TonyL wrote:

Don't know. But if I can't use the spare section on the existing quad op-amp
to drive the headphone then I may as well go for a purpose designed heaphone
amp as suggested by others in here, or add a bipolar complementary pair.

Yes, buffer the op-amp with a complementary pair. Don't forget to bias them on
either and set the current with emitter resistors. You can even take the
feedback from the junction of the emitter resistors but be advised to include a
load 'isolating' resistor of say 47 ohms.

Graham