ADDAC System 504 (2014–)

A module that randomizes a sequence, argues with it, and sometimes lies to it.

Overview

The ADDAC System 504 is a sequencer that doesn't wait for instructions. It flips coins behind your back and feeds the results to your oscillators. It was born in 2014 at ADDAC System in Lisbon, and it sits in the CV Generation section of their Eurorack lineup. Calling it a CV source undersells it. It is a probabilistic engine, a gate manipulator, a tuning tool and a memory keeper, all in a 10 HP brushed aluminum panel.

At its core, the 504 is a five-output gate/trigger generator. Each of the first four outputs has a user-defined probability of firing, and the fifth catches whatever is left. Turn all four probability knobs to noon and you don't get equal chances. You get a cascading decision tree. Gate 1 has a 50% shot, Gate 2 gets half of what's left (25%), Gate 3 takes half of that (12.5%), and so on. The result is a hierarchy of likelihoods that feels like a rhythm section where one drummer is confident, another hesitant, and the last one shows up when no one else wants the job. It's not random. It's weighted, and it's musical once you learn its quirks.

The 504 also works as a CV sequencer with five tunable notes, each assigned to a knob. Four CV outputs can give either randomized voltages from the tuned set or the exact pitch tied to a specific gate. Gate 3 can always trigger an F#2 while the other CV outs scatter around it. All three main CV outs can jump between octaves based on which gate fires. Tying gate logic to pitch output lets it build whole melodic phrases from a single clock pulse. Quantization can be toggled per session (though not per output), so you can switch between chromatic precision and free-tuned drift without repatching.

Specifications

Manufacturer	ADDAC System
Production Years	2014–
Original Price	€385
Format	Eurorack
Width	10 HP
Depth	40 mm
Max Current	+100 mA / -40 mA
Bus Board Connector	8×2 IDC (Doepfer style)
Gate Outputs	5 (selectable gate or trigger behavior)
CV Outputs	4
CV Output Range	Jumper selectable: 0–5 V or 0–10 V
CV Inputs	3 (for probability modulation)
Logic Outputs	4 (Gate 1–4)
Knob Functions	Triple-function per knob (Probability, Secondary Function, Tuning)
Menu/Trigger Button	3-mode access: Gate Probability, Secondary Functions, Tuning Page
Memory	Page 2 and 3 settings stored; Page 1 volatile
Lock Mode	Sequence capture up to 16 steps, direction: forward, backward, pendulum
Sector	CV Generation
Swing Output	CV/Gate output for swing clock or fixed 0–4 V
Weight	Not specified

Key Features

The Knob That Lies to You (Then Tells the Truth)

The 504’s five knobs are its soul—and its trickster gods. Each one pulls triple duty, switching roles via a long-press menu button. In default mode, Knobs 1–4 set gate probabilities, while Knob 5 handles swing (X and Y values for fine-tuning rhythmic push-and-pull). Press and hold for three seconds, and you’re in Page 2: now Knob 1 controls Gate 5’s output type (trigger or gate), Knob 2 adjusts trigger size, Knob 3 sets lock mode direction, and Knob 4 defines sequence length in lock mode. Hold it again, and you’re in the tuning page—where each knob sets a pitch, and CV Out 4 becomes your monitor, spitting out the voltage of whatever knob you’re turning. The module detects movement and routes accordingly, which is slick—but it also means you can’t tweak two at once. That’s a trade-off: precision over speed, intention over improvisation.

What’s brilliant is how the 504 remembers your settings. Pages 2 and 3 (secondary functions and tuning) are stored permanently, so your favorite swing curve or tuned scale comes back on power-up. Page 1—the probability settings—is volatile, defaulting to 12 o’clock on boot. But the module uses “soft-recall”: when you turn a knob, it blinks the menu button as you pass through the last known position, letting you re-engage your prior setting before going further. It’s a subtle touch, but one that keeps you from accidentally blasting your patch with a 100% probability reset.

