AMSynths AM8320 (2008–2017)

A cult-favorite Eurorack filter that recreates the raw, singing sound of the Prophet-5 Rev 3. You have to find one first.

Overview

Turn the resonance on the AMSynths AM8320 past noon and it sounds like a 1983 Prophet-5 Rev 3 melting into a sine wave howl. The CEM3320 chip does that, and the AM8320 doesn't just emulate it. It is it. This is not a modernized, tamed version of the classic Curtis filter. It's a faithful re-creation that keeps the quirks, the instabilities and the overtones of the original. It isn't always predictable, and it will never win a spec sheet fight.

The AM8320 was never a mass-market module. It came in fits and starts, depending on the supply of rare and increasingly expensive CEM3320 chips. It first appeared as a PCB kit in 2008, built to fit Eurorack systems while keeping the exact filter topology of the Sequential Circuits Pro-One and Prophet-5 Rev 3. The designer, Rob at AMSynths, didn't just copy the circuit. He refined it, borrowing the CV rejection trimmer from Digisound's modular implementation to improve tracking stability. The core stayed the same: the FET op-amp input stage, the resonance feedback path that gives the Pro-One its snarl, and the raw, unfiltered sound.

It's not a Swiss Army knife. It's a 24dB/oct low-pass filter and nothing else - no high-pass, no band-pass, no multimode. The cutoff knob covers subsonic rumbles up through glassy highs, and the resonance doesn't just peak. It oscillates, cleanly and musically, so it can stand in as a second oscillator in a pinch. Three audio inputs with individual level controls let you blend sources before filtering. Two CV inputs with attenuverters handle modulation.

There were several iterations: the early 2008 PCBs, a tiny 2013 run of 10 assembled modules when NOS CEM3320s briefly surfaced, and a full relaunch in 2017 when the AS3320 (a modern pin-compatible replacement) became available. The 2017 version, known as the V4, simplified the build with a single PCB mounted directly to the panel, switched to vintage-style knobs, and dropped the "Frequency Fine" control from earlier versions. It's the most accessible version, though even this run was limited.

Specifications

Key Features

The CEM3320 Heart

The whole reason the AM8320 exists is the CEM3320 filter chip. Curtis Electromusic Specialties introduced it in 1980, and it became the filter in the Prophet-5 Rev 3, Oberheim OB-8, PPG Wave 2.2, and SCI Pro-One. Unlike later, more stable filter designs, the CEM3320 is somewhat volatile. It's temperature sensitive and slightly nonlinear, and its resonance path can be coaxed into self-oscillation with a musical purity that feels almost analog in the way vacuum tubes do. The AM8320 doesn't hide any of that. The FET op-amps used for input mixing keep the signal warm and slightly compressed before it reaches the filter, preserving the "vibe" of the original instruments.

Pro-One Resonance Circuit

Many CEM3320-based filters take the easy route and use a generic feedback loop. The AM8320 replicates the exact resonance feedback path of the Sequential Circuits Pro-One. That path gives the Pro-One its aggressive, vocal character, especially in the midrange. The Digisound 80-6 filter, while respected, uses a different approach that AMSynths found less effective. By sticking to the Pro-One's design, the AM8320 avoids the "muddy" or "soft" resonance some clones have, and delivers a sharper, more present tone that cuts through dense patches.

CV Rejection & Tracking Trimmers

One of the smartest inclusions on the AM8320 is the onboard CV rejection trimmer, borrowed from Digisound's modular implementation. It adjusts how well the filter rejects control voltage noise, which improves stability. That matters in complex Eurorack systems where ground loops and interference are common. The CV/Oct tracking trimmer lets users fine-tune the 1V/oct response, compensating for slight variations in the CEM3320 chip or power supply. Both are needed to make the module behave reliably in a live or studio setting.

Historical Context

The AM8320 showed up when Eurorack was shifting from experimental DIY kits to polished commercial modules, but there was still a strong appetite for genuine analog character. By the late 2000s the CEM3320 was getting scarce, and companies like Doepfer had discontinued their own 3320-based filters (like the A-121) because of parts shortages. AMSynths stepped into that gap as a small builder rather than a mass producer, run by someone who had built a Pro-One modular system and knew what made the filter sing. The AM8320 didn't try to compete with Doepfer or Analogue Solutions on price or features. It went for the original sound, even if that meant limited runs and high prices.

It also arrived just before the wave of Behringer reissues, which would later make vintage filter chips like the CEM3320 nearly impossible to source at reasonable prices. The 2013 run of 10 modules was only possible because the designer found a stash of NOS (New Old Stock) CEM3320s. By 2017 that was no longer feasible. The switch to the AS3320 (a modern clone produced by Alfa) let production resume, but availability stayed spotty. That scarcity, plus glowing user reports, turned the AM8320 into a cult item.

Collectibility & Value

The AM8320 is not a module you casually add to your rack. Even new, it sold for $258, and used prices today reflect its rarity and reputation. A working V4 module in good condition typically fetches $350 - $450 on the secondhand market, with earlier PCB-built versions sometimes commanding even more from purists who want the "original" experience. Fully assembled units from the 2013 run are rare and can exceed $500 if they come with documentation or original packaging.

The biggest risk is not failure. It's how the unit was built. The AM8320 was sold as a PCB kit for years, so many units in circulation were hand-built by users. That isn't inherently a problem (the designer provided detailed instructions), but build quality varies. Look for clean soldering, secure panel mounting, and proper calibration. The V4's panel-mounted design is sturdier than earlier versions, which used PCB-mounted pots that could crack under stress.

The CEM3320 chip itself is the weak spot. Original chips are decades old and can drift or fail. The AS3320 used in later models is more reliable, but even it can be sensitive to power supply noise. Before buying, test for smooth cutoff sweep, stable tracking across octaves, and clean resonance oscillation. If the filter sounds thin or unstable, it may need recapping or trimmer adjustment. That's simple for a technician, but a hassle if you're not DIY-inclined.

If you're building a vintage-voiced modular system, especially one inspired by 1980s polysynths, the AM8320 is worth the hunt. If you need reliability, consistency, or multimode flexibility, consider alternatives like the Erica Pico DSP or the Intellijel Polaris.

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