AMSynths AM8328 (2012–2019)
A skiff-friendly Eurorack filter that brings back the CEM3328 chip with self-oscillation, Q compensation, and desert-orange lettering
Overview
The CEM3328 never really got its moment. It showed up as analog was peaking and got buried under the digital wave. The AM8328 isn't a recreation of some legendary filter. 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 hands-on control. No menus, no digital menus masquerading as knobs. You get voltage-controlled cutoff, resonance with full self-oscillation, and one clever trick: as resonance increases, the filter compensates for signal loss, so you don't fade out just as things get interesting. That changes how you patch. You can crank the Q into screaming sine waves without reaching for a VCA to make up the level, 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 mount parallel to the front panel, which cuts depth down to just 35 mm. That makes it one of the rare boutique filters that fits tight cases without compromise. It's 14 HP, which is standard. The real story is the build: polystyrene capacitors (the large green Soviet K71-7s, if you like 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 ties to the "mirage" theme, like a shimmer in the heat, and it stands out in a rack full of black and silver.
Don't expect this to sound like a Moog or a Roland. The AM8328 has its own sound - smooth, articulate, and slightly clinical at low resonance, with a throaty growl when pushed. It doesn't emulate any specific vintage filter. It lets the CEM3328 speak for itself, which makes it 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 self-oscillates nicely, but it's also a full-featured filter that behaves predictably in complex patches, whether for subtle tonal shaping or full resonance sweeps. The chip was never really tweaked by human hands in its original life, so every tweak here is new territory.
Specifications
| Manufacturer | AMSynths |
| Production Years | 2012–2016, 2019 (redesign) |
| Original Price | $195 USD |
| Format | Eurorack |
| Width | 14 HP |
| Depth | 35 mm (2019 version) |
| Power | ±12V |
| Filter Type | 4-pole OTA Low-Pass (CEM3328) |
| Resonance | Voltage-controlled, self-oscillating |
| Q Compensation | Automatic signal level boost with resonance |
| Inputs | 2x audio input, 2x cutoff CV, 1x resonance CV |
| Outputs | 1x audio output |
| Controls | Cutoff frequency, resonance, key tracking, input level |
| Capacitors | Polystyrene (K71-7 Soviet) |
| Panel | 2mm aluminum, orange print |
| Hand Built | Yes, in Horsham, UK |
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 digital control locked it down and it was never allowed to self-oscillate. AMSynths implemented the chip as specified in the original datasheet and added front-panel access to resonance and CV control. 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.
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, cutting depth to 35 mm without sacrificing stability or layout. The build quality stayed high: hand-assembled in the UK, with sturdy jacks and precision-cut panels. The orange print is also highly legible, even in dim lighting. It's a limited-run module, with only 10 completed units in the 2019 run, so collectors who like both the engineering and the rarity chase it.
Modular-Ready Circuit Design
AMSynths didn't just drop the CEM3328 into a Eurorack shell. The module includes input and output buffer op-amps, which weren't always present in original implementations, for 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 lets you dial in how much 1V/oct pitch CV affects the filter, which is useful for monosynth-style patches without external mixers. Owners report it fits 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 a component, not a feature. By 2012, the Eurorack scene was exploding, and there was a hunger for both the real thing and novelty. Modules that resurrected rare or underappreciated circuits, like the CEM3372, IR3109, or BA662, were gaining cult status. The AM8328 landed in that spot: not a rehash of a Minimoog ladder, but a deep cut. It wasn't competing with the Moog 904A or the Roland IR3109. It offered something different in a market flooded with filter clones.
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. Some of the best circuits were never widely used, either because they arrived too late or were buried in budget gear.
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 hold up well, and the hand-wired UK build quality is consistent. Still, 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. AMSynths doesn't plan to reissue the module because of chip availability, so this is effectively a closed production run.
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