Tube headphone amplifier

I decided to build a tube headphone amp because I’ve been aware of them for a while, but I wanted to see how much of a difference they actually make to a listening experience.

I decided to go with an amp that uses a twin triode 12AU7 tube, which has two advantages:

  • Runs on 12V, doesn’t require high voltages or big transformers
  • Can amplify left and right channels by itself, due to it being a twin triode design. This means only 1 is required (cheaper tube rolling)

I used this hybrid 12AU7 design by Roger Gomez, as I was on a budget and so wanted to use a tried and tested design.

It is a hybrid design (between tube and solid state), which means that the signal is initially amplified by the tube, before meeting a MOSFET follower which supplies the power.

Time to order parts!

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First, I soldered up the main board.

The only bolts I could find hanging around the house which were of the right size for the heatsinks were far too long, so these got chopped down first. This makes them look pretty ugly but they do the job.

Since I planned to mount the tube to the top of the enclosure, I also decided to put the connections to the tube on Molex KK headers, as this would allow the top of the enclosure to be completely seperate for assembly/future work.

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I then tested the board with some crappy headphones and found there to be a considerable hum. Eventually I diagnosed this to be the result of the heater power wires being on seperate headers, in close proximity to the audio signal wires.

I therefore added an extra header purely for heater power wires and ensured they were twisted.

I also drilled out the center of the tube socket and added an LED, for extra glow!

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I then added a hole to the top of the enclosure so that the tube could stick out of the top.

Lastly, I wired up the switches, pot and sockets before screwing the enclosure together.

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F3
The finished item

The two horizontal toggle switches on the front are for output selection and bypass. The output select allows switching to a rear output if I ever want to use this as a pre-amp for a speaker power amp, whilst the bypass allows me to directly compare the difference in sound, with or without the amp.

I notice a very slight but definite added warmth in tone. Furthermore, it glows.

 

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