Gio, a MAKE subscriber from Winnipeg, Canada, sent us a link to this well-documented low voltage headphone amp project. The tube-based amp uses a single 12AU7 tube for voltage gain and a IRF510 MOSFET to supply current to drive the ‘phones. The small hybrid amplifier operates off a 12V SLA (sealed lead-acid) battery, so there are no high voltage concerns. An LM317 regulator was used as a constant current source to bias the MOSFET into class-A operation. The amp can be built for about $40-50, less if recycled parts are used.
(Psyched to see the builder using the Grado SR60 headphones, my all-around, all-time faves.)