Befaco A*B+C

Befaco A*B+C

Befaco A*B+C is a voltage multiplier and offset, every system should have one of these! This is a great little utility. I put it up there with Maths. It’s a strong recommendation I know. I really feel this module deserves a place in any rack because it could find a place in any patch!

What is it?

Befaco describes A*B+C as:

A*B+C is a dual, four-quadrant multiplier with VC Offset that can act as Dual Ring Modulator, Dual VCA and Four Channel Mixer. Each section can perform the analog multiplication of two signals (A and B) and add an offset (C).

So it’s two four quadrant multipliers with offset. Each of these can multiply two signals and add another signal as an offset. The output of the first multiplier is normalized with the mixer of the second multiplier.

The inputs are also DC coupled. This means that they can be used for both AC and DC signals. In other words, this works with audio and CV signals.


What’s a multiplier? Think about voltage as numeric values. Often modules will output voltages in a range of +5 to -5 volts. VCOs often use this range. Some modules output in a range of 0 to +10 volts or -10 to +10v. A multiplier multiplies two voltages.

For example if you had +5v and +1v the multiplier would output +5v (5 * 1 = 5). If there was -0.5v at one input and +2v at the other the output would be -1v (0.5 * 2 = 1).

Each multiplier of the A*B+C has two inputs A and B. The B input has an attenuverter that multiplies voltage at this input by -2 to +2. The B input is normalized to 5v.

With no input and the attenuverter in the center B has 0v. Turning the attenuverter counter clcockwise the voltage at the B input goes to -10v (-2 * 5v). Turning the attenuverter clockwise and the voltage rises to +10v (2 * 5v).

With only an input at A turning the attenuverter will multiply that signal by anything from -10 to +10. With the knob in the center the multiplier is 0. 0 times anything is 0 so there is no output. Turn clockwise and the signal goes up by the multiplier. This is a VCA.

Turn the knob counter clockwise and you have an inverting VCA. This can be useful with control voltages or audio signals.


When you have a control voltage at B that voltage becomes the multiplier. The knob controls the level of the voltage from * -2 to * +2. This allows you to invert a control voltages if needed, and scale them.

For example, you want to add some tremolo to your VCO. Plug your VCO into A send the LFO into B. Use the attenuverter to control the depth of the tremolo.

If the LFO ranges from positive to negative you’ll get an inverted audio signal when the LFO goes into its negative range.

Ring Mod

With an audio signal at both A and B you are multiplying one signal against the other at audio rate, this is ring modulation. The amount of ring modulation applied to A is control by the attenuverter.


The offset input is C and it is normalized to 10v. The offset knob multiplies voltage at the C input by -1 to +1. Use the offset to move the range of a voltage. This is great for control voltages. Imagine you have an LFO that ranges from -5v to +5v, and you want to add some subtle filter sweep to an audio signal. Send the LFO into A, use the attenuverter to scale the voltage to a good range, then use the offset to move the value to the sweet spot.

Creating interesting modulation signals is easy with A*B+C. Send one signal into A, use the attenuverter to scale it. Add a second signal into C and use the offset to decide how much offset that signal applies.

The two LEDs show the level of the output signal. Then the LEDs are red the output is positive. When they are green the output is negative.

Offset is also useful for when a module does not accept negative CV values. The offset them into the positive range. Use the LEDs to confirm that your signal is all in the positive.


If you want to mix signals you can send a signal into the A input and use the attenuverter to set the level. Sending a signal into the C input and use offset to set the level. Both signals are mixed and sent to the output.

The output of the upper half of the module is normalized to the lower half. So signals sent to A1, C1, A2 and C2 are all mixed to output 2.

Building A*B+C

You can get A*B+C assembled or as a kit. I bought the kit and assembled it myself. It’s all through hole parts and is a fairly easy build. I’ll call it intermediate because there are lots of parts and the parts are pretty close together.

There are two boards and a front panel. Befaco supplies sockets fro the chips. This great if something happens and chip gets ruined they are easy to replace.

The circuit is made up of a few op-amps and two AD633 four quadrant multipliers.

The two boards are held together by threaded standoff which makes for a solid build. The outer board has a notch for the power cable. This makes for a compact build but makes the cable a tight fit.

The panel is a good thickness and aluminum.

Here are some pictures of my build.

Would I recommend this module? Definitely! As they say you can never have too many VCAs. Here are two more VCAs with some more options. It’s only 6hp so it fits well in small systems that may be lacking utility.

If you have Maths and and a few VCAs you might not need this. Remember you’re also getting the attenuverters and the offsets.

Would I build it? I like to build this stuff so I built mine. Looking at reverb I see a used one for $89, the kit is $95 and new they go for $165. You make the call.

Leave a Reply

Your email address will not be published.