Here is a schematic of a very sensitive microphone preamp circuit, which can be used to increase gain of microphone or boost the audio signals coming from microphone. The circuit is very simple contains only few components and can be assembled in few minutes if all parts are available.
This is a versatile mic preamp circuit and can be used for many purposes. The 10K potentiometer is used in the circuit to increase or decrease the gain. Transistor 2N3904 is a general purpose transistor. The circuit can be operated between 3 to 9 volt DC.