How important is really the headphone output? It is important! Most headphone outputs are either attenuated loudspeaker outputs (tends to be noisy!) or poor opamps. They are not very often serious because the headphone output is a low priority feature.
As always you can use NE5534, LT1115, OPA627, AD797, AD8610 (many good opamps to choose from) plus a buffer stage, etc. and achieve incredible good results. If you use NE5534 the price/performance ratio is unbeatable.
My goal here was not a cheap nor a simple headphone amp. It has been really fun to design every part in the amp. The whole design is good but totally insane. I guess this amp would be impossible to sell if it was a commercial product, but as DIY everything is possible.
How does it sound really? People out there on the internet just dismiss this amp right away of one simple reason: Too many parts. It's a theory out there that every part has influence on the sound and many parts therefore degrade the sound more. I say that they are not completely right. Noone of those persons has listened to my monster. In fact noone who has listened to this amp has disliked it, on the contrary!
With my Yamaha HP-1? (don't remember the exact number) headphones the original amp sounded splendid. With my Sennheiser HD545 reference it sounds really good! I haven't got so many words for how I experience the sound. I have connected my headphones directly to my DAC and then compared the difference between the DAC and DAC plus this amp. The DAC is capable of driving headphones directly. I must say that I can't hear any difference at all. It's totally transparent! Because of the internal speed, the amp sounds splendid even in class B!! Unbelievable! In class A, the sound is just wonderful, very very small difference compared to class B. The amp sounds a lot better than it looks like (according to some).
What about the mosfet output stage compared to ordinary bipolar? The difference is none, very similar, although I simply like mosfet's more. Can't explain why.
The performance is outrageous in headphone amp terms. The technical data speaks for itself.