ADDAC 506 Stochastic Function Generator (2018–)

Four channels of analog envelope and slew generation, with digital randomness laid over the top.

Overview

Patch in a gate, turn a few knobs, and the synth starts swelling slowly and irregularly, never quite repeating. That is the ADDAC 506 working. It is a licensed reimagining of Teia's cult-favorite Stochastic Function Generator. The 506 takes the original's two channels and makes four, rebuilt from the ground up with a new microcontroller. Every square millimeter of the 20HP panel is occupied, but it isn't cluttered. Four independent analog rise/fall generators sit at the core, each able to go from snappy drum triggers to six-minute drone glides. The random engines are slaved to the rise and fall times. They let you dial in a minimum and maximum window within which the randomness operates. Want subtle humanization? Nudge the spread just a few percent. Want full stochastic collapse? Crank the max and let the module decide when the next envelope fires.

And improvisation is where the 506 thrives. It’s not content being a one-trick envelope box—flip it into slew mode and it becomes a lag processor for incoming CV, perfect for softening quantized sequences into something more organic. Feed it a stepped melody from a sequencer, let the random slew times smear the transitions, and suddenly your rigid arpeggio sounds like it’s melting in the sun. The module’s digital control doesn’t sterilize the analog core; it enhances it, adding layers of behavior that would otherwise demand multiple utility modules. Each channel has dedicated attenuverters for amplitude and offset, so you’re not hunting for extra modules to invert or scale the CV output. The three gate outputs per channel—Rise Gate, End of Rise, and End of Fall—can be jumper-configured to spit triggers or gates, making it trivial to chain envelopes together in cascading sequences. Patch the End of Fall from voice one into the trigger input of voice two, and you’ve got a self-advancing modulation chain that can loop back on itself or fan out into multiple destinations. The sum and average CV outputs across all four voices open up even more possibilities, letting you generate complex control voltages from the interaction of multiple randomized envelopes. It’s the kind of module that rewards deep patching, the kind that makes you forget to eat because you’re too busy watching your synth evolve on its own.

Specifications

Manufacturer	ADDAC System
Production Years	2018–
Original Price	375.00 €
Width	20HP
Depth	40mm
Power Draw +12V	200mA
Power Draw -12V	150mA
Power Draw 5V	0mA
Number of Channels	4
Envelope Types	AD, LFO (triangle), Slew
Randomization	Built-in random generators for Rise and Fall times with Min/Max controls
Gate/Trigger Outputs per Channel	3 (Rise Gate, End of Rise, End of Fall)
Additional CV Outputs	Sum and Average of all 4 voices
Random CV Outputs	4 (via ADDAC506B expander)
Random Trigger Inputs	4 (via ADDAC506B expander)
Attenuverters	Dedicated Amplitude and Offset per channel
CV Controllable	Rise Time Min/Max, Fall Time Min/Max
Mode Switches per Channel	Trigger/Slew, Loop/One Shot, Speed Range (Low/Med/High), Lock
Maximum Envelope Duration	6 minutes
Audio Rate Capability	Up to 1kHz in Loop mode

Key Features

Stochastic Control with Surgical Precision

The 506 constrains its randomness. Each channel's rise and fall times can be modulated by internal random generators, but only within user-defined minimum and maximum limits. You set the range, and the module fills in the variation. If the max time is shorter than the min, the randomizer disengages, so you can switch between deterministic and stochastic behavior on the fly. The random engines are CV-controllable via the Min and Max inputs, so external sources like sequencers or other LFOs can shape the randomness itself.

Self-Contained Modulation Workstation

The 506 packs a lot of utility into one module. The per-channel attenuverters for amplitude and offset mean you aren't reaching for extra modules to invert or scale CVs. The sum and average outputs across all four voices work as built-in mixers for modulation signals, blending envelope behaviors into a single control voltage. These outputs are always active, so you can use them alongside the individual channel outputs. The three gate outputs per channel, configurable via rear jumpers, give you timing options: use End of Rise to trigger the next stage in a sequence, or End of Fall to signal completion. That makes complex, interdependent patches possible without external logic modules. The ADDAC506B expander adds four trigger inputs to reseed the random engines mid-cycle and four dedicated CV outputs for the random voltages themselves.

Multi-Mode Flexibility: Envelope, LFO, Slew

In One Shot mode, the 506 is a classic AD envelope - trigger it, and it rises, holds, then falls. In Loop mode, it becomes a triangle LFO, and the rise and fall times can still be randomized, so the LFO stutters rather than repeating mechanically. At high speeds it reaches audio rates (up to 1kHz), which lets you modulate pitch or filter cutoff with irregular waveforms. In Slew mode, it becomes a voltage lag processor, useful for portamento on sequenced melodies or for smoothing stepped CV. The random generators still apply here, so the slew time itself keeps moving. A speed range toggle (Low, Medium, High) on each channel scales the timing behavior.

Historical Context

The ADDAC 506 descends directly from the Teia Stochastic Function Generator, a limited-run module from 2013 that built a cult following for injecting controlled randomness into modular patches. When Teia's module went out of production, ADDAC, already established in the Eurorack scene since 2009, stepped in with an officially licensed redesign. Rather than clone the original, they doubled the channel count from two to four and completely rewrote the firmware to add deeper control and more flexible routing. The 506 was released in 2018, as modular users were looking for ways past static sequences and predictable modulation. Competitors like Mutable Instruments' Tides or ALM's Quaid offered complex modulation, but few combined analog signal paths with such deep digital control over randomness. It wasn't the first module to randomize envelope times.

Collectibility & Value

The ADDAC 506 trades in a tight range, typically between €300 and €375 depending on condition and region. It is a specialized but highly capable module rather than a speculative collectible, and its value comes from utility, not scarcity. Units in mint condition with original packaging and the ADDAC506B expander can fetch closer to the upper end, especially in North America where shipping from Portugal adds cost. Failures are rare, but the dense layout means front-panel components, particularly the small toggle switches, are vulnerable to accidental toggling or physical damage during patching. There are no known widespread reliability issues, and the all-analog signal path with digital control logic keeps points of failure low. The expander is the real sticking point. The 506 works perfectly standalone, but owners report that the ADDAC506B's functionality isn't always intuitive. The random trigger inputs and CV outputs need specific patching to be useful, and some users end up with the expander mounted but underused. For generative patches, though, reseeding randomness mid-cycle or outputting random CVs directly is invaluable. When buying, check that all toggles click cleanly and that the attenuverters don't crackle.