About
This very useful vactrol-based module can crossfade between things. Both CV signals and audio rate signals can be crossfaded with it. The two inputs, the - and + in, are blended in a ratio that corresponds to the sum of the initial crossfade knob and the incoming CVs: the more negative it is, the more the - input will be heard at the output, up to hearing only it. Likewise, the more positive the sum gets, the more you will hear the + input, until only it is audable. Inbetween, it's a smooth fade from one to the other.
It also can be used as a VCA, although in this sense it would mean "attenuator" rather than "amplifier". Just input your signal to the + input and leave the - input empty for the usual VCA response, and vice versa for an inverted, "ducking" one (vactrol compressor, anyone?)
The circuit is very simple, and it's basically a voltage controllable voltage divider using vactrols. It bleeds a tiniest bit in VCA mode if you listen closely, but it's negligible and i had to amplify the signal a lot to actually hear it. I made my module a dual crossfader, but you can make as many as you'd like: one unit is depicted on the schematic.
Schematic
As most of my ideas are, this one is pretty straightforward and simple. You probably already know what an attenuator is. To make it into a crossfading potentiometer, we'd need to hook up another signal to the grounded leg instead of grounding it: it's that simple. Now, to make this potentiometer into a voltage controllable device, i replaced it with two vactrols. The LEDs of the vactrols are in series, and are driven from +12v to -12v. Imagine that the junction between the 2 LEDs was grounded instead of being connected the op-amp output. Both LEDs would be bright constantly. Now, if this ground could "move' up and down voltage wise, it would make the voltage difference across one LED bigger and the other LED smaller, it would increase one light dependant resistor's (LDR) resistance, and decrease the other LDR's resistance. So, a voltage reference from the CV summing amplifier is plugged between the LEDs as a "reference voltage point", or that sort of "moving ground" thing: That's the way CV works here. The rest is easy: two LDRs form a voltage divider, which is basically voltage controllable. We put the two signals to the free legs of the LDRs, then pick up the mix from the connected LDR legs and buffer it up through another op-amp. It's that simple!
As with most things that i do, i didn't care because i'm a punk. So, first of all, at maximum + and maximum - voltage, the output will still be a bit quieter than the original input. You can fix it by turning the output buffer into a non-inverting amplifier with 2 resistors and a trimmer, and then carefully trimming the op amp's gain to match the input. Also, as always, vactrol devices have their drawbacks: it cannot be modulated at audio rate, and the sharp edges of CV are slightly smoothed out by the LDR physics.
Please note, how the input jacks are normalized to ground! Without that, you won't be able to use it as a VCA, and it will do umpleasant thumps into your mixer.
Media
A basic sweep between the SFP21 triangle and the SFP18 LFSR noise. First i fade between the two by simply turning the initial crossfade knob, then i put an LFO into the cv input.
A demo of using this module as a VCA. Nothing is inserted into - input, and a triangle wave playing a melody goes to the + input. First i ramp the knob up and down with hand, and then provide a pluck envelope synched to the melody to the CV input. It almost doesn't bleed, but as any vactrol device, it has this 'ringing' aftertail and a slightly smoothed attack, like you or not.
The whole signal path is DC coupled, so it can process CVs as well. Here, i fade between the two LFOs controlling the same vco, first by hand, then i let the SFP14's flux output control it.
A little demo patch summing most of its features up: the upper half crossfades between an FMed triangle wave and its suboctaves obtained with a subharmonic divider, the result goes to the bottom half which acts as a VCA controlled by a free running pluck envelope, as well as another one synched up to the sequencer.
Pictures
