Behringer Abacus
Four-channel analog function generator and modulation computer
20 HP Eurorack module
The ABACUS is a four-channel analog music computer for Eurorack that combines dual function generators (channels 1 and 4) with two attenuverters (channels 2 and 3), offering envelope, LFO, slew limiter, and cycling oscillator capabilities. Its musical character is deeply expressive and modular-friendly, with voltage-controllable rise, fall, and response curves ranging from logarithmic to exponential, plus analog OR, SUM, and inverted SUM mixing outputs for complex modulation architectures.
Features
- Two full function generators (channels 1 and 4) with independent Rise and Fall time controls
- Voltage-controllable Rise time, Fall time, and combined Both (total time) per function generator
- Rise/Fall response curve continuously variable from logarithmic through linear to exponential
- Cycling mode per function generator for LFO and audio-rate oscillator use
- External cycle trigger input per function generator (separate from main trigger input)
- Two attenuverters (channels 2 and 3) with independent CV inputs and internal voltage generation
- Channel 2 attenuverter covers -10V to +10V internally generated voltage range
- Channel 3 attenuverter covers -5V to +6V internally generated voltage range
- Analog OR output combining all four channel attenuverter outputs
- SUM output with LED indication (green positive, red negative) combining all four channels
- Inverted SUM output for complementary modulation
- End of Rise output (+9V) for channel 1 that remains high until end of Fall cycle
Controls & Jacks (37)
- IN 1 — Signal input for channel 1. Accepts audio or CV for processing through the function generator.
- Trig 1 — Trigger input for channel 1. Fires the rise/fall envelope on each pulse.
- IN 2 — Signal input for channel 2, a simplified attenuverter channel.
- IN 3 — Signal input for channel 3, a simplified attenuverter channel.
- Trig 4 — Trigger input for channel 4. Fires the rise/fall envelope on each pulse.
- IN 4 — Signal input for channel 4. Accepts audio or CV for processing through the function generator.
- Rise 1 — Controls the rise (attack) time for channel 1. Sets how quickly the output ramps up.
- Rise 4 — Controls the rise (attack) time for channel 4. Sets how quickly the output ramps up.
- Rise 1 CV In — CV input for modulating the rise time of channel 1.
- Rise 4 CV In — CV input for modulating the rise time of channel 4.
- Both 1 CV In — CV input that modulates both rise and fall times simultaneously for channel 1.
- Atten. 1 — Attenuverter knob for channel 1 — scales and inverts the input signal.
- Atten. 4 — Attenuverter knob for channel 4 — scales and inverts the input signal.
- Both 4 CV In — CV input that modulates both rise and fall times simultaneously for channel 4.
- Fall 1 — Controls the fall (decay) time for channel 1. Sets how quickly the output ramps down.
- Fall 4 — Controls the fall (decay) time for channel 4. Sets how quickly the output ramps down.
- Fall 1 CV In — CV input for modulating the fall time of channel 1.
- Fall 4 CV In — CV input for modulating the fall time of channel 4.
- Atten. 2 — Attenuverter knob for channel 2 — scales and inverts the input signal.
- Atten. 3 — Attenuverter knob for channel 3 — scales and inverts the input signal.
- LIN 1 CV In — Linear CV input for channel 1 response curve.
- LIN 4 CV In — Linear CV input for channel 4 response curve.
- Curve 1 — Sets the response curve for channel 1 from logarithmic through linear to exponential.
- Curve 4 — Sets the response curve for channel 4 from logarithmic through linear to exponential.
- Cycle 1 — Enables cycling (LFO) mode for channel 1. The function generator loops continuously.
- Cycle 4 — Enables cycling (LFO) mode for channel 4. The function generator loops continuously.
- Out 1 — Output for channel 1 — outputs the generated function (envelope, LFO, or processed signal).
- Out 2 — Output for channel 2 — outputs the attenuated/inverted input signal.
- Out 3 — Output for channel 3 — outputs the attenuated/inverted input signal.
- Out 4 — Output for channel 4 — outputs the generated function (envelope, LFO, or processed signal).
- EOR — End of Rise gate output for channel 1. Goes high when the rise phase completes.
- Integrator 1 — Integrator output for channel 1, summing the input signal over time.
- OR — Logical OR of channels 1 and 4 outputs — goes high when either channel is active.
- SUM — Sum output — adds channels 1 and 4 together.
- INV — Inverted sum output — outputs the inverted sum of channels 1 and 4.
- Integrator 4 — Integrator output for channel 4, summing the input signal over time.
- EOC — End of Cycle gate output for channel 4. Goes high when the full rise+fall cycle completes.
How-To Guides
- Basic Voltage-Controlled Envelope
- Ultra-Slow LFO for Evolving Pads
- Ping-Pong Envelope Cross-Modulation
- Four-Channel CV Mixer and Inverter
- End of Cycle Sample and Hold Melody Generator
- Voltage-Controlled Portamento / Slew Limiter
- Analog Audio Rate Oscillator
- Precision Offset Voltage Generator
- Cascading Envelope with EOR Gate
- Analog OR Maximum Voltage Selector
Explore the interactive Abacus panel on BleepDex — tap any control to learn what it does.
