DIY headphone amplifier project using AD844 and some special transistors.
http://headwize.com/projects/jorgen1_prj.htm
You can build a high-quality portable amplifier to boost the output of Sony's MiniDisc portables.
http://www.connact.com/~eaw/minidisc/headbanger.html
This simple amplifier is ideal for adding a headphone jack to equipment that lacks this feature. The Headphone Buffer circuit board is small enough (1.2" X 1.4") to squeeze into even the smallest spaces and power requirements are so low that existing supplies in retrofitted gear can be used.
http://www.paia.com/headbuff.htm
The class A headphone driver is ultra simple, except it does need a regulated supply and input and output capacitors.
http://headwize.com/projects/showfile.php?file=szeke1_prj.htm
This little headphone amp from SDS Labs uses a push-pull Class A MOSFET output stage, being driven by an op-amp. The output stage is included in the op-amp feedback loop, so the distortion figures are amazingly low.
http://www.web-ee.com/Schematics/HeadphoneAMP/head_amp.htm
low output impedance to drive several pairs of phones
the active gain stage is, almost, perfectly logarithmic, is independent of the absolute value of the pot, has excellent channel tracking, the O/P noise reduces with gain reduction.
http://www.sound.au.com/project24.htm
a low power, mains powered stereo amplifier capable of driving a pair of headphones.
http://www.web-ee.com/Electronic-Projects/projects/headphone_amp/index.shtml
drives up to 4 headphones from a single amplifier, has volume control and mono switch.
http://www.procosound.com/images/HJ4P.PDF