FilterBox: CMOS abuse for fun and profit

The CD4069UB  hex inverter is an interesting and versatile piece of silicon – you can use it for its intended purpose of inverting a signal, but it is far more fun to put it to uses that its designers probably never thought of. I’ve used it in a number of past modules: a twin-T bass drum module (which was sorta-kinda OK but I’ve fi

nessed the design to use op-amps now), the Buzzsaw VCO, which is based on a design by Rene Schmitz that I’ve fiddled with and the 69er resonant low-pass filter which uses the inverters as linear amplifiers to create a wonderfully unruly and shrieky resonant filter.

The FilterBox is an extension of the 69er module – in effect, it’s two of them in a nice, compact package which can live ‘off rack’. This is the second iteration – the first run sold out (gasp!) so after a decent interval I rejigged the board layout and the revision 2 was born. Like its predecessor, it is 100% through-hole construction so you can either buy a PCB and build it up with your own choice of components or you can go for a full or partial kit, with or without top and base plates if you want to mount it in your own enclosure. The new PCB looks a lot nicer, too.


A most curvaceous PCB …

You can either run it as a pair of mono filters, which is a good recipe for weird sounds, or you can chain the two together (ie. feed the output of one into the input of the other) to get a rather more aggressive filter. Whilst this does work, the ‘stock’ configuration of 100nF capacitors does tend to flatten the sound quite considerably. Whilst there’s nothing to stop you chaining the two channels together, I feel that it works better as a stereo/dual mono filter.

Since it is an ‘off rack’ device it runs off a wall-wart power supply – anything supplying 12V through a 5.5/2.1mm barrel jack will do the trick, although if you’re DIYing it you could easily cobble something together. With three pots per channel (two for cutoff, one for resonance) the end result looks eminently tweakable.

In terms of sound, this isn’t massively aggressive although push it hard enough and it will self-oscillate – rather, it gives a nice, buzzy resonance to sounds and works really well with sawtooth waveforms for that squonky acid-bass sound beloved of the 80s and 90s. Since the trend is for people to put things like this in their own enclosures I don’t supply top and bottom panels by default – if you want these, submit a custom order and we can sort something out. If you want to roll your own 


then the panel files are available for download.

You can get the BOM, a couple of audio samples and whatnot from the GitHub page:


PCBs and suchlike can be ordered from the Etsy store:


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