ADDAC 216 Sum & Difference
A dual-channel module that does signal math with voltage, one polarity flip at a time.
Overview
The ADDAC 216 Sum & Difference Eurorack Logic Module (ADDAC216) doesn't blend sounds politely. It does arithmetic with voltage - adding, subtracting, flipping and rectifying. It's a Eurorack logic module, and it works on both audio and CV signals. At first glance it looks like a modest dual-channel processor. It has switches and knobs to invert polarities, rectify waveforms and offset voltages on the fly.
It has two identical, independent sections labeled A and B. Each takes two input signals, X and Y. Instead of just summing them like a standard mixer, the 216 lets you subtract one from the other. That opens up phase-cancellation tricks, differential modulation and clean CV math. You can fold audio-rate waveforms with it, or generate control voltages from the difference between two LFOs. Because it handles both audio and CV, it sits somewhere between a utility and a sound generator.
The odd part is how it processes the result. After summing or differencing the inputs, the signal hits a switchable full-wave rectification stage labeled "Absolute." That flips the negative halves of the waveform to positive and creates new harmonic content. Then it runs through an Amplitude attenuverter (a bipolar gain control), followed by an AC/DC coupling switch and a bipolar Offset knob. Those give you control over both the signal's baseline and its excursion above or below zero.
With just one input per section - say, only X or Y - the module becomes a dual attenuverter and CV processor, with rectification and AC coupling options. That means it earns its keep even in small systems. The front panel has LEDs for each section that monitor output polarity and gain. They're handy when patching in the dark or debugging a wonky modulation chain.
Specifications
| Manufacturer | ADDAC |
| Model | 216 Sum & Difference Eurorack Logic Module (ADDAC216) |
| Format | Eurorack module |
| Width | 8HP |
| Depth | 36mm |
| Power consumption | 80 mA +12V / 80 mA -12V / 0 mA 5V |
| Sections | Two independent sections (A & B) |
| Function | Mixer for audio and CV signals |
Key Features
Two Independent Signal Math Engines
Each section, A and B, is a self-contained processing block with two inputs (X and Y). Each input has a bipolar level control, an attenuverter. You can scale each input positively or negatively before mixing, and that is what makes subtraction possible. Feed a signal into X and another into Y, turn Y's level down into the negative range, and you're subtracting Y from X. That's useful for phase tricks, and also for generating control voltages that respond to the difference between two sources, like envelope followers or sequencer offsets.
Rectify, Then Shape
After the sum/difference stage, the signal hits the "Absolute" switch, a full-wave rectifier that converts any negative portions of the waveform into positive ones. Switch it on and you double the frequency content of periodic signals. Switch it off and the original polarity is preserved. This is where the 216 goes from utility to sound design tool. Rectify a slowly modulating CV and you get a bouncy, always-positive wave. Rectify an audio signal and you create new harmonics, sometimes metallic, sometimes gritty, depending on the source.
Post-Processing with Amplitude and Offset
After rectification, the signal moves through the Amplitude attenuverter. It's another bipolar control that scales the output up or down, including into negative territory. Next is the AC/DC coupling switch, which decides whether the output includes a DC offset (useful for CV) or blocks it (better for audio). After that, the bipolar Offset knob shifts the entire signal up or down in voltage. You need that when you have to re-bias a waveform to fit within 0-10V for a VCO or sequencer step.
Dual Outputs and a Global Average
Each section has a normal output and an inverted output, so you get the flipped version of your processed signal without an extra inverter module. That works for push-pull modulation or for driving opposite parameters, like one filter opening while another closes. The module also has an Average output that sums the outputs of A and B and divides by two. It's a true average, not just a sum, which matters for clean voltage mixing.
Compact and Efficient
At just 8HP wide and drawing a modest 80mA on both +12V and -12V rails (with zero draw on 5V), the 216 is a space- and power-sipper. That makes it easy to justify in even the most crowded systems. The 36mm depth is reasonable for most cases, though deep builds should still check clearance. It doesn’t do anything flashy on the power side — no digital processors, no microcontrollers — which likely contributes to its stability and low noise floor.
Collectibility & Value
The ADDAC 216 Sum & Difference is available new at a current market price of $152.03, with retailers offering a 2-year warranty and a 30-day money-back guarantee. Original MSRP was listed at $181.59, so the price has shifted over time. There is no data on used pricing, common failures, or long-term maintenance issues, likely because it's a modern, solid-state design with no moving parts or known failure points. As of now, and accessible, making it more of a functional purchase than a collectible. Owners tend to keep it patched in permanently.
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