The resistors in BMP tone circuit

[helloesposito]

http://www.beavisaudio.com/techpages/BigMuffToneControl/

For those unfamiliar ^^^^

I built this into a fuzz I made and realized it worked great but noticed a loss in volume due to passive parts. I built another with a recovery stage so it's no problem now but I was just curious if anyone can explain to me the point of the resistors in their respective places. With the changing of the capacitors comes the change in tone, but would the resistors values change depending on the circuits use (like using it in a fuzz and then using it in a standard overdrive.)

Just from assuming, which I shouldn't do  , I'm guessing the resistor going into the lower cap is to limit a volume of the low end before filtering. Am I way off? 

[GGBB]

That is part of it, but they do more. Think of the pot as a blend control. Remove the pot and you have a signal that is split into two. Each leg has both a resistor and a capacitor, so they are both RC filters. One is configured as high-pass, the other low-pass and are connected to either end of the pot accordingly. In an RC filter, both the capacitor and the resistor affect the cut-off frequency. So the resistors not only affect the relative level of the filtered signal, but also affect the frequency range.

[rocket8810]

If the volume loss is that noticable you can easily correct that by installing a small boost after the BMP tonestack, and have the output of the boost as an internal trimmer to bring the level back up to where it was before the tonestack was added. This would make the boost essentially be a gain recovery stage, just like in the BMP.

[Mark Hammer]

The BMP tone circuit is essentially two filters that the control pans between.  On one side is a highpass filter (series cap, resistor to ground).  On the other side is a lowpass section (series resistor, cap to ground).

The corner frequency of each section is set by the joint action of the cap and resistor values, according to the formula Freq = 1 / (2 * pi * C * R)

Change the resistance values, and you change where the corner frequency is.  Same way that if you change the cap values you change the corner frequency.


In theory, you could achieve the same tone response by changing the component values.  So replacing the .004 (which would be .0039 using modern values) and 22k with .0082 and 12k, would get you close to the original response, as would .0033 and 27k.

[GibsonGM]

Wouldn't scaling those resistances down also load the previous stage more, tho, Mark?   And possibly result in more of the same complaint of loss of signal?   Just a thought.   I'd have to 'sim' it to see what the result of that would be.    No free lunch and all that...

I'd suggest simply adding the BMP recovery stage after the tone stack...bing, bang, done in one....

[helloesposito]

Thank you all for the info!

I actually already fixed it in the circuit I did, I'm just trying to learn more about how everything works still. I've been reading a lot, watching tutorials, but I just couldn't find anything about the tone circuit. So again, thanks! I'm getting a better grasp of it now 

[Mark Hammer]

Wouldn't scaling those resistances down also load the previous stage more, tho, Mark?   And possibly result in more of the same complaint of loss of signal?   Just a thought.   I'd have to 'sim' it to see what the result of that would be.    No free lunch and all that...

I'd suggest simply adding the BMP recovery stage after the tone stack...bing, bang, done in one....

Yeah, you're probably right....sadly.

That said, people CAN feel free to experiment with valuies and substitutions. Just remember to pay for your lunch when finished! 

[GibsonGM]

As long as you pay, experiment away!!      Maybe you'd get loading, and then decide to reduce the cap values or something. 
We all know that playing 'modder' means having to make compromises!    The loading effect might also be NICE, if there were some shrill things going on before it and you still had enough level (?)

Hey Brian - have you discovered Duncan's Tone Stack calculator (free, search net) and/or LT Spice (also free) yet?   You can play with tone stacks like the BMP to your heart's content and dig right in there, see what changing things does!   IMO you're doing it the best way tho, by DOING - and hearing!   A few years back that one really bothered me when I couldn't figure out what the 'opposite parts' did, ha ha...

[rocket8810]

Can't agree more. I forgot that by changing values you can alter the load on the previous stage. I'm glad you mentioned that.

[Transmogrifox]

 IMO you're doing it the best way tho, by DOING - and hearing!   

I second that!  I have incidentally been playing with this in LTSpice a lot recently watching the effects of input and output loading on the frequency response.

However, I am finding that what appear to be minor changes on a frequency plot are more than subtle to my ears. 

It's interesting to correlate the "scientific" data with what the ears perceive, but it really takes the time with the breadboard, guitar and amp to understand what happens.

With the BMP tonestack, the main things to keep in the back of your mind are:
On the "treble" side (series capacitor), the smaller cap will mean higher frequencies pass through. 
On  the "bass" side, larger cap means this path filters off high freqs at lower frequency.

Little cap on treble and big cap on bass leaves a mid scoop.  If you make the R's and C's equal on both sides, you get a flat response centered at 1/(2*pi*R*C), but the "R" in the circuit is not so obvious.

The "R" in the circuit is a combination of input and output impedances in parallel/series networks.

Then when you start messing with resistor values, you can change the relative balance (max gain on highs vs max gain on lows).  This is where something like LTSpice can help.  It gives some understanding about what effect a certain type of change will have.

That way if you want "less mud", you will know how to change R's and C's to notch out the mud.

Likewise if you want "more body" you will find out what changes will do that.

If the nice "center" balance seems to be at 70% on the tone knob, then you can discover how to change the relative resistor values to more heavily weight one side or the other to balance it in the middle.

[helloesposito]

Might start a new thread about this, but maybe some of you guys will see this:

When pedals have a eq boost, like a high boost, does the signal still split in two and is something like a spdt used to change the direction, just so it runs into one resistor and one cap, then meet where the unaffected signal would have?

Hope that's clear 

[antonis]

It depends on the EQ circuit..

Some raise the gain of the specific frequency band...  Others attenuate it less than the other frequencies...

If you spilt the signal and then mix the unaffected one with the other (passed through a passive filter) then you "boost" the clean signal...
(unless the filtered signal has it's own gain - active filter..)