ARP 4035 Filter (1974–1976)

Name: ARP 4035 Filter
Brand: ARP

The short-lived filter that dared to sound like a Moog—before ARP got too close for comfort.

Overview

Plug a sawtooth wave into an ARP Odyssey from 1975 and turn up the resonance—just before it starts to sing, there’s a warmth, a slight sag in the low end, a rounded growl that feels more organic than electronic. That’s the 4035 filter talking. It doesn’t scream like a Minimoog, but it whispers in the same dialect. This wasn’t ARP’s usual sound. It was a deliberate detour into Moog territory, a transistor ladder design so clearly inspired by Bob Moog’s work that it didn’t last more than a couple of years before ARP quietly phased it out. Whether there was an actual lawsuit or just the threat of one, the 4035 became known among synth nerds as the “lawsuit filter”—a brief, beautiful anomaly in ARP’s otherwise distinct sonic lineage.

The 4035 replaced the original 2-pole 4023 filter in the Odyssey lineup around 1974, marking the transition from the early “white-face” and “gray-face” models to the “black-and-gold” MkII era. Where the 4023 had a smoother, gentler slope and a reputation for clarity, the 4035 brought a thicker, more aggressive character—24dB per octave of Moog-style filtering with a resonance that could howl when pushed. It was ARP’s first serious foray into ladder topology since the 4012 used in the 2600, but now updated with CA3086 transistor arrays for tighter matching and improved stability. It shared DNA with the 4034 filter found in the Pro Soloist and Explorer I, though the 4035 was tailored specifically for the Odyssey’s architecture, with integrated CV summing and buffering via dual FETs and LM301 op-amps.

But its reign was short. By 1976, ARP had already begun replacing the 4035 with the 4075—a completely different beast, based on a cascaded integrator design that avoided Moog’s patented ladder circuit. The 4075 was cleaner, more precise, and legally safer, but many players felt it lacked the raw character of its predecessor. The 4035, for all its legal baggage, had soul. It overdrove gracefully, retained low-end punch at high resonance, and could self-oscillate with a smooth, musical sine wave. It wasn’t perfect—temperature stability could be finicky, and the original CA3086 chips were sensitive to mismatched transistors—but when it worked, it sang.

Specifications

Manufacturer	ARP Instruments, Inc.
Production Years	1974–1976
Original Price	Not sold separately (submodule)
Filter Type	4-pole transistor ladder low-pass filter
Slope	24 dB/octave
Resonance	Voltage-controlled, feedback-based
Control Voltage Input	Exponential (1V/oct)
Temperature Compensation	Yes, via tempco resistor
Transistor Array	4 × CA3086
Op Amps	LM301 (dual)
Buffering	Dual FET input stage
Signal Path	DC-coupled
Cutoff Frequency Range	Approx. 0 – 35 kHz
Self-Oscillation	Yes, at maximum resonance
Module Designator	4035
Primary Use	ARP Odyssey MkII (2810–2813)
Replacement Module	4075 (later models)
Interchangeable With	4034 (Pro Soloist/Explorer), with modification

Key Features

A Transistor Ladder with Attitude

The 4035’s core is a four-stage transistor ladder, the same topology that made Moog filters legendary. Each stage contributes 6dB of roll-off, totaling 24dB per octave—a steep enough slope to carve through a mix but still preserve harmonic richness. Unlike the OTA-based 4023, the 4035 used discrete transistors arranged in a carefully matched array, which gave it a warmer, more nonlinear response. The CA3086 chips were key: ARP reportedly hand-selected them to ensure matched transistor pairs, minimizing drift and improving tracking. When fed a rich oscillator source—especially the Odyssey’s own dual VCOs—the 4035 could go from a soft purr to a snarling beast with just a twist of the resonance knob. It didn’t just filter; it colored, it breathed, it interacted with the input signal in a way that felt almost alive.

