DIY will be the death of me

I’m building a headphone amplifier to use at my desk at work. A headphone amplifier with honest-to-goodness vacuum tubes.

This of course is the side project I began while the family was out of town for a week. I wanted to build something satisfyingly quickly while I took a break from planning that preamp. I’m pretty sure at the time I figured it would take a month total. Heh. Nine and counting.

It’s what they call a “class A” tube headphone amplifier, which means it runs hot. And it’s an OTL design, which stands for “output transformerless” (or something), which means the tubes do the job of pushing the music into your headphones, instead of using a transformer to convert some of that voltage into the current your headphones need. So naturally it now has a pair of audio transformers in there, custom wound for me by an industry legend, and a knob that lets you select the output impedance that best matches your headphones — OTL (600 ohms), 32 ohms or 60 ohms.

Oh, and a very, very fancy volume control: a collection of six separate circuit boards (eleven, if you count the ones piggy-backing on the main board) that combine to use light to vary the voltage (LDR, light-dependent resistor). And provide a dual panel LED (light-emitting diode) to show what the volume for each ear is set to. And relay-based input selection. And a 12 volt trigger, which means there’s a big power switch in the back that you leave on all the time, and push the volume control knob to switch the tubes off and on. And of course a remote control. Every headphone amp needs a remote control, I’m pretty sure of that.

Almost forgot the built-in DAC.