Still a relatively new face on the Eurorack scene, Petr Horáček, from Brno in the Czech Republic, is fast making a name for himself under the Zlosynth banner. Incredibly, the Arplus is only the third module in the company catalogue, following the Achordion and Kaseta, but once again it demonstrates Horáček’s deftness with digital, not to mention an aptitude for cleverly reappropriating long‑standing musical principles in a fresh and interesting way.
A quick recap on those preceding modules: the Achordion is a 10HP chord‑generating wavetable oscillator, providing up to 18 voices that can be quantised to various scales. It offers various playing modes, from single tones to chords, and its 37 wavetables range from simple waveforms to bell‑like sine shapes to harsh distortion‑infused tones. The Kaseta, meanwhile, is a smartly designed tape‑style delay and looper with four virtual heads, each of which has its own volume, feedback and panning controls. Add to this a rather lovely adjustable saturation and wow and flutter, and you’ve got a highly versatile looping, pinging, tone‑shaping machine. It’s all DIY, but the builds aren’t difficult (trust me: if I can build it, so can you).
Karplus‑Strong Experiment
Why do I bring these up? Because in many ways the Arplus draws on both, completing an initial triumvirate of modules quite beautifully. “Arplus started as a Karplus‑Strong experiment on the Achordion,” Horáček said in our Modular Profile in the May 2026 issue. “A mix of things I found interesting and complementary: scale quantisation, arpeggio patterns and string voices all in one place.” Karplus‑Strong being based on quick delays, this module presents six voices of string synthesis that can be set to arpeggiate, and also be excited by an external audio signal. It takes the Achordion’s notion of scale selection to another level with an astonishing 31 different scales to choose from, allowing for chords of up to eight notes. It has adjustable polyphony, filtering and envelope‑adjacent contour, meaning in practice that its behaviour can feel crisp and stilted, long and languid or anything in between.
The first thing to say is that the Arplus’ raw sounds are every bit as good as you could hope for from a Karplus‑Strong‑based module. From twangy to plucky to woody to metallic; as a loving user of modules like Mutable Instruments’ Rings, I can say with some confidence that it capably matches the best modules in its category for sonics. At the same time, I found it to make the best of friends with spatial effects — particularly delay. I tend to think physical‑modelling synthesis benefits from the implication of a physical space anyway, but the way the Arplus moves through notes in a chord or scale lends itself very nicely to residual echoes of what’s come before. Also, fair to say Karplus‑Strong sounds can by nature become quite harsh and stark at times, in which case a soft analogue delay becomes the perfect emollient.
As for those 31 scales, this is one area where the Arplus really flexes. It’s also a good opportunity to make mention of the fact that recent years have seen an interesting and important move toward what is often called the ‘decolonisation’ of music production environments. Put simply, a vast majority of the time, production software and hardware skews toward Western understandings of rhythm, tonality and temperament, yet in reality a majority of the world does not conceive music in this way. So, suffice to say, variety of this kind is a welcome sight.
Instead of bludgeoning you with scales, the Arplus organises them nicely into groups, with dedicated buttons for both group and individual scale selection. The groups consist of Western diatonic, microtonal Arabic maqam, Indian melakarta and Japanese pentatonics, with a further Full category for quarter‑, semi‑ and whole‑tone scales. Within these are an array of scales that I won’t try to list in full here, but let’s just say that whether you’re looking for a Mayamalavagowla scale, a Hirajoshi scale or a Nawa Athar scale, the Arplus has you covered. A number of arpeggio shapes are also on offer, along with controls for the root note (Tone), number of notes (Size) and number of ‘strings’ used.
These three first fruits from Zlosynth together complete a satisfying three‑piece ecosystem.
As I say; these three first fruits from Zlosynth together complete a satisfying three‑piece ecosystem. Whether Petr Horáček chooses to make that triangle a square or to turn the page on to a different synthesis department entirely, I’m very much looking forward to whatever’s next.