This intercom uses an ordinary speaker to function as its microphone. Once the button is pressed, the relay toggles, and the
speakers interchange their functions. One works as a normal speaker and the other one works as microphone.
The circuit is composed of a complementary final amplifier (T3 & T4) and two voltage preamplifiers (T1 & T2). Pot P1 controls the volume.
The voltage at the common junction of resistors R5, R6 & P2 must be set to one half of the supply voltage.
A lower impedance speaker with a series resistor can replace the suggested type, but will deliver lower sound quality.
Electronic Circuits volume 1.0 - Circuit Nr. 26
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.