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Circuit Design · Interactive

Tone Stacks

The passive EQ networks that give guitar and hi-fi amplifiers their voice. Adjust the controls and watch the frequency response change in real time.

Fundamentals

What Is a Tone Stack?

Passive EQ networks that shape the frequency response of an amplifier

A tone stack is a passive filter network placed between gain stages in a tube amplifier. Unlike active equalizers that can boost and cut, passive tone stacks can only attenuate — they shape the signal by removing frequencies rather than adding energy.

The classic guitar amp tone stack uses three potentiometers (Bass, Mid, Treble) and a network of resistors and capacitors to create frequency-dependent voltage dividers. The interaction between these components produces the characteristic sound of each amplifier brand.

Fender
Deep mid scoop, bright and scooped character. The sound of clean American tone.
Marshall
More midrange presence, aggressive character. The British crunch.
Vox
Cut-only treble control, chimey top end. The jangle of British invasion.
H(f) = Zload(f) / (Zseries(f) + Zload(f))
where Z depends on R, C values and pot positions
Interactive · Fender

Fender Tone Stack

The classic American tone circuit — famous for its mid scoop

InR1250k TrebleC1250pFR21M BassOutC2100nFC347nFR325k Mid
R1250k
C1250pF
R21M
C2100nF
R325k
C347nF
Bass5.0
Mid5.0
Treble5.0
Notice the characteristic mid scoop around 400-800 Hz. With all controls at 5, the Fender stack produces roughly 10-15 dB of insertion loss with a pronounced dip in the midrange. This is what gives Fender amplifiers their “scooped” clean sound.
Interactive · Marshall

Marshall Tone Stack

Different component values produce more midrange presence

R1220k
C1470pF
R21M
C222nF
R325k
C322nF
Bass5.0
Mid5.0
Treble5.0
The Marshall tone stack uses a smaller bass cap (C2 = 22nF vs 100nF) and larger treble cap (C1 = 470pF vs 250pF). This shifts the mid scoop frequency and reduces its depth, giving Marshall amps their characteristic midrange push and crunch.
Interactive · Vox

Vox Cut Control

A simpler treble cut-only circuit with its own character

The Vox AC30 uses a fundamentally different approach: instead of a three-knob tone stack, it employs a simple treble cut control. The “Cut” knob progressively rolls off high frequencies without the dramatic mid scoop of the FMV topology. This gives the Vox its distinctively full, chimey tone.

Cut5.0
At minimum cut (10), the signal passes relatively flat. As you turn the cut control down, high frequencies are progressively attenuated while the bass and midrange remain largely unaffected. This preserves the fullness of the guitar signal.
Interactive · Hi-Fi

Baxandall Tone Control

The active feedback approach used in hi-fi amplifiers

Peter Baxandall's 1952 design revolutionized tone controls by using negative feedback around an amplifying stage. Unlike passive tone stacks that can only cut, the Baxandall circuit can both boost and cut bass and treble symmetrically. It has only two controls and produces a much flatter response at center position.

This is the standard tone control in hi-fi amplifiers, mixing consoles, and most active audio equipment. The key difference: the flat position truly is flat, with near-zero insertion loss.

Bass5.0
Treble5.0
With both controls centered (5.0), the response is essentially flat. Turn Bass up to boost low frequencies, or down to cut them. The symmetrical boost/cut behavior is a hallmark of active feedback EQ design.
Av(f) = −Zfb(f) / Zin(f)
Inverting amplifier topology — gain set by feedback/input impedance ratio
Analysis

Comparison Mode

See all passive tone stacks on the same plot with identical settings

Compare the Fender, Marshall, and Vox tone stacks at the same knob positions. The controls below set all three stacks simultaneously.
Bass5.0
Mid5.0
Treble5.0
Fender
Marshall
Vox
At the same control settings, the three stacks produce markedly different curves. The Fender has the deepest mid scoop, the Marshall retains more midrange energy, and the Vox produces a gentler overall roll-off without a mid dip.
Explorer

Component Value Explorer

Change individual component values and see how each one shapes the response

Start from the Fender values and experiment. The dashed line shows the stock Fender response at the same pot positions for reference. Each component has a specific role in shaping the overall frequency response.
Bass5.0
Mid5.0
Treble5.0
R1 — Treble Pot250kΩ
Controls treble bleed. Higher = less treble loss
C1 — Treble Cap250pF
Sets the treble frequency range. Larger = lower treble corner
R2 — Bass Pot1.0MΩ
Controls bass response range
C2 — Bass Cap100nF
Sets the bass frequency range. Larger = deeper bass
R3 — Mid Pot25.0kΩ
Controls mid scoop depth. Lower = deeper mid scoop
C3 — Slope Cap47.0nF
Sets the mid scoop frequency. Larger = lower scoop center
Try these experiments: increase C1 to shift the treble range lower. Decrease R3 to deepen the mid scoop. Increase C2 to extend bass response. Each component interacts with the others, creating the complex response curves that define each amplifier's character.
Reference

Key Equations

The mathematics behind passive and active tone networks

Capacitor Impedance
ZC = 1 / (j2πfC) = −j / (2πfC)
Capacitive reactance decreases with frequency. At DC, a capacitor is an open circuit. At high frequencies, it approaches a short circuit.
Voltage Divider (Complex)
Vout / Vin = Z2 / (Z1 + Z2)
When impedances are frequency-dependent, the voltage division ratio changes with frequency, creating the filter action that tone stacks rely on.
Parallel Impedance
Zparallel = (Za · Zb) / (Za + Zb)
RC Corner Frequency
fc = 1 / (2πRC)
The −3dB point of a simple RC filter. In a tone stack, multiple RC combinations interact, creating the complex multi-pole response curves seen above.
Baxandall Gain (Inverting)
Av(f) = −Zfeedback(f) / Zinput(f)
The active Baxandall circuit uses frequency-dependent impedances in both the input and feedback paths of an inverting amplifier. When both paths have the same impedance, the gain is unity (0 dB).
Decibel Conversion
dB = 20 log10(|H(f)|)
All frequency response plots show the magnitude of the transfer function in decibels. 0 dB = unity gain. −6 dB = half voltage. −20 dB = one tenth.
Quiz de synthèse

Test Your Knowledge

Validate your understanding of tone stack circuits before moving on.

Question 1 / 7

How does a passive tone stack shape the signal?

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