Some things I did forget in the schematic:
1. An optional switch at the junction of the 560k and 120k resistors and the 47nF cap (connecting to the JFET gate)
This could be a 3-position SPDT switch where it's completely open in the central position (Chopper on), connected to Vcc in one position and ground in the other to select between fuzz and all-out Butt rock to Thrash Metal.
2. The green boxed capacitor and 100k resistor has an error. The 10 uF should be connected to ground instead of "V/2". The 100k resistor should still remain connected to the "V/2" triangle. This setup is based on a guess that some of the clock feed-through is coming into the signal path through the 1Meg resistors from "V/2". Since no op-amp is ideal, I fear that the oscillator is taking a toll and chopping up the V/2 reference in a less than acceptable way. The 10 uF capacitor to ground is intended to filter any wiggle in the V/2 reference so to supply a steady 4.5 V reference to the JFET switch.
Description of the Device:
The concept behind the circuit comes from a single coupling capacitor. I found when using the .47 uF input coupling cap, the circuit produces a mushier BMP like fuzz sound. When it is decreased to 10 nF, the circuit becomes a raging metalizer.
Now imagine having a metal distortion in series with a fuzz distortion (guitar--->>metal--->>Fuzz--->>Amp) on you FX chain. Play along and stomp the fuzz on and off. Now imagine a stompbox that does this automatically. Now imagine this stompbox doing this anywhere from once every 3 seconds to 30+ times a second. This is the Chopper Fuzz.
10 Hz to 30 Hz modulation time is when this effect gets weird. It reminds me of Jimi Hendrix with a Univibe and another part of it like having a chorus on max rate and depth fed into a distortion pedal. Professor, please please please don't make me derive the transfer function of this thing. If the frequency response and feedback analysis isn't bad enough, combine that with square-pulse modulation and non-linear distortion...it could be a homework assignment from bad ol' Lucifer himself.
For those of you who are used to seeing an input buffer, you may think this circuit badly needs one. However, this circuit is designed to use the volume pot in your guitar in the feedback loop of the MOSFET amplifier stage. In simple terms, you can drastically change the amount of distortion just by rolling off the volume pot on the guitar. When the volume is wide open, the impedance looking back into the guitar is smallish, minimizing feedback, thus approaching open-loop gain on the MOSFET stage. When the volume pot is rolled back, the impedance looking into the guitar increases, which increases the feedback....gain goes down.
To say the least, the gain control is very nice. It doesn't do nothing....do nothing...do nothing...then suddenly ramp up. The distortion increases gradually with the gain pot. On the low gain setting, hard-wired to the tight metal tone (coupling cap NOT bypassed), this produces a nice tight "crunch" hard rock distortion. It makes me think of men with long hair and make-up. I think they call that but rock. This device generates some wicked thrash metal tone when the gain is cranked up. I hope all of you enjoy it...whoever builds it.
Ummm....if you post it on other pages, please leave credit to me...just to be courteous. You would have to purposely remove the source from the picture to get rid of it, so you're already twisted if you repeat it somewhere. It's nice to get credit when it takes 3-4 hours to draw up the schemeatic not to mention the 20+ hours of tinkering, calculating, and experimenting for taste....but that's the fun part.
transmogrifox@yahoo.com is a good place to send comments, suggestions, mods and other improvements...etc.