This is a simple form circuit for a 2-station intercom using common 8R mini speakers. The press-to-talk switches should have a spring-return so the intercom can never be left ON. This is the figure of the circuit; Power the speaker from a separate power supply is the secret to preventing instability (motor-boating) with a high gain circuit like
this. An extra station (or two extra stations ) can be connected to this design.