AI Synthesis AI004 OTA VCF Black
It screams, squelches, and smells like patch cables and solder—this is the MS-20’s wild heart stuffed into 8 HP of DIY Eurorack bliss.
Overview
If you’ve ever cranked the resonance on a Korg MS-20 and felt your ribcage vibrate from the raw, animal howl that came out, the AI Synthesis AI004 OTA VCF Black will feel like coming home. This isn’t some polite reinterpretation—it’s a full-blooded, no-disclaimers clone of that legendary filter topology, shrunk down and reborn for the Eurorack era. Built as a DIY kit or bought pre-assembled, it’s a hands-on ticket to acid-soaked resonance, gritty overdrive, and that unmistakable MS-20 character: aggressive, musical, and just a little unhinged. It’s not trying to be clean or clinical. It wants to distort, self-oscillate, and make your basslines sound like they’re tearing through reality.
And the best part? You don’t need a second mortgage to get it. Marketed as a tribute to the MS-20 filter, the AI004 brings that classic sound into modular rigs without the vintage price tag or the fragility of 40-year-old hardware. It’s compact—just 8 HP wide—and draws modest power, making it an easy fit even in crowded cases. Whether you’re a seasoned builder or just dipping your fingers into the hot end of a soldering iron, this module straddles the line between accessibility and authenticity. Owners report it’s “never leaving the rack,” and one user flat-out declared, “I’m really loving the filter as it really very musical and the acid sound of the resonance.” High praise from someone who owns ten other filters.
But let’s be real: this isn’t a precision instrument. It’s a character module. It overdrives in both low pass and high pass modes, the resonance drops signal level as it increases (a known behavior, not a defect), and the whole thing leans into its imperfections like a punk rock guitarist leans into feedback. If you’re chasing surgical EQ or transparent filtering, look elsewhere. But if you want something that growls, sings, and occasionally snarls when provoked, the AI004 earns every decibel.
Specifications
| Manufacturer | AI Synthesis |
| Dimensions | 8 HP, 35 mm deep |
| Current Draw | 15 mA +12V, 15 mA -12V, 0 mA 5V |
| Price | $160 |
| Available as | Assembled module and DIY project |
| Filter slopes | Low Pass slope is 12dB/octave, High Pass slope is 6dB/octave |
| Filter modes | Low pass and High pass |
| CV inputs | A CV input, with attenuverter, controls the cutoff frequency and a second CV input with no attenuation is available for raw voltage |
Key Features
MS-20 DNA, Eurorack Size
At its core, the AI004 uses the exact same filter structure and topology as the Korg MS-20—a fact proudly stated across multiple sources and confirmed by those who’ve traced the circuit. That means the same aggressive, resonant character that defined early industrial and acid music is baked right in. The filter is switchable between low pass and high pass modes, giving you two distinct flavors of sonic mangling. The low pass delivers that classic 12dB/octave roll-off with a throaty, singing resonance, while the high pass offers a 6dB/octave slope—smoother, but still capable of biting through a mix when pushed.
Extended Resonance & Hidden Modes
One of the more intriguing quirks is the hidden “wild” or “crazy” mode, accessible via a physical switch on the back of the module. This isn’t just marketing fluff—it unlocks an extended resonance behavior that pushes the filter into more extreme, unstable territory. Combined with the updated diode gain stage in the resonance section, this lets the AI004 scream with a richness that feels alive. Both low pass and high pass modes can self-oscillate when resonance is cranked, turning the filter into a crude but effective sine wave oscillator—perfect for tuning other modules or generating eerie drones.
CV Control with Attenuversion
The primary CV input for cutoff frequency includes an attenuverter, meaning you can scale and invert incoming control voltages directly on the module. This is a huge win for patch flexibility, letting you use LFOs, envelopes, or random sources to modulate the filter without needing external processors. There’s also a second CV input with no attenuation—ideal for applying raw, full-range voltage when you want 1:1 response. This dual-input approach gives builders and patchers room to experiment without cluttering their rack.
Build Philosophy: No Wires, All PCB
Construction is straightforward: every jack, knob, and component is soldered directly to the PCB. There’s no point-to-point wiring, which simplifies the build process and reduces failure points for DIYers. This design choice makes it a favorite among newcomers to module building—no need to route cables or worry about cold joints in hidden connections. The kit includes everything needed to assemble the module, and pre-built versions arrive fully calibrated and ready to use. It’s a testament to thoughtful design: complex in behavior, simple in execution.
Historical Context
The AI004 is the fourth module released by AI Synthesis, a boutique outfit founded by Abraham Ingle, who clearly has a soft spot for classic analog circuits. Positioned as a loving tribute to the MS-20 filter, its purpose is singular: bring that iconic sound to Eurorack in an affordable, buildable format. It follows the AI001 Multiple, AI002 Mixer, and AI003 Looping ADSR—modules that suggest a deliberate progression for beginners. In fact, the manual itself recommends the AI004 as “the third module you should build” if you’re new to DIY electronics. That tells you everything: this isn’t just a product, it’s part of a curriculum. A hands-on education in analog synthesis, one solder joint at a time.
Collectibility & Value
New units are listed at $160, though the market shows a wide range from $15 to $160 depending on configuration—likely reflecting the difference between bare kits and fully assembled modules. The full DIY kit was priced at $85 as of March 2018, making it “deliciously cheap” by today’s standards. Reviewers have called it “such amazing value,” and users consistently praise its sound and build ease. There’s no data on common failures or long-term reliability issues, but the simple, wire-free PCB construction suggests fewer points of failure than more complex modules. Since it’s still in production and widely available, it’s not a “collector’s item” in the traditional sense—more of a modern staple. But given its cult following and the enduring appeal of MS-20-style filtering, it’s unlikely to disappear from racks anytime soon.
eBay Listings
As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.