Tone control question (reducing or moving range in the SWTC)

[midwayfair]

I don't usually mess with tone controls unless there aren't any good settings on them. Most of the time it's set and forget. However, I recently worked on something using AMZ's version of the Stupid Wonderful Tone Control, and I found that in the circuit I was working on that I really liked at most the first quarter of the knob's travel (CCW, so the darkest setting) with one guitar, and maybe the first half with another guitar that has flat wounds. Anything past noon with my gear and it was simply too bright (even for me, and I like treble) and also let through more noise from the circuit itself, which is pretty high gain. It was also a fairly abrupt change from very bright to full cutoff. So I was thinking about finding a way to "move" the tone so that in future builds, fully clockwise is where noon is right now, and there's more fine control over the fully counterclockwise positions that already exist. I used the stock values here:

http://www.muzique.com/lab/swtc.htm

Here are the problems I ran into trying to correct this:

1) It seems that just changing the value of the tone knob won't get me what I want. It just made the swing between "no capacitor in the circuit at all" and "all capacitor all the time" even more abrupt. Maybe I did it wrong? I tried increasing the pot's value, thinking that the increased resistance would direct more signal toward ground.

2) I know that changing the capacitor value isn't the solution. It starts cutting frequencies I don't want to cut when and it doesn't spread out the range of the control. It was simply too dark in the fully ccw setting when I breadboarded it.

3) I thought one thing I could do was to hardwire in a resistor and capacitor in series before the tone pot with the same values as 1/2 of the tone knob's travel. My one issue with this is that I think it will change the cutoff frequencies, right?

So to sum up, what I'm looking to do is make a SWTC spread what's normally the first half of the control's sound over the full travel of the pot. Here's the real challenge: I'd like to do this with two, maybe three parts maximum.

[Mark Hammer]

There are two challenges to be confronted here: pot taper, and width/spread of range.  for the "classic" SWTC, you are basically constructing a single-pole LP filter whose highest and lowest rolloff points are dictated by the value of not just the cap, but the series resistor just before the Tone pot, and the value of the tone pot itself.

Example: We have a 1k resistor going to a 10k linear pot, and a .01uf cap to ground from the wiper.

This lets us produce a lowpass filter formed by a .01uf cap and as little as a 1k resistance, or as much as an 11k (10k+1k) resistance.  With minimum resistance, the rolloff starts around 15.9khz.  With max resistance, the rolloff starts around 1.45khz.  In essence, an 11:1 ratio, which is what you'd expect from the resistance values used.

Let's make the series resistance smaller - 470R, and see what happens.  The ratio of the total resistance (10k+470R) to the lowest resistance (470R) is just over 22:1, and the range of rolloffs corresponds to that, going from a low of 1520hz to a high of 33860hz.

Let's move the cap value up a bit to .022uf (likely to be in most parts bins).  Our rolloff range now goes from 492hz to 15392hz.  the same overall ratio of range, just shifted downwards.

Use of different tapers will allow which portion of that range corresponds to which part of the pot's rotation to be varief.  You may not actually need all of that range (e.g., the part from 10khz to 15.4khz).  So consider using either a non-linear taper to "compress" that range into a few degrees of rotation, or creating a custom taper via parallel resistors (see "The Secret Life of Pots" over at GEOFEX) to produce a taper with the right amopunt of control for you.

[midwayfair]

There are two challenges to be confronted here: pot taper, and width/spread of range.  for the "classic" SWTC, you are basically constructing a single-pole LP filter whose highest and lowest rolloff points are dictated by the value of not just the cap, but the series resistor just before the Tone pot, and the value of the tone pot itself.

Example: We have a 1k resistor going to a 10k linear pot, and a .01uf cap to ground from the wiper.

This lets us produce a lowpass filter formed by a .01uf cap and as little as a 1k resistance, or as much as an 11k (10k+1k) resistance.  With minimum resistance, the rolloff starts around 15.9khz.  With max resistance, the rolloff starts around 1.45khz.  In essence, an 11:1 ratio, which is what you'd expect from the resistance values used.

Let's make the series resistance smaller - 470R, and see what happens.  The ratio of the total resistance (10k+470R) to the lowest resistance (470R) is just over 22:1, and the range of rolloffs corresponds to that, going from a low of 1520hz to a high of 33860hz.

Let's move the cap value up a bit to .022uf (likely to be in most parts bins).  Our rolloff range now goes from 492hz to 15392hz.  the same overall ratio of range, just shifted downwards.

Use of different tapers will allow which portion of that range corresponds to which part of the pot's rotation to be varief.  You may not actually need all of that range (e.g., the part from 10khz to 15.4khz).  So consider using either a non-linear taper to "compress" that range into a few degrees of rotation, or creating a custom taper via parallel resistors (see "The Secret Life of Pots" over at GEOFEX) to produce a taper with the right amopunt of control for you.

Thanks, Mark. I'll experiment on the breadboard with the ratios.