AMSynths AM8328 (2012–2019)

A skiff-friendly Eurorack filter that resurrects the long-overlooked CEM3328 chip with self-oscillation, Q compensation, and desert-orange flair

Overview

You don’t often get to hear a chip that never really had its moment—buried under the digital wave just as analog was peaking. The AM8328 isn’t a recreation of some legendary filter; it’s a resurrection. AMSynths dug up the CEM3328, a 1984-era 4-pole OTA low-pass filter that barely saw the light of day outside the Ensoniq Mirage and a few obscure drum machines, and gave it the hands-on control it was always meant to have. No menus, no digital menus masquerading as knobs—just voltage-controlled cutoff, resonance with full self-oscillation, and a clever trick: as resonance increases, the filter automatically compensates for signal loss, so you don’t fade out just as things get interesting. That’s not just convenient—it changes how you patch. You can crank the Q into screaming sine waves without reaching for a VCA to compensate, which makes it feel more alive than most filter replicas that stick rigidly to the original circuit behavior.

Originally built between 2012 and 2016, the AM8328 was revived in 2019 with a redesigned, skiff-friendly layout—dual PCBs mounted parallel to the front panel, cutting depth down to just 35 mm. That makes it one of the rare boutique filters that actually fits in tight cases without compromise. It’s 14 HP, which is standard, but the real story is in the build: polystyrene capacitors (specifically the large green Soviet K71-7s, if you’re into that sort of detail), differential signal inputs, and a circuit that follows the CEM3328 datasheet faithfully, with AMSynths’ own refinements for modular use. The front panel’s bright orange lettering isn’t just aesthetic—it’s a nod to the “mirage” theme, like a shimmer in the heat, and it stands out in a rack full of black and silver. It’s not flashy, but it’s memorable.

Don’t expect this to sound like a Moog or a Roland. The AM8328 has its own character—smooth, articulate, and slightly clinical at low resonance, but with a throaty growl when pushed. It doesn’t emulate any specific vintage filter; instead, it lets the CEM3328 speak for itself, which means it’s more versatile than nostalgic. It tracks well across 14 octaves thanks to its accurate exponential response, and the key tracking pot works cleanly with 1V/oct sources. It’s not a one-trick self-oscillator—though it does that beautifully—but a full-featured filter that behaves predictably in complex patches, whether you’re using it for subtle tonal shaping or full-on resonance sweeps. And because it’s based on a chip that was never really tweaked by human hands in its original life, every tweak feels like a discovery.

Specifications

Key Features

The CEM3328 Chip: A Latecomer Finally Gets Its Due

The CEM3328 was one of the last analog filter chips developed before the digital takeover, and it showed promise—differential inputs, wide control range, and that smart Q compensation. But it landed in the Mirage, where it was locked down by digital control and never allowed to self-oscillate. AMSynths didn’t just copy the circuit—they liberated it. By implementing the chip as specified in the original datasheet and adding front-panel access to resonance and CV control, they turned a forgotten component into a performance tool. The result isn’t just accurate—it’s expressive. You can modulate resonance smoothly from zero to full oscillation, and the output level stays consistent, which is rare in OTA filters. Most designs lose level as resonance increases, forcing you to compensate downstream. The AM8328 avoids that by design, making it more musical in motion.

Skiff-Optimized Redesign

The 2019 update wasn’t just cosmetic. The original AM8328 was deeper—fine for standard cases, but a problem for skiffs or travel racks. The redesign stacked two PCBs horizontally, cutting depth to 35 mm without sacrificing stability or layout. That’s a big deal for modular users who value portability and density. The build quality remained high: hand-assembled in the UK, with robust jacks and precision-cut panels. The orange print isn’t just thematic—it’s highly legible, even in dim lighting. And because it’s a limited-run module (only 10 completed units in the 2019 run), it’s become something of a grail for collectors who appreciate both the engineering and the rarity.

Modular-Ready Circuit Design

AMSynths didn’t just drop the CEM3328 into a Eurorack shell—they adapted it. The module includes input and output buffer op-amps, which weren’t always present in original implementations, ensuring clean signal handling in a modular environment. The two audio inputs are differential, which helps reject noise in complex systems, and the cutoff CV inputs are normalized but can be overdriven for nonlinear effects. The key tracking pot is a small but thoughtful addition, letting you dial in how much 1V/oct pitch CV affects the filter—useful for monosynth-style patches without needing external mixers. It’s not over-engineered, but every detail serves a purpose, which is why owners report it integrates seamlessly into diverse systems, from vintage-style ARP clones to modern digital-analog hybrids.

Historical Context

The AM8328 exists in a strange historical pocket. The CEM3328 chip was released in 1984, right as analog synthesis was being eclipsed by digital samplers and FM. The Ensoniq Mirage, one of the few instruments to use it, was a budget sampler that prioritized digital control over hands-on tweaking. The filter was never meant to be a star—it was a component, not a feature. AMSynths flipped that script. By 2012, the Eurorack scene was exploding, and there was a hunger for both authenticity and novelty. Modules that resurrected rare or underappreciated circuits—like the CEM3372, IR3109, or BA662—were gaining cult status. The AM8328 landed right in that sweet spot: not a rehash of a Minimoog ladder, but a deep cut with real sonic potential. It wasn’t competing with the Moog 904A or the Roland IR3109—it was offering something different. And in a market flooded with filter clones, that mattered.

AMSynths has always focused on niche, historically grounded designs—replicas of ARP 2500 modules, Roland System 100M circuits, and obscure chips like the CEM3328. The AM8328 wasn’t a commercial blockbuster, but it cemented their reputation among purists. It also highlighted a trend: the idea that “vintage” doesn’t just mean “famous.” Some of the best circuits were never widely used, either because they arrived too late or were buried in budget gear. The AM8328 proved that a forgotten chip, given the right implementation, could become a modern classic. It’s not a nostalgia play—it’s a restoration of potential.

Collectibility & Value

The AM8328 is not a common module. The original run of 40 units (2012–2016) is long gone, and the 2019 redesign was limited to just 10 completed modules. That scarcity, combined with the rising profile of AMSynths’ ARP 2500 replicas, has made the AM8328 a sought-after item. Used units rarely appear on the market, and when they do, they sell quickly. Expect to pay between $350 and $500 depending on condition and whether it’s the original or 2019 version. The skiff-friendly revision tends to command a premium, especially among touring musicians or those with space-constrained racks.

There are no widespread failure points reported, but the reliance on the CEM3328 chip is a concern. These are no longer in production, and NOS (new old stock) chips are expensive and finite. If a unit fails, repair may be difficult or cost-prohibitive. Owners report that the polystyrene capacitors and buffer op-amps are robust, and the hand-wired UK build quality is consistent. Still, it’s wise to test any used unit thoroughly—especially the resonance sweep and self-oscillation. Some early forums mention a key tracking alignment issue in a few units, but it’s not systemic. Because AMSynths doesn’t plan to reissue the module (due to chip availability), this is effectively a closed production run. For collectors, that makes it a stable hold; for players, it means you’re buying a piece of modular history that won’t be reissued.

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