ADDAC System ADDAC216 (2019–)

A utility module that mixes, computes, inverts and rectifies. It does more than a summing circuit usually does.

Overview

The ADDAC216 is not a module that gets plugged in for fireworks. It is not a lead synth or a bass monster. It is a small utility with a lot of signal math inside. At first glance it is a dual-channel analog utility: two sections, each with X and Y inputs, amplitude control, offset, and a few switches. Dig into the patch bay and it turns out to be a real-time math processor for modular synthesis. Instead of only summing two signals, you can subtract them, take their absolute value with full-wave rectification, invert polarity, or derive an average of both channels. It handles both audio and control voltage, so it works for warping pitch CVs, phase-cancelling audio, or generating complex modulation waveforms from basic LFOs.

You can start with it as a basic dual attenuverter with offset, which scales and shifts control voltages to fit your oscillators or filters. Then you find the Absolute switch, which full-wave rectifies the summed signal. Flip it on and a triangle wave becomes a double-frequency pulse, and a random CV becomes a bouncing ball of positive voltage. It works like a tiny analog computer doing real-time Boolean operations on waveforms. Pair it with a dual LFO, subtract one from the other, rectify the result, and you get a modulation source that evolves in unpredictable, organic ways, with the slight imperfection of analog circuitry.

Then there are the inverted outputs. Every section gives you both a normal and an inverted version of the processed signal. Feed one oscillator's pitch CV into section A, send the inverted version to a second oscillator, and you get mirrored tuning, with one rising while the other falls at the same rate. You can also use the inverted output to feed a filter's modulation input with opposite polarity, so one parameter opens as the other closes. The Average output sums both section A and B outputs. It is a built-in passive mixer for combining the two processed channels, useful for layering complex modulation or blending audio signals after individual processing.

ADDAC System designed and built the ADDAC216 in Lisbon. The front panel is minimalist, labeled in white text on black anodized aluminum, with dual LEDs per section showing signal polarity and gain. It is skiff-friendly at just 8 HP wide and runs on 80 mA from both the +12V and -12V rails.

Specifications

Manufacturer	ADDAC System
Production Years	2019–
Original Price	135€ (kit: 100€)
Format	Eurorack
Width	8 HP
Depth	40 mm
Current Draw +12V	80 mA
Current Draw -12V	80 mA
Function	Dual Sum & Difference Mixer, Attenuverter, Offset, Rectifier
Channels	2 independent sections (A and B)
Inputs per Section	2 (X and Y)
Amplitude Control	Attenuverter per section
Offset Control	Bipolar knob per section
Signal Processing	Sum, Difference, Full-Wave Rectification (Absolute), AC/DC Coupling
Switches per Section	Absolute/Bipolar, AC/DC
Outputs per Section	Normal and Inverted
Additional Output	Average (sum of A and B outputs)
Indicators	2 LEDs per section (polarity and gain)
Special Features	Works with audio and CV, DIY kit available, custom front panel options

Key Features

The Math of Sound: Sum, Difference, and Beyond

Most mixers in Eurorack do one thing: add voltages together. The ADDAC216 also subtracts one signal from another (Y - X), which opens up phase manipulation, differential modulation, and rudimentary waveform synthesis. Subtract two slightly detuned LFOs and you get a slow, evolving beat frequency that can modulate filter cutoff or panning. Subtract two audio-rate signals and you are in ring modulation territory, generating sum and difference tones without a dedicated modulator.

Full-Wave Rectification as a Creative Tool

The Absolute switch engages full-wave rectification, a feature rarely seen outside of dedicated wavefolder or distortion modules. When active, it flips the negative portion of the waveform to positive, which doubles its frequency and gives a more aggressive, harmonically rich signal. For control voltages, a slow triangle LFO becomes a rapid zigzag, good for stuttering gate sequences or jittery filter sweeps. For audio, it turns sine waves into something closer to a square wave, adding bite and presence. The analog implementation has a slight softness compared to digital rectification, which can sound clinical. Edges are sharp but not harsh. It is not meant to be a distortion effect, but it can dirty up a signal in a musical way.

AC/DC Coupling and Bipolar Offset: Precision Control

Each section includes an AC/DC coupling switch, which lets you choose whether the output includes the DC offset or only the alternating component of the signal. DC coupling preserves the full voltage range, which matters for pitch or modulation depth. AC coupling strips away any offset, which is useful for audio signals or for centering a fluctuating CV around zero. The bipolar offset knob can add or subtract up to ±5V, so you can position signals precisely. Need to shift a random voltage from 0-5V to -2.5V to +2.5V? That works. So does centering an LFO around 3V to keep a filter in its sweet spot. This level of precision is rare in utility modules.

Historical Context

The ADDAC216 arrived in 2019, when Eurorack was full of digital modules: complex sequencers, algorithmic generators, and modules that could emulate entire studios in a few HP. ADDAC System stayed with analog fundamentals and made the ADDAC216 part of their 200 Series, a line of skiff-friendly utility modules built for the working modular musician. The functionality could have existed in a 1970s Buchla or Serge system, but this module has modern reliability, consistent calibration, and a form factor that respects rack space.

It competes with other analog utilities like the Mutable Instruments Shades, Intellijel Quad VCA, or ALM Busy Circuits Pam's New Workout. Those modules focus on amplification or logic, while the ADDAC216 specializes in arithmetic. It is less a sound generator and more a signal architect, enabling patches that would otherwise require multiple modules or complex workarounds.

Collectibility & Value

The ADDAC216 is not rare, not discontinued, and not prone to skyrocketing prices. It is a current-production module, available from ADDAC's website and dealers like Perfect Circuit or Reverb. New units list for around $169 USD (or 135€), with DIY kits available for about 100€. Used prices hover between $120 and $150, depending on condition and whether it includes the original packaging or documentation. Custom-colored front panels, available in red, green, blue, bronze, and more, can command a small premium, especially in limited runs, but they don't change the value much.

The value here is utility, not scarcity. Modules like this don't depreciate because they solve real problems. It gets recommended in forum threads because someone just spent 20 minutes trying to invert a CV and realized they could have done it in one patch with an ADDAC216.

Failures are rare. The circuit is passive-heavy, with no delicate processors or memory chips. The most common issues reported are loose jacks or LED dimming over time, which is typical of any Eurorack module after years of plugging and unplugging. Because it is analog and not firmware-dependent, there is no risk of bricking. The DIY kit is well-documented and popular among builders, with a detailed assembly guide available online. Even if a unit fails, it is relatively easy to diagnose and repair, and there is no need to ship it back to Portugal.

When buying used, check that both sections respond symmetrically, that the LEDs light correctly with signal, and that the switches (especially Absolute and AC/DC) click cleanly without crackling. A quick test with a known CV source, like a steady LFO, can confirm that rectification and inversion behave as expected. As a utility, any malfunction can ripple through a patch. Overall, the ADDAC216 is one of the most dependable modules in its class.