Are the diaphragms/ capsules for microphones, like the C-480's, all made from a few manufacturers, like the electret capsules
Does AKG make their own capsules and or microphone diaphragms?
As far as I know, the big manufacturers (Neumann, Scheops, AKG, etc) all make their own capsules. Some manufacturers, like AT may have things made for them, and only them.
If you look at some cheaper stuff, like Studio Projects, ADK, Rode, etc, those are made in China, for a number of people. The key with these is quality control, and one expects those bigger companies to have it. Not the cheaper ones like Apex, Behringer, etc. Those are probably the same Chinese mics, but a crap shoot.
As for electrets, those are made by many companies, like Primo, Transsound, and many many others.
This is all discussed on the Yahoo group "micbuilders". Both circuits and sources can be found there. I've learnt most of my stuff (3-wire, FET circuits, etc) over there too...
Richard