Tone Stacks - Part 1a - Passive Filters

[Chuck D. Bones]

Tone stacks show up in the vast majority of pedals and amps. This article will discuss at a top level what they do, how they do it, how to design them and how to mod them. You'll want to have Duncan's Tone Stack Calculator (TSC) handy so you can follow along. Hardcore enthusiasts will want to run some LTSpice simulations. Good news everyone! There won't be any math!

Low-Pass Filter
The simplest tone stack is a low-pass filter. One way to implement it is how it's done in the Rat. The FILTER control varies the corner freq and R7 sets an upper bound to the corner freq.

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Each line in the plot below corresponds to a setting of the FILTER knob from 0 to 10.



We can easily retune this filter by changing C8. Making C8 larger lowers the corner frequencies, allowing darker settings.

Another version of the low-pass filter is used in the Boss Blues Driver, Marshall BluesBreaker, M.I. Audio Crunch Box and some of the Cornish pedals. This is called a shelving filter; take a look at the curves below and you'll see why. The corner freq is constant, but the amount of cut is variable. This is the Cornish G-2 tone stack:

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We can cajole TSC into simulating this tone stack by removing some parts from the Big Muff tone stack. I deleted R1 and R2 by making them very large and effectively jumpered C1 by making it very large.



This filter can be retuned by changing C2 (in the TSC schematic). Larger darkens the filter and smaller makes it brighter. Changing the TONE pot to A-taper will make the bottom end of rotation less touchy.

Big Muff
The Big Muff tone stack shows up in more pedals than any other tone stack. It's a simple and effective combination TREBLE - BASS control. In its earlier incarnations, it had a mid notch that caused some users, myself included, to dislike it because we couldn't find a satisfactory setting. Here it is with the original component values and the dreaded mid notch. It's basically a high-pass filter (C1 & R2) and a low-pass filter (C2 & R1) in parallel with a blend control.



Boutique pedal builders will replace R2 with a pot or add switches to change the capacitors so that users can adjust the high-pass and/or low-pass corner frequencies. If the two corner frequencies are made equal, then the notch disappears.



C1 & R2 affect the high-pass side of the filter. Making R2 smaller increases the bass cut. Making either or both C1 and R2 smaller moves the high-pass corner freq up, which deepens the mid notch and makes the tone hotter above noon. C2 & R1 set the low-pass corner freq. Making either or both smaller moves the corner freq up, increasing the midrange content. It's possible to create a mid hump by making the low-pass corner freq higher than the high-pass corner freq.



A variant on the Big Muff tone stack removes R2. The result is less bass cut when TONE is set above noon. The notch can be deepened or eliminated by varying the capacitor values. I use this in several of my pedal designs.



I'm almost at the limit for # of attached files, so on to part 1b...

 

[cooder]

Sweet walk through, on point as usual! Cheers!

 

[jubal81]

Great stuff. I literally smacked my face reading that bit about flattening the Muff stack by making the values match. It's so obvious when you put it that way, but I never thought about it like that before.

 

[cooder]

We defintely need a big as facepalm emoji....

 

[zakco]

Thank for this very helpful tutorial!
Beginner question...if I were to use the first example (LPF), adding it to the output of a Rangemaster circuit, would this simply replace the 10n output cap?

 

[Chuck D. Bones]

No, we would have to keep the coupling cap because we need the coupling cap to block DC. Also, we would need to either install a buffer before the LPF or increase the impedance of the LPF because otherwise the LPF would load the output of the Rangemaster. These examples are not stand-alone functional blocks. We need to take into consideration their interaction with the surrounding circuitry.

 

[zakco]

No, we would have to keep the coupling cap because we need the coupling cap to block DC. Also, we would need to either install a buffer before the LPF or increase the impedance of the LPF because otherwise the LPF would load the output of the Rangemaster. These examples are not stand-alone functional blocks. We need to take into consideration their interaction with the surrounding circuitry.

Ah, ok. I was thinking that might be the case. Which approach would be preferable? I’d like to learn how to try this on a breadboard.

 

[Chuck D. Bones]

Go with the buffer. Use an emitter follower or opamp. Keep in mind that the example LPF came from the Rat, and in that circuit the LPF was followed by a buffer.

 

[zakco]

Go with the buffer. Use an emitter follower or opamp. Keep in mind that the example LPF came from the Rat, and in that circuit the LPF was followed by a buffer.

Thank you. I tried the first two examples (LPF and High Shelf) without a buffer and they work pretty well but as you predicted, there is a small signal loss even when the filter is at it's minimum setting. I noticed that you suggested putting the buffer before the tone stack while the RAT circuit places the buffer after the tone stack. I'm curious what it is about the rangemaster vs the RAT that requires the buffer to be before the tone stack instead of after it?

If this thread is not an appropriate place for this digression, please let me know and I will create a new thread.

 

[Chuck D. Bones]

One could make a case for putting buffers in both places. The Rat does not need a buffer before the TONE control because the impedance at the hard clippers is sufficiently low. That is not the case with the Rangemaster, which is why there was some noticeable signal loss. Not only that, but the Rangemaster's output impedance is part of the filter circuit.