AMSynths AM8035 (2002–Present)
A hand-built recreation of ARP's 24dB/oct ladder filter, now in Eurorack, with the snarl, grit and instability of the original.
Overview
Turn the resonance knob on the AM8035 past 12 o'clock and the filter starts a slow, throaty howl on its way to self-oscillation. It isn't a clean, clinical filter. It comes straight out of the ARP 2600, reverse-engineered into a 6HP Eurorack module by Rob Keeble under the AMSynths name. Keeble used the same topology, component choices and layout philosophy as the original. The vintage character here is the circuit, not an imitation of it. It isn't trying to be a Moog or an Oberheim. It's an ARP.
The AM8035 is based on the 4012 filter used in the ARP 2600, a 24dB/oct low-pass design known for its aggressive tone, pronounced resonance peak, and tendency to drift when warm. Keeble's version keeps all of that. The filter sings under high resonance, colors even simple waveforms with a gritty, organic texture, and responds to CV with a slight lag that feels expressive rather than broken. It's not a precision instrument. It's a performance tool. This is the filter that made ARP synths scream on records from Parliament-Funkadelic to Gary Numan, and it's now hand-soldered in small batches in West Sussex.
In the AMSynths lineup, the AM8035 sits above the simpler diode-ladder clones and below the more complex multi-mode designs. It isn't the most flexible filter in the catalog. There's no high-pass, no band-pass and no notch. It does one thing. For anyone building a 2600-style voice, it's the filter section. Pair it with an AM8027 Dual ARP 2600 VCO and an AM8019 VCA, and you have a module-for-module recreation of one of the most famous semi-modular synths ever made, now modular and expandable.
Specifications
| Manufacturer | AMSynths |
| Production Years | 2002–Present |
| Module Format | Eurorack |
| HP | 6 |
| Depth (mm) | 40 |
| Current Draw +12V | 60 mA |
| Current Draw -12V | 40 mA |
| Filter Type | 24dB/oct Low-Pass (Transistor Ladder) |
| Resonance | Voltage Controlled, Self-Oscillating |
| Cutoff Frequency Control | Manual Knob, 1V/Oct CV Input, Resonance Modulation Input |
| Audio Inputs | 1x Normalled, 1x Inverted (with internal jumper) |
| CV Inputs | 1x 1V/Oct, 1x Resonance Modulation |
| Audio Output | 1x |
| Filter Slope | 4-Pole (24dB/oct) |
| Filter Core | Discrete Transistor Ladder (ARP 4012 Clone) |
| Manual Cutoff Range | 10 Hz – 20 kHz |
| Self-Oscillation | Yes, at high resonance |
| Build Type | Hand-Assembled, Point-to-Point and PCB Hybrid |
| Country of Origin | United Kingdom |
| Original Price (Approx.) | £149 (as of 2023) |
Key Features
The ARP 4012 Circuit, Uncompromised
Most "inspired by" filters only approximate the original. The AM8035 follows the ARP 4012 design closely. Keeble didn't model it in SPICE and approximate the response. He built it the way ARP did, with the same transistor ladder configuration and supporting circuitry. So it behaves like the real thing, not just sounds close. The resonance isn't a simple boost around the cutoff. It's a sharp, singing peak that can be coaxed into oscillation with fine control. When driven, the filter adds a subtle saturation that thickens signals without muddying them, which gives basslines a chewy, almost vocal quality. It works especially well on square waves, turning them into growling, resonant tones that cut through a mix like a chainsaw through fog.
Signal Summing & Inversion
The AM8035 has dual audio inputs, one normalled and one inverted, so you can do basic signal mixing right at the filter stage. This mirrors the ARP 2600's main panel, where users could blend oscillators or external sources before filtering. The inverted input is handy for phase manipulation, or for pseudo-wavefolding effects when mixed with a non-inverted signal. A small jumper on the PCB disables the inversion, which turns the second input into a plain passive mixer.
CV Summing & Modulation Flexibility
The AM8035 has dedicated CV inputs for 1V/oct tracking and resonance modulation, with no attenuation knobs. That's intentional. The module expects you to control those parameters upstream, either with attenuverters or with dedicated modulation sources. This keeps the front panel clean, but it also means you'll want a good CV processor in your system. The resonance CV input responds dramatically to modulation, which suits rhythmic filter sweeps or chaotic, LFO-driven textures. Because the filter core is nonlinear, modulating resonance doesn't only change the timbre. It alters the harmonic content in complex ways.
Historical Context
The AM8035 exists because Rob Keeble fell in love with the sound of the ARP 2600 and couldn't find a modern filter that matched it. In the early 2000s, Eurorack was still in its infancy, and while there were plenty of Moog-style clones, ARP's filter voice was underrepresented. Keeble, an electronics engineer and lifelong synth tinkerer, decided to fix that. His first AM8035 was built in August 2002, as a personal project and not a product. Only after fellow modulargrid users and forum regulars began asking for builds did AMSynths become a proper venture.
At the time, most boutique Eurorack modules were either utility-focused or based on Roland or Moog circuits. The AM8035 put ARP's filter in front of that crowd. It arrived just as the DIY synth movement was gaining momentum, and its hand-built, no-compromise approach went over well with a growing community of purists. Each AM8035 was assembled by hand, often with through-hole components and point-to-point wiring where it mattered.
The filter's roots go back to ARP's early days, when engineers like Dennis Colin were pushing analog design hard. The 4012 ladder filter was never as stable as Moog's. Its slight unpredictability, meaning the drift, nonlinearity and harmonic richness, was part of its sound. Keeble didn't "fix" these quirks. He kept them.
Collectibility & Value
The AM8035 has never been a mass-market module. Production runs are small, typically a few hundred units at a time, and they often sell out before shipping. Because of this, used units command a premium, especially those from earlier batches where more of the construction was hand-wired. As of 2025, a new AM8035 retails for around £149. Used prices on the secondhand market range from £120 in good condition to £180 for mint, boxed units with original documentation.
There are no widespread failure points in the AM8035, thanks to its sturdy discrete design and conservative power draw. Owners do report that early units (pre-2010) sometimes suffer from cold solder joints, particularly on the potentiometers and jacks, due to hand-soldering variability. A quick reflow usually fixes it. The pots are high-quality Alps units, known for longevity, but like any analog gear they can develop crackle over time with heavy use. No electrolytic capacitors are used in the audio path, so there's no need for preventative recapping.
For buyers, the real challenge is availability, not reliability. The AM8035 isn't listed on major retailers. It's sold directly through the AMSynths web shop during limited drops. If one turns up on eBay or Reverb, check the serial number and ask the seller about its provenance. Units from the 2002-2008 period are considered "vintage" by collectors and may appreciate slightly, though not as dramatically as original ARP hardware.
The module’s value lies not in speculation, but in utility. If you’re building a 2600-style voice or chasing that ARP scream, the AM8035 is worth every penny. It’s not the cheapest filter on the market, but it’s one of the few that delivers genuine vintage tone without compromise. And unlike original ARP gear, it won’t cost you thousands or spend half its life in the repair shop.
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