Stupidly Wonderful Tone Control: cap value?

[Bucksears]

In the diagram for the SWTC, what is the common value for capacitor 'C'? .0033uF? Mark sorta recommended R=1k, P1=10k and P2=50k when using this circuit with the MXR Dist+, but I'm curious about the cap.
What's in YER circuit?

- Buck

[Mark Hammer]

In the world of guitars, you've more or less achieved maximum brightness once any filtering rises above 7khz or so, simply because the average 10-12" speaker does not crank out nearly as much above that range.  Conversely, once you start rolling treble off around 500hz or so, you are having an effect on the harmonic content of many if not most of the notes played on guitar.

So, to calculate a suitable cap for the SWTC, you need to plug those approximate ranges into the equation along with the value of the tone pot you want to use and the fixed series resistor.  There should be a fixed resistor in series with the input side of the tone pot such that there is an identifiable upper treble rolloff point.  Also note that the tone pot value selected should be such that it does not dramatically attenuate the signal in conjunction with the volumepot.  For example, if you had a 100k tone pot in series with the input lug of a 100k volume pot, you have already chopped the output level by half.  Some pedals have enough gain build into them (e.g., any of Joe Davisson's molten lava offerings) that you can manage that attenuation, but others (e.g., a Distortion+) don't have enough signal coming out of them that you want to  risk losing that much.  So, for high gain, you can likely manage a 100k tone pot with a 100k, 250k, or 500k output pot.  For moderate gain I personally wouldn't want to go higher than a 4 or 5:1 ratio.  So, a 10k tone with a 50k volume, or 25k tone with a 100k volume.  The fixed resistor in series should not be any more than 5-10% the value of the tone pot (i.e., 1k with a 10k pot)

Okay, calculating cap value.

Assuming we had a monster output and a 500k volume pot, lets go for a 100k tone pot with a 4k7 fixed resistor in series.  Max resistance from circuit side of the 4k7 to tone-pot wiper will be 104k7.  Using F = 1 /[2*pi*R*C] we see that a .01uf cap to ground will produce a treble cut starting around 150hz at max cut, and a treble rolloff at 3.4khz at full treble.  Too low.  Let's chop that cap value down to .0047uf.  That gets us a lowest rolloff around 320hz, and a highest rolloff at 7.2khz.  Halfway there.  Drop the cap value a little more to the next standard value of 3900pf and we end up with upper and lower points of 8.7khz and 390hz.  Not bad.  Remember that even though 390hz sounds really low, it,s a 6db/oct rolloff, which isn't very steep.  There will still be plenty of signal in the 1khz zone, just not as much.

I hope that example gives you a workable heuristic.

[Ucho]

Mr Hammer's SWTC is a ... ehm ... wonderful but still very simple tone shaping control that works very good and, as He said, without interacting with volume (apart form the obvious psycho-acoustic effect of 'darker' sound having less volume than 'trebly' ones). There's some magic in simple ideas that work that good... 

By the way, If you don't feel you want to do the math, you can use the RC Filter Cutoff Calculator at AMZ.

I'd like to share an other example.

Using a 100k output pot, 22k tone, 1k R1, and 22nF cap.
Using a 22k tone with a 100k volume you should get about 80% volume (20% volume drop, more like 18% to be exact, if i am not wrong)
At minimum resistance ( 1k (R1) + 0k (P1 'left side') = 1k ) you get a cutoff at about 7.2kHz
At max res ( 1k (R1) + 22k (P1) = 23k ) the cutoff is at 314Hz

Yet another example:
Using a 100k output pot, 10k tone, 680R R1, and 33nF cap.
Using a 10k tone with a 100k volume you should get about 9% volume loss [ (10k + 680R) / (10k + 680R + 100k) ]
Min ( 680R ) cutoff is about 7kHz
Max ( 10680R ) cutoff is about 450Hz

Have fun (and please excuse me if my examples are wrong  )

[Bucksears]

Thanks guys.
I'm going to start with 10k tone and 50k volume with a .0033uF cap and go from there.

- Buck

[Mark Hammer]

That may end up being much subtler than a tone control needs to be, but let your ears tell you.  Thankfully, it is a change that is easy and painless to make.