audio coupling circuit help needed

Hi all,
Iasking the same old 2 wire full duplex question i have placed a link showing three type of coupling circuits now i want to do the following please consider the web link i have placed.


**broken link removed**


1) In my Room i have ,

8 Ohms Speaker,
Condenser Mic
9 Volts Battery.


2) On my Door Side:-

8 Ohms Speaker,
Condenser Mic


Now suppose if iam using direct coupling circuit as shown on the website also , how i can connect the room side circuit and door side circuit using 2 wires so that when ever i turn on the circuit communication can occur in full duplex mode. don't consider the bell thing...that can be worked out at letter stage, just consider that there will be a Push Button on the door side circuit which when ever pressed will cause bell to ring on the room side...now how to procced using all this stuff.

I think on both side transformers are needed as a type of "balancing" transformers which are used for sidetone suppression and getting enough power for suppluying loudspeaker from the line.

I guess you are awhere that your standard telephone is a 2-wire/full duplex system with signaling?

iam talking about door intercom

no telephony stuff iam trying to design a simple door intercom