Made this a couple of months ago. Every so often I load it up and try to figure out why it works at all, how it works so well, and what on Earth was I thinking? :) Maybe there's a clue to the first two points in the DC offsets at the input and output.
It really is an audio amp. It works fine at 20kHz, but at 50kHz the output goes straight to one of the power rails.
I see why it breaks: it's a very unstable circuit. At higher frequencies it develops these little ripples. When the ripples exceed some threshold, the output goes to the rails. Noise can send the output to the rails too, but it takes rather a lot of noise.
Fixed it with an extra stage, making the whole amp inverting. It's much more stable.
http://everycircuit.com/circuit/4642039691214848
