AMSynths AM8320 Prophet Filter (2008–2017)

It rewires the Prophet's sound for the modular age, with a resonance that will either sing you to sleep or scream like a jet engine.

Overview

The AMSynths AM8320 Prophet Filter puts an early '80s Prophet filter into a Eurorack rack. It isn't a vague approximation of the Sequential Circuits Prophet 5 Rev 3 or SCI Pro-One filter. It's a close recreation of the CEM3320-based circuit behind that era's lush, singing leads and punchy basses. AMSynths refined it, too. The AM8320 blends the core topology of the Pro-One's 24dB/oct low-pass filter with the CV rejection improvements from Digisound's modular designs. That makes it both authentic and practical for modern patching.

The resonance is where this filter earns its keep. Turn it up and you get the expected sweep into self-oscillation, but the result is a smooth, musical sine wave that holds its pitch with surprising stability. That's no small feat for a chip-based filter. It comes from AMSynths' decision to replicate the Pro-One's feedback path accurately instead of relying on the less effective Digisound resonance circuit. The filter goes from warm, rounded pads to screaming, feedback-laden leads. It's aggressive when you want it to be, but never harsh. There's richness in the overtones and a slight softness in the attack that keeps it from turning brittle, even at full resonance.

The filter is built around the CEM3320 (and later the AS3320). The front panel has three audio inputs with individual level pots, a cutoff knob with no fine/coarse split (a simplification from later versions), resonance, and two CV modulation inputs with dedicated attenuators. The layout feels more like a vintage modular than a boutique Eurorack module. The signal path uses a FET op-amp to condition the input, so the CEM3320 sees the right level, and another op-amp buffers the output. Simple, effective, and transparent.

Specifications

Key Features

The CEM3320 Heart, Faithfully Rendered

The foundation is the CEM3320, a chip that for a brief moment in the early 1980s was the gold standard for polyphonic synth filters. It was found in the Prophet 5 Rev 3, Oberheim OB-Xa, PPG Wave 2.0, and the SCI Pro-One. Its internal compensation design gave it temperature-stable performance, a big deal when competing filters would drift out of tune with a room-temperature shift. AMSynths didn't just drop the chip into a socket. They studied the Pro-One's implementation, particularly its resonance feedback loop, and replicated it exactly. The filter's character is present even at low resonance: a slight warmth in the mids, a rounded top end, and a bottom that stays tight without getting flabby. It's not as raw as a Moog ladder, not as glassy as an Oberheim SEM. It's a clean but musical filter that sits well in a mix.

Hybrid Design: Best of Both Worlds

The AM8320 is a hybrid. The core filter path follows the Pro-One, but the CV handling borrows from Digisound's modular philosophy, specifically a CV rejection trimmer. This lets you dial out unwanted modulation from the control voltage, so when you modulate cutoff with an LFO or envelope, you get clean, predictable sweeps without wobble or offset. That matters in Eurorack, where CV sources can vary wildly in level and stability. The module also has op-amp summing for the CV inputs, which keeps the control voltages clean and isolated. You can patch in multiple modulation sources, say an envelope and an LFO, without them interfering with each other. The original Pro-One never had this.

Three Inputs, One Voice

Three audio inputs with individual level pots might seem like overkill on a filter. In practice you can mix two oscillators before filtering, blend in a noise source for percussion, or route a drum signal through it for dynamic processing. The inputs are DC-coupled, so you can use them for CV mixing if needed, though that's not their primary purpose. The level pots are smooth and linear, so you can balance sources without tweaking output levels on the sending modules. And because the input stage uses a FET op-amp, there's no loading effect on your oscillators, so you don't lose amplitude or high end when chaining modules.

Historical Context

The AM8320 emerged from a specific moment in the Eurorack renaissance. In the late 2000s, as modular synthesis was shaking off its niche status, builders like AMSynths began filling gaps in the market with high-quality, historically informed designs. The CEM3320 chip had long been out of production, and original Prophet 5 filter boards were either too expensive or too fragile for widespread use. AMSynths' founder, Rob, had already built a version of this filter (the AM3320) for his own Pro-One replica in 2004, but the shift to Eurorack in 2008 made the AM8320 viable. By then, the modular community wanted authentic analog voices, and the Prophet sound was in high demand.

In 2013, a small batch of NOS (new old stock) CEM3320 chips surfaced, allowing AMSynths to build a limited run of 10 modules. Then, in 2017, the AS3320, a modern, pin-compatible reissue of the CEM3320, became available in volume. That prompted a full reissue of the module with a revised, single-PCB design that was easier and cheaper to build. This version also upgraded the knobs from "mixer style" to a more vintage-looking type. The AM8320 wasn't competing with Behringer's 904A or other budget filters. It was aimed at the high end, for builders who wanted the real sound without the vintage risk.

Collectibility & Value

The AM8320 is discontinued. On the secondhand market it trades quietly but consistently, usually between $300 and $450 depending on condition and version. The 2017 AS3320-based units are more common and generally more reliable. The earlier CEM3320 versions are rarer and sometimes command a premium from purists, though in blind tests most can't tell the difference.

There are caveats. The early 2008-2010 PCB kits were sold as DIY projects, and some were poorly assembled. Check for cold solder joints, especially around the power header and pots. The 2013 run of 10 units is essentially mythical, and if one ever surfaces, expect it to go for over $600. The 2017 version is the most practical buy, but even then, inspect the panel for scratches and the pots for crackling. The red lettering can fade in direct sunlight, and while the AS3320 is more available than the CEM3320, it's still not a chip you want to fry. There are no known firmware issues, since it's all analog, but verify that the CV rejection and tracking trims are properly calibrated. Misadjusted trims can make the filter respond erratically to 1V/oct signals.

The AM8320 is a "set and forget" module. Once it's in the rack, you'll probably leave it there. For a vintage-style voice, this filter pairs well with oscillators like the Mutable Instruments Shruthi-1 (in analog mode) or the Intellijel Dixie II. It's not cheap, but it's cheaper than restoring a Pro-One.

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