AMSynths AM8075 (2016)
A Eurorack version of the ARP Odyssey Mk3 filter, down to the last slider and capacitor.
Overview
The AM8075 is a hardware clone of the ARP 4075 24dB/oct low-pass filter. That is the circuit that gave the late-’70s ARP Odyssey Mk3 its bite, and the later ARP 2600s their updated roar. The Mk3 could snarl a bassline one second and coo like a theremin the next. This module is not a digital model. It is built to sound like the real thing, not a cleaned-up version of it, including the subtle bass roll-off at high resonance that purists either curse or cherish depending on the day.
Rob Keeble of AMSynths, a UK-based engineer who loves vintage analog circuits, designed the AM8075. It started in 2007 as a personal project, built with rotary pots for a DIY ARP 2600 replica. By 2016 it had become the Eurorack module: 16 HP wide, with smooth-sliding potentiometers that mirror the look and feel of the original ARP Mk1 front panel. The sliders give immediate control, handy for sweeping resonance in real time or layering modulation sources. The vintage styling hides some modern changes. Keeble upgraded the design with Linear Systems matched transistor arrays and precision 2.5% polypropylene capacitors in the filter block, plus audio-grade electrolytics in the signal path. That keeps the character of the original while reducing drift and improving consistency across temperature changes.
The AM8075 uses the LM3900 Norton amplifier, a key part of ARP’s filter topology and the source of its “smooth” yet aggressive tone. It is not a 20dB Moog-style ladder, and it is not a diode ladder like the SH-series Rolands. The LM3900-based design resonates without turning brittle and self-oscillates cleanly up past 14kHz. It also keeps a slightly organic instability. Feed it a sawtooth wave and turn up the resonance, and the filter starts to howl. It does this with a slightly wobbly, almost vocal quality, not the laser-cut precision of a modern SEM filter. That quality is why ARP synths were the go-to for expressive leads and gnarly effects. Patch in noise or modulate the cutoff with an LFO and it responds well.
Specifications
| Manufacturer | AMSynths |
| Production Years | 2016 |
| Original Price | $213 |
| Module Width | 16 HP |
| Depth | 40 mm |
| Current Draw | +12V: 60mA, -12V: 40mA |
| Filter Type | 24dB/oct Low-Pass VCF (4-pole) |
| Core Chip | LM3900 Norton Amplifier |
| Resonance | Adjustable up to self-oscillation |
| Cutoff Control | Manual slider, CV inputs with attenuators |
| Keyboard Tracking | Switchable via Doepfer Bus CV |
| CV Inputs | 3 (CV1, CV2, CV3), CV3 direct, no attenuator |
| Audio Inputs | 2 (INA, INB) with individual level sliders |
| Audio Output | 1 (Out) |
| High-Pass Filter | Adjustable HPF cutoff slider |
| Input Impedance | 100kΩ |
| Output Impedance | 600Ω |
| Weight | 280 g |
Key Features
The ARP 4075 Heart, Reborn
The AM8075 is the 4075 in Eurorack format. This filter was the centerpiece of the ARP Odyssey Mk3 (introduced in 1978). It replaced the earlier 4034 filter with a more stable, OTA-based design that kept ARP’s aggressive tonal character. AMSynths refined it as well as copying it. The Linear Systems matched transistor arrays give tighter tracking and lower noise than vintage units, where component drift could make filters go out of tune or behave unpredictably. The precision polypropylene capacitors in the filter block further stabilize the response, and they avoid the cap degradation that plagues old ARP synths. Keeble kept the quirks: the filter loses a bit of low end at maximum resonance, and it self-oscillates with a slightly uneven but musical tone.
Sliders That Sing
Many Eurorack modules use compact knobs. The AM8075 uses full-length sliders in the ARP style. The physical length gives finer control over cutoff and resonance, which makes subtle filter movements and live sweeps easier. The input level sliders for INA and INB double as mini mixers, so two sound sources can be blended before they hit the filter. A drum loop can go into one input and a drone into the other, with the sliders balancing them while the filter shapes the combined output.
Three Flavors of Modulation
The AM8075 has three CV inputs: CV1, CV2, and CV3. CV1 and CV2 have dedicated attenuators, so the amount of envelope or LFO reaching the cutoff can be set by hand. CV3 is direct, with no attenuation and full voltage. That is a deliberate choice for 1V/oct tracking or an unfiltered modulation spike from a sequencer or sample-and-hold. It is also the input used for keyboard tracking when patched to the Doepfer Bus CV. An envelope, an LFO, and a random voltage can all modulate the cutoff at once, with the attenuators setting the balance.
Historical Context
The ARP Odyssey Mk3 arrived in 1978, when the synth market was shifting. Polyphony was on the rise, and ARP, already struggling with internal management issues, needed to modernize. The switch from the 4034 to the 4075 filter was part of that effort. The 4075 was more stable, easier to manufacture, and less prone to the tuning drift that plagued earlier models. It kept its edge. Herbie Hancock, Patrice Rushen, and Vince Clarke used the Odyssey’s aggressive filter to cut through mixes with searing leads and punchy bass. The 4075 also went into later ARP 2600s (specifically the 2601 and 2603 models). Those semi-modular workhorses got a brighter, more responsive filter than the earlier 4012 transistor-ladder design.
In the 2010s, as Eurorack grew, demand rose for recreations of these classic filters. Synthesizers.com and Steffen Music offered clones, but AMSynths focused on getting the look and feel of the panel right. The AM8075 recreated the circuit and the feel of ARP’s golden era. Keeble restored vintage gear, including E-mu samplers and ARP systems, and that background informed the design. He built the filter for musicians who wanted the real circuit without the risk of a $3,000 repair bill when a transistor fails.
Collectibility & Value
The AM8075 is discontinued, and AMSynths has no plans to reissue it, citing the crowded market for ARP filter clones. That scarcity has driven collector interest, though prices have not reached those of rare vintage hardware. On the secondhand market, expect to pay between $250 and $350 depending on condition. Units with clean sliders, no wobble in the controls, and full functionality across all CV inputs sit at the higher end. The module uses high-quality components and was built in low volume, so failure rates are relatively low. A few things are still worth checking.
The sliders can accumulate dust over time, leading to crackling or intermittent signal. A treatment with contact cleaner usually fixes this. Heavily used units may need slider replacement, which is a non-trivial task given the front panel layout. The LM3900 chip is sturdy, but like any vintage-style IC it can fail if subjected to voltage spikes or improper power sequencing. Check that the module powers up without noise and that resonance sweeps cleanly to self-oscillation. Also verify that the keyboard tracking switch (if used) engages properly and that all CV inputs respond as expected.
For an ARP-style voice in Eurorack, the AM8075 pairs with AMSynths’ own AM8027 Dual VCO (a clone of the ARP 2600 oscillator) and AM8019 VCA. Together they form a near-complete ARP 2600 voice path. It also works with Behringer’s 2600 or Odyssey clones, adding an authentic filter stage to them. This is not a “safe” filter. It won’t give the polished sheen of a Roland or the warmth of a Moog. It is raw and slightly unpredictable.
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