841 DHT Hybrid Headphone Amplifier

[L0rdGwyn]

Finished up another DIY project. This is a tube-FET hybrid headphone amplifier. Voltage gain is done by a thoriated tungsten filament directly-heated triode, the 841. Here are its A1 operating curves from my curve tracer (it has A2 functionality as well).



It is filament biased, filament supply is a Rod Coleman regulator. The 841 is direct-coupled to a Alpha & Omega AOTN160 source follower. This is then cap coupled to the Sowter 8665A headphone output transformers. Here is a schematic of the amplifier circuit. It is essentially a filament biased, CCS-loaded DHT spud with a FET buffer in the middle.



The power supply is Maida regulated. Here is the interior. Switch in the center alters the turns ratio of the output transformers for various headphone impedances.



The finished product. Mains and filament transformers are on top with matching shrouds.



Sounds very nice, distortion is quite low, here are the numbers into different dummy loads.

THD 1mW into 32ohm (low impedance switch) - 0.017%
THD 1mW into 120ohm (medium impedance switch) - 0.007%
THD 1mw into 300ohm (high impedance switch) - 0.0054%

And the frequency response.



The filaments of the 841 have a characteristic glow, looks very nice in the dark, sounds great with my Sennheiser HD650 and ZMF Auteur.



Thanks for checking it out!

 

[watchnerd]

That looks really neat.

I have boxes full of small signal tubes (6DJ8-type and 12AX7-type) I really need to do something with....

 

[L0rdGwyn]

Thanks 6DJ8 tubes are a hot commodity these days, sounds like the makings of a tube phono stage.