Tone Stack Calculator

An interactive tool to model and visualize the frequency response of the classic Fender/Marshall/Vox (FMV) style passive tone stack circuit used in many guitar amplifiers.

Component Values

Potentiometer Values & Settings

Calculated Characteristics

Formula: This calculator models the circuit as a complex impedance voltage divider. The gain at each frequency is calculated as 20 * log10(|V_out / V_in|), where the output voltage (V_out) is determined by the interactions between the resistors and frequency-dependent capacitor impedances.

What is a Tone Stack Calculator?

A tone stack calculator is a simulation tool used by guitar amplifier designers, technicians, and enthusiasts to predict and visualize the frequency response of an amplifier’s tone control circuit. The “tone stack” is a passive filter network, typically comprised of three potentiometers (pots) for Bass, Mid, and Treble, along with a specific arrangement of resistors and capacitors. This calculator specifically models the ubiquitous Fender/Marshall/Vox (FMV) topology, which is known for its interactive controls and characteristic “mid-scoop.”

Instead of physically soldering and swapping components, you can use this tool to instantly see how changing a capacitor or resistor value will affect the guitar’s tone. It helps in understanding why a Marshall sounds different from a Fender and allows you to design your own custom voicings. A common misunderstanding is that these controls “boost” frequencies; being passive, a tone stack can only cut frequencies from the signal provided by the preamplifier.

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The Tone Stack Formula and Explanation

There isn’t a single, simple formula for a tone stack. Its behavior is defined by the principles of circuit analysis, specifically using complex numbers to represent impedance. The entire circuit acts as a frequency-dependent voltage divider. The output signal is tapped from the wiper of the Treble pot.

The core components are:

• High-Pass Filter: Formed primarily by the Treble Capacitor, which allows high frequencies to pass to the top of the treble pot.
• Low-Pass Path: The Slope Resistor works with the Mid and Bass capacitors to control which lower frequencies pass to the bottom of the treble pot.
• Mixing Point: The Treble pot then blends these two frequency paths. The Bass and Mid pots further shape the low-frequency path before it reaches the Treble pot.

Because all components are interconnected, changing one control (like Bass) affects the range and response of the others (Mid and Treble). This interaction is a defining feature of this circuit. For more details on amp design, you might check out resources on amplifier gain stages.

Table 1. Key variables in the tone stack circuit.

Variable	Meaning	Unit (Auto-Inferred)	Typical Range
Slope Resistor (R_slope)	Sets the overall balance between highs and lows.	kΩ (kilo-ohms)	33kΩ – 100kΩ
Treble Capacitor (C_treble)	Filters high frequencies. Smaller values are brighter.	pF (pico-farads)	250pF – 500pF
Mid Capacitor (C_mid)	Influences the center of the mid-scoop.	μF (micro-farads)	0.01μF – 0.047μF
Bass Capacitor (C_bass)	Controls the amount of low-end frequencies passed.	μF (micro-farads)	0.022μF – 0.1μF
Potentiometers (Pots)	Variable resistors used for Bass, Mid, and Treble controls.	kΩ (kilo-ohms)	10kΩ (Mid) – 1000kΩ (Bass)

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Practical Examples

Example 1: Classic “Fender Blackface” Tone

A quintessential clean tone, known for its glassy highs and scooped mids. This is achieved with a specific set of component values.

• Inputs: Slope Resistor: 100kΩ, Treble Cap: 250pF, Mid Cap: 0.047μF, Bass Cap: 0.1μF. Pots: 250k (T), 10k (M), 250k (B).
• Settings: Bass: 3, Mid: 7, Treble: 8
• Result: This produces a deep mid-scoop around 400Hz, providing space for vocals in a mix. The high treble setting gives the characteristic “sparkle.” You can model this using the “Reset to Fender Defaults” button and adjusting the sliders.

Example 2: Classic “Marshall JCM800” Crunch

A more aggressive, mid-focused tone perfect for rock and hard rock. The component choices shift the frequency response significantly.

• Inputs: Slope Resistor: 33kΩ, Treble Cap: 470pF, Mid & Bass Caps: 0.022μF. Pots: 250k (T), 25k (M), 1M (B).
• Settings: Bass: 7, Mid: 8, Treble: 6
• Result: The smaller slope resistor and different capacitor values raise the mid-scoop frequency and make it shallower, resulting in a pronounced upper-midrange hump. This is the sound that cuts through a dense band mix. Exploring different preamp tube characteristics can further enhance this tone.

