ADDAC System ADDAC220 Dual Envelope Follower +
It doesn't make sound. It listens to audio and turns it into control voltage.
Overview
The ADDAC System ADDAC220 Dual Envelope Follower + is a quiet module, but it changes how a system responds to dynamics. It's an analog/digital hybrid. It takes the immediacy of analog envelope detection and adds the stability and filtering of digital processing. It doesn't generate anything. You feed it audio, and it gives back CV: a voltage that tracks how loud or soft the signal is over time, cleaned up, smoothed, and ready to modulate filters, amps, delays, or anything else that takes control voltage.
It's a Eurorack module in the ADDAC200 Series lineup. It's a dual-channel unit, so you can track two separate audio sources independently, or run in stereo mode where the left side's controls govern the right. The left input is normalled to the right, so if you only plug into the left, the same signal feeds both sides. That helps for mono operation or quick stereo linking. Use it to pull rhythmic envelopes from drum loops, have vocals modulate a filter, or build self-regulating feedback patches.
The hybrid architecture is what sets it apart from simpler analog followers. It starts in the analog domain: incoming signals hit a [GAIN] control, get fully rectified, and pass through an analog envelope follower with a fixed short attack and decay. Then that analog envelope is sampled by a 12-bit ADC inside a microcontroller unit (MCU). A Savitsky-Golay filter reduces noise and jitter, which gives a cleaner, more stable CV output than many purely analog designs. The MCU also keeps a history of signal peaks and maps the minimum and maximum levels to 0V and the full output range. That keeps modulation depth consistent even if your input volume wobbles.
It doesn't scream "vintage," but its design is clean, precise and flexible, and it follows the envelope followers that shaped early synth dynamics.
Specifications
| Manufacturer | ADDAC System |
| Model | ADDAC220 Dual Envelope Follower + |
| Format | Eurorack |
| Dimensions | 10 HP |
| Depth | 45 mm |
| Current Draw | 70 mA +12V, 40 mA -12V, 0 mA 5V |
| Channels | dual/stereo |
| Outputs | CV and gate outputs |
| Price | $355 |
| CV OUT | 0 to +10V |
| INV. CV Output | -10V to 0V |
| AVG. OUT | 0 to +10V |
| INV. AVG. | -10V to 0V |
Key Features
Analog front end, digital refinement
The ADDAC220 doesn't just average voltage. After the initial analog envelope stage, the signal is digitized and filtered with a Savitsky-Golay algorithm, a technique borrowed from data smoothing that preserves shape while eliminating noise. You get a responsive envelope without the jitter or instability that can plague analog followers, especially on complex or quiet signals. Filter sweeps come out smoother and amplitude modulation more consistent.
Precision control with [ATTACK] and [DECAY]
Once digitized, the envelope is slewed using dedicated [ATTACK] and [DECAY] controls. They set how quickly the module responds to incoming transients and how long it holds or releases. Some followers are locked into fixed response times. The ADDAC220 lets you dial in snappy drum detection or slow, swelling pad tracking. The [RESPONSE CURVE] control adjusts the behavior like a traditional AD envelope, offering logarithmic to exponential characteristics so you can match the feel to your source material or desired modulation effect.
Flexible gain and offset shaping
The [ENV. GAIN] attenuverter doesn't just scale the CV. It can amplify it up to 2x, so you can overdrive downstream modules or stretch modulation ranges. The [OFFSET] control shifts the entire CV range up or down. Together they let you position the envelope within the control voltage range of your system. Need a filter to start closed and open only slightly? Offset down and boost gain. Want a VCA to ride just above silence? Offset up.
Smart stereo and dual-mono operation
The [STEREO / DUAL MONO] switch handles the two modes. In stereo mode, the left channel's [ATTACK], [DECAY], [RESPONSE CURVE], and [ENV. GAIN] controls govern both sides, and only the right channel's [GAIN] stays independent. That suits stereo material with unified dynamics processing. In dual mono, both channels operate completely independently, so you can track two different sources with separate settings. The normalled left-to-right input means you can start mono and expand later without patching extra cables.
Threshold-based gate generation
There's also a [GATE] output that triggers when the envelope signal exceeds a user-set [THRESHOLD]. That turns dynamic peaks into triggers, such as snare hits into clock resets, kick drums into sequencer advances, or vocal bursts into effect toggles. It makes the module a timing source as well as a modulator, all derived from the audio you feed it.
Visual feedback and signal monitoring
A red LED monitors the ENV. GAIN level and shows the envelope's amplitude in real time. It's not a full waveform display, but it's enough to confirm signal presence, check for clipping, or see how your response settings affect the envelope's shape.
Collectibility & Value
The ADDAC220 is a modern module without vintage scarcity, and it stays in steady demand. New units list around $439 on Reverb and $422.84 on eBay, slightly above the $355 price listed in the manufacturer's channel data. That spread reflects retailer markup and platform fees. Used examples do appear, with one listing in the EU priced at €269.00 in December 2025, which suggests moderate depreciation but continued value retention for a functional utility module.
Condition appears well-maintained in the market. One eBay buyer noted their unit arrived sealed in the original box with all manuals, cabling, and warranty information, ideally packaged to prevent shipping damage. No data exists on common failures, maintenance needs, or long-term reliability. Since it's a hybrid design with both analog circuitry and a microcontroller, firmware issues or ADC failures could theoretically arise, but there's no evidence of such patterns in the available data.
It’s not a grail module, but it’s not disposable either. For Eurorack users building a dynamic, responsive system, the ADDAC220 is a trusted tool — and that kind of quiet reliability often translates to stable resale value.
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