Behringer Abacus

Name: Abacus
Brand: Behringer

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.