Sunday, August 29, 2010

Vacuum Tube Headphone Amplifier

So what does an engineer do to stay up on their electrical engineering and earn a little respect with his coworkers?  Take on a little electrical engineering project of course.  So what project could I take on that I could show off at work on my desk that looks cool and has a practical application?  The most interesting thing that I found was a vacuum tube headphone amplifier - basically something that takes the output from your CD player or iPOD and makes it louder.  Yes, both such devices already have a circuit in them to power your headphones however they usually aren't of the best quality and generally only work well with cheaper headphones that don't sound all that great to begin with.  This thing should make a huge difference.

So after quite a bit of research I settled on the SOHA II headphone amp which utilizes a vacuum tube input stage and solid state output stage.   All of the info including schematics and building instructions can be found here at and PC boards can be purchased from

This is a pretty clever little design utilizing inexpensive and easy to find 12au7 preamp tubes, the schematic is shown below (this is the schematic for one channel).  The beauty of this design is that these tubes only require 60-90 volts to run properly as opposed most tubes requiring 200+ volts to work so it is much safer for a first time builder such as myself.

Here is the power supply schematic as well:

While the schematic looks pretty complex at first look it is really quite an elegant design making use of relatively inexpensive parts and is a rather robust design allowing for easy tube swapping.

Once I did all of my homework I ordered up most of the parts including the circuit boards from and purchased in the rest from Mouser, Digikey, and some scraps I was able to scrounge up at work.

Step 1:  Design the Amplifier Housing
I started out with plans to use a commercially available metal enclosure but I decided something a little more unique would be more interesting.  So I sketched up a little design:
Then a quick CAD model - Google Sketch is very cool!

Step 2: Build up the Circuit Board

From a box of parts to the nearly completed board
Step 3:  Build the case and and put it all together

The corners and top piece are Bocote while the main body of the amp is Zebrawood

I put LEDs under the tubes to get them to glow

Listening Impressions:
Now that I have had this for a while I find this amp is quite incredible, its almost like switching to a high def TV from std def.  The bass has a lot more punch, the midrange/vocals are amazingly smooth and the highs shimmer and are exceptionally clean.  Another  observation is that the music takes on an almost 3D quality. With such clarity your ear can roam to single out individual instruments in a way that I had never experienced before.  Bottom line - I love this little amp!

I'm thinking maybe this coming winter I may just have to try another one of these!

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