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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. As the op-amp directly drives headphones some care should be given when choosing an op-amp, some op-amps are not suitable for such low impedance loads and will result in poor performance. (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".
Circuit
A typical CMoy consists of two identical AC coupled, non-inverting operational amplifier circuits each with a 100kΩ input impedance.
Power is supplied to the opamps using a dual power supply, which effectively divides the input voltage source in half to create a virtual ground. Many virtual ground circuit options are presented in the various CMoy tutorials found online.