Do you know about that overdrive? No, not that one, that one, this one!
You might know I like to be very straightforward or to clean up other designs usually, this began as a test to see if I could be more flamboyant with my designs. Also I wanted to take on the difficult challenge of making an overdrive pedal which doesn't borrow heavily from known designs. In the end my minimalist approach is still there, this isn't a physically big circuit and the values are all common: despite extensive breadboard testing, I didn't have to give up on that.
We all know which is the predator in this category, and we know it's green :). Now, I wanted to refresh on that a bit. For example, we know the pre-gain low shelf is very important to keep a clear distortion, and it's very important to me, but what if we could make that roll-off steeper, and maybe that way being able to push it lower? The most obvious solutions would be a tiny input cap or an active high pass filter at the input, but I'm not a fan of either approach, either because of interactivity or because of having the noise of an active stage before all of my gain. Instead I thought, what if we feed back more of the low frequencies? what if instead of the usual shelving low-pass formed by the feedback network in a non-inverting op amp we try to make it second order?
Same thing for the post-gain low-pass. First order passive filters are ubiquitous, and they work OK. Even the green pedal, with its active tone control, in practice doesn't go far off at all from the response of a cut-only first order low-pass, once you combine the active stage with the fixed low-pass before it. A second order filter here is relatively unexplored, and can be used, among the other things, to preserve more of the nice lower treble while still keeping the bees in the nest with the steeper roll-off.
Before getting here, I've messed with some things while getting ideas, like parallel resistors+capacitors in the feedback for more of a custom-fit frequency curve (but for the grounded leg that would have given me more bass not less!), switchable clipping, fixed frequency gyrator mid control... In the end I tend always to simplify though. For the feedback I settled for two parallel networks, one traditional with variable gain, one a fixed second order low-pass, which then get mixed together. The response of the gain stage looks like this (stepped gain pot, you can see the bass cut is steeper than with a maxed-gain traditional green pedal):
Do you think this is too much? I know glassy and clean bass isn't for everyone, so I added a bass control, which in practice isolates the additional network, leaving you with something more familiar (again green pedal for reference):
Finally, there's the S-K low-pass. A trick here is to switch between the usual buffer and an amplifier, which increases the final Q. That's what positions 1 and 3 of the resonance switch do. The small peaking near the cutoff is very very effective in giving you more sounds. The boosted Q value has been chosen to be still low and usable, but different enough. The 100KC dual pot is a bit unfortunate, but other values can be used with scaling, and A taper if wired in reverse. In resonant mode, the output is taken after R12, which makes the output impedance closer to 1k. This is still very good, and what you get for it is that the volume remains the same, so you don't have to adjust it when changing mode.
Mode 2 is a nice little addition for no cost. By disconnecting C6 the filter becomes a more standard passive first-order. I usually find this mode the more lackluster, but for the sake of being able to replace the green pedal, other than going beyond it, you can have it with an on/off/on switch, or leave it out with a two-position.
Here are sweeps of all three modes:
Build picture:
Vero layout:
Demos: