Karplus-Strong Synthesis

When senior DSP engineer and Strymon co-founder Pete Celi finished tuning the monophonic Karplus-Strong voice in our StarLab Experimental Reverb module a few years ago, he immediately started thinking about how to build a standalone polyphonic instrument.

But what exactly is Karplus-Strong synthesis? Simply put, a very short delay line with high feedback is excited by a sharp transient or burst of noise to produce a pitched sound, which is very similar to the principle of plucking a string. Longer excitations can create bowed string sounds, and when the polarity of the feedback structure is reversed a pipe or tube-like timbre can be produced instead. In SuperKar+ this method of synthesis almost seems to be alive, responding dramatically to slight changes in attack and decay times as well as damping (high frequency filtering that can dampen the string or pipe’s vibration almost like a palm mute). Even clay drum textures can be found with certain settings.

From Brightly Ringing To Palm Muted

The Damp control is more than just a filter—it’s part of the delay feedback structure, affecting how freely the string or tube can vibrate. In combination with the Decay control it can produce anything from bright ringing textures to darker sounds with deep, resonant bass, extending even to palm-muted effects. CV control of the Damp parameter can create very lifelike and nearly human-performed sounds.

Play The Feedback

Coupled with a short Attack time, the Decay control allows you to create super short and tight percussive sounds near its minimum setting. Sounds ring out freely at higher settings, and tuned self-oscillation is possible near the maximum setting. The interaction between Damp, Decay and Attack can create an infinite variety of intriguing results, and all three are controllable via both knob and CV.

Attack of the Tones

Unlike traditional ADSR amplitude envelopes where a longer attack time means that a note takes significantly longer to fade in, with SuperKar+, longer attack times can produce bowed or blown sounds, ranging from short strokes to more sustained bowing/blowing. Very short attack times create plucked or struck sounds. Like the Damp and Decay parameters, you can control the Attack time via knob or control voltage, with the knob acting as an offset for the CV input.

Solo Voice Detune

Available via CV input, the Detune feature on the Solo Voice allows for random detuning for every note that is played. This can take string-like timbres to a whole new level of realism, or add additional movement or even a haunting aspect to a patch.

A Tale of Two Timbres

At the push of a button, SuperKar+ allows you to choose an independent timbre for each voice. The green LED denotes positive feedback for string-based sounds, and the red LED denotes negative feedback for hollow-tube-based sounds.

Portamento To The People

You can use the variable Glide setting to slow down the transition between pitch changes, with the effect being more noticeable and pronounced the further away the notes are from one another in pitch.

Doubling, Octaves, Smart Harmonies And More

With the Harmony control you can create fat doubled notes, lower and higher octave and fifth harmonies, or choose from a selection of smart harmonies that can create advanced and expansive chords in conjunction with the Scale selection.

Even though the Voices are labeled “Solo” and “Chord,” you can easily play chords with the Solo Voice via MIDI while using the Chord Voice to create basslines with doubling or lower octave harmonies. You’re only limited by your imagination.

You can even create a unique tremolo-like effect by applying an LFO to the Harmony CV input, where instead of varying the amplitude, the number of harmony extensions is increased or decreased.

Super Scale+

When a smart harmony is selected via the Harmony parameter, the Scale setting influences the flavor of the chords that will be produced. Choose from: Harmonic Minor, Aeolian, Major, Expanded Voicing Major Scale (for harmonies spaced further away from the root), Dorian, Lydian, Phrygian, or All Major Chords.

Solo Voice Pitch

Connect an LFO to the Solo Voice Pitch CV input to create vibrato or trills, or connect an envelope for envelope-based pitch effects. You can also use this input for offsetting the pitch by a fixed amount. The range of the Pitch CV input is  +/- 1 octave.

Wide Stereo, Independent Processing, and More

SuperKar+ is capable of creating a strikingly expansive stereo image. For both of its two stereo output modes, the Chord Voice creates the same type of satisfying stereo image based on the number of harmonies being used. The Solo Voice behaves differently, however.

For both stereo modes, if the Solo Voice has the polyphony set to 1, Solo Voice notes will be heard in the center of the stereo image. Once Solo Voice polyphony is raised to two or more voices, alternating voices will be panned left and right in the stereo image. In Wide Stereo mode, they are panned hard left and right, and in Narrow Stereo mode they are panned near 10 o’clock and 2 o’clock.

But that’s not all: SuperKar+ has two additional output modes for increased flexibility. Split Outputs mode sends the Solo Voice to OUT L and the Chord Voice to OUT R, so that you can process each signal separately. Mono Sum mode sends the same mono mixed output of both voices through both outputs, allowing you to send the same mix to multiple destinations.