In most cases my line preamplifiers can drive headphones without any problem. But certain headphones with 32 Ohms impedance are a too heavy load for a preamp. Typically they are too sensitive to be connected to a power amp, even if it is a flea power single ended one. So in such cases a dedicated headphone amp which acts as buffer after the preamp is the ticket.
This is how the E55L single ended headphone amp was born. It is meant to drive the Grado RS1 which is very sensitive (1mW for 96dB SPL) and has a 32 Ohms impedance. The E55L triode connected delivers more than enough power for that and has just the right gain. So the amp can be kept rather simple with just a single E55L stage per channel.
The circuit follows my usual approach in power amps. Output transformers are Hashimoto. The E55L runs at a moderate 30mA. The power supply is tube rectified with a 6BY5 augmented with UF1007 diodes to create a full wave bridge. Several LC filter stages after rectification. This guarantees a hum free operation even with these sensitive headphones.
There are 3 outputs connected to the 4, 8 and 16 Ohm taps of the output transformers. The headphones work on either of them. Selecting different output provides some additional gain control.
Just one tube per channel:
View from the back:
The E55Ls proved to be just right for this application.