AI Synthesis AI004 OTA VCF Black
It screams, squelches, and smells like patch cables and solder - this is the MS-20's wild heart in 8 HP of DIY Eurorack.
Overview
Crank the resonance on a Korg MS-20 and the howl that comes out is the sound the AI Synthesis AI004 OTA VCF Black is after. It's a full clone of that filter topology, shrunk down for Eurorack. You can build it as a DIY kit or buy it pre-assembled. It gives you acid-soaked resonance, gritty overdrive, and the aggressive, musical, slightly unhinged MS-20 sound. It isn't trying to be clean or clinical. It wants to distort and self-oscillate.
It's also cheap. Marketed as a tribute to the MS-20 filter, the AI004 puts that sound into modular rigs without the vintage price tag or the fragility of 40-year-old hardware. It's just 8 HP wide and draws modest power, so it fits in crowded cases. Seasoned builders and first-time solderers can both handle it. Owners report it's "never leaving the rack," and one user declared, "I'm really loving the filter as it really very musical and the acid sound of the resonance." That user also owns ten other filters.
It 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, which is a known behavior and not a defect. If you want surgical EQ or transparent filtering, look elsewhere.
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
The AI004 uses the exact same filter structure and topology as the Korg MS-20. Multiple sources state this, and people who have traced the circuit confirm it. That gives it the same aggressive, resonant character that defined early industrial and acid music. The filter switches between low pass and high pass modes. The low pass has a 12dB/octave roll-off with a throaty, singing resonance. The high pass has a 6dB/octave slope. It's smoother, but it can still bite through a mix when pushed.
Extended Resonance & Hidden Modes
There's a hidden "wild" or "crazy" mode, enabled by a physical switch on the back of the module. It unlocks an extended resonance behavior that pushes the filter into more extreme, unstable territory. The updated diode gain stage in the resonance section adds to that. Both low pass and high pass modes can self-oscillate when resonance is cranked, so the filter works as a crude sine wave oscillator for tuning other modules or making drones.
CV Control with Attenuversion
The primary CV input for cutoff frequency has an attenuverter, so you can scale and invert incoming control voltages on the module. That lets you modulate the filter with LFOs, envelopes, or random sources without external processors. A second CV input has no attenuation, for applying raw, full-range voltage at 1:1 response.
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 and reduces failure points. Newcomers to module building like it for that reason: no cables to route and no cold joints hiding in connections. The kit includes everything needed to assemble the module, and pre-built versions arrive fully calibrated and ready to use.
Historical Context
The AI004 is the fourth module released by AI Synthesis, a boutique outfit founded by Abraham Ingle. It's positioned as a tribute to the MS-20 filter, with one purpose: bring that sound to Eurorack in an affordable, buildable format. It follows the AI001 Multiple, AI002 Mixer, and AI003 Looping ADSR. The manual recommends the AI004 as "the third module you should build" if you're new to DIY electronics.
Collectibility & Value
New units are listed at $160, though the market shows a range from $15 to $160 depending on configuration, likely reflecting bare kits versus fully assembled modules. The full DIY kit was priced at $85 as of March 2018, which one source called "deliciously cheap." 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. It's still in production and widely available, so it isn't a "collector's item" in the traditional sense.
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