This circuit 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 3v supply. The second and third transistors are not turned on during idle conditions and the quiescent current is just 5mA.
The project is ideal for listening to conversations or TV etc in another room with long leads connecting the microphone to the amplifier.
source : http://www.talkingelectronics.com.au/projects/200TrCcts/200TrCcts.html
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