This is the circuit diagram of dynamic mic amplifier. This simple and easy built circuit can be used as a subtitute of an electret microphone. It will turn an ordinary mini speaker to become a pretty sensitive microphone.
Any NPN transistors can be applied for this circuit, for example: BC547. The circuit will operated from 3v to 9v DC supply. The amplifier configuration used in this dynamic mic amplifier is a common-base amplifier, it accepts the low impedance of the speaker to deliver a gain of more than 100 times.