Circuits

Build Your Own Vacuum Tube Headphone Amplifier

In this class you’ll solder together your own vacuum tube stereo headphone amplifier.  Use any audio source with 1/8″ or RCA stereo jacks–iPod, iPhone, stereo, etc.  Output is to headphones with 1/4″ or 1/8″ stereo jack.  A 12 volt wall transformer (provided) provides power.  (Yes, there are no high voltages in this circuit!)  A simple acrylic enclosure will be provided (see below).

Assembly will consist mainly of soldering.  This would be a do-able first soldering experience, and we can have some other boards around to practice on first.

We’ll also go over the circuit and discuss what each component does and how the vacuum tube in particular works in this circuit.

Note to audiophiles:  This amplifier uses a vacuum tube, but we’re not making any claims that this will magically change your life.  The project is just for fun.  If you’d like much more detail about the project, see the project page referenced above, and the related forum (108 pages!).  Also, note that you can buy similar tube-based amplifiers (many based on a dual-triode like the 12AU7) in kit or assembled from overseas, often quite cheaply.  Check e-Bay for some examples.  This class is about the fun of building your own.

Details:  Based on a dual-triode 12AU7 vacuum tube, this “OTL” (Output Transformer-Less) hybrid design uses a MOSFET-follower to eliminate the need for a bulky output transformer, and a LM317 voltage regulator serves as a constant-current sink for the follower.  This amplifier is based on the popular NP-100V12 design from Rogers Gomez (see http://diyaudioprojects.com/Solid/12AU7-IRF510-LM317-Headamp/), with the addition of a 30-second power on output delay relay, an indicator LED (red when warming up, green when ready), and a few other small modifications.  It features Wima polyfilm input capacitors, Nichicon audio grade output capacitors, Alps volume control, and NEC output relay–based mostly on the recommendations from the project above and related forums.  The PCB is a professional 2-layer board with solder mask and silkscreen.  The tube is the dual-triode 12AU7 from Electro-Harmonix (Russia), but you can substitute a tube of your own.  (See a selection of other 12AU7 options at http://tubedepot.com/12au7.html.)

1 class:  3 hours

Class size:  5 people

$135

The NP-100V12 design is used with the kind permission of Rogers Gomez.

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