Here is a basic but really efficient pocket headphone amplifier circuit based on OPA2227P high precision and low noise operational amplifier IC. As seen from the diagram, there are only a few components; an op-amp, handfull of resistors, a potentiometer and a capacitor. Since the circuit is for only one channel, you should construct two of this to use in stereo mode.
Two 9V battery is used to power the amplifier. A small battery circuitry is given in the project page to get positive and negative 9V for the op-amp from two batteries.
Additionally the project page provides all detailed information about the circuit operation, components, assembling, boxing and troubleshooting.
The article is written especially for the people who are new to the audio DIY world.