This headphone amplifier can output 4W - enough to drive some loudspeakers. It has a FET opamp front-end with complementary MOSFET outputs and a toroidal transformer-based regulated power supply.
http://headwize.com/projects/showfile.php?file=stokes_prj.htm
the output stage uses a triode-connected pentode.
http://headwize.com/projects/showfile.php?file=ahammer1_prj.htm
Fully-balanced, discrete transistor front end and tube output stage using EL34s in grounded grid configuration. This amp is the author's answer to the legendary Stax T2 amplifier.
http://headwize.com/projects/showfile.php?file=gilmore4_prj.htm
this headphone amplifier features an opamp front-end with complementary bipolar outputs for excellent linearity and high-drive capability. It exhibits low noise, can drive headphones of any impedance and protects against shorts.
http://headwize.com/projects/showfile.php?file=eaton_prj.htm
The Futterman output stage in this tube amp offers the choice of DC- and AC-coupled outputs. The class AB design can output up to 24mW into a 30-Ohm load or nearly 500mW into a 600-Ohm load.
http://headwize.com/projects/showfile.php?file=stratum1_prj.htm
A simple yet high performance design using two Burr-Brown OPA132 FET input opamps. It is powered by a single 9V battery configured as a virtually-grounded dual supply and is small enough to fit in a shirt pocket. Includes a balanced input and audio limiter option for turning the pocket amp into a personal monitor.
http://headwize.com/projects/showfile.php?file=cmoy2_prj.htm
This portable DAC can improve the sound from portable devices, such as PCDPs, and other digital gear that has a TOSLINK optical or coax digital output. It features the Burr-Brown DIR1703 receiver and the Analog AD1866 DAC ICs for digital conversion and a high-quality multi-loop headphone amplifier that can drive low-impedance loads with authority. This PDAC will accept input digital streams up to 24-bit/96kHz.
http://headwize.com/projects/showfile.php?file=ifkovic_prj.htm
A 2-stage transformer-coupled tube headphone amplifier with a DAC front-end. The amplifier can drive headphones of 300 - 600 ohms impedance. The DAC is from a Crystal Semiconductor CS4390 evaluation board and has been modified to use a transformer-based output stage instead of the on-board active filter.
http://headwize.com/projects/showfile.php?file=ciuff3_prj.htm