DIY SYNTH VCF Part 3: Resonant High-Pass & Vactrol-Based Voltage Control

(Note that the LDRs don't react to light here – you can only change their resistances via the sliders.)

My phone decided to unlearn proper white balancing at some point during filming. Sorry. Time to get a camera I guess.

In this series, I'm taking a detailed look at how to build an analog VCF from scratch. Things get spicy this episode, as we begin to tackle the trickiest part of synthesizer circuit design: voltage control.

If you want to follow along – which I strongly recommend –, here's a bill of materials:

1x TL074 quad op amp
3x 5mm red LED
2x Light dependent resistor
1x 100k linear potentiometer
2x 100k resistor
3x 600 ohm resistor
2x 33 nF foil capacitor
1x Pack of FIMO

Chapters:
0:00 Resonant High Pass
6:00 Voltage Control - Difficulties
9:04 Vactrol Basics
13:10 DIY Vactrols
16:03 Basic LED Driving
19:30 Offset Voltages & Voltage Mixing
22:38 Fixing the frequency range
26:50 Sound Demo & Outro

[slick]

Quick Heads up Fimo is a brand of Polymer clay. Another popular brand is Sculpy. There are plenty of other brands too.

[annonymousname.]

the production of these videos is insanely high quality, and the information is absolutely brilliant

[Rob_]

I am an electronics engineer so the basic stuff you explain is all known to me. But electronics is like Lego; knowing how to stack blocks does not make you a good builder.
Analog synth circuits are fun: they defy all rules of what I have learned of audio circuits. Square- and saw tooth waves, harmonics and resonance are things I surely do not want in high quality audio gear but in synthesizers this is a complete different story.
That's what I love about designing my own analog synthesizer modules: the combination of all those techniques, or how to create a nice country house from those colorful Lego bricks. And that is what makes your series of videos so good to watch and follow along with your journey.

[MattNeighbour]

I'm enjoying these videos, thanks. A couple of thoughts :
1) DIY vactrol allows you to put two LDRs with one LED. You could engineer an opamp drive with feedback from the 2nd LDR to linearise the response.
2) You neglected the LED forward voltage 😛 (resistor calculation should be 10.3V/20mA)

[Barnaclebeard]

This is like learning from the cool big brother I never had.

[Aleph_Null_Audio]

Just found your channel and have been shotgunning your VCF series. I've been into DIY for a while. Your videos are the most lucid explanations I've ever encountered!

[marknesselhaus]

Now you have my full attention. I have been making homemade Vactrols for a few years now to use in my transisterless processor. I already use several hundred of them. I encapsulate them using clear hot glue then black out the outside with black nail polish. Results in a very small package just a little larger than the led used. I also already have a working VCO using just my vactrols and a few resistors and caps. Has a very rich tone due to many harmonics generated. I am just recently following these videos and hoping to proceed with building my own synth. Oh yes, I also have a working 7 step sequencer using just my vactrols and a few caps.

[asciisynth]

The common visible-light LDRs are based on CdS, which are most sensitive to green light. But, I've used all colours of LEDs when making home brew vactrols. I love vactrols because they are so simple, and seem to work even when I make them really shoddily. If the side of the circuit with the LDR in it can respond to very small changes in current, that can mitigate the problem of slow LDR response. EDIT: I see that you did modify the circuit to respond to those small changes, and it sounds great!

[freeelectron]

That last sequence in high pass sounded great with the reverb.

[wsc]

Really lucid explanations in this series, great work. Filter sounds awesome already!

[nickygrillet]

I had no idea vactrols were a thing and that's absolutely brilliant! I was really hoping to learn enough to try to mod a few small synths, and this looks both easy and effective! thank you so much for your videos!!

[jakobhalskov]

Thanks for making such detailed videos with great content - very pleasant to watch and listen to :) It is very inspiring for my own upcoming electronics videos. Take care Moritz!

[grantharris]

Thanks for posting this series. Really informative and helped fill in a lot of the gaps in my knowledge of filter design.

[dannyroessler]

different capacity values on C1 and C2(C1>C2) are drasticaly changing the tone of the filter resonance, great fun and damn, I spend the whole last night with tweaking input intensity and feedback gain values and didn't found the "right" spot so far, so it will stay as trimmers in my design

[nram]

Hilariously enough, one might consider the vectrol a type of low pass low frequency filter for the CV due to the change resistance of the LDR...
Absolutely phenomenal series.
Using this to get into filter design for a custom bass guitar preamp I had in mind for a while.

[stevesmyth]

Silonex NSL-32SR3 LED/Cds optoisolator has a rise time of 5 mS and a fall time of 10 mS, purchase from ebay and online component stockists.

Thanks for the brilliant series of videos, Moritz, a great help to my understanding of analogue electronics.

[AFlippingBudjunky]

Now the stereo potentiometer i extra ordered after i watched the last episode is usless ...thanks

[achmedabadoba]

Thanks a lot for providing this information! Hope to hear from you in another video!

[ManfredGerhard]

Super hilfreich. Danke für die Mühe, die du für die Videos aufbringst :)

[Proudbmodest]

For a good quality LDR look at Fairchild's H11F1M. These have a very wide resistance range and are readily available from Digikey.
Response time is also reasonably quick.