AMSynths CEM3328 (2020–2023)

A eurorack resurrection of the forgotten filter chip that almost defined the '80s—until digital swept it aside.

Overview

You plug in the AMSynths CEM3328 and twist the resonance up slowly, expecting the usual analog growl—and then it happens: a smooth, glassy sweep into self-oscillation that doesn’t crackle, doesn’t waver, doesn’t apologize. It’s not aggressive like a Moog ladder, not plasticky like a Korg MS-20, not nasal like a Roland 303. It’s something else entirely—refined, almost clinical, but with a warmth that sneaks up on you. This is the sound of the CEM3328 filter chip, resurrected from near obscurity by a UK boutique builder who saw potential in a silicon orphan. And while the chip itself dates back to 1984, the AMSynths module that bears its name didn’t arrive until decades later, giving modern modular users their first real chance to hear what might have been—if history hadn’t moved on too fast.

The CEM3328 wasn’t just another filter chip. It was Curtis Electromusic’s final analog hurrah, packed with features that should have made it a staple: differential inputs for noise rejection, voltage-controlled resonance with built-in Q compensation, and a frequency response stable across 14 octaves. It was technically superior to the CEM3320 it evolved from, with better temperature stability and a wider power supply range. But timing is everything. Released in the mid-'80s, just as digital synthesis and MIDI were taking over, the CEM3328 landed with a thud. It only ever appeared in a handful of instruments—the Ensoniq Mirage sampler, the BIT01 drum machine, and a few obscure Japanese synths—most of which locked the filter down with digital control, never letting it scream like a proper analog filter should. In the Mirage, it was deliberately detuned to prevent self-oscillation. It was like giving a racehorse a saddle and then parking it in a garage.

AMSynths didn’t just dust it off—they set it free. Their Eurorack module, released in the early 2020s, is a love letter to the chip’s unrealized potential. No microcontrollers, no digital menus, no menu diving. Just knobs, CV inputs, and a circuit that lets the CEM3328 behave like the analog star it was meant to be. It’s not a recreation or an emulation—it’s the real chip, wired for rebellion. And in doing so, AMSynths didn’t just build a filter; they rewrote a footnote in synth history.

Specifications

Key Features

The Chip That Time Forgot

The heart of this module is the CEM3328 itself—a chip that collectors and engineers now treat like a rare artifact. Unlike the CEM3320, which powered classics like the Prophet-5 and Oberheim OB-X, the 3328 added automatic resonance compensation, meaning the output level stays consistent even as you crank the Q. That’s a big deal in practice: most analog filters get quieter as resonance increases, forcing you to ride the levels manually. The 3328 handles it internally, giving you a more predictable, studio-friendly response. It also features differential signal inputs, a pro-level touch that reduces noise and crosstalk—something you’d expect in a high-end console, not a synth chip. And with a stable 1V/oct response over 14 octaves, it tracks pitch better than many discrete filter designs. It’s a chip that was built for precision, not character—but in the right hands, it develops plenty of both.

Analog Liberation

What AMSynths did was simple but radical: they gave the CEM3328 analog control. In the Ensoniq Mirage, the filter was digitally addressed, its parameters stepped in increments, never allowed to sweep continuously. The AMSynths module tears that limitation away. The cutoff and resonance are fully voltage-controlled, with smooth front-panel pots and attenuated CV inputs. You can modulate it with an LFO, envelope, sequencer—whatever you want. And crucially, it *can* self-oscillate, something the original Mirage implementation actively prevented. That changes everything. Now the filter isn’t just a tone shaper—it’s a sound source. At high resonance, it sings with a pure sine wave that’s stable enough to use as a VCO substitute. It’s not as rich or gritty as a Moog doing the same, but it’s smoother, more laser-like, and eerily precise.

Build and Integration

The module follows AMSynths’ “more analog than analog” philosophy—no microcontrollers, no firmware, no hidden digital layers. It’s a discrete analog circuit centered on the CEM3328, with supporting components chosen for stability and sonic transparency. The panel is powder-coated steel, with a clean, industrial layout that puts function first. The knobs are sturdy, the jacks are recessed to avoid cable strain, and the power connector is standard Eurorack. At 12 HP, it’s not the smallest filter on the market, but it’s not bloated either. It fits comfortably in most cases, and the 35mm depth won’t cause issues in shallow racks. It draws modest current, making it easy to integrate into any system. And because it uses the actual CEM3328 chip—not a clone or recreation—its behavior is authentic down to the silicon level.

Historical Context

The CEM3328 was released in 1984, the same year Yamaha dropped the DX7 and synth history pivoted hard toward digital. Analog was suddenly “old news,” and manufacturers scrambled to cut costs, streamline production, and embrace microprocessors. Curtis Electromusic, once a key supplier to Sequential, Oberheim, and ARP, was on the ropes. The 3328 was their most advanced filter chip yet, but it arrived too late to matter. Ensoniq used it in the Mirage sampler, a budget-friendly machine that undercut the Fairlight and Emulator, but they neutered the filter’s potential with digital control and conservative tuning. A few other companies dabbled—Elka, for example, used it in the EK-44 organ—but none gave it the analog love it deserved. By the late '80s, Curtis was gone, and the 3328 faded into obscurity.

Fast forward to the 2020s, when the Eurorack revolution created a hunger for forgotten circuits and obscure chips. Builders like Electric Druid began sourcing NOS (New Old Stock) CEM3328 chips, testing them, and selling them to DIYers. AMSynths saw an opportunity: take this overlooked component and give it the platform it never had. The result wasn’t just a filter module—it was a correction of history. In a world where every other vintage chip had been celebrated, the 3328 had been ignored. AMSynths changed that, not by exaggerating its quirks, but by letting it speak for itself.

Collectibility & Value

The AMSynths CEM3328 is discontinued, and original units in good condition now trade for £300–£400 on the secondhand market, depending on availability. That’s a significant premium over its original £201 price, but not outrageous by boutique Eurorack standards. The main driver of value is scarcity: AMSynths produced it in limited runs, and the supply of genuine CEM3328 chips is finite. Once those are gone, reproductions will have to rely on clones or discrete builds—which may sound similar, but won’t be the same.

Buyers should be cautious of counterfeits or modules claiming to use the CEM3328 but actually using modern equivalents. The real chip has a distinct date code (most commonly 8619, indicating 19th week of 1986), and the module should include documentation confirming the chip’s authenticity. Also, check that the power draw matches specs—some clones run hotter or draw more current, which can cause issues in crowded cases.

Common failures are rare, but not impossible. The CEM3328 chip itself is robust, but it’s sensitive to voltage spikes. Modules powered by unstable or poorly regulated supplies may suffer premature failure. There are no user-serviceable trimmers on the board—AMSynths designed it for plug-and-play stability—so if it stops working, repair usually means replacing the chip or PCB. Given the chip’s rarity, that could be costly. For that reason, owners are advised to use high-quality power supplies and avoid hot-plugging.

If you’re hunting for one, prioritize units with clean panels, no bent jacks, and verified working status. Since it’s a passive filter (no digital brains), testing is straightforward: sweep the cutoff, modulate resonance, check for oscillation. If it responds smoothly and doesn’t crackle, it’s likely fine. Given how few were made, finding a dead one isn’t uncommon—so don’t assume every listing is functional.

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