AMSynths AM8328 The Mirage Filter (2013–2019)
A rare 4-pole OTA filter built around the forgotten CEM3328 chip. Bright orange panel, self-oscillating, and a dry, hypnotic sound.
Overview
The CEM3328 chip at the core of the AM8328 was one of the last analog filter ICs before digital took over. It was 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 set it free. The AM8328 The Mirage Filter gives this orphaned chip its first real analog front-panel control. It's a 4-pole low-pass design built exactly to the CEM datasheet, using polystyrene capacitors (those big green Soviet K71-7s you'll spot on the PCB). It also has a trick most filters don't: as resonance increases, the signal level is automatically boosted to compensate for the usual volume drop. So 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 drier and more precise, with a crystalline clarity that cuts through dense patches. Dial in resonance and 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. No frills. The design trusts the chip to do the work. 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
| Manufacturer | AMSynths |
| Production Years | 2013–2016, 2019 (redesign) |
| Original Price | $195 USD |
| Form Factor | Eurorack |
| HP Width | 14HP |
| Depth | 35 mm (2019 revision) |
| Power Connection | 16-pin Doepfer bus board |
| Voltage | ±12V (also tested at ±15V) |
| Filter Type | 4-pole OTA low-pass (CEM3328 chip) |
| Response | 24dB/octave |
| Frequency Range | 0.02 Hz – 20 kHz |
| Exponential Range | 14 octaves |
| Resonance | Voltage controlled, self-oscillating |
| Q Compensation | Automatic signal boost with resonance |
| Inputs | 2x audio in, 2x CV cutoff in (with attenuverters), 1x resonance CV in |
| Outputs | 1x audio out |
| Front Panel Controls | Cutoff frequency, resonance, key tracking, CV attenuverters |
| Capacitors | Polystyrene (K71-7 Soviet) |
| Panel Color | Black with bright orange lettering |
| Availability | Discontinued |
Key Features
The CEM3328 Chip: A Forgotten Analog Relic
The CEM3328 was designed by Doug Curtis, the same engineer behind the CEM3372 and CEM3396. 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. By building the AM8328 exactly to the CEM datasheet, AMSynths 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, which suits melodic patches that need to stay in tune. It doesn't "color" the sound aggressively. It shapes it with clarity and leaves the source material room to breathe.
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 handles 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 built into the CEM3328's design and implemented faithfully here. You can sweep the resonance from zero to self-oscillation without the sound thinning out or collapsing. That helps in live performance, where dramatic filter sweeps stay loud and present. It also makes the filter more forgiving in complex patches, since there's no 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. That 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. The 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.
Historical Context
The AM8328 arrived during the early boom of Eurorack modular synthesis, when boutique builders were digging into obscure analog designs to set themselves apart. Most were cloning classics like the Moog ladder or Oberheim SEM filter. AMSynths went another way. The CEM3328 had never been used in a standalone modular context - never given pots, never patched, never let oscillate. 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 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.
Collectibility & Value
Only about 50 units of the AM8328 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 sturdy.
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