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)

First build

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.

Demo

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





video of the module in action

Schematics

Vactrol VCF schematics

You can add it to your virtual rack here: modular grid

Buy

Build notes

An interactive build guide and BOM can be found here.

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. First, close the filter on the front panel, then adjust the offset until the filter closes, you’ll need to trim a fair bit to achieve this and it will happen rather suddenly. Set it where you want the filter to be when fully closed. Then open the filter and adjust the range if needed. Go back and forth between both until you’re happy.

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/red 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.

Leds (D1,D2) should be installed with the positive lead in the round hole and the negative lead in the square hole.

gut-shot

Troubleshooting

If things don’t seem to work, here’s a little troubleshooting guide.

Measure your power supply voltages on the IC’s. Pin8 should have +12V and Pin 4 -12V.

In case you reversed the power connector, both IC’s will have blown, in which case they both should be replaced – after rectifying the problem of course. If it was connected wrongly for too long the electric capacitors will have blown, but you will certainly hear, see and smell that when it happens.

See whether the input and output jack are properly connected. What’s what it written on the PCB in small print.

Reflow your soldering connections (re-heat them)

Measure the voltage on the led side of the vactrol with the filter completely open, you should measure a positive voltage. If not, check whether adjusting the trimmers make this happen.

Measure the voltage on the led side of the vactrol wih the filter completely closed, you should measure a low positive voltage. If not, adjust the offset trimmer, it should go down.

Use a scope to follow the signal path. On audio input, U1 pin 5, U1 pin 7, U1 pin 3, U1 pin 1, audio out. If you don’t have anything on U1 pin 5 check whether the leds in the vactrols work, they either don’t get power or they could be placed backwards.

Links

modular grid

Build guide and Bill of Materials (REV1 board)

Build guide and Bill of Materials (REV2 board)

Vactrol DIY guide