AMSynths CEM3328 (2020–2023)

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

Overview

Turn the resonance up slowly on the AMSynths CEM3328 and it goes into a smooth, glassy self-oscillation. It doesn't crackle and it doesn't waver. It isn't aggressive like a Moog ladder, plasticky like a Korg MS-20, or nasal like a Roland 303. It sounds refined, almost clinical, with some warmth underneath. That is the CEM3328 filter chip, brought back from near obscurity by a UK boutique builder. The chip dates back to 1984, but the AMSynths module that carries its name didn't arrive until decades later. It gave modern modular users their first real chance to hear what might have been if history hadn't moved on so fast.

The CEM3328 was Curtis Electromusic's final analog filter chip. It had differential inputs for noise rejection, voltage-controlled resonance with built-in Q compensation, and a frequency response stable across 14 octaves. It beat the CEM3320 it evolved from, with better temperature stability and a wider power supply range. But it was released in the mid-'80s, just as digital synthesis and MIDI were taking over, and it landed with a thud. It only appeared in a handful of instruments: the Ensoniq Mirage sampler, the BIT01 drum machine, and a few obscure Japanese synths. Most of them locked the filter down with digital control. In the Mirage, it was deliberately detuned to prevent self-oscillation. That's a racehorse with a saddle on, parked in a garage.

AMSynths dusted it off and let it run. Their Eurorack module, released in the early 2020s, has no microcontrollers and no digital menus. It has knobs, CV inputs, and a circuit that lets the CEM3328 behave like the analog part it was designed to be. It isn't a recreation or an emulation. It's the real chip.

Specifications

Key Features

The Chip That Time Forgot

The CEM3328 itself is the heart of this module, and collectors and engineers now treat the chip like a rare artifact. The CEM3320 powered classics like the Prophet-5 and Oberheim OB-X. The 3328 added automatic resonance compensation, so the output level stays consistent as you raise the Q. Most analog filters get quieter as resonance increases, and you have to ride the levels by hand. The 3328 handles that internally, which makes the response more predictable. It also has differential signal inputs, which reduce noise and crosstalk and are more at home in a high-end console than in a synth chip. With a stable 1V/oct response over 14 octaves, it tracks pitch better than many discrete filter designs. It was built for precision, and in the right hands it gets plenty of other qualities too.

Analog Liberation

What AMSynths did was simple: they gave the CEM3328 analog control. In the Ensoniq Mirage, the filter was digitally addressed. Its parameters moved in steps and never swept continuously. The AMSynths module removes that limit. Cutoff and resonance are fully voltage-controlled, with smooth front-panel pots and attenuated CV inputs. You can modulate it with an LFO, an envelope or a sequencer. It can also self-oscillate, which the original Mirage implementation actively prevented. That makes the filter a sound source as well as a tone shaper. At high resonance it produces a pure sine wave, stable enough to use as a VCO substitute. It isn't as rich or gritty as a Moog doing the same. It's smoother, more laser-like, and very 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. 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 most cases, and the 35mm depth won't cause trouble in shallow racks. It draws modest current. Because it uses the actual CEM3328 chip and not a clone or recreation, its behavior is genuine down to the silicon.

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 move to 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. Ensoniq used it in the Mirage sampler, a budget-friendly machine that undercut the Fairlight and Emulator, but they limited the filter with digital control and conservative tuning. A few other companies tried it. Elka, for example, used it in the EK-44 organ. By the late '80s, Curtis was gone, and the 3328 faded into obscurity.

Fast forward to the 2020s, when the Eurorack boom 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 opening: take this overlooked component and give it the platform it never had. Every other vintage chip seemed to have had its turn, and the 3328 had been ignored. AMSynths built the module without exaggerating the chip's quirks.

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. Scarcity drives the value. 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 watch for counterfeits, or modules claiming to use the CEM3328 while 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 is genuine. Also check that the power draw matches specs. Some clones run hotter or draw more current, which can cause issues in crowded cases.

Failures are rare, but not impossible. The CEM3328 chip itself is sturdy, but it's sensitive to voltage spikes. Modules powered by unstable or poorly regulated supplies may fail early. There are no user-serviceable trimmers on the board, since AMSynths designed it for plug-and-play stability. If it stops working, repair usually means replacing the chip or PCB, and given the chip's rarity, that could be costly. Owners are advised to use high-quality power supplies and avoid hot-plugging.

If you're hunting for one, go for units with clean panels, no bent jacks, and verified working status. It's a passive filter with no digital brains, so 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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