ADDAC107 ACID Source ()

A 9 HP Eurorack module with saturated bass and squirming resonance. It started as a drum module and ended up an acid voice.

Overview

The ADDAC107 ACID Source is a compact analog synth voice with a lot of aggression packed into 9 HP. ADDAC System built this Eurorack module with a full signal chain: VCO, VCF and VCA, all tuned for grit and punch. It isn't polite. If you want clean pads or pristine leads, look elsewhere. If you want fat, distorted basslines and screaming acid squelch, the Source delivers.

Owners report it does best in acid, industrial, noise and anything that benefits from a little sonic violence. The design leans hard into saturation. The VCA distorts when pushed, and the filter howls when resonance climbs. It's not a Roland 303 clone, but it's clearly after the same spirit. It doesn't nail every detail of the 303 experience (more on that later), but it has a raw, immediate character that feels alive, unstable and dangerously fun.

A lot is packed into a narrow footprint. For a single module, it offers waveform blending, filter type switching, an accent input and a built-in AD envelope. It's not trying to be a full synth. Like any good weapon, it has quirks. It demands attention and it fights back a little. When it clicks, it's electrifying.

Specifications

Manufacturer	ADDAC System
Product type	Eurorack module; a full synth voice (VCO/Mixer/VCF/VCA)
Width	9 HP
Depth	40mm
Power Consumption	80mA +12V, 80mA -12V
VCO tunable across	four octaves
DIY availability	Available as a DIY kit

Key Features

VCO: Simple but Effective

The oscillator is straightforward. A slide switch picks between triangle and sawtooth waveforms. There's no sine and no pulse width modulation. The interesting part is the waveform mixing: the selected waveform (triangle or saw) can be blended with a square wave to thicken the sound or add bite. The FREQUENCY and FINE TUNE knobs cover a range of four octaves, and there's a CV input with an attenuverter for external pitch modulation. It's not the most flexible VCO out there, but it's stable enough and tuned for character over precision.

Filter: Switchable and Screamy

The filter is where the Source really starts to snarl. It's a resonant VCF with a 3-position slide switch to select highpass, bandpass or lowpass, which is unusual flexibility in such a compact module. The CUTOFF and RESO knobs sweep from murky rumbles to piercing shrieks, and at high resonance the filter can self-oscillate and act as a second tone source. There's a CV input for cutoff with its own attenuverter. There's no CV control over resonance or input gain, which limits expressive possibilities. Owners also report a quirk: cranking the cutoff can affect the oscillator's pitch, creating an unstable, wobbly interaction that's either a bug or a feature depending on your mood.

VCA and Envelope: Fast, Fierce, and Fuzzy

The VCA is where the distortion happens. It accepts an external input signal and has an INPUT GAIN knob that can boost the signal by up to 2x. Push it too far and the VCA circuit overloads, intentionally. The AD envelope has a fixed short attack and an adjustable decay. When it opens, it slews the control voltage, which is also sent to a CV OUTPUT and an LED for visual feedback. The envelope only goes up to +5V for unity gain. Beyond that it saturates, adding harmonic grit. This isn't clean amplification. The Accent input lets you inject a transient boost, good for punchy bass hits or snappy leads.

Signal Flow and Normalization

The signal path is normalized: the CV OUTPUT is patched to both the Frequency and Cutoff inputs by default. The envelope from the VCA can shape pitch and filter at the same time in a classic "acid" contour. That gets you a 303-style sweep without patching a single cable. There's no dedicated filter envelope generator, so the VCA's AD envelope has to handle both amplitude and filter modulation. That limits polyphonic or complex modulation setups. It's a trade-off: instant results for monophonic acid lines, less flexibility for advanced sound design.

Historical Context

The ADDAC107 ACID Source didn't start life as a synth voice. It was originally conceived as a complex drum source, aimed at generating rich, evolving percussive tones. During development the design pivoted, and the result was a compact, full-featured analog voice built for aggressive, modulated bass. That shift explains some of its quirks: the accent input, the emphasis on saturation, the tight integration of envelope and filter. The experimental origins are still audible in its unstable, raw character.

Collectibility & Value

As of now, there's no confirmed data on current market prices or original retail cost. The module remains available as a DIY kit, which keeps it accessible to builders and tinkerers. This availability likely keeps resale prices in check, though condition and build quality will vary widely among user-assembled units.

Collectors and users note several pain points. The slide switches used for waveform and filter selection are a common complaint, and some find them clunky or unreliable over time. The trim pots, particularly for resonance and CV offset, have been described as "wiggly," which suggests potential drift or instability in live use. Tuning and tracking can be inconsistent, especially when pushing the VCO across its range or modulating pitch via the filter cutoff. At high resonance, the filter's cutoff sweep becomes limited, which hurts when you're trying to nail a smooth acid glide.

There's also a design critique: the lack of a dedicated filter envelope, no CV over input gain or resonance, and the shared modulation path mean it's not a true 303 replacement. As one user put it, "you'll have to find some workarounds for an acid-typical filter envelope." Another noted it "feels like an idea that needs more development and design work (and product testing) before launch." Even with these flaws, the consensus is that for the price, a full analog voice in 9 HP with this much character is hard to beat.