They use a stereo pot for the cutoff. I just used vactrols for the voltage control. I recently started using some LM 13700s. I realize that there is a way to use them for the voltage control on my filters but I have not been able to figure it out. I have one of the filters on breadboard still and I’ve been experimenting.
For a voltage controlled filter using LM13700 you can look at the LMNC Performance Filter and the MS-20 filter clone by René Schmitz that it’s based on. Though looking at it recently I think it really needs some values modified to use with modern synth levels — otherwise as people who’ve used it will tell you, you need to attenuate your input to avoid distortion. You might also be a fan or not a fan of the (deliberately) distorting resonance. If not you could try cleaning up the resonance feedback by removing the diodes and modifying resistor values. The Barton Sallen-Key filter is a similar design, though with I think more appropriate voltage division on the input.
I like the ms20 clone with the lm13700. Mine has an attenuator for the input. It definitely adds another level of sound control aside from making it useable at all for some of my diy vcos. I just really like the all around sound of MKs filter. I have a highpass, low pass, and I kept one on breadboard that I just move jumpers to switch between the two. My intention is to make it permanent on strip board with a double switch. I remembered hearing in the video that it’s possible to get voltage control with an ota, and I’d like to have that in my final module. It seems like it should be possible, anyway. I’m gonna study the ms20 schematic a bit and compare to MKs schematic and see if I can make it make sense.
Basically to make it voltage controlled, each pot gang gets replaced by an OTA, which functions sort of as a current controlled resistor. Take the Kleins schematic, erase the first voltage follower because it’s not needed, replace the resistances by OTAs (and associated passives), and it’ll start to look a lot like the Schmitz and Barton schematics. Probably the difference in sound is mainly due to the resonance feedback which is cleaner in Kleins’s version. Though the OTA itself does color the sound somewhat.
I had a feeling that might be the case! Thank you for confirming that. I did a quick side by side comparison of the schematics and tried to visualize something along the lines of what you’ve described. You’re description has helped piece together my puzzle for sure. In the spirit of experimentation I may try to do this. I may just try a better vactrol setup. I’m just using electrical tape, a couple light dependent resistors and an led. I could get better response with a different resistor on the led, but I may try a commercially produced fancy vactrol.
Here’s a straightforward way to build a vactrol if that helps.
I found high-intensity green diode + GL5528 worked out best from a kit of 7 LDRs I found on eBay.
Very important to stop any background light finding its way in down the leg-holes:
Sorry, looking back again I realise that what @TimMJN was actually talking about was plastic domes with multiple LDRs lit by the same LED and all hiding inside the same little igloo.
I think I remember seeing another implementation of this idea somewhere with a mirror on the roof of a closed box with LED + multiple LDRs all mounted on the same PCB.
But that wouldn’t glow like a dalek thinking naughty thoughts.
Which, btw, I did manage to do. However, they don’t really work for a LPG. @lookmumnocomputer calls the covers custom, but I found out they aren’t. They’re Fresnel motion sensor covers and they can simply be bought of aliexpress.
I think I have the switching figured for this when I actually get the switches delivered. I’ve never used dpdt so I had to study up. I’m gonna throw a switch in to bypass the clipping on the resonance as well. I like having the option. I’m also gonna go ahead and build a permanent version of MKleins low pass with a high pass option switched in with better diy vactrols I will be experimenting with.
Along with switching out the diodes, you’d probably need to switch the feedback resistor to something smaller to compensate. Maybe make it something like 4.7k. Or if it’s easier leave the 10k in place and switch in a parallel 10k?
I found a schematic for a version of the Rat distortion pedal that has a switch for selecting between three different types of diode for the clipping circuit–regular silicon, germanium, and LEDs. I plan to test it with an additional bypass option to cut out the clipping entirely when I get around to building it. I thought having the option to vary the sound that way was really clever.
I also ADDED clipping diodes to this one like it is in MKs high pass circuit, AND added a bypass switch as well. Options it sounds dope with an Atari punk with a sample and hold. I haven’t built the ms20 mods yet, but it is on the way. had to order another power source.