Single Transistor Microphone Pre-Amplifier for Both Dynamic and Condenser Type Mics
Condenser microphone and dynamic microphone (moving coil microphone) has different characteristics. For dynamic mics, a biasing current is not needed because the transducer generate the electrical signal. In condenser mics, a bias current is needed to compensate the leakage in the microphone, and because of the pre-amplifier loading that discharge the condenser mic’s electrical charges. The microphone pre-amp described here provides input terminal for both microphone types, and uses only a single transistor as the active components.
The circuit uses 9V battery, but you can use it for 6-15V with no modification. Use 25V rated electrolityc capacitors for all caps. You can see the input for dynamyc mic is decoupled with C3, and thus DC-isolated from any DC volatge coming from R2 and R1. If you need only the dynamic mic input then you can omit R1, C1, and C2 components. If you need only the electret/condenser mic input then you can omit C3.
Looking further at C1 and C2, you can see the caps is wired back-to-back, and this configuration prevent the leakage current in the reversed 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.