I was trying to find a suitable headphone amplifier to go with the signal digger regen circuit. It needs to have a high input impedance. reasonable gain and some kind of audio limiting to limit the effect of impulse noise etc.
This circuit is working well for me:
The most R2 can pull the emitter voltage of Q2 up to is 2 base emitter voltage drops. That is about 1.4 V less than the supply rail. The saturation voltage of Q2 might be around 0.2 V. The maximum voltage swing available at the collectors of Q1 and Q2 then is 1.4-0.2=1.2 V which provides some audio level limiting. Probably you could put a resistor in series with the headphones to limit that further but I haven't worked that through yet.
Anyway, practically it works well. I would kind of compare it to Selenium's TL431 audio amplifer circuits but a bit better it seems.