AMSynths AM8328 Mirage Filter (2010–2024)

A 14 HP Eurorack module that resurrects a forgotten analog filter chip with surgical precision and soulful character.

Overview

Plug in the AM8328 Mirage Filter for the first time, twist the resonance past 12 o’clock, and you’ll hear something rare: a filter that doesn’t collapse under its own feedback. Most analog filters—especially those with voltage-controlled resonance—tend to eat their own signal as resonance climbs, leaving you with a thin, wheezing shriek. Not this one. The Mirage doesn’t just sustain level as it resonates; it *boosts* it, thanks to an internal compensation trick baked into the CEM3328 chip. That means when you push it into self-oscillation, you get a full-bodied sine wave that cuts through a mix like a theremin in a cathedral. It’s not just stable—it’s *musical*. And that’s the magic of the AM8328: it takes a chip that barely saw the light of day in the analog era and gives it the hands-on control it always deserved.

AMSynths didn’t just slap the CEM3328 into a Eurorack panel and call it a day. They built a shrine to it. This module is one of the last analog filter designs from the golden age of discrete synthesis, released in 1984—just as digital synths were taking over. The CEM3328 was used in only a handful of products: the Ensoniq Mirage sampler, the BIT01, and a few drum machines. In most of those, it was tucked behind digital control, never touched by a knob, never pushed into the red. The Mirage Filter module flips that script. Here, it’s front and center, with dedicated knobs for cutoff, resonance, and envelope amount, plus CV inputs that respond with laser precision. It’s not a clone of a famous filter—it’s a rescue mission.

In a market flooded with recreations of Moog ladders, Oberheim SEMs, and Roland IR3109s, the AM8328 stands out by being obscure. It doesn’t sound like a 2032 or a 4075. It’s smoother than a diode ladder, tighter than a state-variable, and more articulate than a transistor-ladder. The response is 24dB/octave, four-pole, with a silky top end that doesn’t get harsh, even when driven. The exponential scaling over 14 octaves means tracking is dead-on across the keyboard range—no tuning tricks, no offset fiddling. And because it uses differential inputs, noise rejection is excellent, making it a solid choice in mixed-signal systems where ground loops and crosstalk can plague lesser designs.

But let’s be clear: this isn’t a “character” filter in the way a Korg MS-20 or a Serge resonator is. It doesn’t growl or bark. It’s more like a scalpel—clean, precise, but capable of surprising warmth when you feed it rich waveforms. Pair it with a gritty VCO, and it carves out space like nothing else. Use it on drums, and you get that punchy, rounded-off transient that sits perfectly in a mix. It’s the kind of filter that makes you rethink your patches, not because it’s aggressive, but because it reveals detail you didn’t know was there.

Specifications

Key Features

The CEM3328 Chip: A Latecomer That Deserved Better

The heart of the AM8328 is the CEM3328—a chip that arrived too late to the analog party. Released in 1984 by Curtis Electromusic Specialties, it was one of the last filter/VCA combo ICs designed before the digital wave crested. Unlike earlier chips that required external components to stabilize resonance, the CEM3328 had built-in compensation. That’s what allows the AM8328 to increase signal level as resonance rises—something most filters can’t do without external circuitry. In the Ensoniq Mirage, this feature was ignored; the filter was digitally controlled and never allowed to oscillate. AMSynths unlocked it. Now, for the first time, you can *play* this filter like an instrument. Turn up resonance, tweak the cutoff, and you’re not just filtering—you’re generating tones.

Differential Inputs: Quiet, Clean, and Resilient

Most Eurorack filters use single-ended inputs, which are prone to picking up noise, especially in larger systems. The AM8328 uses differential signal inputs, a feature more common in professional audio gear than in modular synths. This means the input stage compares the difference between two signals, rejecting any common-mode noise—like ground hum or RFI—that sneaks in along the way. The result is a noticeably cleaner passband, even when driving the filter hard. It’s a subtle advantage, but one that becomes obvious in complex patches where signal integrity matters. You don’t realize how much noise you’re losing until you hear how much you’re *not* losing here.

