LFO Version 1.0
The LFO seemed to be the best place to start for a Modular Synth. There appears to be SEVERAL circuits to choose from. The best part is the generally, these do not require odd parts.
Doing some digging around I found a few similar circuits.
- Here is one from an ASM page. And another ASM page.
- Embedded here is a simple one…or here.
- Here is the oldest looking version of the circuit from a digisound site (alt site)
- Note that the Diodes are in the feedback loop to create a pulse/ramp signal.
- Another version is here.
So as you can see, there is not a lot of innovation needed here.
I kinda picked around all the versions and ended up with this.
The only real differences was that I really wanted the output to be buffered so that the LFO frequency would be SOLID. The Digisound circuit does this, but it has only one output. This is not so bad because when you switch in those diodes, the triangle changes to a ramp and the square turns to a pulse. I suppose its simply easier to just make the user flip a knob to pick one of them. That design uses three op amps….might as well add the extra one so that there is two outputs.
I added an LED display as well.
Im still learning about the ins and outs (literally) of modulars. This circuit tries for an approximate -/+ 5 volts swing. This is ok for “ADDING” to a CV voltage, but makes it bad as a CV voltage. In other words, I cant put this into a VCA as only the postive voltages would have any effect.
Well…I could..which is what I did and this is what I got.
As you can see, the vca puts out a pretty nice triangle. (the positive part).
Originally, I had a problem. It sounded odd and looked like this:
Not much of a triangle of all.
This was because I did a straight swap of a tl074 in the circuit. To be honest, Im not sure WHY it was an issue…Imput impedance…whatever? Increasing the input resistor to 20k did the trick.
Here is the LFO controlling a VCA. Note that the top is a little clipped…I need some attenuation pots im sure but the basic thing works well.
