About

I wanted my 20th module to be a (finally!) fully featured VCO, with variable waveshape outputs, exponential CV and linear FM, and such. But the fate decided otherwise and i got into a trade with a servermate. As a result, i had to sketch up and build something cool and passive for him, and ended up with this slightly improved version of your usual vactrol passive low pass gate. Of course, i would not be me if i didn't make one for myself, thus so far, this is the only SFP module that exists in two copies. This little module is a cheap and dirty volume control, basically. Something along the lines of a VCA, but also as the signal becomes quieter, it also turns dimmer.

My module has two such low pass gates. Each has two CV inputs, one attenuated. They are not summed, but rather diode OR'd, making the maximal of the two the active one. Then it has a tone switch to choose between three different frequency responses of the module - bright, dim and dark. I chose my own certain values for the capacitros which determine the 'colour', but you are free to experiment and go even brighter/darker. Finally, it has an input and output jacks. It's that simple.

My unit has insignificant bleedthrough on higher frequencies and sharper waves. A unit i am sending over for trade has almost no bleedthrough. I have no idea why is that so, but also it means it's possible to get close to no bleeding from such a simple LPG circuit. It's a simple and useful module, so i think it's a good SDIY beginner project.

Schematic

As you can see, the schematic of this little thing is very simple. First, we have two CV inputs that are OR'd through a pair of diodes. One of them is passed through a potentiometer in voltage divider mode (attenuator) before that. The result of the diode OR is the maximal of the two voltages, and it drives a orangeish yellow LED through a 560 ohms resistor. I suggest using red or yellow LEDs, and testing different resistors from 220r to 1K to see which gives the curve most 'linear' to the eye. The audio part of the cirtuit is essentially an RC low pass filter, but the static R is replaced with a light dependant resistor (LDR), which should be packed together with the LED in a lightproof casing, like heatshrink tube. The C is represented by a small capacitor that's always connected to ground from one side and to the LDR on the other, and two bigger capacitors that are connectable with the SPDT toggle switch (either one of them is connected, or nothing is connected. Be sure to use an SPDT/on-off-on switch!). That's it, that's the circuit. The volume attenuation happens because of how huge the LDR resistance gets in dark (about 1-1.5 megaohms) + the cutoff frequency of the RC filter gets to low ranges, cutting off most of the signal. Very easy, smart and useful circuit!

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