When I needed a simple audio oscillator for, for instance, continuity testing or Morse practice, I wanted to make it as simple as reasonably possible.
I thought I would try to use a standard telephone earpiece's inductance as the L of a Colpitts oscillator, and in so doing eliminate the need for a separate amplifer/buffer.
This is the circuit. I think the transistor is a ZTX237, but any common one should work.
And here is a picture of one.
