A single-ended, class A MOSFET headphone amplifier with an active current source that results in increased efficiency (50%) and lower distortion.
http://headwize.com/projects/showfile.php?file=pellerano_prj.htm
This reverse-engineered version of the famous EarMax headphone amplifier has just 3 tubes, uses no global feedback, and is powered from an 19VAC wall-wart.
http://headwize.com/projects/showfile.php?file=cmoy5_prj.htm
This OTL mu-follower design uses paralleled tubes (2 per channel) to minimize noise. It also makes an excellent preamplifier.
http://headwize.com/projects/showfile.php?file=strain1_prj.htm
This OTL amp features a single-ended class A input stage and a paralleled-tube cathode follower output stage with 33 ohms output impedance. It can drive any headphone from 300 to 600 ohms. Now includes two "high-end" versions based on the 6SN7 and the 3A5 direct-heated triode.
http://headwize.com/projects/showfile.php?file=ciuff_prj.htm
Designed and built for the author's Stax Omega II headphones, this amplifier can be used with other Stax models. It is fully DC-coupled.
http://headwize.com/projects/showfile.php?file=gilmore2_prj.htm
A discrete all-bipolar, zero feedback class A amplifier with a new opamp-based servo. The servo operates outside the audio feedback loop by controlling a pair of current mirrors at the input stage.
http://headwize.com/projects/showfile.php?file=jorgen2_prj.htm
Design principles and circuits for building opamp-based headphone amplifiers. This article reviews selection criteria, feedback configurations, capacitor coupling, RF shielding, output stage topology, short circuit protection, equalization, acoustic simulation, headphone distribution amplifiers and power supply options.
http://headwize.com/projects/showfile.php?file=opamp_prj.htm
The gain of this voltage-feedback design can be set by changing the values of two resistors. The authors presents versions optimized for low and high impedance headphones as well as one that performs well with a wide range of loads.
http://headwize.com/projects/showfile.php?file=balkishan1_prj.htm