I’ve been asked if I could do a workshop about synth building. While there are no solid plans to do so yet, I was thinking about what to build during such a workshop. It would be cool to have something to take home you’ve build yourself during the workshop, and even better if it was actually useful. While I was thinking more along the way of simple CMOS based noise-making machines, I was asked whether a filter would be possible. People like filters and if it could be CV controlled, it could be useful even for more advanced synth enthousiasts.

Building a simple CV controlled filter however isn’t that easy, there are plenty of nice designs floating around the internets. Most of them however aren’t exactly ‘beginner’ projects you can build on the cheap. Plenty of designs feature expensive or obsolete components or have a high part count. Until I stumbled upon this page on the Doepfer website outlining some simple vactrol based VCF schematics which was an ideal starting point to build something that’s simple enough to do in a single evening and yet yielded some good results.

Vactrols are a cheap – at least if you build them yourself – and are an easy way around the voltage controlled resistor problem. An LDR – Light dependent resistor – , a red led – you can use other colours, but from what I’ve tried the red ones gave me the best results – and some shrink tubing is all you need to build your own. I found a very good video on youtube documenting the process without rambling on for 10 minutes.

There are a few issues with vactrols though, the unreliability when it comes to response curves being the most prominent one. Especially when building your own you should be aware that your choice in LED and LDR are important. An LDR with a high ‘closed’ value and a low ‘open’ value is ideal. You should also take the response time into consideration which is the time it takes to react to the light source. The faster, the better.

I took the basic filter as outlined on the Doepfer site, breadboarded it, tweaked it, put it on prototype board and implemented it in a current project I’m doing. In the end, I came up with this pretty ‘simple’ yet useable design which doesn’t even sounds all that bad either.

Update 1.2

After doing some extensive testing on the prototype in my modular. I’ve came to the conclusion the results, while useful, could be a lot better. Resonance was dependent on the frequency setting and got out of control easily, which was annoying in real life usage. So I set out to fix this issue. In the end I ended up with some modifications to the schematics making things behave a lot less erratically.

Audio demo

simple 808 beat and a single oscillator running through the filter.

Schematics

RV1 & RV2 are 2 trimmers which allow you to adjust the offset and range of the CV signal. These are needed due to the unreliability of home made vactrols. Depending on the leds and LDR’s you use, response can vary wildly, hence the need to be able to do some adjustments. It really helps if you have some kind of way to visualise the output when trimming those although it’s certainly doable by ear.

Mods

A simple modification would be to replace the resonance pot with another vactrol and make that CV controlled. Personally I hardly use CV controlled resonance on the filters I have which offer such a feature. So I haven’t implemented the feature in the design above, but you should be able to figure it out pretty easy enough.

Different types of leds (D1,D2) have different levels of forward voltages. You could play around with these and see which ones you like best. Or you could replace them by regular diodes in which case distortion will occur sooner and you might want to change the value of R1 to a lower one and at the same time increase the gain of U1A by putting in a slightly higher value for R8.

The voltage divider defined by R4 and R1 can be made variable by a potentiometer. That way you could drive the input harder and distort the filter intentionally.