AMSynths AM1031 (2024)
A Eurorack filter that resurrects a myth—five modes of voltage-controlled tonal sculpting, born from ARP’s lost blueprint and refined for the modern modular rig.
Overview
The AM1031 Multipurpose Filter from AMSynths is a 14HP Eurorack module. It is built on ARP's 1970 Tonus Inc. catalog promise: a dual-state variable filter with high-pass, low-pass, band-pass, notch, and the rarest of the lot - peaking mode. It is not a clone, and it is not a loose reinterpretation. It is a reverse-engineered reconstruction of a circuit that never officially existed. For decades, synth historians speculated about what the ARP 1031 might have sounded like. AMSynths' Rob Keeble reverse-engineered it, drawing from Dennis Colin's later work at Aries and Oberheim to reconstruct a topology that feels both vintage-correct and sonically advanced.
Unlike many modern SVFs that lean on textbook designs, the AM1031 is built around a re-engineered circuit inspired by the Aries 327 Multipurpose Filter, a forgotten design from 1975. It uses CA3080-style OTA chips with LM301 op-amps and JFET buffers. The sound is clean but not sterile. There's a slight organic sag in the resonance ramp, and a softness in the peaks that digital filters often miss. The peaking mode is the unusual part. Instead of boosting a narrow band, it inverts the notch behavior, which gives a focused, almost laser-like tonal spike that cuts through dense mixes without harshness.
The AM1031 sits in the AMSynths 2500 "Lost Modules" series. Below it is the AM1006 Low Pass Filter, a simpler 24dB workhorse. Above it, conceptually, there's nothing. It is the flagship multimode filter in the lineup, not in size, but in versatility and historical weight. It doesn't replace the AM8060-based designs Keeble experimented with in 2015 (which used IR3109 chips and Jupiter-6 DNA), but it supersedes them in faithfulness to the original idea. That earlier version worked, but it lacked the fifth peaking mode and relied on a different architecture. The 2024 AM1031 is the answer to a 50-year-old question.
Specifications
| Manufacturer | AMSynths |
| Production Years | 2024–present |
| Format | Eurorack |
| HP | 14 |
| Depth | 40 mm |
| Current Draw +12V | 80 mA |
| Current Draw -12V | 70 mA |
| Filter Type | State Variable Filter (SVF) |
| Filter Modes | Low-Pass, High-Pass, Band-Pass, Notch, Peaking |
| Slope | 12dB/octave per stage |
| Frequency Control | Voltage-controlled (1V/Oct) |
| Resonance Control | Voltage-controlled with dynamic resonance compensation |
| Inputs | 1x Audio In, 1x FM CV, 1x Resonance CV |
| Outputs | LP, HP, BP, Notch, Peaking |
| Power Supply | ±12V, 3.5mm power connector |
| Trim Controls | 7 trimmers for calibration (frequency tracking, resonance response, CV offset) |
| Construction | SMD components, green PCB, front panel with silkscreen labeling |
| Special Features | Dynamic resonance control to prevent distortion at high Q, peaking mode as inverse notch |
Key Features
The Fifth Mode: Peaking as Sonic Spotlight
Most multimode filters stop at four: low-pass, high-pass, band-pass, and notch. The AM1031 adds a fifth, peaking, which behaves like a mirror image of the notch. A notch filter removes a narrow band of frequencies. Peaking amplifies it, creating a resonant spike that's surgical yet musical. Run a drum loop through it, dial in peaking mode, and the snare sits forward in the mix without EQ or compression. It's also self-oscillation adjacent. Push the resonance and the peak will sing, but the dynamic control circuit keeps it from breaking up, so it stays clear even at extreme settings. This behavior was lifted directly from the Aries 327 design, where Dennis Colin implemented it to avoid the need for input attenuation during resonance sweeps.
Voltage-Controlled Everything, Calibrated for Stability
The AM1031 demands CV. The 1V/Oct frequency input tracks accurately across ten octaves, thanks to seven onboard trimmers that allow fine-tuning of response, CV offset, and resonance scaling. Seven trimmers sounds like overkill, and it is. This is not a plug-and-play module. First-time users report spending 30 minutes with a tuner and oscilloscope just to dial it in. Once calibrated, it holds pitch like a precision instrument. The resonance CV input includes dynamic compensation, a feature Keeble admired in the Aries design: as resonance increases, the circuit subtly adjusts gain to prevent the output from distorting. That means resonance sweeps can be automated from zero to self-oscillation without clipping.
From 5U Dream to Eurorack Reality
The AM1031 began as a 5U prototype in 2015, built around dual AM8060 circuits and IR3109 OTAs. That version was scrapped because of front panel durability issues and a lack of historical fidelity. The 2024 Eurorack version is a complete redesign. It drops the Jupiter-6 DNA in favor of a topology closer to what ARP might have built had they ever manufactured the module. It's smaller, more efficient, and more faithful to the period. The shift to SMD components in 2024 allowed for tighter layout and improved thermal stability, but Keeble kept through-hole op-amps (LT1012, OPA2134) for their sonic character. The module feels like it could have existed in 1974, yet performs like a modern boutique design.
Historical Context
The AM1031 exists because ARP teased it. In a 1970 Tonus Inc. catalog, the company listed a "Multipurpose Filter" (Model 1031) with five modes, but no known unit was ever produced. Unlike the ARP 2500's famous 4012 or 4034 filters, the 1031 remained vaporware. Keeble, a veteran of vintage synth restoration and a key designer behind Behringer's ARP reissues, saw the gap and filled it. His research led him to Dennis Colin, ARP's chief engineer and later designer of the Oberheim SEM. Colin's work on the Aries 327 in 1975, another multipurpose filter with peaking mode, became the blueprint. The AM1031 isn't a copy of the Aries, but a spiritual successor, built with the same design philosophy: maximum flexibility, minimal compromise.
In the early 2020s, as Eurorack matured, demand grew for modules that were historically significant as well as functional. The AM1031 fits that niche, part restoration, part archaeology. It competes not with mainstream filters like the Mutable Instruments Steiner or the Make Noise QPAS, but with boutique reissues like the 4ms Dual VC Octave Switch or the Intellijel Polaris. Those are original designs. The AM1031 claims lineage.
Collectibility & Value
The AM1031 is not a mass-market module. Produced in small batches by AMSynths, it sells out quickly through the company's web store, often within hours of release. As of 2025, new units retail for £249 / €295 / $329, with no significant depreciation on the secondhand market. Used units in good condition trade for 80-90% of retail, reflecting scarcity more than demand spikes. There are no known manufacturing defects in the 2024 SMD version, though early adopters are advised to check calibration, especially if buying used. The seven trimmers mean a poorly set-up unit can drift or respond unevenly to CV.
Failures are rare but not unheard of. The most common issue is misaligned front panel silkscreen wear, a problem from the 2015 prototype that was resolved in the 2024 run. The SMD components are sturdy, but the module's reliance on precise calibration means DIY repairs are discouraged unless you have access to a calibration jig and test equipment. Owners report that once set up, the AM1031 is stable for years. No firmware, no updates, just analog circuitry.
If you're hunting for one, prioritize units sold directly by AMSynths or authorized dealers. Avoid listings that don't mention calibration status. This isn't a beginner's filter. It's for the collector who wants a piece of synth history that never was, or the sound designer who needs that fifth mode to cut through a mix. It's not the cheapest SVF, nor the easiest to use.
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