A bit more complicated than usual…

Most of my projects tend to be quite simple designs. Apart from the obvious reasons of making the builds smaller, easier and cheaper, I very much like the thought of being able to make something play music with just a few parts. However, every so often I find myself attracted to designs that are more complex. One of these is this headamp/preamp.

This a variant of a couple of well-known designs by “Mr. FET” Erno Borbely, yet another of the greats of audio design. Sadly, the Borbely webshop closed up a couple of years ago, but many of the designs are still used today. This one is a discrete JFET input/BJT output headphone/preamp coupled with a Sulzer-style series-regulated discrete voltage regulator (phew! 😉 ). The boards were purchased from ebay seller al_tsankov in Bulgaria. The boards are decent enough in quality but very importantly, they come accompanied by detailed and clear instructions and BoMs/schematics you can easily read and understand – a welcome change from many of the chinese ebay sellers to be honest…

As with (nearly) all of Borbelys designs, this one incorporates some parts that are quite hard to find these days (Toshiba JFETs). I happen to have a small stock of these, but if you don’t this layout incorporates the option of using BJTs for the input as well (which is also why the board isn’t fully populated in the pictures). The circuit is fairly complex for what it does  (but no doubt, as Einstein said: “as simple as possible, but not simpler” :D) with constant current sources, LEDs to keep noise down and so on. Building it wasn’t too difficult as long as you concentrate on the parts and check everything before fitting. Turning something like this on for the first time is always with a sense of trepidation for me because you know the troubleshooting is going to be a pain if it doesn’t work, but in this case everything was fine the first time 🙂

As you can see, the boards are assembled and tested and working fine. All that is missing is final case assembly when the front and rear panels arrive from Schaeffer (FPX for the Americans) in a couple of weeks.

EDIT 24th Jan. 2014: I got a message from Alex (the PCB designer) correcting my text a little. The Sulzer regulator is a series-type, not a shunt-type as I had written above – sorry. Alex also asked me to attribute the regulator design, i.e. the modifications to the original Sulzer circuit, to Angel Despotov of Analogdomain which I am of course happy to do 🙂