Single Ended Modular Headphone AmplifierA Transformer coupled, single ended, modular headphone amplifier.
Back in the early 90's -- the summer of '94 to exact -- I read Corey Greenberg's review of Melos' SHA-1 in Stereophile and also started seeing ads from Headroom for headphones and headphone amps. I had spent much of my high school years staying up too late listening to Dead Kennedys albums on headphones, and I knew then that high end headphones were for me. I bought a Headroom Little amp and some Grado SR-80 headphones, and I was in heaven.
Cut forward a few years, and after being largely absent from the audio world, I was reintroduced by finally owning a Melos SHA-1. I bought some better headphones, loved the amp, but always felt that there was something missing with it. New megabuck tubes made it better, but the Melos ate those (fortuitously on day 89 of their 90 day warranty.) I also started thinking about DIY as a) there were not a lot of tube amps on the market at the time, and b) it seemed like something fun to do. I build one of Pete Millett's original low voltage headphone amps and was hooked.
Several years, and a lot of reading, tinkering, and learning later, I was finally ready to build a real headphone amp. The result is this amp. It is, in reality, a quite simple design. It uses a single tube transformer coupled to the headphones. The tube is biased via LEDs. Typically, a tube's cathode is connected to ground with a resistor. As current flows through the tube, a certain amount of voltage is dropped across the resistance, and the cathode is biased up from ground by that amount. In this configuration, however, typically the resistor must be bypassed by a capacitor -- usually an electrolytic in the 100uF to 1000uF range depending upon the tube used. The capacitor provides a low impedance path for the signal, but, it is directly in the signal path. Using an LED, instead of the resistor-capacitor combination, drops a certain amount of voltage (with almost no impedance) to bias the tube but does so in a less harmful way. It essentially does less damage to the signal than the capacitor. There is more on this in Morgan Jones' book, so I won't get into it here.
The basic schematic is as below.
In addition to this bit of cleverness, perhaps the neatest part of this amp is that it is modular. The tubes are mounted on little replaceable circuit boards that can be swapped in and out. There is one set of boards for 5842's (which are shown in the pictures and which are my favorites thus far), one set for 6c45pi, one set for 5687's, another set for 6N6p/6H30/6DJ8/etc. The dual triodes run with both sides in parallel. There are also more tubes to try (WE437, 6688, etc.) which I will get to at some point.
In order to accomplish the modularity and not have to do an enormous amount of math, the power supply is regulated at 160V. This is a pretty decent compromise for all of these tube types, and it allows the different tubes to draw different currents without the supply drooping.
Other notes that might be of interest are that the transformers are all custom made by Electra-Print. The output transformers are 5K:32R, so this amp is really made for Grado headphones. Custom transformers from Jack at Electra-Print are generally no more expensive than other company's off the shelf models, they are arguably of high quality than most of these, and in the process of ordering them you'll get a nice lesson in transformer design from Jack, which is a nice side benefit.