This is Ringer:

It’s a Kosmo format four quadrant multiplier, basically. That means it can take two voltages as input and multiply them together for the output. A VCA does that, sort of, but it takes a bipolar signal and multiplies it by a control voltage that has to be non negative — if you give it a negative CV it’ll just treat it as zero. That means it’s a two quadrant multiplier. A four quadrant multiplier can multiply one bipolar signal by another: Both signals can be either positive or negative, and it will multiply them.

If the two signals are waveforms with a mix of harmonic frequencies, then the output is a mix of frequencies that are sums and differences of the input harmonic frequencies. That means generally the output frequencies are not all whole number multiples of a single fundamental, so it produces more complex sounds rather than simple tones. Applications include making sounds like bells, or making your voice sound like a Dalek.

In fact the original Dalek voices were created using a ring modulator, which produces results similar to a four quadrant multiplier. I built a ring modulator a while ago, and it works but it’s kind of quirky, and I wanted something cleaner, so I built this. The name “Ringer” is in part to hint at its ring modulator-like nature. There are other reasons for the name but they’re even more stupid.

There are three inputs, labeled `A`

, `B`

, and `C`

. There are two attenuverters, labeled `AB`

and `C`

. Whatever voltages are on inputs `A`

and `B`

are multiplied together, divided by 5, and attenuated and/or inverted by the `AB`

attenuverter. The voltage on input `C`

is scaled and/or inverted by the `C`

attenuverter, and then the two results are added to form the output.

`A`

is normaled to 0 V, and `B`

and `C`

are normaled to +5 V.

So if, for example, the `C`

attenuverter is centered (which attenuates the `C`

input right down to zero) and the `AB`

attenuverter is fully clockwise, the output is `A`

times `B`

divided by 5: Four quadrant multiplication.

There are additional applications. If nothing is plugged in on `B`

it’s 5 V, so `A`

times `B`

divided by 5 is just `A`

. Then the output is `A`

, attenuverted, plus `C`

, also attenuverted — it behaves as an attenuverting 2-channel mixer.

Or if the `C`

attenuverter is zeroed and you put a signal into `A`

and a positive control voltage into `B`

, it acts just like a VCA.

So if you find yourself short a mixer or a VCA to finish up a patch, you can use this to do it.

This module is inspired by the Befaco A*B+C , and it functions similarly. Both are based around the Analog Devices AD633 analog multiplier chip. Actually the Befaco is based around two AD633 chips, because it’s a dual module, with two sets of inputs and two outputs. Ringer is a single module almost entirely for one reason: The AD633 in DIP format sells, currently, for about $18.50 at Mouser or Digi-Key. (There are “AD633” chips selling for much less on AliExpress, but I can guarantee at those prices they’re not legitimate Analog Devices AD633s.) I asked myself how likely it was I’d really want to do two A*B+C operations simultaneously and was it worth spending nearly $20 on one chip to allow that, and I decided not very and no. If I really need to at some point I can just build a second module.

Another difference with the Befaco is the attenuverter circuits. Befaco’s attenuverters strike me as complicated and confusing. I had to get out pencil and paper and do a bunch of algebra to convince myself they really were doing what they were supposed to do. There’s a simpler attenuverter design I’m used to and I decided to use that. Both designs are discussed more here .

A third difference is that I included trimmers to null out offsets, as discussed in the AD633 datasheet. I suspect you wouldn’t notice any significant offset without the trimmers, but what the heck, I put them in.

There isn’t much to the design other than the AD633, the attenuverters, and the trimmers, so that means my circuit’s pretty much completely different from Befaco’s.

As usual, schematics, design files, and Gerbers are available in a Git repository: https://gitlab.com/rsholmes/ringer . It’s open source, so sell your firstborn, buy an AD633, and have fun with it.