ARP 4023 (1972–1975)

The original whiteface Odyssey filter. Less polished than a Moog, more aggressive than a Prophet, and built on ARP's 2-pole design.

Overview

Take a whiteface ARP Odyssey from 1972 and turn the filter cutoff just past 12 o'clock. You get a growl, thick with harmonics and just shy of oscillation. That's the 4023. It is not the later 4035 or the 4075. It's the original 2-pole voltage-controlled filter that defined the early Odyssey before ARP smoothed out the edges. It's a dual OTA (operational transconductance amplifier) bi-quad design. It trades the 24dB/oct slope of its successors for a looser 12dB/oct roll-off. The filter wobbles and bites when pushed, and the resonance howls.

The 4023 was found only in the first run of ARP Odyssey Mk I units (Model 2800). ARP wanted a compact, stable filter with its own sound that could handle the aggressive output of the Odyssey's dual oscillators. Where Moog went for the deep, rounded ladder, ARP went for speed and attitude. The 4023 responds to modulation with a nervous energy. Add some keyboard tracking, crank the envelope, and you're wrestling with it. It isn't the most precise filter ever made. This was a gigging synth in the early '70s, built to cut through a mix with sawtooth waves, ring modulation, and PWM.

Later revisions of the Odyssey swapped in the 4035 (a 24dB/oct ladder) and eventually the 4075. Both tamed the wildness in favor of smoother sweeps and better bass response. Collectors still chase the 4023 for what it doesn't do, which is play it safe. It's the filter behind Herbie Hancock's "Chameleon" lead, and the one that gave Jan Hammer his early fusion bite. It's also the reason a well-maintained whiteface Odyssey commands a premium today.

Specifications

Manufacturer	ARP Instruments, Inc.
Production Years	1972–1975
Original Price	Not available
Filter Type	2-pole voltage-controlled bi-quad
Filter Slope	12 dB/octave
Topology	Dual OTA (CA3080 or equivalent)
Resonance	Voltage-controlled, self-oscillating
Frequency Range	Approx. 20 Hz – 20 kHz
Control Voltage Input	Exponential, temperature-compensated
Signal Path	DC-coupled
Key Components	CA3080 OTAs, 2N5459 FETs, matched TO-92 transistors, temperature compensation resistor
Potentiometers	Six 20mm Alpha slide pots (on original module)
Module Designator	4023
Used In	ARP Odyssey Mk I (Model 2800), whiteface and early grey/gold variants
Replaced By	4035 (in Odyssey Mk II), 4075 (in later models)
Dimensions	Not available
Weight	Not available
Power Requirements	±15 VDC (via host synthesizer)

Key Features

A 2-Pole Rebellion in a 4-Pole World

By the time of the 4023, 24dB/oct filters were becoming the standard - Moog, Oberheim, even ARP's own later designs. The 4023's 12dB/oct slope went the other way. It doesn't scoop the mids as dramatically, doesn't tighten the bass as aggressively, and doesn't give you the "locked-in" sweep some players want. What it gives you is movement. The transition from open to closed is slow rather than abrupt. That suits evolving pads, squelchy leads, and anything that needs to feel unstable. The bi-quad topology means it can be patched for multiple outputs (low-pass, band-pass, even high-pass with external circuitry), though in the Odyssey it's hardwired to low-pass only. The secondary band-pass output on pin 9 of the module has become legend among modders, and some quietly tap it for extra grit.

OTA Grit, Not Ladder Warmth

The 4023 is built around dual CA3080 OTAs, running in a transconductance configuration. That gives a faster, more dynamic response than the integrator-based ladder filters ARP would later adopt. OTAs are inherently noisier and less linear, but they're also more immediate, with no lag between your envelope and the filter's reaction. That's why the 4023 feels so "present," even at low resonance settings. It also modulates, distorts, and interacts with the input signal in ways that ladder filters avoid. The trade-off is a higher noise floor, especially as the module ages and the OTAs drift. Many argue the noise is part of the charm, and compare the slight hiss under a sawtooth lead to tape saturation on a vintage console.

Drop-In Legacy, Still Alive Today

The 4023 was a module, a self-contained black box designed for easy replacement and service. That has kept it alive decades later. Boutique builders like Synthchaser, Frequency Central, and G-Storm Electro offer faithful recreations, often with modern reliability tweaks like temperature compensation resistors and matched transistors. Eurorack users can drop a 4023 clone into a 5U or 3U case and patch it like a standalone processor. The original schematics are well-documented (thanks to ARPtech and the DIY community), so even failed modules can be rebuilt from scratch. It's rare for a 50-year-old filter to have an active clone ecosystem.

Historical Context

The 4023 emerged in 1972, when ARP was racing to capitalize on the Odyssey's surprise success while fending off cheaper competition from Roland and Korg. The original whiteface Odyssey was a scaled-down 2600, but it had its own identity. Where Moog emphasized stability and musicality, ARP leaned into aggression and versatility. The 4023 was central to that.

By 1975, ARP had shifted course. The Mk II Odyssey introduced the 4035, a 24dB/oct ladder filter that brought the synth closer in tone to the Minimoog, likely to broaden its appeal. The move made commercial sense, but it alienated purists who missed the 4023's raw edge. Later models like the Omni and Pro Soloist would use the 4075, a cost-reduced version of the same ladder design, cementing ARP's pivot toward smoother, more predictable tones.

Today the 4023 is revered for its sound and for what it represents: a brief window when ARP prioritized character over conformity.

Collectibility & Value

If you're hunting for a 4023, you're buying a piece of a whiteface or early grey/gold Odyssey (Model 2800). These synths are rare, and the ones with original, unmodded 4023 modules command serious money. A fully serviced whiteface Odyssey with its original filter intact can fetch $2,000-$3,000, sometimes more if it's in exceptional condition. Beware, though: many have been recapped, slider-upgraded, or even retrofitted with 4035 modules. A "whiteface" with a 4035 isn't a whiteface in spirit, no matter what the case says.

The 4023 itself is prone to a few failure points. The CA3080 OTAs degrade over time, leading to loss of resonance, instability, or complete cutoff failure. The 2N5459 FETs can drift, causing tuning issues in the filter's response. The module is DC-coupled, which is great for CV processing, but it also means any power supply ripple gets amplified straight through. Tantalum capacitors in the surrounding circuitry are notorious for leaking or shorting, so a full recap of the host synth is strongly advised, even if the 4023 module itself is sealed and untouched.

For the budget-conscious, standalone 4023 modules on Reverb or eBay offer a way in. Expect to pay $250-$400 for a tested, working unit, less if it's untested or cosmetically worn. Clones from Synthchaser or Frequency Central run $180-$280 and often outperform originals in stability, though some argue they lack the "magic."

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