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RJM's 3 Channel Mosfet Headphone Driver

There are a lot of headphone amps out there these days, and lots of "DIY" projects with circuit boards and documentation and all that. One of the earliest non-opamp designs specifically for low impedance phones was the so-called Szekeres amp. It is a basic single ended Mosfet follower. It is a unity gain current buffer, and so long as there is sufficient voltage on the input it can push low impedance headphones better than many amps. The down side of the design is that it requires both input and output capacitors, the output needing to be large which means electrolytics in most cases.

The original designer suggested various ways to make the amp DC coupled (that is, ways to remove the output cap) but in the end most of these turned out to not work due to thermal changes in the mosfets changing how they perform. However, rjm figured out a clever way to make the amp DC coupled. By adding a third mosfet to the ground channel, the mosfets more or less balance against each other. The design is simple, elegant, and sounds great. I have found it to work especially well after a tube preamp stage.

The details of the design, and instructions on how to build it can be found at http://www.geocities.com/rjm003.geo/rjmaudio/diy_sze.html.

I threw together a simple PCB, used a power suply from an old gainclone, and built one in an afternoon (plus the time for the PCB to be manufactured, of course.) It is highly recommended as the sound is smooth, relaxed, and detailed. You can email me if you are interested in the gerber files for my PCB, though in hind sight it is just as easy to build on perf-board.