Electronic Ear

Circuit below is a very sensitive 3-transistor amplifier using a speaker transformer. This can be wound on a short length of ferrite rod as show above or 150 turns on a 10mH choke.

The biasing of the middle transistor is set for 3 V supply. During idle conditions and the quiescent current is just 5mA, the second and third transistors are not turned on. This device is very ideal for listening to conversations or TV etc in another room with long leads connecting the microphone to the amplifier.