Having been in the community for a few weeks now, lurking for a while before that, and developing a few Kosmo format modules I have decided now is the time for me to start sharing.
I have a few modules on my github that I shall announce over the coming days and should be on Kosmo modular grid soon.
My first module is a Quad Slow LFO, in a 100mm module.
Each has a cycle duration between a few seconds and a few minutes, approx 6 sec 12 minutes
Each LFO has its own speed control
A single global speed control
CV control for the global speed (not 1V/Oct)
Each LFO has both unipolar and bipolar outputs
I have been using the lfo for slow evolving movement of sounds, it is not a replacement for a standard lfo, normal tremolo and vibrato can not be realized with this module.
It is based on the Single-cell VCO on the LM13700 datasheet, this is a triangle wave with a big step so I added an LPF to each oscillator.
Contained are schematics & Gerber files for pcb fabrication of both electrical and front panel. This is suitable for entry-level however a basic level of electronics, soldering, fault finding, and some basic tools are required.
You will see the 6 + 1 logo in places, however, I am moving away from this imaginary brand name as I used it on a previous opensource project that I have currently put up for adoption, I have decided I’m too old to use a pseudonym and an now just going to use my name. I due course I shall be removing the old logo and not replacing it, however as I wont publish anything I have not built and tested you will see my old logo until I need to get more panels fabricated.
As with all my projects constructive criticism is welcome, all I ask is it is kept public so all the community can benefit.
Nice! I too used one of the lm13700 datasheet triangle oscillator in my LFO, but not this version with the back to back diodes.
Personal preference, but I consider an unused op-amp a sin. Maybe there is something interesting to be done with it? Maybe summing a quarter of all separate LFOs to make a funky waveform? Or a Schmitt trigger to provide a square wave LFO from one of the others. Or maybe something else as yet unthought of…
I’m curious what exactly makes this module inherently “slow”? How hard would it be to modify this to have a wider range, or to be switchable between a “fast” and “slow” mode?
The inherent slowness of the lfo, is determined by the 470uF capacitors, one for each stage, C3, C4, C15, C16. You could experiment with these, find values you like. Maybe having different values for each lfo, or using a switch to make them selectable.
Just changing those caps isn’t enough, because the 0.15 Hz lowpass filters (R12/C7 etc.) that are being used for removing the distortions that come from the single OTA oscillators used would filter out the whole LFO. To increase the frequency you also need to change the cutoff of those accordingly.
I am seeing a strange behavior after building this module. At first everything seems to be working fine - all 4 LFOs cycle once after powering it on. However after that first cycle, LFO3 (and its LED) stops and the rest continue cycling normally. I hooked LFO3 up to my scope and I could see that it would get to the “bottom” of its cycle and stay at that voltage indefinitely. I then noticed that if I tweak the LFO3 knob, it will then perform another cycle, and stop at the low side of that cycle again.
I swapped the 13700s and their associated opamps to try and determine if the problem followed one of the ICs, but it remains isolated to LFO3. I’m tempted to swap out the pot for LFO3, but I honestly can’t understand what might be going on that would explain why merely “twiddling” that knob a little would kick another cycle into motion.
Replacing the pot may solve the issue. I would first try to resolder it and any components nearby, as the act of touching the knob could be enough to make or break a problematic joint.
There are only three passive components that need to have the correct orientation, so it could be worth having a check of these: the two signal diodes, D3 & D4, and the timing capacitor C4.
It would be good to know what has caused the problem, as it may help improve any future designs.
