ARP 1023 Dual VCO Module ()

Two gritty, idiosyncratic oscillators in one 2500 slot - great sound, awkward tuning, and a design that still raises eyebrows.

Overview

The 1023 was built for anyone who wanted more oscillators in the limited matrix slots of an ARP 2500 system. It has no sweeping filters and no modulation tricks. It is two plain VCOs on a single panel, meant to pack more voices into ARP's rare and sprawling modular system. It sounds great and it is an unusual design, and it leans into the raw, slightly unpredictable nature of early analog synthesis. It was built for function, not finesse, and the ergonomics show it.

The 1023 is a module for the ARP 2500 modular synthesizer system, and its reason for being seems to be efficiency - more oscillators in a system where every slot counts. It is essentially 2 ARP 1004 oscillator boards on one chassis, sharing space but not compromising the circuitry. You get the same core sound engine as the standalone 1004, just doubled. It lacks pulse width modulation, which is a notable omission. What it does offer is raw waveform presence, with that distinctive ARP edge that cuts through a mix without turning brittle.

Owners report it's especially useful for filling up 2500 slots that lack matrix switches, since the module only uses 6 of the 10 available. That leaves routing flexibility for other functions. It's a utilitarian upgrade, not a luxury feature, and it's the module you buy when you need more oscillators.

Specifications

Manufacturer	ARP Instruments Inc.
Product type	Dual Voltage Controlled Oscillator module
Matrix switch usage	The module uses 6 of the 10 matrix switches available in each slot.
PWM	There is no PWM.
Internal design	The module was essentially 2 ARP 1004 oscillator boards.
Waveform selection	It had a rotary panel switch to select the output waveform.
Circuit construction	The 1023 uses 2 modified boards from the 1004 VCO module.
Submodules	Each board has 2 potted submodules.

Key Features

Dual 1004 Core, One Panel

The 1023 is a space-saving hack that ARP made official. It mounts two modified 1004 VCO boards in a single module, so 2500 owners could double their oscillator count without doubling the real estate. Each side operates independently, with its own waveform output and tuning, so nothing is lost but panel space. Because it uses actual 1004 boards (modified), the tone carries the same ARP authority: stable enough for melodic work, with a slight grit when pushed, especially in the triangle and sawtooth waves. It's not as silky as a Moog, nor as clinical as a later Roland.

Rotary Waveform Selector

One of the more visible departures from the standard 1004 design is a rotary panel switch that selects the output waveform, rather than the "T" or "P" boards used in the 1004-T/P modules. That simplifies the panel layout and makes waveform selection quicker, with no internal boards to swap and no patching around for basic shapes. You get the standard ARP set: sawtooth, triangle, square, pulse, all selectable with a twist. It's a small quality-of-life improvement that helps in live use, though it doesn't change the underlying circuit behavior.

Unusual Triangle Wave Generation

Like the 1004, the 1023 uses an unusual method of generating its primary triangle waveform. This isn’t just a technical footnote — it affects the character. Most oscillators build a triangle by integrating a square wave, but ARP’s approach here diverges, resulting in a waveform that feels slightly asymmetrical, even when trimmed. Some users describe it as “lumpy” or “breathing,” especially at lower frequencies. Whether that’s a flaw or a feature depends on your taste. For experimental patches, it adds movement; for precise tuning tasks, it can be a distraction. But it’s part of what makes ARP’s 2500-series oscillators stand out in a sea of more textbook designs.

CV Linearity and Tuning Quirks

Stock units have CV linearity limitations, which is a polite way of saying they don't track perfectly across the keyboard. It isn't a dealbreaker, but owners need to manage it, especially when using the 1023 for melodic leads or basslines. You'll likely need to trim the response at both ends of the scale, and even then there might be a slightly sour spot in the middle register.

Then there's the tuning. The concentric fine tune knobs are difficult to tune, and that's a common complaint. They're small, stiff and easy to bump, so micro-adjustments take patience. Once they're set, they tend to stay put, as long as nobody nudges them during a patch session.

Build and Component Details

The internal construction is functional, serviceable and built to be repaired. Each board houses 2 potted submodules, likely protecting sensitive timing or wave-shaping circuits from interference. The parts list reads like late '60s/early '70s analog design: 1/4 watt, 10% carbon composite resistors; tantalum, ceramic disc, and polycarbonate capacitors; LM301AH op-amps; 2N5172 and A2803-004-A transistors; and linear taper trim pots. Nothing exotic, and nothing cutting corners either. These are industrial-grade components chosen for stability and availability.

The lack of pulse width modulation is a real limitation. If you want to animate the pulse wave or create classic PWM swells, you're out of luck. You can modulate pulse width via CV if you patch into the oscillator core directly, but the module doesn't support it natively. That makes the 1023 better suited for static, harmonic-rich tones such as drones, pads, or square-wave stabs than for evolving, modulated leads.

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