FREAQ

Voltage-Controlled LP Filter prototype

Eurorack 'Freaq' filter prototype - voltage-controlled(!) Sallen-Key LPF

A Sallen-Key LPF with a bit of a difference - both of the cutoff resistors have been replaced with 'ghetto Vactrols' (the old 'LDR+LED' combo) and both can be controlled independently (or multed together ... your choice). Take into account the natural 'relaxation' time of the LDRs and some crazy sounds can result. Through experimentation I've found that you can get some nice 'percussive' sounds out of it by using two separate ADSR envelopes gated at different intervals - to take advantage of the LDR relaxation time you really need to hit it with a trigger, though.

This started out as a hunch, and it worked so well that I've decided to do a run of circuit boards with 'proper' Vactrols and possibly CV attenuators too - as a result, the prototype unit is up for grabs. It shouldn't be too difficult to hack a couple of pots onto the CV inputs, and by replacing the capacitors you can change the character of the filter quite a bit.

As for the CV inputs themselves, pretty much anything goes, although the LEDs will only respond to the positive part of any signal - there are current-limiting resistors on the inputs but don't go nuts. If you're feeling brave, you could hack the 'vactrols' to use bipolar LEDs.

Power connector is the standard 16-pin (2x8) Eurorack connector - panel is made from 3mm birch ply and is approximately 6HP (30mm) wide. If you don't like it, or want to make something tidy, then you can unscrew the jacks and LED bezels and fit your own panel. Mounting is taken care of by a pair of Ettinger right-angle brackets.

Depending on how your rack is set up, you may need to take a file to the mounting holes - for best results, use M3x10 machine screws for mounting.

NOTE: this module uses 60/40 lead/tin solder - please wash your hands after handling it.

https://www.etsy.com/au/shop/YoMoCircuits?ref=l2-shopheader-name