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.
(edit: a workshop has been planned @ the pulse modular music festival in Opwijk, Belgium on 15/06/19)
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.
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.
- RV1 & RV2 are 2 trimmers which allow you to adjust the offset and range of the CV signal. So you can scale the controlling CV to fit neatly into the range needed by the vactrols.
- Depending on how well your home-made vactrols behave, and thus how close the 2 filter poles are together, you might find the filter unable to self-oscillate at full resonance. In that case, a lower value of R7 is needed, not by much, say 39k or even 42k should do. In revision 2 this resistor is replaced by a 100k trim pot to make life easier.
- Both yellow leds (D1,D2) can be replaced by standard 1n4148 diodes, this will make the filter distort much earlier on and you’ll end up with a more aggressive sound not very unlike the Korg MS-20.