This is my latest project, I decided to make a DC-coupled headphone amp. As I prefer OTL amplifiers due to the obvious advantages these have compared to normal transformer coupled power amplifiers the natural choice for me was to build a OTL type headphone amp.
This is an easy project that didn't took so long time, After deciding the circuit and layout I built it in less than 2 days including mechanical machining. It was not so difficult to make it work either, the biggest problem in a headphone amp that runs on AC power is probably to eliminate hum as much as possible. I use AC for heaters but still the hum is not noticeable, (it is less than 0.1mV Pk-Pk). The amplifier use 2 tubes per channel. one 12AX7 and one 6DJ8. The 12AX7 is used as voltage amplifier and phase splitter and the 6DJ8 as single ended power amplifier. I use feedback from the ouput to the grid of the first tube.
The power supply is of normal design and give + and - 150V for the power stage and + 200V for voltage amplifier and phase splitter.
The amplifier gives enough output power to drive most headphones, at 30 ohm load it gives ~600mV out at ~1% distorsion which is what is expected for a power stage with 6DJ8.
The distorsion is mostly 2nd order and any component above 5th is neglible. Frequency response is ruler flat up to 300kHz. The sound is open with good transient response and very clear sound without any tiring effects, I have already tried this amp with several headphones, Sennheiser, Audio Technica and Yamaha.
I will put a picture here later.
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Created on ... October 6, 2002