Here’s the Face MKI schematic:
I really like this one, it’s a booster/overdrive with a nice Fendery feel and voice to it. As you can see there are some values which you can alter on the schematic that allow you to tailor the tone to your exact needs. Firstly the supply voltage – the lower the voltage the less overall gain and clipping you’ll get, more voltage = more gain and clipping. The jfet choice also makes a difference to the tone of the circuit; 2N5457s give more gain but are more compressed sounding (better for the Fender “tweed” emulation) than the MPF102s (better for the larger Fender “twin” type emulation) which have more headroom but less gain. You can use almost any jfet type in this circuit 2N5458, J201 (I didn’t like these, they were too compressed and gainy – almost fuzzy!), you just need to find your type! If you want to learn a little about jfet’s and their properties check out this article over at runoffgroove: http://www.runoffgroove.com/fetzervalve.html. Finally the 3n3 tone cap can be altered to your preference, a higher value will lower the high end gain, a lower value will allow more high end drive.
In my final build I’ll be using MPF102 jfets running at 18V with a 4n7 tone cap to keep the high end under control. This setup provides a nice “cranked twin” tone and is really dynamic and reactive to your playing, a decent low end is retained while it still fizzes out at higher gains.
Here’s the schematic for the version I built up (note the altered tone control which, I find, works better with the MPF102 jfets):
I took inspiration from a range of sources for this little circuit; firstly this patent from 1943: http://www.muzique.com/amz/SRPP.pdf
And this microphone preamp (1997):
And one of Tim Escobedo’s excellent circuit snippets – the Syrupp Booster (2002):
All comments are welcome, for the freestompboxes.org topic check here: http://www.freestompboxes.org/viewtopic.php?f=13&t=16321