This is a simple audio pre-amplifier circuit using single transistor 2N3904.
This easy circuit provides good gain to weak audio signals such as electret microphone. Use it in front of an RF oscillator to make an RF transmitter that’s very sensitive to sound. Power supply used for this circuit is wide range from 3VDC to 9VDC, you may choice the middle range about 6VDC to minimize the risk, safety purpose 🙂
R1 = 10k R2 = 100k R3 = 10k C1 = 0.1uf or if you prefer 104 or 100n C2 = 0.1uf or if you prefer 104 or 100n Q1 = 2N3904 MIC1 = a common electret microphone