About
This one's just a simple utility module mainly made for CV processing, but capable of weird filtering of sound as well. It adds "glide" to stepped voltages, making them more smooth. It can also smoothen up sharp non-stepped voltage spikes, too. It can be used as a portamento processor for a keyboard or sequencer that controls pitch, or as a simple attack-release envelope for controlling the amplitude. There are two identical sections in the module, each has a rise rate and fall rate knobs that are not voltage controllable, a linear/exponential response type toggle switch, a 3-position range toggle switch (slow, fast, moderate) and a cool dual-LED output monitor, as well as, of course, the input and output jacks. It's not a hard build at all and i mostly did it right on the faceplate, having the board as just an op-amp + pin header thing to which the circuit connects - thus, i think it's simple enough for beginner SDIY people. Of course it's possible to build just one of these, so the schematic i provide is for just one half.
Schematic
This is your most basic "slew limiter with two op amps as followers" circuit, but on steroids.
The input signal goes through the first op-amp, which either is in the voltage follower configuration (- shorted to out) or is connected to the output of the slew limiter (- shorted to the other op amp's out), which makes the response logarithmic or linear responsively. You probably could replace this switch with a potentiometer to morph between the two.
It is then passed through two diode-potentiometer combos in parallel, one diode facing forwards (the capacitors will charge through it), the other facing backwards (the capacitors will discharge through it). This way, we separate the controls over lag time for rise and fall.
The capacitors are selectable - there always is one small 33n cap going to ground from the second op amp's + input, and it will do well with audio frequencies, but won't work well with CVs. For this, there's a single pole dual throw RANGE switch which either floats, or connects one of the additional, bigger capacitors to the same point, effectively increasing the overall capacitance and making the available lag time range increase a lot.
The smoothed-out signal at the capacitors-potentiometers joint is buffered through the second op amp in voltage followe mode and passed on to the output, and to a simple double-LED output monitor (very handy).
You possibly could add a bunch of other mods to this circuit, but i decided to keep it clean and simple, since i just wanted two slew limiters/lag processors that do their job. This circuit is reliable enough for CVs if you're a bleep bloop noise trash musician like i am, but you may have to dig around with it if you want to glide a quantized 1v/oct signal through it, as for me it introduces a slight voltage drop overall.
It's worth mentioning that the linear mode of operation for such a simple circuit was invented by Harry Bissell, and i found his circuit in this thread, yet found no other trace of this amazing engineer. Truly a beautiful solution, and honestly i can't even tell you for sure why the f%#k it makes the slew limiter act linear, and that's why i kinda skipped it. If someone finds either the man himself or the explanation, i'll be very happy to credit more of his resources here, as well as pass on the knowledge on this module's page.