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How to Use This Tone Stack Calculator

1. Enter Component Values: Start by inputting the values for the resistors and capacitors in the “Component Values” section. You can use the dropdowns to select the correct units (e.g., kΩ for resistors, μF, nF, or pF for capacitors).
2. Set Potentiometer Values: Enter the total resistance for each of the three control pots (Treble, Mid, Bass).
3. Adjust the Tone Controls: Use the “Treble,” “Mid,” and “Bass” sliders (0-10) to simulate turning the knobs on an amplifier.
4. Analyze the Graph: The chart will update in real-time, showing the gain (in decibels, dB) across the audible frequency spectrum (20Hz to 20kHz). A value of 0dB represents no change, while negative values indicate a frequency cut.
5. Interpret the Results: The “Calculated Characteristics” section provides a summary, including the frequency of the deepest mid-scoop. This is key to understanding the voicing of the amp.

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Key Factors That Affect a Tone Stack

• Slope Resistor: This is arguably the most critical component. A lower value (like Marshall’s 33kΩ) allows more bass and mids through, creating a fatter, more mid-forward tone. A higher value (like Fender’s 100kΩ) creates a deeper scoop.
• Treble Capacitor: Directly controls the “brightness.” A smaller value (250pF) passes only very high frequencies, resulting in a crisper sound. A larger value (470-500pF) lets more upper-mids through for a thicker treble response.
• Mid Capacitor: In conjunction with the Mid pot, this determines the frequency of the mid-scoop. Changing this value moves the scoop left or right on the frequency chart.
• Bass Capacitor: This determines the low-end roll-off point. Larger values (e.g., 0.1μF) allow for a deeper, fuller bass response.
• Mid Pot Value: A smaller value mid pot (e.g., 10kΩ) provides a much wider range of mid control and a deeper potential scoop compared to a larger value (e.g., 25kΩ).
• Source Impedance: The circuit driving the tone stack (typically a tube gain stage) has an output impedance. A lower source impedance (from a cathode follower circuit) results in less signal loss and a slightly different response curve. For accurate modeling, consider our guide on output impedance matching.

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Frequently Asked Questions (FAQ)

What is a “mid-scoop”?

It’s the characteristic dip in the midrange frequencies created by the FMV tone stack. Fender amps are famous for a deep scoop, which sounds pleasing on its own, while Marshall amps have a shallower scoop, helping the guitar cut through a band mix.

Why is the frequency axis on the chart logarithmic?

Human hearing perceives frequency logarithmically. A jump from 100Hz to 200Hz sounds like the same musical interval (an octave) as a jump from 1000Hz to 2000Hz. The log scale accurately represents how we hear pitch differences.

Can I use this for a Big Muff Pi pedal?

Yes, conceptually. The Big Muff Pi uses a very similar passive tone control, though it’s not identical. You can approximate its behavior by using a very large bass cap and small mid cap to get its characteristic massive scoop. The principles of a passive filter design calculator are the same.

What are typical component values?

Use the “Reset to Fender Defaults” button for a classic starting point. Marshall-style values are typically: 33kΩ slope, 470pF treble cap, and 0.022μF for both mid and bass caps.

Why do the controls interact?

The components are not isolated. The bass pot, for instance, is in the signal path that the mid pot also affects. Therefore, turning up the bass changes the total resistance that the mid-circuit “sees,” altering its behavior. This is a hallmark of passive designs.

What does “Pot Taper” mean?

It describes how a potentiometer’s resistance changes as you turn the knob. “Linear” (B-taper) means 50% rotation gives 50% resistance. “Audio” or “Log” (A-taper) changes logarithmically to better match human hearing perception of volume. This calculator assumes linear pots for the tone controls for simplicity, but bass pots are often audio taper.

How accurate is this tone stack calculator?

This calculator provides a very accurate model of the ideal circuit. However, in a real amplifier, component tolerances (a 100kΩ resistor might actually be 95kΩ or 105kΩ) and the complex impedances of surrounding tube stages will cause slight variations.

What is the difference between a passive and active EQ?

A passive EQ, like this tone stack, can only cut frequencies. An active EQ uses powered components (like op-amps) to both cut and boost frequencies, offering more dramatic tone-shaping capabilities. For more on this, see our active vs passive filters comparison.

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