Founder of nascent Czech Eurorack boutique Zlosynth, Petr Horáček is the kind of developer that makes the world of Eurorack — in particular DIY — go round. With a flare for clever DSP, he recently unveiled what he considers to be his best work yet, the Arplus quantiser, arpeggiator and string synthesizer; just the third instalment of an already impressive range featuring the Achordion ‘chord‑crafting’ wavetable oscillator and the ingenious Kaseta tape delay and ‘feedback sculptor’.
On his entry into modular
I have a tendency to get obsessed with new projects. My dad got me into DIY and programming from an early age, so I was always comfortable picking up new things and building them. Around 2012 I found out about YuSynth modules. I sourced parts for the modules and built a case, but then it got overwhelming and I never finished the project. Years later, a Look Mum No Computer YouTube binge pulled me back into thinking about synthesizers. From there it was a short step to Eurorack. I picked some basic circuit designs from the Internet, learned how to redraw them in KiCad, ordered the PCBs and built my first modules. Once things started working, I was hooked.
On his go‑to modules
My rack is mostly a test bench! There is a Joranalogue Test 3, a Konstant Lab PanoramaMIX, a Bastl CV Trinity, and the rest are my own Zlosynth modules and various prototypes. I use it for testing new designs — sometimes for playing around. Designing and building takes up most of my free time, which does not leave much room for actually playing... I hope to set aside some time in the future to get deep into Eurorack as a musician.
On the story of Zlosynth
My day job changed and I needed a new project to satisfy my builder’s brain. Eurorack seemed like a good fit. I set myself a goal to keep the motivation alive: design a module from scratch and sell five kits. The five kits sold, and then more did. People asked for assembled modules, so I added those too. Six years on, it is still my evening hobby business! Every time someone shares a patch built around one of my modules, or cares enough to email me their feedback, it makes my day. Zlosyn, without the ‘th’, is an archaic Czech curse word meaning villain, or literally a son of evil. A Czech curse word is probably not a great idea for a global brand... but it stuck.
On the Arplus
Arplus started as a Karplus‑Strong experiment on the Achordion. I liked how the plucked strings combined with filter resonance sounded, then I tried spreading the chords into arpeggios, and soon it grew into its own module. It ended up being a mix of things I found interesting and complementary: scale quantisation, arpeggio patterns and string voices all in one place. You can plug in a clock and it just runs.
I find it quite engaging when you push its filter resonance to the edge of self‑oscillation and then you balance it there. Or when you vary the contour between plucking and bowing. And if you connect an external sound source, it gets a whole new dimension. I think it is my favourite Zlosynth module. One difference compared to my previous modules is the focus on making Arplus self‑explanatory, with the manual being purely optional. I think it came out quite well!
On the culture of modular
I see myself more as a builder who got here through making rather than playing, so I cannot claim to know much about modular culture broadly... But what I do know is the DIY side of it. People share their schematics, PCB files and firmware openly, for others to learn from. The amount of available material and the simple format makes Eurorack really accessible to new makers. I find that pretty special. Everything I make is open for the same reason.