AMSynths AM8109 JP8 LPF (2011–2022)
A hand-built Eurorack version of the Roland Jupiter 8's creamy, self-oscillating filter, with the analog parts intact.
Overview
Plug in the AM8109 JP8 LPF and crank the resonance. The first sine wave howl is warm and round, and a little unstable. It is not an algorithmic approximation or a DSP emulation. It is a component-level rebuild of the Roland Jupiter 8's low-pass filter, the one behind the lush pads of 1980s pop and the punchy basslines of early synthwave. Rob Keeble, the engineer behind AMSynths, sourced the same IR3109 OTA chip that Roland used in the Jupiter 8's voice. The module follows the frequency response, the way the resonance blooms, the smoothness of the cutoff sweep, and a slight sag in the envelope when you push it hard.
The AM8109 launched in 2011 and was refined over more than a decade. It is a workhorse filter for modular players who want the vintage sound without the fragility of a 40-year-old keyboard. It comes as a full module and as a PCB kit for DIY builders. The original version used actual Roland IR3109 chips, which became increasingly scarce. By 2022, AMSynths moved to the RPar AS3109, a modern clone that is sonically indistinguishable.
Many Eurorack filters scream or break up when pushed. The AM8109 stays musical even at high resonance, and it self-oscillates into a clean sine wave that can act as a secondary oscillator. The 2-pole (12dB) and 4-pole (24dB) modes are selectable with a small red push-button, and an LED shows the slope. The difference between them is subtle. The 2-pole mode is silkier, with a gentler roll-off, while the 4-pole delivers the classic Jupiter punch. It is not a radical change, but it shifts the texture, which is how the original behaved. There is no menu diving. The panel has three pots (cutoff, resonance, and CV mix) and three audio inputs.
Specifications
| Manufacturer | AMSynths |
| Production Years | 2011–2022 |
| Original Price | £135 (completed module, 2022) |
| Filter Type | Low Pass Voltage Controlled Filter (VCF) |
| Poles | 2-pole (12dB/octave) and 4-pole (24dB/octave) selectable |
| Core Chip | IR3109 OTA (original), AS3109 (later versions) |
| Q Compensation | Yes, maintains signal level at high resonance |
| Self-Oscillation | Yes, at high resonance settings |
| Inputs | 3x Audio (3.5mm), 2x CV (3.5mm) |
| Output | 1x Audio (3.5mm) |
| Power Consumption | 22 mA +12V, 22 mA -12V |
| Width | 14 HP (original), 10 HP (v2) |
| Depth | 80 mm (original), 35 mm (v2) |
| Front Panel | 2 mm machined aluminum, color printed |
| Controls | Cutoff Frequency, Resonance, CV1 Level, CV2 Level, 3x Input Level |
| Construction | Through-hole and SMD components, Doepfer-compatible power |
| Availability | Discontinued (2022), succeeded by AM8109SE |
| Special Features | Based on Roland Jupiter 8/4 filter design, uses original IR3109 or AS3109 chip |
Key Features
The IR3109 Difference
Roland introduced the IR3109 in 1980 with the Jupiter-4, to shrink its discrete filter designs into a compact, reliable package that could scale across polyphonic synths. It went into the Jupiter-8, Juno-60, SH-101, and even Boss phasers. AMSynths did not simulate it. They sourced it. Early AM8109 units used genuine Roland IR3109s, often pulled from dead or decommissioned synths. No clone could match that until the supply dried up. The later switch to the AS3109 was a necessity, but testing confirmed it behaves identically in-circuit. Collectors pay a premium for the original IR3109-equipped modules, not because they sound better, but because they come from a specific moment in the analog revival.
Smooth Operator: Resonance and Q Compensation
Many Eurorack filters go wild with resonance, screaming, distorting, or collapsing. The AM8109 doesn't. Its resonance builds gradually and smoothly, on a natural curve. That is due in part to the Q compensation circuit, which keeps the signal from dropping as resonance increases, a feature of the original Jupiter 8 design. You can crank the resonance without losing presence. When it hits self-oscillation, it does so cleanly, producing a stable sine wave that can be voltage-controlled like any oscillator. That makes it a good fit for basslines, where the resonant "whistle" needs to cut through a mix without fizzing out.
Practical Design for Real Use
The AM8109 was never about flashy features. It is a filter, built for patching. The three audio inputs allow mixing multiple sources before filtering, which helps for layering oscillators or processing external gear. The dual CV inputs modulate cutoff from two sources (say, an envelope and an LFO), with dedicated attenuators to set the amount. The front panel uses high-quality potentiometers and Erthenvar jacks (upgraded from Cliff in v2). The original 80 mm depth was a skiff-killer. The 2019 redesign halved that to 35 mm, which is far friendlier to a rack. The v2 also introduced a 10 HP width, though the 14 HP version stayed in production for a time.
Historical Context
The AM8109 arrived around 2011, when the Eurorack boom was pulling modular out of its niche and into the kit of professional musicians. Most filter clones then focused on Moog or Oberheim topologies. The Jupiter 8's filter, while beloved, hadn't been widely replicated in modular form. AMSynths filled that gap by rebuilding the sound from the ground up using the original silicon. This was before Behringer's deep dives into Roland IP and the flood of affordable clones seen today. The AM8109 was part of a wave of boutique builders, like TipTop, Make Noise, and Intellijel, who treated vintage circuits as sacred texts worth preserving and adapting.
Roland had long moved on from the IR3109, and by the 2010s the Jupiter 8 was a six-figure collector's item. The AM8109 offered a way to own a piece of that sound without the price tag or the risk of maintaining a 40-year-old keyboard. It also arrived as software synths were hitting their stride, which made the tactile, unpredictable nature of analog filtering more valuable.
Collectibility & Value
The AM8109 is now discontinued, with AMSynths shifting focus to the AM8109SE and other designs. On the used market, condition and chip type matter. Original units with genuine Roland IR3109 chips command a premium, often 20 to 30% more than later AS3109-equipped versions, despite no audible difference. Modules in excellent condition, with clean panels and no bent jacks, typically sell for £150 to £200 in the UK, or $180 to $250 in the US. The v2 skiff-friendly versions are slightly more desirable because of their reduced depth.
Failures are rare but not unheard of. The most common issue is cracked solder joints on the power connector, especially in early units that saw frequent plugging and unplugging. The IR3109 chip itself is tough, but if a module isn't powering correctly, check the power rail voltages before assuming the chip is dead. There are no known firmware issues (it's entirely analog), and no maintenance beyond standard dusting and jack cleaning.
For buyers, the decision is the AM8109 or the newer AM8109SE. The SE version adds a high-pass filter and uses slide pots for a sleeker look, but it drops the discrete 2/4-pole toggle in favor of a switchless design. Purists prefer the original for its tactile controls. Modernists might lean toward the SE for its expanded functionality.
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