be omitted. When the dynamic mic is needed, R1,C1, and C2 components can be omitted as well.
The caps is wired back-to-back at C1 and C2. This configuration prevents the leakage current in the reserved direction of the capacitor polarity. This back-to-back capacitors emulate a non-polar capacitor with two low cost electrolityc capacitors. Any small signal NPN transistor with high gain (hfe > 150) should be compatible to replace Q1, for examples: 2N3904, FCS9014, C828, or C945.
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