Man, studied those at the university, those things are very complicated...

I mean, simple ones are easily explained, but they sound awful, so to get a usable one it takes lots of theory.

Unlike speakers, which you can build at home and can sound OK.

That said, wikipedia is a surprisingly good source on this. Also, this circuit simulator (http://www.falstad.com/circuit/) comes with some DAC/ADC circuits builtin, and you can watch them live.