Q&D VCF Filter - Tim Escobedo -- cut off frequency

[wax+wire]

Hi all

I have built multiple (maybe 13 in total!) Voltage Controlled Filters based on Tim Escobedo and G Meredith's version of the Q&D filter design for a bunch of different modified and circuit bent CASIO SKs. Anyway, I always find the range not quite what I want. What part(s) of the circuit should I look at to increase the frequency range - both how low the cut off goes, and how high the frequencies are that are let through at the upper extreme.

[blue hell]

I'd try to lower the 100 k CV related resistors to get more range (but leave some resistance, say 10 k minimal). When it won't go low enough anymore after that you could make the 0.01 µF (or 0.02 µF, depending on version) filter caps larger. Not sure if it would work, but it is what I would try.

[elektrouwe]

the LED is used as a voltage controlled "resistor" here.
the best range spreading of this "R_led" would be to drive it from a voltage source. Then R_led would change exponentially over many decades which is
what we want. BUT:
the CV series resistor of the LED drive is parallel to R_led for signals !
so reducing the 100k to small values will not help.
The solution is to replace the LED by 2 LEDs in series and connect the center of the LEDs to the center of the C's.
Now you can reduce the 100k CV-resistors to very small values and get
a wide almost expo. sweep range.
2 LEDs in series need a lot of CV. if you replace them by 1N4148 you can bring down CV

[Cfish]

I have used the filters you listed a few times my self.

Have used this one too. For an easy filter it isn't too bad.

Pretty much a PAIA 4700 low pass filter with a couple of mods

The 100k pot on the left is nice for manual control.

I used the 500k trim on the opamp feedback loop to get the gain right where I wanted it.

You can leave the opamp out and just use the voltage controlled low pass filter. Higher value caps lower the cutoff.

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[elektrouwe]

[Cfish]

Pretty much sounds like sweeping the tone knob on a guitar.

I however use it a lot. Usually with 3 octaves of square wave mixed, then sweep it with an inverted AD envelope signal.

[elektrouwe]

[Cfish]

I think that's a great idea Elektrouwe. Can never have too many basic circuits to experiment with. Basic VCF schematics have not been as easy for me to find as some circuits are

[sourcery]

Just wanted to say thank you for the tip on the 2 1N4148 diodes! I built it all three ways (the two on the schematics and then the lower schematics with the 2 1N4148s instead) and the result went from "meh" to "wow"

Here is a video if you want to hear what it sounds like in the three different configurations: https://www.youtube.com/watch?v=fLR1B6JqyNA

Thanks for sharing this information, the video refers/links to this post for any viewer who is interested in the subject.

[elektrouwe]