Resonance That Doesn’t Lie

Voltage-controlled resonance is a blessing and a curse. On paper, it’s great—modulate resonance with an LFO, and you get wah-wah effects, sweeping textures, dynamic movement. In practice, most filters lose level as resonance increases, forcing you to compensate with makeup gain. The AM8328 sidesteps this with its internal level compensation. As resonance climbs, the signal is gently boosted to maintain consistent output. This isn’t a limiter or a compressor—it’s baked into the chip’s design. So when you modulate resonance, you’re not fighting a disappearing signal. You’re shaping tone with confidence. And when you push it into oscillation, you get a pure, stable sine wave that holds pitch beautifully. No wobble, no drift—just tone.

Historical Context

The AM8328 didn’t emerge in a vacuum. It arrived in 2010, right as the Eurorack revival was gaining momentum. Modular wasn’t just coming back—it was mutating. Builders like Doepfer, Make Noise, and Intellijel were reimagining analog synthesis, but most were focused on either cloning classics or inventing new architectures. AMSynths took a different path: they dug into the footnotes. The CEM3328 was a footnote—a chip used in a few obscure machines, never celebrated, never mass-produced. Ensoniq used it in the Mirage sampler, but only as a fixed filter, never under real-time analog control. The AM8328 gave it a second life.

At the time, most boutique filter modules were chasing the sound of the 1970s—Moog, ARP, Oberheim. AMSynths looked to the early 1980s, a period often dismissed as the “dark age” of analog, when cost-cutting and digital integration diluted the purity of earlier designs. But the CEM3328 was different. It was a final, refined statement from the analog era—engineered for stability, precision, and integration. By building a module around it, AMSynths wasn’t just preserving history; they were correcting it. They asked: what if this chip had been given the spotlight? What if it had knobs, patch points, and a place in a musician’s hands?

The answer was immediate. The AM8328 became one of AMSynths’ most popular modules, alongside the AM8109 JP8 filter and the AM8140 Dual ADSR. It wasn’t the flashiest, but it was the one builders reached for when they needed reliability without sterility. It found its way into experimental setups, live rigs, and studio systems where consistency mattered. And because it was based on a chip that never had a “vintage” pedigree, it avoided the price inflation that plagued clones of more famous filters. It was, in every sense, a hidden gem.

Collectibility & Value

The AM8328 is discontinued, but not rare. AMSynths produced steady runs from 2010 until 2024, and units pop up regularly on the used market. That said, condition matters. These modules were built with high-quality components—WIMA capacitors, precision resistors, and custom slide pots—but they’re not indestructible. The most common failure point is the power connector, especially if the module was frequently swapped between cases. Bent pins or cracked solder joints on the power rail can cause intermittent operation or complete failure. Always check for clean power draw before buying.

Another issue: the CEM3328 chip itself. While robust, it’s no longer in production, and replacements are scarce. If the chip fails—due to static discharge or voltage spike—repair becomes a sourcing challenge. Some techs have reported success with NOS (new old stock) chips, but they’re expensive and not guaranteed to match spec. There’s no drop-in modern replacement, so a dead chip could render the module a paperweight.

That said, most AM8328s out there are healthy. They were built conservatively, with overrated components and solid construction. Owners report years of trouble-free use, even in touring setups. The included nylon washers and compatibility with both Doepfer and ASYS cases speak to a builder who cared about real-world usability.

On the market, expect to pay $225–$275 for a working unit in good condition. Scratches or dents don’t hurt value much—this is a utilitarian module, not a display piece. But a unit with a replaced chip or questionable service history might drop to $175. Bargain hunters should watch for bundles; the AM8328 often appears in full AMSynths racks, where its value gets absorbed into the package. If you’re after one, buy standalone, test thoroughly, and verify the resonance sweep and self-oscillation.

It’s not a “grail” module, but it’s a keeper. For anyone building a precision analog system—especially one focused on clean, musical filtering—the AM8328 is worth the hunt. It’s not loud, it’s not flashy, but it’s the kind of module that makes you wonder why more people didn’t talk about it when it was new.

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