Okay, single-end version is done. Couple of things:
The C10 mystery gets a little deeper. I decided to socket this thing so I could understand what it does. Turns out, not only does it provide some frequency dependent filtering as Baja mentioned, but it actually stabilizes the circuit when it's in buffer mode. Without out a cap there it is warble city. Massive chop, esp. when you have the gain up high and roll the volume down on the guitar. With it in there, it seems to minimize the problem. Whatever engineering principle is behind this (which I'm still guessing is impedance related?), I can't say. Whether or not he arrived at this design on purpose or by accident is also a mystery. I can say that different values work. A 1n provides the stabilization, but I didn't care for it. A 4n7 is what I ended up with...I really like the mid-range you get with it.
Second, the tone control is definitely a bit odd. It seems to have a more pronounced effect in buffer mode, when you can really hear that massive 1kHz roll-off when the knob is all the way down. You can get a fairly decent range of mid cut in buffer mode. In "reactive" mode, the tone control didn't seem quite as flexible. Not useless, just different.
I found a 500kC works best for the main gain control. A 500kA works better for gain2.
About the trims: what I ended up doing was wiring the Gain2 control externally, and I left the trim off for the treble control. My thought here was that it would be interesting to try out different values for C4 to obtain a nice frequency boost with the Gain2 control. I don't like the 1n5 stock, but a 10n sounded really good to my ears. When I box it up, I will probably use a center off SPDT and have the two ends tie to different values caps with the middle position being just a full range second gain control.
With the second gain control wired externally, you will get some pretty useful variations of the OD. With gain1 turned down, and gain2 up, you can actually get a different feel. With both maxed, the effect is pretty noisy and quite unstable, but it also gets very fuzzy and interesting. You can even get into metal territory. There is a massive amount of gain on tap in the circuit, but you can get some decent low gain sounds as well.
All in all, I think it is a good design. I think this version suits my needs and tastes better, but obviously there was some thought put into the design. It is, at least, more unique than anything else I have recently built.
Have fun!
Mysterioso_JR.pdf
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