ADDAC320 Servo Control ()

It doesn't make a sound, but it makes everything else move. It turns knobs by voltage.

Overview

The ADDAC320 Servo Control isn't a synth voice, an oscillator or a filter. It's a puppeteer. ADDAC System of Lisbon, Portugal builds it as a 4HP Eurorack module with one job: physically turn knobs on gear that was never designed to be automated. Take a compressor, a reverb unit or a synth with no CV input. The Servo Control uses a small, precise servo motor to reach out and twist the knob in real time, driven by a 0 to +5V control voltage signal. It descends from one-off contraptions like Neil Young's Whizzer, which inspired its creation.

At 4HP wide and 45 mm deep, it fits in tight systems. You patch in a CV, and the microcontroller inside translates that voltage linearly into angular movement across a 270-degree range. The motion goes through a flexible metal coupler, basically a springy wire with metal connectors, to the shaft of whatever knob you want to automate. It doesn't clamp down or force anything. When set up right, it's a gentle, responsive link between voltage and motion. Set it up wrong and 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

The ADDAC320 uses a digitally controlled 270-degree metal gear servo motor, the kind you'd find in robotics or RC vehicles. Unlike a regular motor that just spins, a servo can be told exactly where to point, like a needle on a meter. The microcontroller takes any incoming CV between 0 and +5 volts and maps it linearly to the full arc of the servo's range. So 0V sends it to one extreme, +2.5V to the middle, and +5V to the opposite end. 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 with a spring wire and metal couplers. The flexible coupler allows for slight misalignment and absorbs minor tension. It isn't a rigid drive shaft. When aligned properly, it turns knobs cleanly and reliably. The fact sheet carries a quiet warning, though. If the servo is poorly coupled, because the angles don't match or the knob resists, it will keep trying to reach its commanded position. It applies full torque and can overstress both the motor and the potentiometer it's turning. That's a consequence of the design, which assumes the target gear cooperates. 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], define the usable arc of the servo's movement. Instead of always using the full 270 degrees, you can narrow it to a sweet spot that matches the useful range of a particular knob. If 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. If you turn the [MIN ANGLE] knob higher than the [MAX ANGLE], the servo's response inverts. Rising voltage then pulls the knob backward instead of forward. That helps with non-linear or reverse-taper pots, or for flipping the modulation direction without patching an inverter.

Attenuation for Precision

The module includes an attenuator that scales down the incoming CV before it reaches the servo. This sets how much movement results from a given voltage change. For a subtle wiggle instead of a full sweep, turn the attenuator down. To modulate the modulation, patch a CV into the attenuator for dynamic scaling.

Historical Context

The ADDAC320 Servo Control is a compact, Eurorack-friendly answer to an old problem: most audio gear doesn't have CV inputs. Long before modular automation became common, 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. It has been described as "way ahead of its time." The ADDAC320 shrinks that idea into a 4HP module that doesn't require welding or custom fabrication.