Want to connect two headphones with mic so that two persons can talk over it as over a phone call. Please Help. Thank you
This is fairly easy IF you are wanting to use a maximum of two headphone / mic combinations. We call this combination a "Headset". You will need to build two identical circuits as follows:
Each headset's circuit contains a mic preamp and a headphone amplifier. The preamp design depends on whether you are using a headset with a dynamic mic or electret mic. The dynamic mic needs more gain but the electret mic needs a quiet bias supply. Both of these things are easy.
The headphone amplifier doesn't need much gain but it does need to supply a bit of current to drive the headphone speaker. I would also add a volume control to the input of the headphone amplifier so that you can adjust the volume.
To use these blocks, simply connect the mic preamp output of one headset to the headphone input of the other headset. Repeat for the other headset.
You can use a shielded pair to connect the headset electronics to each other. Shield is ground, one conductor is the talk for one headset, the other conductor is the talk for the other headset.
This is easy. If I were doing this, I would use my ancient standard op-amps NE5534. These are single op-amps but that is useful because of the signal level differences between mic and headphone.
If you want to emulate a professional intercom system that has multiple stations, have a look at RTS / Telex intercom systems. Both of these systems use what is called "bilateral current sources" (otherwise known as Howland current pumps) and are described very well in an old National Semiconductor app note. The idea is that there is a standard termination resistor at one location in the system and each station provides talk audio via a current source. This minimizes (but does not eliminate) the loading effect as stations are added or removed.