ADDAC 307 (2019–present)

A pulse from your finger becomes a gate, an envelope, a living rhythm.

Overview

You clip a small sensor to your fingertip, and your heartbeat becomes a trigger that fires a synth note, or an envelope that swells with each pulse. The ADDAC 307 Heart Sensing module doesn't generate sound. It generates response. It turns the pulse under your skin into a control voltage that shapes a patch in real time. Most modular gear leans on precision clocking and mathematical sequencing, and the 307 is imperfect on purpose. It wavers, it hesitates, it speeds up when you get excited, because it is you, translated into voltage. It's a biofeedback instrument and a performance tool, in a 4HP panel.

The 307 comes from ADDAC's Expressive Controls Series, the same line that gave us the muscle-sensing ADDAC303. It follows a lineage of experimental controllers from the 1970s and 80s, but with modern reliability and Eurorack integration. The module uses a clip-on optical sensor, similar to a pulse oximeter, that detects blood flow changes with each heartbeat. That signal is converted into a 0-5V gate and a corresponding 0-10V AD (attack-decay) envelope, both of which can be shaped and offset. The rhythm it produces is irregular, because it comes from a person.

It's not the module you reach for when you need a metronomic pulse. When you want a sequence that breathes, a drone that swells with your anxiety, or a live performance that responds to your physical state, the 307 fits. Artists working in generative music, bio-art installations, or immersive performance have found it especially compelling. It works in a gallery piece where audience members wear the sensor, and in a dark studio where a composer lets a resting heart rate modulate a filter cutoff.

Specifications

Manufacturer	ADDAC System
Production Years	2019–present
Original Price	$201 / €130.90
Format	Eurorack
Width	4 HP
Depth	35 mm
Current Draw +12V	60 mA
Current Draw -12V	10 mA
Current Draw +5V	0 mA
Trigger Output	0 to +5V
Envelope Output	0 to +10V
Envelope Controls	Attack, Decay, Amplitude, Offset
LED Indicators	Trigger and Envelope status
Sensor Type	Optical clip-on (included)
Sensor Connection	3.5mm jack
Outputs	Gate, Envelope (normal and inverted)
Power Connector	Standard Eurorack 16-pin ribbon
DIY Kit	Yes (SMD and through-hole components)
Panel Color	Black anodized aluminum

Key Features

The Pulse as a Performative Element

Most envelope generators start with a gate and end with a shape. The 307 starts with a biological event and ends with a modulation signal. The optical sensor detects each pulse by measuring changes in blood volume under the skin and converts that into a clean gate signal. The gate feeds an internal AD envelope with user-adjustable attack and decay, so you get a dynamically shaped response to your physiology, not just a trigger. The amplitude control scales the output voltage, which sets how much sway your heartbeat has over a filter or VCA. The offset knob shifts the baseline, useful for matching the envelope to modules that respond to different CV ranges. A resting pulse might give you a slow, meditative swell. A raised heart rate from physical movement or emotional intensity gives tighter, more urgent bursts.

Human Rhythm vs. Machine Time

The 307 doesn't lock to a tempo. The slight gaps between beats and the natural accelerando when you get excited are features, and in a patch they make sequences feel organic rather than programmed. Pair it with a quantizer and a sequencer, and you have a rhythm generator that follows your state. Use it to modulate a delay feedback knob, and the repeats grow denser as your pulse quickens. The inverted envelope output lets you create complementary movements, so one parameter opens up while another closes down, all in sync with your circulation. The two front-panel LEDs give visual feedback. One blinks with each trigger, the other pulses with the envelope's rise and fall, so you can see if the sensor is picking up a signal, even in low-light performance settings.

Integration and Limitations

The 307 is straightforward in operation. It requires no firmware, no USB connection, no software - just power and the sensor. The 3.5mm sensor input is not a standard CV input. It supplies power to the sensor and expects a specific analog signal in return. Forum reports confirm that plugging a regular LFO or gate into the sensor jack can damage the module, as the input is designed to source voltage, not receive it. This means you can't use the 307 as a standalone envelope generator without the sensor attached. Some users have experimented with alternative sensors, like modified contact mics or photodiodes, but these are unreliable and not recommended.

Historical Context

The ADDAC 307 emerged in 2019, when Eurorack was deep into its expansion, in sonic capabilities and in how sound could be generated and controlled. Many manufacturers focused on complex digital algorithms or hyper-precise analog circuits. ADDAC carved a niche with the Expressive Controls Series, built around physiology instead of voltage. The 307 followed the ADDAC303 Muscle Sensing module, which used EMG signals from muscle contractions. Together they formed a pair of bio-responsive tools that felt more like instruments than utility modules.

This wasn't the first time biometrics entered electronic music. In the 1960s, Alvin Lucier used brainwaves in *Music for Solo Performer*. In the 1970s, David Rosenboom composed works using EEG feedback. Those setups required rooms full of gear and custom patching. The 307 brings the concept into the modular world with plug-and-play simplicity. It arrived alongside other experimental controllers, like the Critter & Guitari EYES or the Sensel Morph, but unlike those, it doesn't generate sound or MIDI. It generates control data, which makes it more akin to a sensor than a synth.

Collectibility & Value

The ADDAC 307 has settled into a stable secondary market, with used units trading between $160 and $200 depending on condition and region. New old stock or sealed DIY kits occasionally appear on eBay for around $220. Because it was sold as a DIY kit (though pre-built versions exist from third parties), condition varies widely. Factory-assembled units are rare, since most were built by users, and signs of poor soldering or misaligned jacks are not uncommon. The optical sensor is generally reliable, but the clip can become brittle over time, especially if stored in direct sunlight or extreme temperatures. Replacement sensors are available from medical supply outlets, but the exact model used by ADDAC (a standard 3.5mm pulse oximeter probe) must be confirmed before substitution.

Failures are uncommon but usually traceable to incorrect assembly, particularly in DIY builds. The most frequent issue reported is no signal from the sensor, often due to a cold solder joint on the input jack or a misoriented LED/photodiode pair on the sensor itself. Power draw is modest (60mA on +12V, 10mA on -12V), so it won’t stress a bus board, and the 35mm depth makes it skiff-friendly. For buyers, the key is verifying that the sensor works: ask for a demo video showing the LEDs responding to a pulse. Avoid units advertised as “tested without sensor”—if the seller didn’t test it properly, there’s no guarantee it functions. Given its specialized role, the 307 isn’t a must-have for most racks, but for performers and installation artists, it’s a unique tool worth the investment.