The CMoy headphone amp is a popular headphone amplifier that is small enough to carry everyday and powerful enough to drive headphones at higher levels than normal. Most headphones and most sources will get along ok, but there are players with weak output, and high end headphones typically give up efficiency for the best sound.
The CMoy amp was originally designed by Chu Moy, and at this point there are hundreds of variations with different components, layout, and features. Sometimes, users simply swap op-amps for a different sound and performance. This circuit uses op-amps to directly drive headphones, something they are not designed to do, also, some of the op-amps used are not even designed for audio applications!
It provides decent audio fidelity from just a few small and inexpensive components. In the last few years, CMoy style headphone amplifiers have been very popular with the DIY audio crowd to increase the audio output to some of the larger and higher impedance headphones available.
DIY audio enthusiasts typically stuff these components into an Altoids tin, other tins, or other small project boxes. The CMoy style of amplifier is versatile enough to fit into many different form factors. Users also determine the container based on what features they choose to include with their circuit. They might add features like bass boost, DC input, and rechargeable batteries.