TONE in Drive Pedals - It's In The CIRCUIT! | Too Afraid To Ask

There’s never just one way to implement tone…

Recently a question popped up in the CSGuitars Discord which asked about tone controls on drive pedals: What are they doing? Cutting treble? Boosting Bass? Or something else entirely?

An excellent question! Understanding what’s going on in a circuit as we turn the knobs can assist in using our equipment far more effectively and eliminates the frustration when we can’t get it to do what we think it should.

For the video above I gathered together 4 iconic drive pedals - ProCo RAT, EHX Little Big Muff, RYRA The Klone (Klon Centaur clone), and Ibanez Tube Screamer - which each implement their tone control in a different way. This is by no means an exhaustive list of tone circuits, but it should demonstrate some of the most common methods used in pedal tone controls.

The most simple of our 4 is the RAT. It features a basic low pass filter which bleeds treble out of the signal beginning at a frequency decided by the position of the tone knob.

Using the oscilloscope we can perform Bode plots to see the frequency response of the pedal at various stages of the control. As the knob is turned clockwise more and more high frequencies disappear from the plot.
This is the same tone arrangement we find inside our guitars - passive low pass filter which removes treble from the signal.

Next up is the Little Big Muff - this fuzz pedal also features a passive filter based tone control, but this time there are two different filters connected to either end of the potentiometer.

The lower filter is a low pass - just like we saw with the RAT - removing high end above 408Hz.
The upper filter is high pass, it allows all frequencies above the threshold dictated by the component values to pass, and instead attenuates low end below 1.8KHz.

The tone knob blends between these two filters - fully anticlockwise only the lower filter takes effect, fully clockwise only the upper filter takes effect, and in the middle where the two filters meet a mid scoop is formed at around 1kHz.

This gives a lot of adjustability to a single control, but keeps the circuit simple without any need for active components.

To go beyond attenuation we need to start using active circuits with powered components than can amplify frequencies.

The Klon Centaur is a pedal wrapped in myth, magic, and mojo - however the reality is a lot more scientific and quantifiable. I’m using a RYRA recreation in this example as it is an accurate analog for the rare and expensive original.
This circuit features a TL072 opamp which amplifies everything above 400Hz, with the tone control acting as a gain control of the treble boosting.

The treble boost provides up to 18dB more high end at the highest extreme of the control, added to the enormous headroom of the pedal which can cleanly crank the signal level to amplitudes other pedals can only dream of, the Klon becomes an extremely useful effect for turning a dark, vintage amp into a ferocious high gain monster.

Finally we come to our good friend - the Ibanez Tube Screamer.

This is combines a couple of passive filters with an active circuit to control how much treble above 720Hz gets pumped to the pedal output.
The tone knob sits within the opamp feedback network and behaves in a similar fashion to the presence control on an amplifier; bleeding treble out of the feedback loop in order to allow high frequency to be amplified untouched.

This is a very elegant solution to a tone control, keeping the whole control within a useable range - never becoming too dark or too bright - and shifts the mid peak of the effect into whatever range suits best the band mix.
It’s no wonder the Screamer, or some variant of it, is on almost every guitarist’s pedalboard.