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).

New/Old Modular Case Total Organ Transplant

I recently won a second hand case in a online competition hosted by Muffwiggler forum. The case was previously owned by forum member Strettara, he posted it to me from Italy. Oddly enough, he used to live in Newport where I live now and the case was exactly what I needed, portable and exactly the same slightly odd size as my previous case.

Yesterday I got to work making some minor structural improvements and transplanting the sketchy linear power supply over from my old modular setup – it was a tight fit but fit it did! Not without the usual blood, sweat and tears hands on DIY usually bears for me of course. Here are some pictures of the ordeal, the patient has made a remarkable recovery from the surgery, some would say miraculous!: (left click to enlarge)

And of course a short video:

Injury patching

Yesterday I finished a new case for my modular system which means I have space again to upgrade it. I have really wrecked my back and my hands are covered in cuts and blisters, but it is worth it!! I managed to trade some bits for a Polish XOAC Systems Tirana sequencer, which is featured in the video below, and also and Expert Sleepers ES-4 module which will allow me to connect my laptop via SP/DIF to the system. I had a play before breakfast this morning: