This is the link that I found on Makezine’s website a few weeks ago. Of course, the actual link is on the page I linked to. It is definitely worth checking out. I pretty much had everything available with the exception of a 12au7 tube. I just happened to win an eBay auction with about eight 12au7 tubes. I paid $25 dollars for 14 tubes.
I liked the idea of making the amp using batteries, but I figured that it would take a fairly large battery, and I didn’t want to spend the money on that and a charger. I decided I would use a wall-wart and just regulate the voltage to 12.6V. Well, that was a great idea, but also a frustrating idea.
Most of the circuit was already designed. There’s no need to reinvent the wheel, although you certainly could if you wanted to, but how much can it differ at just 12 volts?
My first step was to check my wall-warts with a multimeter. I found one labeled 12V @ 1.2A. that should be more than sufficient. I assumed that the total circuit would use 500mA at 12.6V. The transformer measured about 17 volts, so I knew it would be necessary to regulate it with an LM317. I simply used an LM317 calculator that I found on Google and made some guesses. I used two 120 ohm resistors in series for resistor #1, and I used a 2.2k ohm resistor for resistor #2. This should give me about 12.7 volts at the output. I added a couple of capacitors to smooth the supply out. I’m done!
All I had to do was build the circuit as laid out in the schematic and mount it in a box. It was great until I got it turned on. I had some major 60Hz buzz. I tried high-pass filters, a 4700uF capacitor across the supply rails. I tried it all. I even busted into the wall-wart to see what was in it: 3300uF cap, 4 diodes and a transformer. Nothing special.
I worked on it for two hours, which carried me into the early morning, trying to kill the hum. It was at work the next day when it dawned on me: “I have the power supply that I built a few years ago…Why not try it? I came home at lunch and hooked it up using gator clips. Super silent. The hum was reduced to almost inaudible. My house has a lot of EMF! I also tried a Radio Shack wall wart, it made a high-pitched whining sound similar to a jet engine starting up, but that’s not bad. I think the cheap wall warts are just naturally noisy. I’ll build my own portable power supply later.
Well, the whole thing is built. The 12au7 and a dual-gang volume pot were the only things I didn’t own. I obtained the tube because it was a need. I didn’t think I would need a volume. Wrong. This thing can get loud depending on the source. I dug in my parts cave and found two 50k pots. I just made individually adjustable volumes. Now I can single out one side of a track without removing the headphones. So cool!