Powered by a single AAA cell,this amplifier is scarcely thicker than the audio cable and fits between the line out socket of your PC sound card and your headphones. Most sound cards have line level outputs, at high impedance, and will not drive headphones directly at full volume. It is based on the headphone amplifier chip from Maxim, the MAX9725, which can work at voltages as little as 0.9 volts (range 0.9 to 1.8 volts). This chip does not need capacitors on its output since it has directly coupled output stages and it generates a negative supply rail by using an internal switchmode power supply stage. This is a small surface mount chip and can be placed by automatic equipment onto a suitable printed circuit board. I am using wires on a hand carved board, and am presenting my steps here in the hope that it will inspire somebody out there to try something similiar with the tiny chips those guys insist on manufacturing - great features, but a tiny package.
http://www.instructables.com/id/In-line-headphone-amplifier/