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

[johnlondon]

Hi All,

I'm hoping someone can help, apologies if my first post has ended up in the wrong forum. 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.

My question is: how do you add these signals together? Do you simply connect the outputs of the output op amps/line drivers at each station together onto the wire (via some form of AC coupling [not shown in the diagram above], perhaps?).

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

 

[DF96]

You need something called a 'hybrid', which knows the difference between putting a signal out and getting a signal in. In the olden days this required a transformer but op-amps can do something similar. Examine telephone circuits.