Wikipedia
A CMoy is a pocket headphone amplifier originally designed by Chu Moy on the headwize.com Headphone Resource Site and public discussion Forums.
The headphone amplifier is designed around single or dual-channel op-amps such as Burr-Brown's OPA2134 or OPA2132PA, however, a wide variety of op-amps have been successfully implemented. The op-amps directly drive headphones, even though most are not designed for such low-impedance loads, leading to poor performance and audible differences between op-amps that would not exist in a good design. (See Op-amp swapping.)
The amplifier's design is quite simple. It consists of only a few components, can be assembled on a small section of protoboard, has a lower parts cost than other headphone amplifiers, and can run for many hours on a single 9 volt battery. The CMoy headphone amplifier can be fit into Altoids tins and can often be found for sale on eBay by searching for "CMoy".