Multi-dimensional one-knob tone controls

[Mark Hammer]

Jack Orman has an excellent article at the AMZ site on the standard "Big Muff" type of tone control and how to customize it to achieve different sounds.

The Big Muff control is essentially a circuit that pans between the outputs of two simple filters, combining a highpass and lowpass output in different proportions to provide bass and treble emphasis on a single control.

This control, while flexible, is not the only option out there, so I'm posting this because I'd like to learn more about what a single pot can do.  At least part of the reason is because these things intrigue me, but certainly another reason is that I like effects that are very flexible tonally without a lot of effort, and I also like powerful controls that allow one to build something into a smaller space, occasionally so you can conserve panel space for other controls that *can't* be condensed into single controls.

A lot of these controls are to be found in amplifiers that have a "Shape" or "Contour" control, although occasionally they are referred to as "Mid" controls.  Most often, they seem to be part of the overdrive or "dirty" channel on such amps.  Here, I'm thinking of some of the solid-state Marshall, Fender, and Crate amps like the 8020, VS230, Bullet Reverb,  Frontman, and GFX-120.

In other instances, they can be found in pedals.  For instance, Alexandros Charissis' "Ballz Booster" pedal has a tone control that does a few things at once.  I liked the idea of it and adapted it a bit on the "Chaos" pedal.  On another lamebrained cascaded-clipper adventure of mine, I took a lowpass-filtered output from one stage and a mid-scooped output from another, and tried to tie them together, à la BMP, to pan between two different characters of tonal balance AND distortion.  I'm sure there are others out there, but these are the examples that come to mind easily, and I have the schematics for handy.  Of course, there are simple versions to be found in any of the Superfuzz derivatives or the Bee-Baa, where switches are used to select between tonal presets, but I'm thinking more in the way of variable controls.

For some reason, the sites I go to for Crate and Fender schems seem to be inaccessible at the moment, or else I'd link you to them.  For those of you with these schems, though, I'd be curious about what your thoughts are on these do-several-things-at-once controls.  For instance...
- do they do things that are normally unachievable with a single resonant boost/cut circuit?
- do they incur more passive signal loss than other types of controls, and roughly how much?
- do they share any common design features?

[Ben N]

As you alluded, very often the most useful single thing a tone control can do is midrange, if it has a wide enough range to go from scoop to hump, or "smile" to "frown", like the Boss GE-7s that David Gilmour used to have in his pedalboard but now uses his EMG active controls for.  This doesn't just fine-tune the tone, it pretty radically alters it.  A simple way to do that might be to adapt the Dan Torres mid-cut/boost control, which uses, IIRC, a 1.5 Henry inductor.

Ben

[Tim Escobedo]

One effect I'm working on now is a bit like the Phozer. It's a twin T circuit. however, rather than having the filter in a negative feedback loop, it's in series with the input.

Generally, this sort of thing simply means a passive notch filter. However, I found that if followed by a emitter follower, with one or both of the filter paths to ground getting bootstrapped from the emitter, the circuit is more than unity at some frequencies. Depending on the filter RC values, so much so that it can oscillate. Much of this depends on transistor hfe.

This circuit results in what could be implemented as a single pot tunable tone control where the output simultaneously has a tunable dip and bump. Kind of a neat sound, not quite wah, not quite phase shift-like. And the twin T filter values don't have to follow the typical formula to be sonically useful.

[Paul Perry (Frostwave)]

It wouldn't suprise me if a centre tapped pot enabled even more flexibility.
Because then you could go between A and B, as well as B and C.

[R.G.]

Hmmmm....

How about using an LM3914 to get ten reliable states from one pot, and a VCA arrangement to select how much of the current state and the two on each side of it was mixed? I think that gets thirty mixes into each pot rotation. You could cut that down to nine or ten with judicious use of diodes.

There's a synth setup that does something like this as well.