Welcome to Robot Dialogs. This is a place where I will record my progress, mistakes, errors, failures, lessons learned and hopefully occasional success in my designs and implementations of robots and other technical endeavors. I will try to make it as educational, informative and entertaining as I can...

Tuesday, September 6, 2022

Eurorack LFO Module - VCLFO10

Here's a module that's a completely overkill VC LFO. 

It's based around the Electric Druid VCLFO 10. I picked up three of these (component shortages). This design is pretty much a full breakout of all the functions the chip provides. I laid out the design first on the breadboard. Note, there's a second LFO on the right side, and a VCO on the left side, more on them in other posts. I reused a few things between the two LFOs and didn't want to breadboard that stuff again.


The chip offers 16 waveforms in two sets. To use them the circuit needs a way to select those waves, I've used an 8 position rotary switch. To switch between the wave sets there's another toggle switch. 
I'm using the filter shown in the data sheet to convert the PWM from the chip into the desired wave.

I wanted to try an LED circuit driven by an op-amp that is similar to the circuit in the Befaco Dual Attenuverter. That meant an extra op-amp on board so I used another op-amp from that package to invert the waveform so there are two output jacks. There's two LED circuits on the PCB, because I wasn't sure if I wanted a 1 LED solution or 2 LED. 

The normal and inverted outputs are ±10V because the zero-offset can bias the outputs as needed above or below GND reference.

Simulation of the filter/outputs.

Here's what one of the waveforms looks like on both outputs:


There are several control inputs to the chip: Frequency, Level, Wave Distort, Step Rate, Range, Sync Mode, Wave Smoothing, and Sync. 

Frequency, Level, Wave Distort and Step Rate all have a knob and a CV input. I'm using non-inverted CVs (that's another input option). The circuit for CV's is one I've used before, here's a simulation. It handles 10V inputs and gives the chip the 5V input it needs. It could probably be simplified.

The Sync Mode and LFO Range would be better as rotary switches, but I couldn't source a small 4 position rotary so I've used a pot for it, but there's an option to wire up a rotary off the PCB for both. I just couldn't justify an even bigger panel for this thing so I made mine with knobs.

The Wave Smooth input is the other toggle switch.

The sync input has a circuit between it and the chip, it's the circuit shown in the data sheet. Simulation here.

The Zero Offset knob is actually in the output path, not a control input to the chip. 

I'll leave it to the reader to look through the data sheet for the details about what the chip's functions are.
Bare boards:

Version 1 Issues:
  • The LEDs that are driven by the op amp didn't work initially. I had the input to the op-amp going to the same place as the other LED circuit (the PWM from the chip) but that was a mistake. It should be going to the output of the filter (the same place I tap off the output to invert it). The picture of the PCB above shows the modification need to fix that (R51 should go to R73 or U6 pin 7, use pads for rework with through hole resistor).
  • The switches are backwards from what I intended, so the panel was altered to make it make sense.
  • The zero adjust potentiometer should be reversed, turning it clockwise lowers the zero point which is not the typical direction for that.
As I mentioned before, I only have three of the chips so that's all I've assembled. Maybe when Electric Druid gets more in stock I'll build the rest, but I've got another LFO design that uses another of their chips and less HP that is a pretty good alternative.

Of all the modules I've built so far this one gives some really satisfying bleep bloops when plumbed into a VCO's CV. I'm happy with it.

12V Current: 19mA
-12V Current: 11mA
5V Current: None (has onboard regulator)

Schematic (v1): vclfo10_v1.pdf
BOM (v1): Google Doc