Something I am Working On

I have been fiddling with filter designs lately and ended up knocking together a pedal / standalone unit for a prototype/demo. It is pretty fun, but could use some tweaks:


Yeah the knock-off Hammond case is held together with tape, it is because it is a knock off and no fault of my own!! Since I took the photo I replaced the tape with tape of a better colour.

Currently the unit has the following specs:

  • Sallen-and-Key topology design, low parts count and pretty good sound.
  • 2-pole Low Pass Filter with nice gentle slope.
  • Film capacitors for the RC section seemed to sound better, might have been my imagination!
  • Vactrol based voltage control, pingable (is that a word?) and easy to connect external modulators
  • Internal triangle wave modulation, 60 seconds to 1 second cycle time 🙂
  • Runs on a pair of 9V batteries


  • Filter Cut-off
  • Filter Resonance
  • LFO Rate
  • LFO Depth
  • External CV input

Here is a vid and sound clip playing with some other battery powered bits:

Yeah the vid could be better, but it definitely sounds great, I have been using it as part of an effects loop with my modular setup recently and it really delivers!!

Improvements I would like to make:

  • Gain compensation for more resonance, at the moment it distorts before the Q gets to a nice whistle – I presume this is handled in other filters by adaptive gain compensation, but this is just a guess.
  • I think a pre-gain control could be nice, would be good to be able to overdrive the filter a little. I don’t know if this would sound good yet, but I did notice that the filter behaves quite differently to different levels of input. Perhaps a pre and post gain control?
  • HPF mode should be easy to implement also but needs a slightly complex switch.
  • Some minor improvements for maximum LFO depth and filter frequency range would be good (and easy to implement).