How this tone control works

This is the tone stage made famous by the Tube Screamer. The signal enters through C1, a coupling cap, before anything else. C1 works against R1 and the 100k bias resistor R6 to form a high-pass filter. At the stock 100 nF that corner sits around 16 Hz, below anything you can hear, but shrink C1 dramatically and that corner climbs up into the bass, so even the "housekeeping" cap trims low end if you make it small enough. From there the signal hits R1 and C2, a low-pass filter that rolls off treble. That is the dark half of the tone control.

The bright half is the op-amp. It is wired as a non-inverting gain stage with a large high-pass filter built into its gain leg: R2 and C3, fed through the pot wiper. At low frequencies C3 blocks, the stage sits at unity gain, and nothing changes. At high frequencies C3 conducts and the stage boosts. R2 and C3 together set not just how much boost is available but which frequencies get it: C3 sets where the boost starts, and R2 sets the ceiling (up to 1 + R3/(R2 + R_p), with R3 as the feedback resistor that determines how much boost exists in the first place). Turning the tone pot sweeps you between the two halves: at 0% you hear the darkened low-pass signal, at 100% the treble boost takes over and lifts the highs the low-pass took away.

The part values matter enormously here. Swapping the tone pot alone, say a 20k for a 1k or a 500k, changes how the control loads the low-pass node and how the boost scales across the rotation, and the difference is huge. Changing R2 or C3 moves and resizes the treble boost. Changing R1 or C2 moves the low-pass corner. Same topology, dramatically different tone stacks, all from values. That is exactly what this calculator is for: try it before you solder it.

C5 is the part that separates the SD-1 from the TS. It sits directly across R3, so at high frequencies it shorts the feedback resistor out and drags the stage back toward unity gain. That turns the treble shelf into a treble bump: the boost rises above f_z as before, but above f₅ = 1/(2π·R3·C5) it comes back down. At the SD-1's stock values (10 kΩ against 10 nF) that corner lands at 1.59 kHz, and at the noon setting the boost actually peaks at +3.8 dB near 1.3 kHz instead of climbing to the +5.6 dB the resistors alone would allow. Turn the pot to full bright and the difference gets dramatic: about +10.6 dB peaking near 4.5 kHz with C5 in, versus +27 dB of screaming treble shelf without it. That one small cap is a big part of why the SD-1 sounds smoother up top than a Tube Screamer.

The TS / SD-1 preset buttons load each pedal's stock values in one click, including the bias resistor R6 (100 kΩ in the TS trim, 1 MΩ in the SD-1 trim). R6 matters less than it looks: even at the SD-1's values, where the 10k input resistor makes the loading heaviest, swapping 1 MΩ for 100 kΩ only moves the level about 0.4 dB, and at TS values the difference is a fiftieth of a dB. The presets are starting points, not locks. Every slider stays live afterward, so you can build your own hybrid.

Two assumptions sit under all of these curves. The input is assumed to come from a buffered, low-impedance source: feed it straight from a 10 kΩ source instead and that impedance adds to R1, dragging the stock TS low-pass corner from 723 Hz down to about 66 Hz and darkening everything. The output is assumed to drive a high-impedance stage: hang a 10 kΩ load on Out and the output cap's corner roughly doubles from 16 Hz to about 31 Hz while the level drops a fraction of a dB. The remaining fixed parts do the housekeeping: C4 couples the output and blocks DC, R6 feeds the op-amp its bias voltage (vref), and R4/R5 set the output impedance and give the output cap a load. The dashed curve shows what you would actually hear if you built it with the nearest standard E12 parts.