AMSynths AM8328 The Mirage Filter (2013–2019)

A rare 4-pole OTA filter that breathes analog life into a forgotten CEM3328 chip—bright orange panel, self-oscillating magic, and a sound that’s both dry and hypnotic.

Overview

Plug in the AM8328 and you’re not just adding a filter—you’re resurrecting a ghost. The CEM3328 chip at its core was one of the last analog filter ICs before digital took over, released in 1984 and barely used outside the Ensoniq Mirage sampler and a few obscure drum machines. In those machines, it was digitally controlled, never allowed to self-oscillate, and buried under firmware that kept its character locked down. AMSynths didn’t just dust it off—they set it free. The AM8328 The Mirage Filter gives this orphaned chip its first real analog front-panel control, letting it scream, resonate, and sing like it was meant to. It’s a 4-pole low-pass design built exactly to the CEM datasheet, using high-quality polystyrene capacitors (those big green Soviet K71-7s you’ll spot on the PCB), and it’s got a trick most filters don’t: as resonance increases, the signal level is automatically boosted to compensate for the usual volume drop. That means you can crank the Q all the way up without the sound vanishing into a thin, brittle whine.

It’s not a silky-smooth Roland IR3109 or a throaty Moog ladder—it’s something drier, more precise, with a kind of crystalline clarity that cuts through dense patches. Dial in resonance and it doesn’t just howl; it sustains, oscillates cleanly, and tracks pitch perfectly across 14 octaves thanks to its accurate exponential response. The filter has two audio inputs, two CV inputs for cutoff frequency with attenuverters, a key tracking pot for keyboard control, and a dedicated resonance CV input. It’s a no-frills, no-compromise design that trusts the chip to do the work. And it does—beautifully. The original run started in 2013, with only 40 units built through 2016. A second, skiff-friendly revision in 2019 cut the depth to 35mm by stacking two PCBs side-by-side, making it easier to build and fit into tight cases. That batch sold out fast—just 10 units—and with the CEM3328 now long out of production and selling for premium prices on eBay, it’s unlikely AMSynths will ever make more.

Specifications

Key Features

The CEM3328 Chip: A Forgotten Analog Relic

The CEM3328 was designed by Doug Curtis, the same engineer behind the CEM3372 and CEM3396, and it was meant to be a high-performance, stable OTA filter for the next generation of analog synths. But by 1984, the industry was pivoting hard to digital. The Mirage sampler used it, but only in a fixed, digitally controlled way—no front-panel tweaking, no resonance sweeps, no oscillation. AMSynths didn’t just replicate it; they liberated it. By building the AM8328 exactly to the CEM datasheet, they preserved its original character: tight, precise, and dynamically stable. The differential inputs reject noise, and the exponential converter is accurate across its full 14-octave range, making it ideal for melodic patches that need to stay in tune. This isn’t a filter that “colors” sound aggressively—it shapes it with surgical clarity, letting the source material breathe while still carving out space.

Automatic Q Compensation: No More Volume Drop

Most analog filters lose amplitude as resonance increases, forcing users to compensate with a VCA or mixer. The AM8328 solves this at the circuit level: as resonance rises, the signal level is automatically boosted to maintain consistent output. This isn’t a sidechain trick or a digital fix—it’s baked into the CEM3328’s design and implemented faithfully here. The result? You can sweep the resonance from zero to self-oscillation without the sound thinning out or collapsing. It’s especially useful for live performance, where dramatic filter sweeps stay loud and present. It also makes the filter more forgiving in complex patches, eliminating the need for constant level balancing when modulating Q.

Skiff-Friendly Redesign of 2019

The original 2013–2016 version of the AM8328 had a deeper profile that made it awkward in shallow cases. In January 2019, AMSynths redesigned it with two PCBs mounted parallel to the front panel, reducing depth to just 35mm. This wasn’t just a cosmetic tweak—it made the module significantly easier to assemble and more compatible with travel-friendly skiff setups. The electrical design remained unchanged, so the sound is identical, but the build process became more accessible for DIY builders. Unfortunately, this redesign also marked the end of the line: with NOS CEM3328 chips becoming scarce and expensive, the 2019 run of 10 units was likely the last. There’s no PCB or panel kit currently available, making surviving modules even more desirable.

Historical Context

The AM8328 arrived during the early boom of Eurorack modular synthesis, when boutique builders were digging deep into obscure analog designs to differentiate themselves. While most were cloning classics like the Moog ladder or Oberheim SEM filter, AMSynths went off the beaten path. The CEM3328 had never been used in a standalone modular context—never given pots, never patched, never let oscillate. By building a module around it, AMSynths wasn’t just offering a new filter; they were completing a historical circuit that had been cut short by the digital revolution. The Ensoniq Mirage, where the chip was most famously used, was a budget sampler with a weak synth engine, and its filter was never fully exploited. The AM8328 finally gave the CEM3328 the spotlight it deserved. It also fit into a broader trend of “lost chip” modules—designs like the SSM2040 or CEM3372 that were underused in their time but prized by modern builders for their unique character. In that sense, the AM8328 isn’t just a filter—it’s a piece of analog archaeology.

Collectibility & Value

The AM8328 is a rare bird: only about 50 units were ever made across all production runs, and with no kits or PCBs currently available, the secondhand market is the only option. Used modules typically sell between $450 and $650, depending on condition and whether they come with the original packaging or documentation. The 2019 skiff-friendly versions are especially sought after due to their limited run and improved build. Because the CEM3328 chip is no longer manufactured, repairs are a major concern—replacements must be sourced from old Ensoniq Mirages (which contain eight per unit) or the secondary market, where prices can exceed $150 per chip. There are no known failure-prone components beyond standard electrolytic aging, but the Soviet polystyrene capacitors, while high quality, are decades old and could drift over time. Buyers should test for smooth cutoff sweeps, consistent resonance response, and proper self-oscillation. Modules with bent front-panel knobs or damaged jacks are common, as the original Doepfer-style hardware wasn’t always robust. Given its rarity and the unlikelihood of reissue, the AM8328 is a solid long-term hold for collectors of obscure analog tech—less a utility module, more a conversation piece with serious sonic chops.

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