Tone Stacks Demystified
Passive vs active tone stacks in guitar pedals: how the Big Muff scoop works, where the midrange goes, and when a gyrator-based active EQ makes sense.
Tone controls are filters, not magic
Every 'tone' knob on every guitar pedal is just a filter you can adjust with a pot. The reason a Big Muff sounds different from a Tube Screamer at the same gain setting is mostly the tone stack — the network of resistors, caps, and pots after the clipping stage that decides which frequencies survive to the output jack.
Understand four filter shapes — low-pass, high-pass, band-pass, and notch — and you understand every tone control ever made.
The single-knob passive tone control
The simplest pedal tone knob is one pot and one cap. The pot acts as a variable resistor in series with a cap to ground. As you turn it down, more highs are shunted to ground — voice gets darker.
This is what the Rat's 'Filter' knob does (with the polarity flipped — turning it down adds treble). It's a one-pole low-pass filter with a corner frequency you can sweep with the pot. Cheap, effective, and the reason a lot of cheap pedals sound like a Rat with the lid taped on.
The Big Muff stack and where the mids go
The Big Muff's tone control is the famous one: two filters sum into a single pot. One side is a low-pass to ground (a cap and resistor that pass bass), the other side is a high-pass to ground (a cap and resistor that pass treble). Both halves meet at the wiper of the tone pot, and you blend between them.
In the middle position, both halves are summing — but they're 180° out of phase right where they cross over, around 1 kHz. That's the legendary Big Muff midrange scoop. Want less scoop? Make the low-pass and high-pass corners closer together. Want it to sound like a Triangle Muff? Raise the low-pass cap.
When passive isn't enough
Passive tone stacks can only cut frequencies, never boost them. If you turn 'treble' up, what's really happening is you're cutting the bass — the treble was always there, you're just removing what surrounded it. That's fine for distortion pedals (you have plenty of gain already), but it's limiting when you want a flat-then-boosted EQ.
That's where active tone stacks come in. They use op-amps and gyrators (an op-amp circuit that emulates an inductor without needing an actual coil) to genuinely boost a chosen frequency band by, say, +12 dB. The Vox wah is built around a gyrator. The MXR 10-band EQ is ten of them in parallel.
Designing your own
If you're rolling your own tone stack, draw it in Pedal Bench, then run an AC sweep from 20 Hz to 20 kHz at three or four pot positions (0%, 25%, 50%, 100%). You'll see exactly what the filter is doing across the audio band. The first time you do this, you'll catch a design flaw your ears would have missed for weeks.