befaco C_V4 Two Module Channels User Manual

If you plug the module backward you might burn it out and unfortunately, this is not covered by the warranty.
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WHAT IS A*B+C?

A*B+C is the ultimate utility for audio and control voltage.

It performs multiplication, addition, and attenuation, providing a simple way to create complex signal interactions and timbres.
It is formed by two channels, each of which takes two input signals (A and B), multiplies them (Ax B), and then adds a third signal (C) to the result.

The two channels can be MULTor MIX acting as a signal splitter or a two-channel mixer.

By combining different blocks, many functions can be achieved:

Four-quadrant amplitude modulation (ring mod.)
Linear VCA

Signal mixer
2x gain amplifier

Phase inverter
Attenuator

2 channel Multi
Voltage-controlled attenuator

CV mixer
Attenuator

Inverter
VCA

DC generator

Core Function

The two module channels perform the same mathematical operation: (AX B) + C

Where

A, B, and Care inputs that can accept signals (control voltages or audio).

B and Care normally to 5V, automatically providing a default voltage if nothing is plugged in.
Depending on the Gain switch, the level of B can be kept the same, inverted, or doubled

CONTROLS

Manual Attenuator of 8 IN of channel 1
Adjusting this control changes the amplitude of the signal at IN B1. At the leftmost setting, the amplitude is zero. As you move to the right, the level increases based on the B1 Gain Switch (2)
81 Gain Switch
This switch scales the signal at B 1 by either 1 (· ), 2( •• ), or -1 (-•)
Manual Attenuator of C IN of channel 1
Adjusting this control changes the amplitude of the signal at IN C2. At the leftmost setting, the amplitude is zero. As you move to the right, the level increases based on the C2 Gain Switch (8)
C1 Gain Switch
This switch scales the signal at C1 by either 1 (·), 2(··), or-1 (-·)
Manual Attenuator of 8 IN of channel 2
Adjusting this control changes the amplitude of the signal at IN B2. At the leftmost setting, the amplitude is zero. As you move to the right, the level increases based on the B2 Gain Switch (6).
82 Gain Switch
This switch scales the signal at B2 by either 1 (· ), 2( •• ), or -1 (-·)
Manual Attenuator of C IN of channel 2
Adjusting this control changes the amplitude of the signal at IN C2. At the leftmost setting, the amplitude is zero. As you move to the right, the level increases based on the C2 Gain Switch (8).
C2 Gain Switch
This switch scales the signal at C2 by either 1 ( • ), 2(·· ), or -1 (-·)
A 1 and A2 Inputs
A Input jacks for channels 1 and 2 respectively
B1 and B2 Inputs
B Input jacks for channels 1 and 2 respectively
C1 and C2 Inputs
C Input jacks for channels 1 and 2 respectively
OUTs 1 and 2
Output jacks for channels 1 and 2, respectively. Out 2 functions as a mixed output of channels 1 and 2 if the MIX mode is selected via the Mode Switch (13).
Mode Switch
Selector for Splitter or Mixer modes or combining the two channels
Level LEDs
Level Indicator for the Outputs. Green indicates positive voltage and red indicates negative.

Internal structure

Multiplication (Ax B):
The signal A is multiplied by the signal at B. As a result, the level of B directly affects the A level by multiplying their voltages at any given moment

As it is a multiplication the general rules of multiplication apply

If B=O Volt or A=O Volt, A*B will always be = 0 volt
If A or Bare negative voltages A*B will always have negative sing
If A and Bare negative voltages A*B will always have positive sing

A*B=B*A The order of the factors doesn’t change the result.

Level adjustment
To ensure that voltage levels remain appropriate for modular synthesizers, the voltage resulting from multiplication (A * B) is always divided internally by a factor of 5. Without this adjustment, the output voltage could easily exceed acceptable limits due to the nature of the multiplication operation.

As a result, when 5 volts are applied to either input A or B, the output will produce a unity gain, meaning it will equal the other input This can be understood by applying the identity property of multiplication, which states that

If you multiply any number by 1, the result is the number itself: X * 1 = X.

On top of that, the gain switch, scales the B signal by either 1, 2, or -1:

Setting 1 (•): gain ranges from Oto 1 (attenuator).

Setting 2(••): gain ranges from Oto 2 (doubling amplifier)
Setting -1 (-•): gain ranges from Oto -10 (inverting attenuator)

B is normalized to 5 volts and A is multiplied by B, so if a signal is plugged into A and the B jack remains unpatched, the B pot becomes a manual level control for A

Addition ( +C)
The result of Ax B is added to the signal at jack C.

Just like at point B, if nothing is connected to point C, the voltage defaults to 5 volts. This introduces an offset to the output The voltage of this output depends on the gain switch setting, which scales the signal at point C by factors of either 1, 2, or -1 .

Setting 1 (•): gain ranges from Oto 1 (attenuator).

Setting 2(••): gain ranges from Oto 2 (doubling amplifier)
Setting -1 (-•): gain ranges from Oto -10 (inverting attenuator)

Channels Combination

Each of the two channels can be combined in two ways, depending on the setting of the Mode switch:
MULT: This mode sends the signal present at A on channel 1 to A on channel B, effectively splitting the same signal to both channels and functioning as a Mult. This is achieved by normalizing A 1 to A2; once a jack is connected to A2, the normalization is broken, and the channels become independent.
MIX: In this mode, the outputs of the two channels are blended, functioning as a mixer. This is accomplished by normalizing OUTl to the mix bus; when a jack is connected to OUT2, the normalization is broken, and the channels become independent.

Operational Levels

All inputs accept voltages in the range of+/- 10 volts.
Any voltage exceeding 10 volts will be clipped internally, whether it originates from external signals or results from internal operations. This feature is useful for maintaining signal levels suitable for modular synthesizers or for adding distortion to audio signals.

Each channel is equipped with two LEDs located above the outputs. These LEDs indicate the output levels for each channel: a green LED signifies positive voltage, while a red LED indicates negative voltage.

A*B+C Diagram

SPECS AND CREDITS

Current needs: + 12V: 50mA, -12: 50mA, 5v:0

Width: 6 HP
Depth: Assembled version 25 mm
Depth: 25 mm Assembled version – 35 mm DIV version

Designed with care and love by the Befaco Team in Barcelona – 2025

Documents / Resources

befaco C_V4 Two Module Channels [pdf] User Manual
C_V4 Two Module Channels, C_V4, Two Module Channels, Module Channels

References

User Manual

befaco C_V4 Two Module Channels

Instructions