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There's a twang in modular that probably resonates with all of us. It stands outside the usual blips and bloops of modular; it falls into spaces that analogue waveforms don't seem to find, and rubs up beautifully against the common themes and textures that emanate from your machine. That twang is generated by the Karplus-Strong algorithm, which combines some simple sonic components to electronically produce a remarkably lifelike plucked string sound.
We hear that sound a lot, and you'd be forgiven for assuming it's a sample, but actually Karplus-Strong is a form of physical or acoustic modelling that you can produce yourself with a couple of modules. You need a noise source and a delay module with filtered feedback. Send a short burst of noise into the delay set to a very short delay time. The sound that comes out initially is probably along the lines of a badly resonating pipe or snare drum. But with a bit of manipulation of the filter and feedback the sound starts to resemble a plucked string.
Several clever people, like Alexander Strong and Kevin Karplus, pulled a workable algorithm out of this idea and turned it into a form of physical modelling synthesis, and it's this that forms the basis of the Karplus-Strong modules we find in Eurorack. It came to prominence 10 years ago with the hugely successful Rings module from Mutable Instruments. That module was everywhere and is still in the top 10 most popular modules on ModularGrid three years after it was discontinued. The SuperKar+ from Strymon is the latest module to adopt the algorithm and move into the void left by Rings. Let's see what it brings to the table.
SuperKar+ Overview
SuperKar+ is a 32-voice string synthesis module. It splits the polyphony between two different voice engines that you can see on the module delineated by a slightly wayward dotted line. On the left, we have the Solo voice. It can take single notes via CV/Trigger or multiple notes via MIDI and sustain or overlap them through up to 16 voices. On the right side is the Chord Voice. This works differently, generating intervals and chords that expand through the turn of the Harmony knob. There's no MIDI control on this side; just CV and trigger to set your root note. It's worth noting that a Karplus-Strong module isn't sounding all the time like a regular oscillator. It requires a trigger to fire the burst into the delay, or pluck the string as it were.
The Solo Voice offers three knobs for control, doubled up with an Alt function, plus a couple of other parameters that are only accessible via CV modulation. The most frequently fiddled with are the DAMP and DECAY knobs. Damping shifts the plucked string sound from open ringing to palm-muted thuds, while the Decay gives space to the sound, letting resonances and harmonics drift on even when fully damped. Left unchecked, the falling sound seems to gather an energy all of its own into some really nice feedback.
Attack is a little unexpected. Rather than providing a ramped level change into the sound, it changes the nature of the synthesis towards a more bowed approach. It can be quite raspy and strange, but you can also dig out a believable bowed string sound.
Second-level controls are accessed by holding the Timbre button on the Chord Voice side. This gives us a level control, the ability to add glide or portamento and also control over the amount of polyphony at play. You can reduce this down to a single voice so that each note is stolen for the next, or find a place of realistic overlap, like with an instrument of a few strings, or just unwind into all the notes you could want (assuming that's no more than sixteen).
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In the patch bay, you've got CV control over the physical controls, but there are also a couple of extras. The Pitch input lets you add a bit of pitch bend or vibrato outside of using the 1v/oct input for the notes. The Detune input is really interesting in that it lets you introduce pitch instability and increases the likelihood that notes will be out of tune. It's a great feature except that it's too easy to leave on by accident and find yourself wondering why you can't get the module in tune with the rest of your rack.
Over on the right is the Chord Voice. It has a similar selection of control knobs but adds the central Harmony/Scale knob that governs all the chordal shenanigans. You can select one of fifteen different harmony settings which work with one of eight scales to give you a chord based upon the voltage you patch into V/Oct input. On the left side of the knob, we have various combinations of the root note plus octaves and/or fifths. On the right side, it adds more and more intervals until you get more or less all of them. Strymon calls them "Smart Harmonies", which they don't explain anywhere, but I imagine it's to do with how they respond to the scales. You can use CV control to navigate around the Smart Harmonies, adding intervals with modulation. The scales are all the usual suspects, but there's one setting called Extended Major which adds in a wider range of internals and octaves up and down the keyboard.
The Alt function under the Attack knob on the Chord Voice side gives you control over the two audio outputs. The first mode pans the Solo Voice hard left and right on alternate notes while keeping the Chord Voice in stereo. A second mode softens this into the middle a bit more. A third mode splits the two voices into separate outputs, and a fourth sums everything to mono to both outputs. It's an unexpected feature that creatively deals with what would otherwise be a lot of similar sounds mashed together.
What might not be apparent as you're enjoying the string sound is that there is a whole other sound in here in the shape of a pipe. Apparently, if you do some mad feedback reversal, you can push a vibrating string into sounding like a vibrating column of air inside a pipe. It's a strangely different sound that starts to come alive when you find interesting breath points with the Attack knob. It doesn't quite have the impact or sweetness of the plucked string, but with the touch of the Timbre button you have something different to switch to.
SuperKar+ Impressions + Comparisons
In use, the SuperKar+ is very impressive. If you plug in a MIDI keyboard, you'll find the velocity-driven polyphony turns it into a very expressive playing experience. Unfortunately, there's no other MIDI action available, no CC number mapping or response to modulation, which feels like a missed opportunity. Switching to CV kills the velocity, but that tends to encourage you into fiddling with the Decay and Damping in order to inject it with some feel and variation. The range of those knobs is quite restrictive, but this actually works to our advantage. There's no lurch into squeaks or disappearance into clicks as many other modules find themselves at the extremes. The sound remains fully within the sweet spot of what Karplus Strong can do, and Strymon has tuned and curated it perfectly.
The Chordal side takes a bit more work to uncover its usefulness in a modular context. But once you lock into the intervals and trigger patterns you're after, it can become a rare polyphonic source in a largely monophonic system. Being able to run the Solo and Chord voices simultaneously is fantastic. It could have so easily been one mode or another, but here you can have a texture bed of pulsing chords accompanied by expressive plucks and shimmers.
While Rings can produce gorgeous plucked sounds, its focus is more in the experimental nature of resonators and exploring different inputs and sources of excitation, which is interesting because the SuperKar+ also has an audio input for doing exactly the same sort of sonic experiments, but it's not the module's principal focus. The Qu-Bit Surface is a bit closer in terms of polyphony with up to 8 voices, but only in terms of overlapping sustain. The Surface features many other sounds and is an elegant take on the algorithm, but I find it too easily overwhelms at the extremes of parameters. The 2HP Pluck is probably the simplest example of the sound; it has four voices and no input options. It has a sweetness that is occasionally broken by squeaks and slight mis-tunings. The Intellijel Plonk is also worth a mention, but the focus there is on modelling percussive sounds with mallets and beams.
All of these modules have their strengths, and somewhere in the middle, they can all produce a similarly gorgeous plucked string sound that, when routed through a reverb and delay, can take you off into special ambient places. I feel where the SuperKar+ excels is in its musical expression. While it can experiment, that's not its forte; it would much rather be spinning out impressive arpeggios and leaning into the rasp of that bow. The bottle sound gives it some welcome variation, and you have plenty of modulation options to keep things moving. The knobs hamper each other by being a bit too thick and crowded, which detracts a little from what is otherwise a thoroughly beautiful module.
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