We are developing a portable battery operated class A headphone amplifier with linear phase. This design minimizes the number of transistors in the signal path and does not use opamps or capacitors in the signal path. There are plenty of headphone amplifiers out there with great signal characteristics. This design was undertaken as a scholarly endeavor to see if a class a headphone amplifier could be developed with linear phase characteristics. 
The design of a class A headphone amplifier with some amount of phase distortion is a trivial one, but if you want to be a purist and require that an amplifier reproduces exactly what is input, the design becomes more complicated. The completed schematic will be posted soon...
Northcutt Engineering