|The classic Big Muff Pi tone control pans between the outputs of a simple lowpass and highpass filter. The corner frequencies are set so that: a) there is reasonably equivalent output level from each extreme of the tone control (i.e., changing the tone won't drastically change the volume), and b) the "middle" position delivers a gentle mid-scoop.|
While elegant in its own way as a do-all control, you can't have more bottom without losing bite, and can't get more bite without losing bottom. Personally, I find that a nuisance.
While looking over some schematics from various Shin-Ei fuzzes, various Univox Superfuzz issues, and the Roland Bee-Baa, I saw that many favourite fuzzes incorporated a switch to go from a mid scoop sound (which would accentuate buzz and octaves above by de-accentuating the fundamental) to a "normal" full-bandwidth sound. This started me thinking about replacing the switch with a panning control similar to the Big Muff's. The big question was what would be at the non-scoop end of the control's rotation. To simply fade in a mid-scoop was a bit too plain and the sort of thing you could easily do with a resonant boost-cut control/stage.
At the same time, I started pondering what eventually turned into the Rosey-Ray fuzz (see a few entries below). There, I wanted a control that would pan between two different fuzzes, and I wanted the pan control to blend between the best that each had to offer. What I ended up with was a bizarre control that incorporated a lowpass filter on one side, much like the BMP, plus a mid-scoop at the other side, and a little twist that inserts more variation into the control with a single component.
What I've posted here is the essential design idea, without any posted values. You can look at the Rosey-Ray for one implementation of it, but note that the values chosen were arrived at by trial and error to complement the two distortion stages and produce roughly equal volume. I thoroughly expect other component values to be more appropriate in other contexts. For the Rosey-Ray, the two parallel signal paths to be panned between were coming from different sources. What I've shown in the schematic here assumes that a single source (e.g., the output of a high gain device like any of Joe Davisson's excellent designs) is simultaneously filtered in two ways and you adjust how much of each you want.
The lowpass side should have a rolloff somewhere in the 2-3khz zone. This will yield a nice warm distortion tone, a bit of bite but with all the edges sanded down smooth and round. It will easily stand out in a mix and you will know that it is a distortion, but with a more relaxed bluesier feel suited to a humbucker-equipped solid body or semi-acoustic. The mid-scoop/notch can be whatever you want it to be. The essential thing is that it retain both top end and bottom. I recommend the articles that Jack Orman has at his www.muzique.com site in the lab notebooks, concerning notches and scoops. Myself, I just started with a known quantity: the Superfuzz notch.
The neat quirk I stumbled onto is the addition of the cap between wiper and ground. Bear in mind that cap, in tandem with each leg of the pot, forms a low pass filter whose rolloff is set by the capacitance and pot-leg resistance. Let's say, for instance, the pot was a 100k linear, just like the BMP, and the wiper-cap was .001uf. With the pot rotated completely in any direction, the pot and cap make a LPF with a 1.5khz rolloff which is ADDED to the action of whatever filter precedes it. When the pot is rotated fully towards the lowpass side, the cap is paralled with the existing treble-cut cap to form a single-pole lowpass with a lower rolloff.
Rotate the pot 10k away from the end, and you'd have a 6db/oct rolloff at 2.3khz, followed by a second 6db/oct rolloff at 16khz. Of course, at the same time, the signal coming via the notch filter side is also rolled off at 1/(2*pi*90k*.001uf)= 1768hz.
The nifty thing here is that as you rotate the control, you not only change the relative balance of each source, but you also change the spectral content it is contributing as the LPF rolloff for each side keeps changing. The result is a control where a couple of degrees rotation gets you an audibly different sounding tone, with a different character, not just a little brighter or a little duller, more boomy vs more nasal. It's not quite a "clickless switch" like those pots that double as program switchers in many digital FX boxes, but there is a big increment to the amount of clear variation can be introduced by a single pot. Again, the nice thing is that you don't have to trade off bite for bottom. If you need more bite, you can simply up the value of the cap labelled C1.
It may well require a lot of tinkering to get it to work right in your circuit, but this is a circuit that is well worth tinkering with.