DIY CD4069 state variable filter for modular synth

Lauri published a new video featuring a state variable filter.
You can find it here:

Note, it could be worth your while to have a look at Lauri’s other videos in which he shows the drumkits, chello’s and all kinds of other string instruments he made from scrap wood and such.

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Yes I had seen his video, interesting but I did not understand everything with the CV.
He removes it from the diagram, I found it strange, then afterwards he shows it in demo, but I don’t see a CV IN, just a CV pot … ?
I may not have understood the explanations (not my language :slight_smile: )

And yes some other vid are cool too.

If you look on the top left, the label CV is the CV-input. The use of the potentiometer is not correctly drawn, I think. The input signal should be connected to the top of the potentiometer and the middle connection should be connected to the vactrols.

The same goes for the potentiometer following the IN-label.

Yes i seen it on shem, but after … he do this

and after he show a demo with CV IN

but i don’t see any CV IN on his build (only a CV pot)

?

EDIT : sorry I just saw it , it was banana plugs and I thought that was the GND (the black wire)

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He shows this reduced schematic to make clear which components are the core of the filter. But I think in his implementation he uses the schematic that contains the vactrols and output buffers.

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I contacted Lauri and pointed out the problem with the potentiometers in the schematic. He has changed it accordingly.

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I just found this video too! This sounds delicious. Very clean resonance and very few components :yum:

I’m surprised he puts a 10k resistor before the two vactrols. The current through both LED’s will never be exceed 1.2mA, which doesn’t seem like a lot for two LED’s. :thinking:

I’ve never seen a CD4069 used to drive output stages, is it some kind of simple opamp-kind of thing? Getting curious about them.

Also curious if there are other options to increase the range of the filter except the range switch he added. Perhaps replacing the two 1K resistors by 220R ?

I will play with this design for sure :slight_smile:

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If there would be no resistor then the full CV voltage could be set on the 2 vactrols if the potentiometer would be turned left in full, which might challenge their specs.

Do let us know what your findings are!

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Yes the non schmitt trigger inverter works a lot like an inverting opamp.

If you bias the input at half supply voltage, the output will also be at half supply voltage, so you can bias it like a single rail opamp.

You can also add a feedback loop following the inverting opamp rules. There might be some quirks i’m not aware of.

You can find it in guitar distortion units. It clips very hard when it gets near the voltage rails :slight_smile:

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Hi there,
I want to build this filter, but I’m a bit confused, the question can be seen in the picture (1K different ways)
Did someone build the filter? Which vactrol do you have the best results with?

Thank you in advance

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For symmetry purposes I think I would wire the second vactrol like the 1st one. I’m not sure why the series resistor is there in the 1st place. Connecting output 2 of IC1 to input 3 of IC1 should be OK even if the potentiometer is set to 0 Ohm.

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Sometimes (almost always in my experience Ive been building these filters for a few years) without a resistor in series the high frequency of the cutoff will cause some distortion in the first part of the rotation when you turn the pot. Sometimes it sounds cool but its definitely not something I would want to happen all the time haha.

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