Help SWTC - I stink at math!

[Der Groovemeister]

Can anybody help me to calculate the SWTC for a 386 based distortion pedal? The first resistor is way higher than given in any example, 220k (already in the schematic) and the vol pot is 100K and I stink at math!
http://hammer.ampage.org/files/SWTC.gif
http://www.diystompboxes.com/smfforum/index.php?topic=60157.0

Thanks very much in advance.

[Derringer]

I honestly wouldn't worry so much about the math and would just start with a 1K resistor, 10K pot and a 0.01uf cap.
Use your ear.
If it's still too bright, increase the cap size
If it's too muddy, decrease the cap size.

You'll probably be just fine with the 10K pot.

the equation for the frequency roll off is 1/(2piRC)
and you can just use this boxed up calculator
http://www.muzique.com/schem/filter.htm
to see exactly what the rolloff will be

for instance
1K and thr 10K pot at max treble attenuation=11,000 ohms with a 0.01uf Cap rolls of the high's starting at about 1.4kHz
at the least roll-off it's at about 15.9kHz.

don't mess with the pot or the 1K resistor, just play with the capacitor for right now

[Der Groovemeister]

I did just that, but it doesnt do anything. Probably because the last component of the schematic is a 220K resistor. And what corner frequency is best suited for guitar?

[slacker]

If the 220k resistor is in place of "R" in Mark's schematic then if you just used a 10k pot for P1 that would only give you a very small range of adjustment, which is probably why it doesn't sound like the control is doing anything.

I would either just remove the 220k resistor and start with the values Mark gave or post a schematic of what you're building so we can see what the 220k resistor is there for.

[Derringer]

ahh

yeah, can we see a schem of the circuit you're using ?

[Der Groovemeister]

I built this 386 based distortion from the layout on page 9 in this thread:
http://www.diystompboxes.com/smfforum/index.php?topic=86389.160 (link is to page 9 already)
There is no final schematic posted in the thread. But the last res. is there to take the gain down to acceptable level for the input of an amp. I changed it from 1M to 220K to have enough gain to drive a tone stack.
I know it's hard to discuss without a schematic...
Thanks in advance!

[slacker]

Right so the 220k acts like a voltage divider with the volume pot to reduce the volume.
You could try something like this then, replace the volume pot with a 50k, make R2 a 1k resistor, then use a 100k tone pot and something like 10n for the cap. That will keep the volume about the same as with the 200k resistor, you can change the resistor and cap to get the sounds you want.
Or you can remove R2 and use the values in Marks schematic as a starting point, then put an extra resistor between the tone pot and the volume control to drop the volume to what you want.

[Der Groovemeister]

Thanks very much! I'll try that!

[Gus]

I looked at
http://hammer.ampage.org/files/SWTC.gif

Two things that seem to be missing
One you need to know the output resistance of the circuit before it
Two you need to know the input resistance of the circuit after it

The tone control can load the circuit before and the circuit after can also change how the volume works

One could use a Jfet input dual opamp setup as two buffers with 1meg input resistance and place a buffer before and after to make this a "standalone" circuit

[Mark Hammer]

First, the advice offered so far is spot on. 

Second, what I provided is more of a strategy than a fixed set of values.  The "goal" is to identify a tone pot value (from among standard values available) that does not reduce the maximum output volume appreciably.  A person could use a 500k tone pot in conjunction with a 10k volume pot after it, but you wouldn't really hear much.  A 1k tone pot with a 500k volume pot would not reduce output level very much, but it probably would not provide a desirable range of tone control either.

Once you identify an appropriate tone pot value for your context, you then have to figure out what sort of rolloff frequencies you want to be able to achieve.  That involves establishing two pieces of information: how low do you want it to go, and how high?  Once you know that, you identify a suitable common value of cap and fixed resistor.  The fixed resistor will set the highest rolloff permitted, and will also set the "ratio" of tone control range. 

So, if the tone pot were 5k, and the fixed resistor 1k, then the control would vary frequency over a 6:1 range.  For example, 1k, fixed, 5k pot, and a .022uf cap gives a rolloff ranging from 1.2khz to 7.2khz.  Make that tone pot 10k, and the range extends from 660hz up to 7.2khz, or roughly an 11:1 ratio. 

Quite honestly, there is no reason to need to extend the rolloff of a distortion pedal higher than around 6-7khz, simply because:
a) 10-12" speakers don't go much beyond that in terms of their own bandwidth
b) the rolloff itself is quite shallow so plenty of treble will still be getting through
c) it's often a good idea in any high gain circuit to filter out any acquired noise as low in the spectrum as you can

Indeed, yu probably wouldn't feel any significant "bite" was missing from your pedal if you set the highest rolloff at 4khz

As for how low, I think you'll find that there isn't much point in going below 400hz.  The lowpass section on a Big Muff's tone circuit is set at around 400hz.

So, whatever set of values gets you a 400-4khz rolloff range will probably provide for sufficient tailoring.

[Der Groovemeister]

Thanks everybody for the info! I will fiddle around with some values.

[Der Groovemeister]

I get it now. Installed and working great! I removed the 220K res from the end of the circuit and put a 1k, 10K pot, 22n cap, then the 220K res and then a 50K vol pot. This is a great tonecontrol! Thanks Mark and all who replied for your help.