AMSynths AM8328 Mirage Filter (2010–2024)
A 14 HP Eurorack module built around an analog filter chip that almost nobody used.
Overview
Turn the resonance on the AM8328 Mirage Filter past 12 o'clock and it does something most voltage-controlled-resonance filters don't. It keeps its level. Most analog filters lose signal as resonance climbs, and you end up with a thin, wheezing shriek. The Mirage boosts the level instead, thanks to an internal compensation trick in the CEM3328 chip. Push it into self-oscillation and you get a full-bodied sine wave that cuts through a mix. AMSynths took a chip that barely saw the light of day in the analog era and gave it hands-on control.
AMSynths didn't just slap the CEM3328 into a Eurorack panel. The chip is one of the last analog filter designs of the discrete synthesis era, released in 1984, just as digital synths were taking over. It was used in only a handful of products: the Ensoniq Mirage sampler, the BIT01, and a few drum machines. In most of those it sat behind digital control and never got a knob. The Mirage Filter module puts it 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.
The market is 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, with no tuning tricks and no offset fiddling. It also uses differential inputs, so noise rejection is excellent. That helps in mixed-signal systems where ground loops and crosstalk cause trouble.
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 and precise, with some surprising warmth when you feed it rich waveforms. Pair it with a gritty VCO and it carves out space. On drums it gives a punchy, rounded-off transient that sits in a mix.
Specifications
| Manufacturer | AMSynths |
| Production Years | 2010–2024 |
| Original Price | $195 USD |
| Module Format | Eurorack |
| HP Width | 14 HP |
| Filter Type | 4-pole Low Pass Filter (24dB/octave) |
| Core Chip | CEM3328 |
| Resonance Control | Voltage Controlled, with automatic signal level compensation |
| Frequency Response | Accurate exponential scaling over 14 octaves |
| Inputs | Differential audio input, CV cutoff, CV resonance, envelope amount |
| Outputs | Filtered audio output |
| Power Requirements | ±12V, 30mA per rail |
| Panel Material | Black anodized aluminum with silk-screened labeling |
| Mounting | Compatible with Doepfer and ASYS cases, includes nylon washers |
| Discrete Components | Matched transistor arrays, precision resistors, WIMA capacitors |
| Envelope Response | Adjustable positive or negative modulation |
| Self-Oscillation | Yes, at high resonance settings |
| Weight | Approx. 180g |
| Depth | 35mm (max) |
| Status | Discontinued |
Key Features
The CEM3328 Chip: A Latecomer That Deserved Better
The CEM3328 arrived late to the analog party. Curtis Electromusic Specialties released it in 1984, and it was one of the last filter/VCA combo ICs designed before the digital wave crested. Earlier chips needed external components to stabilize resonance. The CEM3328 has built-in compensation. That's what lets the AM8328 increase signal level as resonance rises, which most filters can't do without external circuitry. In the Ensoniq Mirage this feature went unused, because the filter was digitally controlled and never allowed to oscillate. AMSynths unlocked it. Turn up resonance, tweak the cutoff, and the filter generates tones.
Differential Inputs: Quiet, Clean, and Resilient
Most Eurorack filters use single-ended inputs, which pick up noise, especially in larger systems. The AM8328 uses differential signal inputs, a feature more common in professional audio gear than in modular synths. The input stage compares the difference between two signals and rejects common-mode noise like ground hum or RFI. The result is a noticeably cleaner passband, even when driving the filter hard. It's a subtle advantage, but it becomes obvious in complex patches where signal integrity matters.
Resonance That Doesn’t Lie
Voltage-controlled resonance has a catch. Modulate resonance with an LFO and you get wah-wah effects, sweeping textures, and dynamic movement. But most filters lose level as resonance increases, so you have to compensate with makeup gain. The AM8328 sidesteps this with internal level compensation. As resonance climbs, the signal is gently boosted to keep the output consistent. This isn't a limiter or a compressor. It's part of the chip's design. Push it into oscillation and you get a pure, stable sine wave that holds pitch. No wobble, no drift.
Historical Context
The AM8328 arrived in 2010, right as the Eurorack revival was gaining momentum. Builders like Doepfer, Make Noise, and Intellijel were reimagining analog synthesis, but most were cloning classics or inventing new architectures. AMSynths dug into the footnotes. The CEM3328 was a footnote: a chip used in a few obscure machines, never mass-produced. Ensoniq used it in the Mirage sampler, but only as a fixed filter, never under real-time analog control.
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 earlier designs. But the CEM3328 was a final, refined statement from the analog era, engineered for stability, precision, and integration. AMSynths built a module around it and gave it knobs and patch points.
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 builders reached for it when they needed reliability without sterility. It found its way into experimental setups, live rigs, and studio systems where consistency mattered. Because it was based on a chip that never had a "vintage" pedigree, it also avoided the price inflation that hit clones of more famous filters.
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. Condition still 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 is the CEM3328 chip itself. It's no longer in production, and replacements are scarce. If the chip fails from static discharge or a 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 leave the module a paperweight.
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 are practical touches.
Expect to pay $225-$275 for a working unit in good condition. Scratches or dents don't hurt value much, since this is a utilitarian module and not a display piece. A unit with a replaced chip or questionable service history might drop to $175. The AM8328 often appears in full AMSynths racks, where its value gets absorbed into the package, so buy standalone if you can. 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.
Similar items on eBay
No exact AMSynths AM8328 Mirage Filter listings are live right now — these searches show current AMSynths and similar vintage synth gear on eBay:
As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.