Adding a LPF to a rangemaster circuit

After much fiddling, confusion and disappointment, I stumbled across the idea of using a capacitor to ground via an on/off switch at the output of C4. After trying a few different values, I settled on 3n3, which perfectly rounds off the highest, unwanted frequencies without losing much level or making the circuit sound dark or veiled. I'm super happy with this very simple solution!

I'm have a question about this: C2, C7 and C4 work as high pass filters and I understand how changing the values varies the corner frequency. The 3.3n capacitor (to ground) that I've added after C4 works in the opposite way, acting as a LPF. Could someone please explain (or point me in the right direction) how/why it works this way? Does adding a cap (of roughly this this value range) to ground at the output of a pedal circuit always result in a LPF? Is there a formula or calculator that would show how the value relates to the corner frequency?
 

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Does it get brighter as you turn boost down?
Boost pot an the 13.3n of C4 and the added cap form a LPF.
RC with cap to geound will be a LPF
CR with R to ground will be a. HPF

Corner feeq =1/2(3.14)RC, regardless of orientation.
I like the Caltronics App. Electrodoc is good as well.
 
Does it get brighter as you turn boost down?
It doesn't get brighter as in acting in reverse to the high cut, but listening carefully, I'd say that the effect is less noticeable. The extra harmonics from the boost knob cranked make the high cut more pronounced. I think this non-linearity is actually a benefit in practice.

Boost pot an the 13.3n of C4 and the added cap form a LPF.
By 13.3n, I assume you referring to the combined values of C4. Does the 2nd cap going to ground mean that the 2 caps are considered to be in parallel? (even though C4 doesn't directly connect to ground)?


RC with cap to geound will be a LPF
CR with R to ground will be a. HPF
Very helpful thanks!

Corner freq =1/2(3.14)RC, regardless of orientation.
I like the Caltronics App. Electrodoc is good as well.

In this equation, how exactly is "RC" calculated? (sorry for my very beginner questions!)
 
Wait. No. Yeah. Sorry. Been in the sun all day.
Series so 10n +3.3n is ~2.4n so the 10k pot sweeps from 64.6k to 6.4khz.(At 1k on the pot).
So yeah. There should be a noticeable difference in shrill frq reduction with boost pot maxed .
But I'm not sure how the reactance then applies to R4. Would be something to sim. I'm fairly certain there's something I'm forgetting here.

There are plenty of good write ups of filters all over the web. for pedal specific stuff, see this thread
 
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