Top-Level OTL Tube Headphone Amplifier

This article shows my tube headphone amplifier designs for driving headphones with an impedance of 200 to 600 ohms (I am using these amplifiers with the Sennheiser 580, which has a 300-ohm impedance).

The problem with many tube headphone amplifiers is the high output impedance and the low output current that decrease the sonic performance. I have analyzed many schematics, from a hybrid design with an E88CC voltage gain stage and a direct-coupled IRF610 MOSFET output stage to a design involving a 6080 or 6C33C-B as output stage in a cathode follower configuration.

The hybrid with the IRF610 source follower does not have very good sound, the amp with the 6080 output stage has pretty high output impedance (more devices per channel would lower output impedance, but then the tubes must be carefully selected) and the 6C33C-B is not very linear for a headphone amp (but it is very good in an OTL amp for driving loudspeakers).