Hot Schmitt Fuzz Pedal

My brother and I were talking on the phone a while back about different fuzz pedal architectures and somehow the idea of using a Schmitt trigger to produce fuzz came up. We weren't sure if it had been done before, but we figured it should sound pretty great and thought it would be worth trying out.

From the first prototype, we knew we were on to something. Crunchy and spicy, it just wanted to be played. Through several revisions, we were able to tame it into a useful and useable effect.

This fuzz pedal would be wildly inappropriate for “adding a little fuzz” to your sound. It is all-fuzz all-the-time. It does not, will not, and cannot “clean up” — nicely or otherwise. What it seems to excel at is splitting open skulls so they can be exposed to Universal Truth. And this is the principal purpose for which we recommend it.

A prototype guitar
effect sits on an anti-static mat. It's built on a plank of clear-finished maple
with the electronic components assembled onto a single 3M SuperStrip board.
Adjustments and controls are mounted in a length of aluminum angle which has
been painted black. They connect to the SuperStrip via multi-colored wires.
While the maple plank makes up the bottom of the assembly and the aluminum angle
forms the front, the remaining four sides are enclosed by clear acrylic sheet.
The acrylic sheet is laser-engraged with the words 'Open Heart Effects' and 'Hot

Revision D prototype

We've continued to think about the sound and experiment with the available adjustments, but this revision was mostly about hammering out the power arrangements, switching, and other interface type stuff.

Revision C prototype

We like the benefits of the eighteen-volt supply, but thought it would be nice if the pedal could run on a single nine-volt battery.

This prototype brings back the 9V battery and adds a charge pump to produce a -9V rail locally and adjusts the op-amp stages to take advantage of the split supply.

I've started looking at mechanical design. It might be a tight fit, but I think we can fit this effect in a standard 1590B enclosure.

Revision B prototype

We found the hysteresis control on the first prototype didn't cover as wide of a range as we'd like. The Schmitt trigger fuzz architecture has the interesting property that the sound chokes out suddenly as the sustain runs out. Though it is interesting, we didn't find this to be particularly musical in testing. The tone controls might be useful with further development, but probably not for this particular pedal.

Our second prototype runs on a single eighteen volt supply to get more headroom, reduce the relative effect of offset voltage, and facilitate wider range on the hysteresis control. It also includes a new path from the input buffer past the Schmitt trigger and straight into the level-limit stage, with adjustable drive. At the very end of the sustain, this helps smooth out the transition from Schmitt trigger fuzz to silence. This revision also drops the adjustable low-pass corner frequency, replacing it instead with a fixed-frequency that we arrived at through testing.

I think we're moving in the right direction.

Revision A prototype

Our first prototype is a basic demonstration of using a Schmitt trigger to produce fuzz. It includes an adjustment for trigger hysteresis and an output level control. I also snuck in a — perhaps unusual — tone control consisting of a high-pass and low-pass filter each with an adjustable corner frequency.

Initial testing has convinced us we're onto something, but that additional design work will be required to make it a really great guitar effect.

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Aaron D. Parks
Parks Digital LLC
4784 Pine Hill Drive, Potterville, Michigan