ADDAC System ADDAC501B Eurorack Module

That moment when your random voltages finally get a pulse—three pairs of them, in fact, with a twist you didn’t see coming.

Overview

The ADDAC501B Complex Random Expansion isn’t a standalone module. It doesn’t sit on the bus board. It doesn’t even pretend to be self-sufficient. Instead, it’s the quiet, focused sidekick to the ADDAC501 Complex Random, plugging directly into the back like a nervous system extension. You won’t find it listed as a full module in some directories—because it’s not meant to be. It’s a Eurorack expander in the truest sense: a surgical upgrade that unlocks new behaviors from an already rich source. If the ADDAC501 is the brain generating chaotic, evolving voltages, the 501B is the reflex arc—adding gate outputs that fire when those voltages cross a threshold, turning randomness into rhythm.

The module gives you three pairs of gate outputs, each tied to one of the ADDAC501's random generators. Each pair has both a positive and an inverted output, so you can trigger rising and falling events from the same logic. The gates fire when the CV output goes above the average of its min and max. Sudden shifts in the voltage become triggers.

Then there's the Smooth knob. It sits centered at rest. Turn it clockwise and you get a standard voltage glide, with smooth transitions between random steps. Turn it counterclockwise and you get a "ring modulation"-like portamento effect. The manufacturer doesn't explain how it works, and the fact sheet doesn't clarify either. It's not a feature you'll find in every manual.

Specifications

Manufacturer	ADDAC System
Dimensions	4 HP
Depth	50 mm
Current Draw	20 mA +12V
Current Draw	20 mA -12V
Current Draw	0 mA 5V
Gate outputs	0 - 5v
Format	Eurorack

Key Features

Direct-Coupled Expansion

The ADDAC501B doesn't patch into your system. It patches into the ADDAC501, through a rear-mounted link that bypasses the bus board entirely. There are no input jacks for CV sources and no cables needed. The module reads the state of the ADDAC501's random generators in real time, so there's no latency and no patching clutter. The 501 generates, the 501B interprets. That makes it useless without its parent module, and that's by design.

Gate Logic with Dual Outputs

Each of the three random channels on the ADDAC501 gets a dedicated gate pair on the 501B. When a channel's voltage exceeds the midpoint between its minimum and maximum range, a gate fires. Because each pair includes both normal and inverted outputs, you can use the rising edge to trigger an envelope while the inverted gate resets a sequencer or mutes a channel.

Smooth Knob with Directional Behavior

The Smooth knob is where the 501B stops being predictable. Centered at 12 o'clock, it's inactive - no glide, no smear. Rotate to the right and you get standard voltage ramping between random values, gradual and linear like a portamento circuit. Go left and the behavior shifts into what ADDAC System calls a "peculiar portamento 'ring modulation'." It's not actual ring modulation, since there's no carrier or modulation input. The effect has a resonant, almost metallic character, like the voltage is oscillating briefly as it moves. It doesn't behave like a traditional glide, and it isn't documented in technical terms.

Collectibility & Value

The ADDAC501B carries an original price of $150.00, though some listings show a higher figure of $194.26, likely due to regional pricing or marketplace markups. Current listings hover around $169.14, with a European price of 120 EUR. It's not a rare module, nor is it particularly sought-after on its own, but it holds steady value among users of the ADDAC501 who want to expand its functionality. With only three votes on record, it holds an average rating of Ø 4.33. There's no data on common failures or maintenance issues, likely because it has no exposed connectors or moving parts beyond the potentiometer. As an expander, its collectibility is tied entirely to the ADDAC501.