Newbie tone control how-to, question...

[micro]

So let me see if I understand this correctly. I can add a tone control to any circuit, lets say the Stupidly Wonderful Tone Control, by taking the output of my circuit direct to the first component in the schem, then out of the tone  control (vol pot?) to the output jack?
On this tone control,  are both "R's" potentiometers? Is one tone and one volume? So if I want to add this to a circuit that already has a Vol control then I just omit the  original?

So many questions...

[Mark Hammer]

Look here for an expanded discussion: http://www.muzique.com/lab/swtc.htm

The SWTC provides a variable lowpass filter.  That is, the point where the high frequencies gets rolled off changes.  This is different from your guitar's tone control, where the point of rolloff remains the same but how much gets rolled off at that point changes.

The rolloff point is set by the resistance in series with the signal, and the cap that goes to ground after it.  In the initial AMZ illustration, this is R1 plus whatever portion of R2 is on that same side of the wiper.  R1 is there to set an upper limit to where the filtering takes place and can be any value you find suitable.  A 10:1 filter sweep (e.g., from 6khz down 600hz) will result from having it around 1/10 the value of the pot.  That arrangement is also used in the Proco Rat to filter out high end.  Unfortunately, as that combined resistance gets larger, it interacts with the volume pot after it and results in much less output volume.   The SWTC has the nice feature in that the overall resistance in series with the volume pot never changes.  What changes is how much of that overall resistance sits before the cap andis used with the cap to roll off high end.

Now, the SWTC does have the effect of reducing overall output level.  If you had a 100k SWTC tone pot and a 100k volume pot, your maximum possible output would only be half of what it would be using just the 100k volume pot.  The practical advantage of the SWTC is that the amount of volume reduction is fixed and will not change depending on your tone setting.  It's a convenience thing, not a question of  having more or more sophisticated control over tone.

If you want to add a tone control in the manner shown, you need to consider how potent the existing signal is, what output level you want, and what degree of tonal control you want.  So, if you had a stock Distortion+, you would want a tone pot MUCH lower in value than the volume pot, since the output level on those is not spectacular.  Ideal might be a 5k-10k tone pot with a 50k volume pot or perhaps a 10k-25k tone pot with a 100k volume pot, and even there some might find that additional tone pot detracting too much from the output level.  If you had something like a Red Llama or one of Joe Davisson's fire-breathing high-gain circuits, you could probably afford to use a tone pot equal in value to the existing volume pot because you have enough signal to spare that chopping the maximum value in half poses no hardship.

One needs to work out the math to make the right choices.  So, click on your scientific calculator in Widows (calculator -> view -> scientific) and work it out using 2 * pi * C (in uf) * R (in megohms), then hit the 1/x key. 

Let's say we want to tack it on to a DOD250.  You will note that this pedal already has an RC filter (see R9 and C7 here: http://www.generalguitargadgets.com/diagrams/dist_250_sc.gif ) providing a rolloff at 15.9khz, well above what your speakers normally handle, and really just there to keep out the radio noises.  R9 already interacts with the volume pot (R10) to trim back the maximum level a bit, though not as much as with a 50k output pot.  To add an SWTC, you would take the wire that normally leaves the circuit board to go to the volume pot and route it to a fixed resistor then the added tone pot.  It doesn't really matter which of the outside lugs; either will work, just in opposite directions.  Now run a wire from the remaining outside lug to the input of the volume pot.  If your tone pot's chassis is going to be electronically grounded to the overall stompbox, then you can simply solder a cap between the wiper (centre lug) and the pot chassis, and you're done.

Let's assume we want to maintain as much potential output level as possible, so we'll stick with a 10k tone pot.  If we stuck an additional 1k resistor in series after R9 and then our 10k pot, we will have added 11k resistance overall.  R9 drops the output level down a bit, and our two extra resistances a bit more, but it is still much less than half the value of the output pot, so we're okay.

Assuming a maximum tone control resistance of 11k, what sort of cap would provide a reasonable cut?  A .01uf cap would give you a rolloff  starting at 1446hz at its lowest and a rolloff around 15.9khz at its highest.  Not bad.  Let's say we made the additional fixed resistor 2k2.  That adds 12.2k of resistance beyond R9 which doesn't provide that much more obverall output attenuation.  With a .01uf cap again, our control range would now be 7.2khz at max treble, and 1.3khz at min treble.  You can see that be changing the fixed resistor, the overall range of control is both a bit lower but also narrower. 

That could be a good or bad thing depending on your preference.  The original range is good because it allows you to essentially restore the stock tone where there is no practical restriction on treble....assuming you like it.  The second arrangement allows for a bit more treble taming, but makes the brightest setting just a tad warmer than stock, which some may like.  The nice thing is you can play with the component values and math to settle on something you like.

Although Jack doesn't mention it, I will note in passing that if you set it up to still have an audible rolloff, even in max treble position, you can tame some of the hiss that normally occurs when distortion controls are cranked.

[micro]

Please tell me you had this explanation somewhere on your computer and you just copied and pasted it! Thank you for your time and
effort in answering this question. I totally needed it broken down! I'm sure this will help other beginners in the future who are looking for info on tone control. You rock!

[Mark Hammer]

I try to rock so others can rock even harder!

Glad my ramblings made helped you.  Do make a point of reading Jack's SWTC2 article, and some of his other articles on tone controls.  I'm sure it will make more sense to you now.