Circuit
Controls
Drag knobs up/down, or use the sliders
How it works
The FMV tone stack, used in Fender, Marshall and (in modified form) Vox amplifiers, is a passive network that cannot boost, only cut. With all controls at mid-position it produces the famous mid-scoop, a dip of 10–20 dB somewhere between 300 Hz and 1 kHz. The "treble", "bass" and "middle" knobs interact strongly because every component influences every path.
- Treble path: C1 couples high frequencies directly to the top of the treble pot; the wiper picks them off.
- Bass path: the slope resistor R1 feeds C2 and then the bass pot, which raises the bottom of the treble pot above ground.
- Middle: the mid pot lifts the whole network from ground. At "0" the mid frequencies are heavily shunted.
Rather than using closed-form equations, this calculator builds the actual network (nine elements, six nodes) and solves it with a complex nodal analysis at 200 frequencies. Bass and treble pots use an audio (logarithmic) taper approximated by:
It assumes an ideal (zero-impedance) driving source (accurate for the cathode follower of a Marshall, slightly optimistic for a Fender's plate-driven stack) and a 1 MΩ load.