The Chu Moy Pocket Headphone Amplifier is a very simple, yet, quite good sounding amplifier. As the name says, it is intended for use with headphones. This little amp can be very effective in combination with a discman for example. You could put the discman volume at a very low level, thus using less battery power. The amplifier only needs a minimum number of parts and can be build as small as the 9v battery it needs for power. However, it’s not possible to convert that layout into a PCBsource

The PPA is a high-end headphone amplifier that works for both stationary and “transportable” amplifiers. There is a high-capacity fast-charging NiMH battery supply available, allowing excellent sonics without sacrificing run time despite the PPA amp’s hunger for power. While only the most dedicated audiophile would take it jogging with their iPod, this combination makes for a fine travel amplifier.

This is a simple do-it-yourself (DIY) headphone amplifier project that is fashioned primarily after the Class A MOSFET Headphone Driver project by Greg Szekeres and to some extent Mark’s DIY Class A 2SK1058 MOSFET Amplifier Project. The amplifier concept is simple and follows a typical single-ended class A circuit utilizing an active constant current source (CCS) in place of a passive resistor. A CCS doubles the efficiency of the circuit over that where a passive load resistor is used, bringing it to a maximum of 25%. source

This is a very simple headphone amplifier designed by CMoy.Each channel uses a single Burr-Brown OPA134 opamp in a non-inverting configuration. It has adequate current capability to drive most headphones without an output stage.The amplifier is ideal as a booster for power-conserving stereo sources such as portable CD players and for interfacing with passive EQ networks such as tone controls or a headphone acoustic simulator.source

Portable Headphone Amplifier 3V Battery powered,high Performance unit.It can be directly connected to CD players, tuners and tape recorders. It’s tested with several headphone models of different impedance: 32, 100, 245, 300, 600 & 2000 Ohm. source