Lock Mode: The Memory That Haunts You

Lock Mode is where the 504 remembers. Trigger it via gate input and the module captures the last sequence of active gates, up to 16 steps, and starts looping it. You can set the direction: forward, backward, or pendulum (there and back again). The sequence length is voltage-controllable, so you can modulate how long the loop runs, and the whole thing can be CV-triggered. That turns the 504 from a stochastic source into a generative sequencer with memory. Phrases can start chaotic, settle into something repeatable, then dissolve again when you release the lock.

Lock Mode is also where the limitations show. There's no visual feedback beyond the menu button's blink. No step LEDs, no position indicator. That's fine when you're deep in the flow, but frustrating when you're trying to sync it to a master clock or chase a timing hiccup. And while the lock sequence can be modulated, you can't save multiple sequences internally. For variation you're repatching, using external logic, or relying on probability shifts to morph the output over time.

CV That Thinks in Octaves

The four CV outputs are where the 504 transcends typical random generators. CV Outs 1–3 output voltages based on which gate fires: if Gate 3 is selected, they output a random note from your five tuned pitches, but in the octave corresponding to that gate’s position (Gate 1 = Octave 1, Gate 5 = Octave 5). CV Out 4, meanwhile, always outputs the exact pitch tied to the active gate’s knob. So if Gate 3 fires, CV 4 gives you Note 3’s voltage—no randomness, no octave shift. This creates a natural lead/harmony split: CV 4 for melody, the others for accompaniment. It’s a clever way to generate counterpoint without a second sequencer.

The CV range is jumper-selectable between 0-5 V and 0-10 V, so it works with both Eurorack standards and older systems. Quantization is global, on or off via a front-panel switch, and it doesn't store per patch. Turn it off, tune in microtonal bends, power down, power up, and you're back in chromatic mode until you toggle it again. A per-memory quantize flag would have made this a much better microtonal module.

Historical Context

The ADDAC 504 arrived in 2014, when Eurorack was shifting from boutique curiosity to mainstream synth culture. Producers, pop musicians and bedroom tinkerers were buying modular, not just academics and noise artists. Randomization modules were popular then, but most fell into two camps: simple sample-and-hold noise sources, or over-engineered digital brains buried in menus. The 504 sat between them, complex enough to generate real surprise but tactile enough to feel immediate.

It shared shelves with Mutable Instruments' Pam's New Workout, a Swiss Army knife of clock manipulation that could do probability but buried it in layers. The 504 put probability front and center. No menus, no screens, just knobs, gates, and consequences. Forum threads from 2018 show users torn between Pam's versatility and the 504's immediacy. "I wouldn't be using Pam's for anything but probability stuff," one user admitted, "and I like the idea of all the knobs on the 504 for insta wiggling."

ADDAC System, a small Portuguese outfit, wasn’t trying to beat Doepfer or MakeNoise at scale. They were building instruments for sonic expression—quirky, thoughtful, and slightly academic. The 504 fit that ethos perfectly: a module that rewards study, that demands engagement, that doesn’t just generate randomness but lets you *shape* its soul.

Collectibility & Value

The ADDAC 504 isn't rare, but it's not common either. It's stayed in production since 2014 with no major revisions. ADDAC offers custom panel colors (red, green, blue, bronze), but the standard black is by far the most seen. Used units trade between $250 and $350, depending on condition and whether they include original packaging. New ones still list for around $385.

Failures are few. The menu button is mechanical and frequently used, so it can wear out or develop contact issues over years of heavy use. The IDC power connector is standard, but like all Eurorack modules, it's susceptible to reverse-powering damage if cabled wrong. There are no known firmware bugs or design recalls, though ADDAC provides a detailed firmware update guide for users who want to tinker. The jumpers for CV range are accessible but tiny, and easy to mis-set without magnification.

When buying, test all three knob modes. Check that the menu button cycles cleanly, that CV Out 4 responds to tuning movements, and that lock mode captures and repeats reliably. Ask if the quantize switch clicks firmly and whether the original manual (though not included with the module) has been downloaded. Some users lose the firmware guide and can't update later. It's not a high-maintenance module, but it does want your attention.