ADDAC 401 Gated Envelope Follower (2013–present)

It splits your sound's volume into eight rhythmic pieces, and each one can trigger something.

Overview

Patch in a drum loop, a vocal, a cello, anything with movement. The ADDAC 401 slices it up. Most envelope followers hand back a smooth CV that tracks amplitude. The 401 cuts that amplitude into eight thresholds, and each one puts out a gate the moment your input crosses its level. Swells become stutters, breaths become triggers, dynamics become sequences. The original version hit racks around 2013. The updated vs2 changed the PCB layout (now parallel to the panel, tidier for deep builds), but the core concept stayed the same: turn volume into rhythm.

Patch one of the eight gate outputs to a sequencer clock, another to a sample & hold, another to a noise burst, and an ambient pad ends up generating its own accompaniment. The module doesn't care if you feed it clean sine waves or distorted feedback. It reads the energy and fires accordingly. The LEDs show it working as your input rides the VU-like detection, so you can see which gates light up first, then adjust the decay and input level to stretch or sharpen the response. ADDAC's 700-series modules lean into vintage warmth and saturation. The 401 is clinical in presentation, but wildly expressive in use.

Specifications

Manufacturer	ADDAC System
Production Years	2013–present
Original Price	$339 / €260
Format	Eurorack
Width	8 HP
Depth	4.5 cm (40 mm for vs2)
Max Current Draw +12V	110 mA
Max Current Draw -12V	20 mA
Max Current Draw 5V	0 mA
Bus Board Connector	8×2 IDC (Doepfer style)
CV Output Range	±10V
Gate Input Threshold	2.5V (0–5V range)
Gate Outputs	8 independent, 0–5V
Audio Input	1/8" (3.5 mm) jack
CV Output	1/8" (3.5 mm) jack
Offset CV Input	1/8" (3.5 mm) jack with attenuator
Offset Gate Input	1/8" (3.5 mm) jack
Manual Offset Knob	Yes
Input Level Knob	Yes
Decay Knob	Yes
Envelope Gain and Offset	Yes, for CV scaling
Monitor LEDs	CV output and digital detection

Key Features

The Eight-Gate Fracture

Most envelope followers give you one thing: a voltage that rises and falls with your signal's loudness. The 401 uses a digital detection circuit that quantizes amplitude into eight fixed thresholds, each tied to its own gate output. These are precise, timing-locked events that fire the instant your input hits a level. Think of it as a volume-powered step sequencer that writes itself. You don't program the rhythm. You feed it sound, and it extracts pulses. Musicians have used it to convert live vocals into drum patterns, turn bowed textures into arpeggiated glitches, or let a noise sweep dictate the tempo of an entire patch. The gates are normalled in series, so if your signal crosses threshold 3, gates 1 through 3 all fire. That gives a cascading effect that can drive multiple events in sequence.

Offset Control: Shifting the Grid

The offset function is what lifts the 401 above novelty status. A dedicated knob shifts which of the eight thresholds counts as "zero," which slides the whole gate grid up or down the amplitude scale. Feed a quiet signal and only the higher gates fire. Crank the input and the lower ones join in. You can also patch a random voltage or LFO into the offset input (with attenuation via the front-panel knob), and the threshold grid moves around in real time. The gate outputs then respond to both input dynamics and external control, which gives evolving rhythmic densities. There's also a gate input that steps the offset forward with each pulse, so you can sequence the sensitivity itself.

Envelope CV with Full Scaling

The gates aside, the 401 still delivers a fully functional, voltage-controllable envelope CV. The output isn't fixed. With the envelope gain and offset controls, you can map the detected amplitude to any CV range you need, from 0-5V for standard triggering to ±5V for bipolar modulation. That lets you control filter cutoff, pitch, panning, or even motorized faders on external gear. The input level and decay knobs shape the response, so you can tighten up transients or stretch out decays for smoother results. It's especially useful when tracking complex or quiet sources, like a breath mic or a lightly plucked string, where standard envelope followers might miss nuances. The CV output is buffered and stable, so it stays reliable even in dense patches.

Historical Context

When the ADDAC 401 arrived in the early 2010s, Eurorack was expanding rapidly, but many modules still treated audio and control voltage as separate domains. The 401 blurred that line. It arrived alongside a wave of "audio-to-CV" utilities from makers like Intellijel and Mutable Instruments. Where others focused on precision tracking or pitch-to-CV conversion, ADDAC went for rhythmic disruption. The module wasn't trying to replicate acoustic instruments. It turned them into control sources for electronic chaos. The Portuguese company had already built a reputation for modules that combined clean engineering with playful interactivity, and the 401 fit that: no DSP, no menus, just analog envelope detection feeding a digital logic core. It landed when modular users wanted tools that generated unpredictability without giving up control. Competitors like the MakeNoise Echophon offered envelope-following with built-in effects, but none matched the 401's eight-gate output scheme.

Collectibility & Value

The ADDAC 401 isn't a rare module, but it's far from disposable. Units in working condition typically sell between $250 and $350. The vs2 version (introduced mid-life with the reoriented PCB) commands a slight premium because of its improved layout for skiff-friendly builds. Original red panels are standard, but custom colors (black, blue, bronze, or silver gray) are available directly from ADDAC at extra cost and longer lead time, which makes them minor collector's items among boutique Eurorack builders. Failures are uncommon, and when they occur they're usually power-related. The module draws a modest 110mA on +12V, but incorrect cable insertion or bus board issues can damage the regulator. Technicians note that the IDC connector is sturdy, but older units should be checked for cold solder joints on the power pins. There's no firmware, no moving parts and no regular maintenance, just solid-state circuitry that should last decades if treated well. When buying used, verify that all eight gate outputs fire correctly across different input levels and that the CV output tracks smoothly without jitter. A non-responsive gate or erratic CV usually points to a faulty op-amp or comparator IC, repairs that run $60-$100 at most modular tech shops. The 401 holds its value well, especially in systems focused on generative or experimental music.