Resonance That Feels Like Feedback

Resonance in the 4035 isn’t just a tone control—it’s a performance tool. It’s implemented by feeding the output back into the inverting input of the filter core, a classic Moog technique that allows the filter to self-oscillate at high settings. But unlike some filters that thin out or collapse when resonance is maxed, the 4035 holds onto its low end, giving it a full-bodied howl. This makes it ideal for leads, brass stabs, and acid-like sequences, especially when overdriven slightly at the input. The resonance control is voltage-controllable, allowing for dynamic sweeps, but owners report that it can be touchy—trimmer pots for VCF balance are critical, and replacing them with the wrong value (like a 10k instead of 1k) can kill the signal path entirely. It’s a filter that rewards careful calibration.

Designed for the Odyssey, But Not Alone

While the 4035 was built specifically for the ARP Odyssey MkII, it’s not a one-trick module. Its circuit design—complete with CV summing and buffering—makes it a self-contained unit that can be adapted to other systems. Modern recreations like the Synthchaser 4035 submodule and the AMSynths AM8035 are proof of its enduring appeal, offering drop-in replacements or Eurorack-compatible versions. The original 4035 plugs into the Odyssey’s main board via a 0.100″ header, and while it was later superseded by the 4075, it can still be swapped into later Odysseys or even other ARP synths like the Axxe or Omni with minor modifications. It’s a modular thinker’s dream: a vintage filter with modern adaptability.

Historical Context

The 4035 arrived at a time when ARP was trying to reclaim its edge. The original Odyssey (1972) had been a success, but its 12dB 4023 filter was seen by some as too polite, too clean. Meanwhile, Moog’s ladder filter had become the gold standard for fat, expressive analog tone. ARP, never one to shy from competition, decided to meet Moog on his own turf. The 4035 was the result—a deliberate homage to the Moog sound, updated with ARP’s own engineering flair. But patents loom large in synth history, and while no lawsuit was ever filed, the resemblance was too close for comfort. By 1976, ARP had pivoted to the 4075, a non-ladder design that avoided legal risk but also sacrificed some of the 4035’s character.

This wasn’t just a technical shift—it was a philosophical one. The 4035 represented a moment when ARP was willing to borrow, to experiment, to sound like someone else if it meant better music. The 4075, in contrast, was a statement of independence: cleaner, more precise, and unmistakably ARP. Other companies watched closely. Roland’s early SH-series filters, for instance, also flirted with ladder designs before retreating to safer ground. The 4035, then, is more than a filter—it’s a snapshot of an industry in flux, where innovation danced on the edge of infringement, and where a single submodule could become a legend just by sounding too good.

Collectibility & Value

As standalone modules, 4035s are rare—and valuable. They were never sold separately, so every working unit pulled from a vintage Odyssey is a potential replacement part for another. On the used market, a tested, working 4035 filter board can fetch $300–$500, depending on condition and provenance. Boards pulled from early 2810 or 2813 Odysseys are especially prized, particularly if they’re still in the original chassis with matching serial numbers. Non-working units go for less—$100–$200—but come with serious risk. The CA3086 chips are prone to failure, and the LM301 op-amps can drift or oscillate if not properly maintained. Recapping is almost always necessary, and trimmer pot values must be exact; as one technician noted, installing a 10k pot where a 1k belongs can silence the filter entirely.

For restorers, the 4035 is both a prize and a puzzle. Its sound is coveted, but its reliability is questionable. Many owners opt to keep a 4075 on hand as a backup, or install modern recreations like the Synthchaser module, which uses precision components and eliminates the temperature sensitivity of the original. If you’re buying a vintage Odyssey and the seller brags about the “Moog-style filter,” ask for proof—open the case and check the board label. Some later Odysseys were retrofitted with 4035s, while others had 4075s swapped in. Authenticity matters, and so does function. A 4035 that doesn’t self-oscillate or cuts out at high resonance likely needs a full rebuild. Don’t trust the sound alone—test every control, check for noise, and verify the CV response. This isn’t a filter you fix later; it’s one you verify now.