The road to nirvana

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My journey into Photography, Horology, and Audiophilia

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AK-100 Headphone Amplifier

Using the Western Electric 417A and variants

I have always wanted to create a spud amp, a really small one. While I have several candidates, the 417A meets my main requirements- i) single tube per channel, ii) enough gain, iii) sufficient power output.

Using my "breadboard", I hastily wired a stereo circuit using my trusty Hammond 125ESE as the output transformer. I used a 5Y3-GT as the rectifier, a 300V-0-300V power transformer, a couple of smoothing caps, and resistors so I get 200V for the 417A.

I tested it first using a pair of Kenwood car speakers from my Honda CR-V. This pair has served me well, as they are the speakers I use for testing every amp I made. The source was a cheap Pioneer DVD player. I am pleased with the results, so I hooked it to my main system to enjoy a mighty 0.5W (or less) of tube sound.

Then it occured to me that I have a low impedance headphone- the Philips HP-890, which led me to try the amp as a headphone amp. Using an iPod Shuffle as a source, it was... Not bad! However, this is such a big amp (physically) for headphone duties!


So I decided to turn it into a real headphone amplifier. Using a cute aluminum chassis from Hong Kong, it became home to the various parts needed for the project.


Due to the small size of the chassis, I have to ditch the tube rectifier and go with solid state rectification. The PSU is a very simple CRCRC topology but is good enough for this amp. In the future, I would like to build a MOSFET regulated PSU for this amp.


I asked our local winder to make a custom output transformer with 5K primary and 32, 300 ohm secondary. It is small enough to fit in the chassis, and they're mounted on opposite sides. The power transformer is also custom wound and is mounted at the back. In the future, I would like to try a Lundahl LL1630 10mA line transformer as the output transformer.

Schematic Diagram