Sigmoid Drive

A stereo waveshaping distortion built on a variable-curvature sigmoid. It ranges from warm tube-like saturation to aggressive near-hard-clipping, with drive amount and harmonic character on separate controls so you can dial in grit independently of how hard you push it.

How it works

The signal is pushed through the rational shaper x(1+k)/(1+k|x|). Drive sets how hard you hit it; Shape morphs the curvature k from gently rounded toward a brick-wall clip. A built-in makeup gain renormalizes the peak back to full scale, so sweeping Shape changes the harmonic flavour without changing level — which is exactly what makes it such a clean modulation target. Level then trims that auto-normalized wet output up or down, so you can stage the distortion into whatever comes next. Left and right are shaped independently with shared settings, preserving stereo width.

Controls

• Drive — pre-gain into the shaper, 1x to 10x; turn it up for more saturation and harmonics.
• Shape — curvature of the sigmoid, soft to hard. Low is nearly clean, mid lands in tanh-like warmth, high pushes toward hard clipping. A great modulation target.
• Mix — blends the distorted signal against the dry input. At 100% you hear the shaper alone.
• Level — post-gain on the wet path, 0 to 2x. The default 1x is the auto-normalized makeup level; turn it down to tuck the grit back into the mix, up to push the next module harder.

Inputs & outputs

• In L / In R — the audio to distort; patch one side for mono, both for stereo.
• Drive / Shape / Mix / Level CV — voltage control over each knob, summed with its setting; patch an LFO or envelope to any of them for animated saturation, blend, or output level.
• Out L / Out R — the shaped stereo result.

Tips

• Leave Drive moderate and sweep Shape with an LFO for a vowel-like, level-stable timbral wobble.
• For parallel distortion, run Mix below 100% to keep the dry transients intact underneath the grit.
• An envelope into Drive CV gives percussive bite that blooms and settles with each hit.