ADDAC320 Servo Control ()

It doesn’t make a sound—but it makes everything else move, turning knobs like a ghost hand guided by voltage.

Overview

The ADDAC320 Servo Control isn’t a synth voice, an oscillator, or a filter—it’s a puppeteer. Built by ADDAC System out of Lisbon, Portugal, this unassuming 4HP Eurorack module exists to do one very specific thing: physically turn knobs on gear that weren’t designed to be automated. If you’ve ever stared at a beloved analog compressor, reverb unit, or synth with no CV input and thought, “If only this knob could move itself,” then the Servo Control is your answer. It’s a mechanical bridge between the modular world and the rest of your rack, using a tiny but precise servo motor to reach out and twist things in real time, all from a 0 to +5V control voltage signal. It’s not magic—it’s engineering with a sense of humor, and a direct descendant of wild, one-off contraptions like Neil Young’s Whizzer, which inspired its creation.

This isn’t just a motor on a module; it’s a thoughtfully constrained solution. At 4HP wide and 45 mm deep, it’s compact enough to fit in tight systems, but what it does punches far above its size class. You patch in a CV, and the microcontroller inside translates that voltage linearly into precise angular movement across a 270-degree range. That motion gets transferred via a flexible metal coupler—basically a springy wire with metal connectors—to the shaft of whatever knob you want to automate. It’s not clamping down or forcing anything; when set up right, it’s a gentle, responsive link between voltage and motion. But get it wrong? That’s where things get tense.

Specifications

Manufacturer	ADDAC System
Dimensions	4 HP
Depth	45 mm deep
Current Draw	300 mA +12V, 0 mA -12V, 0 mA 5V
Servo motor type	Digitally controlled 270 degree metal gear servo motor
Control voltage input range	0 to +5v
Price	$133

Key Features

The Servo as a Voltage-to-Motion Translator

At its core, the ADDAC320 uses a digitally controlled 270-degree metal gear servo motor—the kind you’d find in robotics or RC vehicles, but repurposed for audio. Unlike a regular motor that just spins, a servo can be told exactly where to point, like a needle on a meter. The module’s microcontroller takes any incoming CV between 0 and +5 volts and maps it linearly to the full arc of the servo’s range. That means 0V sends it to one extreme, +2.5V to the middle, and +5V to the opposite end—predictable, repeatable, and smooth. It’s not just for on/off movement; you can modulate it with an LFO to slowly sweep a reverb mix knob, or trigger it with envelopes to automate a filter cutoff on a vintage synth that’s otherwise hands-off.

Mechanical Coupling: The Delicate Dance

The servo’s shaft connects to your target knob using a spring wire and metal couplers—a flexible metal coupler that allows for slight misalignment and absorbs minor tension. This isn’t a rigid drive shaft; it’s more like a mechanical extension of your finger. When aligned properly, it turns knobs cleanly and reliably. But the fact sheet carries a quiet warning: if the servo is poorly coupled—if the angles don’t match or the knob resists—it will keep trying to reach its commanded position, applying full torque and potentially overstressing both the motor and the potentiometer it’s turning. This isn’t a failure of the module; it’s a consequence of its design. It assumes cooperation from the target gear. If the knob is stiff, worn, or misaligned, the servo won’t know when to stop. So setup matters. A lot.

Range Control: Sweet Spots and Inversion

Two front-panel knobs—[MINIMUM ANGLE] and [MAXIMUM ANGLE]—let you define the usable arc of the servo’s movement. Instead of always using the full 270 degrees, you can narrow it down to a “sweet spot” that matches the useful range of a particular knob. Maybe your reverb decay only sounds good between 9 and 3 o’clock—set the min and max to cover just that span, and your CV will only move within it. Even cleverer: if you turn the [MIN ANGLE] knob higher than the [MAX ANGLE], the servo’s response inverts. So rising voltage pulls the knob backward instead of forward. That’s useful for matching the behavior of non-linear or reverse-taper pots, or just for creative flipping of modulation direction without patching an inverter.

Attenuation for Precision

The module includes an attenuator, allowing you to scale down the incoming CV before it reaches the servo. This gives fine control over how much movement results from a given voltage change. Want a subtle wiggle instead of a full sweep? Turn the attenuator down. Need to modulate the modulation? Patch a CV into the attenuator for dynamic scaling. It turns a one-dimensional motion controller into something expressive—closer to a performance tool than a robotic arm.

Historical Context

The ADDAC320 Servo Control was built as a compact, Eurorack-friendly answer to a very old problem: most audio gear doesn’t have CV inputs. Long before modular automation became commonplace, guitarists and engineers rigged up mechanical solutions to control their gear hands-free. The module’s direct inspiration is Neil Young’s “Whizzer”—a custom-built device that let him manipulate amplifier knobs via foot pedals during live performances. Described as “way ahead of its time,” the Whizzer was a testament to what could happen when ingenuity met necessity. The ADDAC320 channels that spirit, but shrinks it into a 4HP module that doesn’t require welding or custom fabrication. It’s not just a tribute; it’s a practical realization of that same idea—using motion to extend control—now available to anyone with a Eurorack case and a knob they wish would move on its own.