ADDAC301B Dual Expression Attenuator (2014)
Two channels of passive control. Plug in any expression pedal and shape your modular patch with your feet.
Overview
The ADDAC301B Dual Expression Attenuator brings physical movement into a modular synth rig without any control voltage math. It doesn't generate sound and it doesn't process it. It sits in the rack and turns the slow press of a foot into changes across your patch. Made by ADDAC System (often referred to simply as ADDAC), this 4 HP module is part of a focused family of tools for interacting with analog synthesis through more intuitive, human-scale control instead of more oscillators or filters.
It's a passive module, meaning it doesn't need power to function. No internal amplifiers, no active circuitry. You plug a CV or audio source into the input, route the output to whatever you want to modulate (a filter cutoff, a VCA, a delay time), then connect a standard expression pedal to the module. As you rock the pedal forward or back, you directly adjust the amplitude of that signal. Each of the two independent channels has an invert switch. Flip it, and heel-down becomes full signal instead of zero. That is handy live, when your patch feels backwards.
It's not flashy. No blinking lights, no menus, no patch memory. The ADDAC301B does one thing cleanly and reliably. It's part of the ADDAC300 Expressive Controls Series, a set of compact modules aimed at making modular synthesis more tactile and immediate. It isn't built for automation or precision sequencing. It's built for feel and muscle memory, shaping sound in real time with your body.
Specifications
| Manufacturer | ADDAC System |
| Width | 4 HP |
| Depth | 3 cm |
| Module type | Passive Module |
| Max current | 20mA |
| Bus Board Cable | 8 × 2 IDC (Doepfer style) connector |
Key Features
Passive Dual Channel Design
The ADDAC301B gives you two completely independent attenuator channels, each with its own expression pedal input and signal path. Since it's passive, it doesn't color the signal with noise or distortion. What goes in comes out, just quieter or louder based on pedal position. That suits both control voltages and audio-rate signals, such as fading in a drone or slowly opening a filter over time. The lack of a power requirement simplifies installation, but the Doepfer-style 8×2 IDC bus cable means it still connects to your case's power rail. That is likely to support the invert switch circuitry, though the core attenuation remains passive.
Invert Switch for Expressive Flexibility
Each channel has a physical toggle switch to invert the pedal response. Normally, pressing the pedal forward increases the signal (toe down = full). With the switch engaged, it flips and heel down becomes full signal. Some expression pedals are biased toward heel-down resting positions, and some playing styles favor backward sweeps. Inverting on the fly lets you match the module's behavior to your pedal's mechanics and your own physical intuition.
Compact and Purpose-Built
The module is 4 HP wide and 3 cm deep (some sources list 4 cm, but the 3 cm measurement appears more consistently across documentation), so it fits even tight racks. The front panel is clean, with clear labels and sturdy jacks. There's no attenuation knob. Your expression pedal is the control. You set it up once and patch it in.
Historical Context
The ADDAC301B was introduced in January 2014 as part of a five-module launch in the ADDAC300 Expressive Controls Series. This wasn't a line of sound generators or processors, but a toolkit for expanding physical interaction with modular systems. Eurorack was growing rapidly and becoming more complex at the time. The series included the ADDAC300 Power Starvation (for voltage manipulation), ADDAC301 Floor Control (a larger foot-operated interface), ADDAC301C Dual Sustain Switcher (for pedal-triggered gates), and the experimental ADDAC303 Muscle Sensing (which read EMG signals). The ADDAC301B fit that philosophy: minimalist, tactile, and built for performance.
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