How to calculate the frequency cut off on this tone filter on an NPN Fuzz Face

[bushidov]

Hi Guys,

I asked this in an earlier post, but got into a tangent and never really got to the bottom of this. So I have this schematic:



I've built it onto a PCB and it works perfectly fine. I know the gist of how everything is working except for the tone control.

What I do know is that when the tone is turned all the way counter clock wise, the highs are cut out (or the bass is let in) and the schematic would look like this:



If the tone pot is fully set to clockwise, I get a brighter tone (highs are either brought in or the bass is cut out) and the schematic would look like this:



I know the general rules of calculating the frequency cut by doing 1 / (2 x Pi x R x C), I can guess that C4 and C5 are the value of C, but what is R?

[antonis]

From Q2 Drain point of view, R5 is always in series with whatever comes next..

So, on R5/R4/C3/Tone node there is a voltage divider, partially depended on VR3 setting..
(although, your shemes concering CW & CCW pot settings are mismached..) 

Your tonestack forms together with R5 a 2-pole LPF which only cuts treble..(thanks to Leo..)
Final HPF is formed by C3 & Vol pot setting
(negleting MosFets diode body resistance..)

[nocentelli]

Your tonestack forms together with R5 a 2-pole LPF which only cuts bass..

LPF, low pass filter, allows low end (bass) through and will only cut treble

[antonis]

Eagle eye, Leo...!! 

[j_flanders]

Falstad interactive, online simulator with the schematic from the screenshot:
https://tinyurl.com/y426p2rn

A larger pot (1k - 2,5k log instead of 500Ohm) for the tone control seems to give more range, or at least gives the option of a flat frequency response when it's maxed.

[Rob Strand]

Here's some rough calculations.



Where you see 2*847 that means the pot set at 847 in series with Ro.
Where you see 847/2 that means the pot set at 847 in parallel with Ro.

Unfortunately it would take a while to explain in fine detail.

After simulating the circuit, the 11kHz frequency "f7' looks like it stays at around the f6 mark.  That happens when the approximations aren't quite valid.

[Edit: there's also a mistake.   Ro should not include the 5k6 for that circuit.  So it's just 1k // 500k]

[ElectricDruid]

I'm with j_flanders on this one. Sim for the win.

You can sometimes get a broad overview of what to expect by thinking about it a bit, but for the detail, putting it into a sim beats working it out longhand every time!

[bushidov]

Thanks everyone. I learned a ton from this. I like the simulator and have bookmarked that. I also liked the maths from Rob Strand, as that gave me something to look at. I did recalculate the RO, nixing the R5 from the parallel resistances. Thanks Leo for pointing it out as a low-pass filter, as I figured from my ears that was the case, but wasn't sure from a schematic standpoint. And thanks Antonis for correcting me on the tone knob schematics. That was me just being a bone-head.