ADDAC 205 (2010s)

A dual-channel Eurorack module with two oscilloscopes that also work as waveform generators.

Overview

The ADDAC 205 has two OLED screens, and they do more than display. The module doesn't make noise so much as show you what your oscillators, envelopes, and LFOs are doing at any given moment. The first Lissajous figure on one of its high-contrast displays changes how you patch, because you stop guessing and start looking.

The 205 comes from the early 2010s, from Lisbon-based ADDAC System. On paper, it's a dual oscilloscope: two independent units, each with two input channels, for monitoring voltage or audio signals in real time. It's also an arbitrary waveform generator (AWG), capable of producing 8-bit sine, triangle, and sawtooth waves at a 44.1kHz sampling rate. So it watches and it contributes, which makes it a hybrid instrument and more than a diagnostic tool.

At 10 HP wide and only 5 cm deep, the 205 slips easily into even the tightest skiffs. Its compact footprint belies its functionality: each scope has a 128Hz refresh rate, which isn’t quite real-time video, but is more than fast enough to capture the behavior of audio-rate signals with clarity. The interface is menu-driven, navigated via four tactile buttons per scope, which takes a few minutes to learn but quickly becomes second nature. You can toggle between voltage metering, X/Y mode for Lissajous patterns, and standard time-domain display—features that turn abstract control voltages into visible shapes, making it easier to debug complex patches or simply marvel at the geometry of sound.

Specifications

Manufacturer	ADDAC System
Production Years	2010s
Original Price	€385
Format	Eurorack
Width	10 HP
Depth	5 cm
Power Supply	±12V and ±15V compatible
Current Draw	150mA max
Bus Board Connector	8×2 IDC (Doepfer style)
Screen Refresh Rate	128Hz
Display Type	High-contrast OLED
Inputs per Scope	2 channels (±10V CV)
Outputs	0–5V CV (arbitrary waveform generation)
Arbitrary Waveform Resolution	8-bit
Sampling Rate (AWG)	44.1kHz
Waveforms Generated	Sine, Triangle, Sawtooth
Trigger Input	1 external trigger per scope
Additional Features	Voltage metering, X/Y oscilloscope mode, Lissajous display

Key Features

Two Scopes, One Vision

Two independent oscilloscopes in one module suit anyone who juggles multiple signal paths. One scope can monitor an audio oscillator while the other tracks an envelope, or both can compare phase relationships between two LFOs. Each scope has two channels, so you can overlay signals, compare timing, or use one as a reference while tweaking another. The OLED screens are sharp and legible even in low light, and the 128Hz refresh rate keeps fast-moving waveforms from blurring. It's not a full vector display, but it's responsive enough to catch glitches that might go unnoticed in a dense patch.

Arbitrary Waveform Generation Meets Visual Feedback

The generator side is what sets the 205 apart from typical test equipment. Each scope includes an 8-bit AWG running at CD-quality sampling rates, outputting sine, triangle, and sawtooth waves. These aren't high-fidelity tones. 8-bit resolution means you'll hear some digital grit, a lo-fi edge that suits experimental Eurorack textures. Because the AWG is tied directly to the display, you can see exactly what you're generating. You can check the symmetry of a triangle wave, or confirm a sawtooth isn't folding over, just by looking at the screen. That makes the 205 a compositional tool as well as a monitor.

Menu-Driven Simplicity with Hidden Depth

Four buttons per scope feels minimal at first, but the layout is intuitive once learned. You can switch inputs, adjust scaling, toggle X/Y mode, or engage the AWG with a few presses. The commutable input switches let you route signals flexibly, and the external trigger input lets you sync the display to a clock or envelope for stable readings. In X/Y mode, two periodic signals fed to the horizontal and vertical axes generate Lissajous figures. They also show phase coherence, frequency ratios, and modulation depth in ways meters and ears alone can't.

Historical Context

The ADDAC 205 arrived in the early 2010s, when Eurorack was shifting from boutique curiosity to widespread adoption. As users built larger systems, the need for diagnostic tools grew. Most relied on external gear such as bench oscilloscopes, multimeters, or laptop software. These were either bulky or disconnected from modular patching. The 205 offered a built-in, patchable visualizer on the panel. It wasn't the first module to bring visual feedback to Eurorack, but it was among the first to do it with dual functionality and high-resolution OLEDs at a reasonable size.

ADDAC System, based in Lisbon, had already made a name with hybrid digital-analog designs like the ADDAC111 Ultra .WAV Player and the ADDAC506 Stochastic Function Generator. The 205 fit their philosophy of expanding what modular could do with modern tech. Competitors like Intellijel and Mutable Instruments focused on sound generation or sequencing, while ADDAC explored tools for understanding, interaction, and experimentation. The 205 wasn't a headline-grabbing synth voice, but for users pushing complex modulation and feedback networks, it became indispensable.

Collectibility & Value

Today the ADDAC 205 sits in a quiet but steady market. It's not a "hype" module, so prices aren't skyrocketing on Reverb or eBay. It's also not easy to find new, because ADDAC's legacy modules are no longer in active production and used units reflect that scarcity. Expect to pay between €300 and €450 depending on condition. Mint examples with original packaging and accessories (like the six 12-inch and seven 14-inch patch cables noted in one listing) command the upper end.

Reliability is generally good, with a few things to watch. The OLED screens can degrade over time if left on static images for extended periods. Owners report leaving Lissajous patterns running for hours as ambient visuals, which may shorten display life. The menu system relies on firmware that isn't field-upgradable, so any bugs or quirks are permanent. Power draw is moderate (150mA max), but the module requires both ±12V and ±15V rails, which can be a compatibility issue in some cases. Verify your case supports dual-voltage operation before buying.

Maintenance is minimal. There are no moving parts, and the circuitry is well-protected. If the unit has been stored improperly or exposed to humidity, the button contacts may develop resistance or chatter. Contact spray usually fixes this, but replacement switches are not user-serviceable without desoldering. The most common failure point is the IDC power connector, which can loosen over time from repeated plugging and unplugging. A recrimped or replaced cable is an easy fix for a technician, but check it before purchase.

For buyers, the value is in utility, not resale. If you're building or expanding a Eurorack system with complex modulation, feedback loops, or experimental patches, the 205 is worth every euro. It helps you debug a misbehaving envelope or confirm a delicate FM ratio. The Lissajous and X/Y modes also work as a responsive light show. Don't expect pristine 24-bit audio from the AWG. The 8-bit resolution is a limitation, but it adds a gritty, retro-digital texture that pairs well with analog oscillators.

Market Value

┌──────────────────────────────────────────────────────────────────────────┐
│ ADDAC 205                                                       Aug 2026 │
├──────────────────────────────────────────────────────────────────────────┤
│ High      $61                                     Latest             $61 │
│ Median    $57                                     Trend             ▲ Up │
│ Low       $43                                     Volume           ~2/mo │
├──────────────────────────────────────────────────────────────────────────┤
│   $100 ┤                                                                 │
│        │                                                                 │
│        │                                                                 │
│    $57 ┤.                                                               .│
│        │         .                                                       │
│        │                                                                 │
│        │                                                                 │
│     $0 ┤                                                                 │
│        ├─────────────────────────────────────────────────────────────────│
│    vol │▄        █                                                      ▄│
│        └─────────────────────────────────────────────────────────────────│
│         2026-05                      2026-06                      2026-07│
└──────────────────────────────────────────────────────────────────────────┘

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