Tonight I've built the headphone amp, and tweaked the design to achieve lower distortion (noise is now the
dominant factor). 30R headphones you're looking at around 0.01% distortion at 1kHz when pushing 120mW into the headphone (unlikely!).
For 600R headphones you can go even louder with even lower THD, and then
I'm starting to reach the limits of my HP distortion analyser (below
about 0.005%).
--Neil
