This duplexer can work with a standard telephone line with U_dc = 60 V (idle) and I_dc = 30 mA (short-circuit). The ac voltage source on the left can be a microphone or another input signal source. The 8 Ohm resistor can be a speaker. The part after the switch on the right is the telephone line. Note that the signal from the mic goes to the line but does not go to the speaker. This circuit can be used either as suppressor of the microphone effect in a hands-free setup or as a duplexer to decouple the remote signal from the local signal.
