The Klon Centaur (1994) is a guitar overdrive pedal whose sound is dominated
by three things: asymmetric clipping, a clean-signal blend, and a
treble-cut tone control. The full Turtle is at examples/klon.ttl.
Input ──► preGain (Gain, drive) ──► AsymClip ──► DryWet ──► OnePole ──► Gain ──► Output
▲
Input ─────────────────────────── dry ─────────────┘
The same guitar signal reaches two places: the clipping chain and the DryWet
dry port. Fan-out (one output to multiple destinations) is always fine in
Valis; it is fan-in onto a non-Mixer input that is flagged as an error.
val:AsymClip models two diodes in antiparallel with different forward
voltages - a positive threshold and a negative one. The default values
approximate the Centaur's original component values:
| Direction | Component | Forward voltage |
|---|---|---|
| Positive | BAT41 Schottky × 1 | 0.3 V |
| Negative | 1N34A Germanium × 2 | 0.4 V (two in series) |
The result is that positive peaks clip earlier than negative ones, producing a lopsided waveform with stronger even harmonics - the "warmth" commonly attributed to the pedal.
:clip a val:AsymClip ;
val:drive 1.0 ;
val:posVf 0.3 ;
val:negVf 0.4 .
val:drive scales the signal before clipping. The preGain element ahead of
it is set to 20 dB to bring a typical guitar signal (~100 mV peak) up to the
clipping range (~1 V). Increase drive for heavier saturation; decrease it
to back off the clipping without losing level.
val:DryWet mixes the unprocessed input (dry) with the clipped output
(wet). At mix=0 the pedal is fully clean; at mix=1 it is fully clipped.
The Centaur sits around mix=0.5.
:blend a val:DryWet ; val:mix 0.5 .
This blend is what distinguishes the Centaur from a pure overdrive - the clean signal preserves pick attack and low-end definition that would otherwise be compressed away by the clipping stage.
The original uses a passive treble-cut pot. val:OnePole in lowpass mode
approximates this: at full cutoff (8 kHz) the signal is essentially flat; lower
the cutoff to reduce high-frequency fizz.
:tone a val:OnePole ; val:cutoff 8000.0 ; val:mode 0 . # 0 = lowpass
Harder clipping: replace val:AsymClip with val:DiodePair for a symmetric
silicon pair, or with val:HardClip for the most aggressive square-wave style.
Symmetric clipping: set val:posVf and val:negVf to the same value.
Richer saturation: insert a val:Tanh with val:antialiasing val:ADAA2
between preGain and clip to add tube-like compression before the diode stage.
Stereo: duplicate the chain for a second channel; the Input and Output elements each carry one mono bus in the current engine.
Paste examples/klon.ttl into the Turtle editor tab or use File → Load
Circuit…. Six parameters are exposed: Drive, Positive Vf, Negative Vf, Blend,
Tone and Volume.