Music and Synth DIY

Eurorack, SMD, Synth DIY

VCDLFO – TAPLFO3 based voltage controlled LFO (finished)

Just finished laying out the Eurorack LFO from Tom Wiltshire’s Electric Druid TAPLFO3 datasheet in KiCad (hence Voltage Controlled Druid LFO).

I’ve got a stripped down version working here already – this one has all the CV inputs from the datasheet, and I added:

  • CV in for LFO depth and Clock Multiplier
  • Clamped wave set input to toggle wave banks A and B
  • Clock out
  • Inverted LFO output
  • Two bicolour output LEDs for a bit of visual feedback at slower rates. One is always going to be the inverse of the other, so is largely redundant, but flashing lights, right?

VCDLFOVoltage Controlled Druid LFO

V1 panel
Pots board

Update #1 – version 1 boards built

Couple of errors slipped through the net – a footprint error on the 79l05, bank 1/2 switch the wrong way round, and I’ve added a range adjustment for the clock multiplier in, which has a dead spot above 3.75v to the V2 boards.

Aside from that, it’s all good. V2 boards ordered – I’ll upload the gerbers and kicad files once I’ve built and tested them.

Update #2

V2 boards built and tested. All fully working, but it did take me TIME to work out why neither board worked first time, and I only found the problem by eliminating everything else.

On first power up, the output was just a saturated -10 or so volts. These need to be calibrated to zero volts midpoint between +/-5v with the op-amp bias trimmer in the final stage of the PWM filter, so beforehand, the output could be saturated + or -, or simply not centered around 0V.

However, no joy – just an adjustment range to zero volts but with no LFO output present, and a weird oscillation at the + end of the range before the saturation point.

Now, this bias trimmer TR1 is just outputting -12v attenuated to ground via the wiper, as per this part of the schematic from the TAPLFO3 datasheet:

Turns out I’d flipped the pins when I’d put the footprint on the back of the board, so pin 1 is GND and pin 3 -12v. Reversing the orientation back again fixed it…which for the life of me I still don’t get.

Surely all that happens when you reverse the orientation of a pot set up as an attenuator is you change the direction of the pot from maximum to minimum attenuation and vice versa. But not here for some reason – it simply won’t work with the trimmer placement reversed, and the problem is duplicated on both builds.

If anyone reading this can tell me why, drop me a line ( or leave a comment on this post…

In the meantime, this is built and verified, so I’m going to flip the orientation back and do some cosmetic changes to the panel, and upload the final boards over the next couple of days.