As we all know headphone amps are not cheap to buy. I think this is the main reason that drives the hobbyist to make their own amps. Most people are familiar with the popular C’Moy headphone amp which is cheap to make and it sounds nice.
But there also other designs available, for example this class A MOSFET headphone amp. The author designed and build it because he’s 32 ohm Grado SR80 headphones didnt’t sound good on it’s computer soundcard.
An IRF610 MOSFET was used by the author but there is a wide variety of FET devices that can be used instead. Unlike the C’moy amp this amp was primarily designed to sit on a desk, it’s bigger the MOSFET needs heatsink cooling so this wouldn’t fit the portable category of amps.
A LM317 regulator is used for the constant current source, and the current is limited to about 250mA. In the end the sound is better than on the sound card and it was a small investment as most parts were salvaged.
It’s not too difficult to build something similar but it will sure help to have some experience with similar circuits.
MOSFET Headphone Amplifier: [Link]