DOD-FX25

I really have an itch for envelope filters and this is my way of scratching it. The DOD-FX25 is an OTA based state variable filter with a basic envelope follower detector which started production at the begining of the 80s.

You can find a lot of information about it at page of Alex Petrini. He also has a simplified schematic and a PCB layout.

I couldn't find a veroboard layout for it so I made my own, and for my first layout I'm pretty happy with it. I couldn't avoid a couple of standing resistor and shared holes but if you are tidy it can fit in a 1590B box, although I would recommend using a 125B. I included a filter mode selector switch to select between band pass filter (original) and low pass filter. The low pass mode works very well with a bass. I also included a decay pot that helps to tame the long decay of the original design. This mod was suggested by Mark Hammer at diystompboxes.com, by including a resistor in paralel with C7 to ground we let it discharge faster thus shortening the decay time. By putting a pot in series with that resistor wired as variable resistor we can change the decay time.

This was also my first time working with a raw enclosure. I enjoyed the  drilling and painting and it came out pretty well (aside from the long wires).