Electronic – Party-line / Full-duplex intercom – mixing audio signals

analogoperational-amplifiersumming

Party-line or full-duplex intercoms allow for the simultaneous transmission and reception of audio from all connected stations, i.e. effectively there is an audio 'bus' and each station takes a feed from it and routes this to its headset, and also adds a signal from its microphone that is then available to the other stations. The connecting wire can be either balanced or unbalanced.

schematic

simulate this circuit – Schematic created using CircuitLab

My question is: how do you add these signals together? Do you simply connect the outputs of the output op amps at each station together onto the wire (via some form of AC coupling, perhaps?).

Is that what this guy does in this circuit here?: ComClone2 Circuit Diagram

Best Answer

No, that's not what he does. He feeds each microphone through an amplifier then through an output impedance that feeds the line through R14 and C4 shown in thick red circle below: -

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I've shown the wire that is the actual line used by the intercom in a thin red/brown circle.

Because of the output impedances of several units connected all at once there will be no clash of signals and, within reason, the line voltage will be an approximate sum of the contributions of all active intercoms.

AC coupling via C4 is needed for the op-amps but note that there is a call circuit that needs to put DC onto the line to "ring" the other intercoms.

Side-tone cancellation takes place around U2B and for a 1-to-1 call you won't get feedback in your ear of your own voice. With several of these units connecting at once you may get feedback from mic to ear and this could cause havoc to other users too. So be aware of this.

Balanced Line out with DC signalling: -

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