Here is the schematic of a quality voice amplifier circuit. The maximum output of the circuit is 10 watt and the minimum and maximum operating voltage is 8 volts to 18 volts DC. The circuit is able to deliver 5 watt on 9V supply which is an ideal output power to use it as a speech amplifier for a
big crowd of audience. The circuit is a simple microphone amplifier using only two stages first a microphone preamp circuit stage which is built around a 2N3904 and second is an audio amplifier circuit stage built around TDA2003 audio amplifier IC. The volume of the circuit can be adjusted with the help of 10K resistor between the transistor and IC.
The battery will only connect to the circuit when you press the push switch to talk or otherwise the battery will be disconnected. This will increase the battery life.
Working of this microphone amplifier is simple, the electret microphone receives the sound and changes it in to the electrical signals, these electric signals are then amplified by the 2N3904 transistor and finally amplified by the TDA2003 IC.