Integral Preamp - Modifying the EQ bands?

[Caldo71]

Hey Folks,

So I just finished this fun-and-easy-build, and it sounds great, but it got me thinking about messing around with what frequencies the "bass" and "treble" knobs actually affect.

So much so, that I socketed some of the resistors and capacitors that I THINK are responsible for tonal response, so that I might swap them out for components of different values once I know how to calculate all that—hence my forthcoming question.

Now, I am still very much a novice "schematic reader", but if I understand correctly, these two capacitors and one resistor per EQ knob that I've pointed out in the schematic below are the components that make the magic...



...so those are the ones I socketed.

So here is my question (finally): is there some kind of online calculator or other handy way of easily figuring out how swapping out these resistors and caps will alter the EQ frequency?

I know there's lots of talk about THIS particular online gyrator calculator being handed out on this and other forums...

Guitar Pedals: Gyrator Filter Calculator

Calculate freuqency cutoff for guitar effects pedals with vacuum tubes and solid state electronics. FREE calculator.

www.muzique.com

...and the same guy publishes THIS cutoff frequency one...

Guitar Pedals: R-C Filter Calculator

Calculate frequency cutoff for guitar effects pedals with vacuum tubes and solid state electronics. FREE calculator.

www.muzique.com

...but it seems to me that those aren't quite right, given the fact that the Integral preamp uses ONE resistor and two caps, rather than the "two-and-two" or "one-and-one" that those online calculators utilize? If I am even reading the schematic properly in the first place. OR, am I supposed to actually be factoring in the potentiometers as the second resistor, hence making the gyrator calculator relevant after all?

Any insights that could set me on the right path to being able to do these kinds of mods would be hugely appreciated!

 

[Passinwind]

LTspice is great for modeling stuff like this.

 

[Chuck D. Bones]

R7 sets the limit of how much treble boost you get with TREBLE dimed.
R8 sets the limit of how much bass cut you get with BASS at zero.
C4 & C5 set the freq where the TREBLE control kicks in. Typically, they are 10:1, but don't have to be.
C7 & C8 do a similar thing for the BASS. R9 also affects the bass roll-off freq, but to a lesser extent.

Like Passinwind says, LTSpice will be very helpful in picking values.

If you're not up for running sims, then try subbing both treble caps at the same time and go in 2x steps at first. Bigger treble caps moves the TREBLE control range down into the midrange. The try the same thing with the bass caps. Smaller caps moves the BASS control range up into the midrange.