I'm trying to build some of the above mic amps inside the dotted line labeled CASE. The MFG will not sale them to me. Can anyone identify the part labeled M-4 AMP??? The schematic is a David Clark headset that plugs into a David Clark inter comm system.
That's a rough diagram, not really a full schematic. Even if you had that particular amp, what would you do with it? Clearly all connections aren't show.
This site isn't about grabbing a chip you don't know and plunking it into a circuit you don't understand. If you want help here, it has to be about real electrical engineering. Therefore, the real solution is to decide what exactly you want this circuit to do, then come up with one that does it.
Electret microphones generally need up to about 1000x voltage gain to get to line level audio. With a volume control, the actual gain can be anything from there down to 0.
It looks like the main feature of the amp you show is that it has two complimentary outputs that can drive at least 600 Ω between them. Start with that spec and see what you find. If you can't find such a thing that can do a gain of 1000 at audio frequencies, then just put another stage in front of it.