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Eurorack LFO Module - Stomp LFO

Here's another LFO module, this time in only 7 HP. Like the previous post about an LFO, the chip at the heart of this one comes from Electric Druid's line of LFO chips. This one is the STOMPLFO chip.

The design started on the breadboard as usual. I ruined one chip by putting a 5V regulator backwards.

With the testing on the breadboard I gained a better understanding of how this chip works, the most interesting thing was the PDM output rather than PWM like the VCLFO10 in the previous design. I hadn't come across using PDM before. 

In this scope image it's possible to see the PDM and the resulting wave (after amplification to 10V).

The output stage just has a filter (passive filter from the data sheet) to tame that PDM signal. Much easier to implement that the PWM filter for the VCLFO10. I like my CVs 0-10V so I've amplified the output of the chip and that meant another op-amp was free to have an inverted signal too, so that's the outputs. 

As for inputs, there's a pot to select one of the 8 available waveforms, and two more pots to change the offset and depth of the output. There's a sync input and tap tempo button to manually set the frequency.

Here's a simulation.

Unlike that simulation the actual circuit's LED is driven by the PDM signal. It behaved better than when driven by the filtered output.

Issues on v1:

This was a very straight forward design. In general everything I've checked on the v1 boards works, there are just a few component that weren't quite the correct values.

  • Filter from the data sheet that I used with the output buffer/amp's R4 created a voltage divider that makes the output 0-5V instead of 0-10V.
    • Using (R8+R5=430k and R7=30k) cranks it up to 0-10V as desired.
  • R10 could be two 1k resistors instead of a 2k resistor to reduce component variety.
Other stuff in the schematic:
  • There are two 5V regulator footprints, of course only one is needed at a time, I wanted to try some SMD options.
  • There are alternate capacitor footprints for through hole and surface mount. I wasn't sure I'd have the right values for the filter so I wanted options.
  • I've used a BC547 instead of the data sheet's 2N3904 because that's what I have on hand, many others would likely work fine.


The front panel needed a revision, the first attempt (v1) in the repository wasn't quite correct. v1.1 is what's shown here.

12V Current: 16mA
-12V Current: 4mA
5V Current: None

Schematic: lfo_stomp_v1.pdf