[sdiy] simple vcf

[Seb Francis]

Hi Gavin, Tim,

There's a circuit for this kind of VCF here ...
http://www.armory.com/~rstevew/Public/SoundSynth/00_PenfoldSynth/penvcf.html

It's one of the more simple VCF circuits that I've seen.

I built it with an LM13700 which are much easier to get hold of than LM13600.  Sounds ok, although it has quite a soft cutoff being only a 2-pole (12dB/Octave) filter, and it doesn't self-resonate.  You could use a 2nd LM13700 to make it a 4-pole (24dB/Octave) filter.

BUT: Note that the freq control is an exponential current, not a linear voltage, so you would probably need some additional components (e.g. a couple of transistors and an opamp) - this slightly spoils the simplicity - Tim is the circuit you are thinking of controlled by a linear CV?

René's got a similar circuit on his site here, with the expo-converter included, and with a opamp to boost the resonance:
http://www.uni-bonn.de/~uzs159/ms20.html

But for simplicity I do like the look of the 4007-based circuit which René just posted :)

Seb




Tim Escobedo wrote:

> Why no OTA? The simplest decent VCF I can think of uses a single
> LM13600. One chip, a few resistors and caps. Just about every other
> design I can think of is significantly more complicated.

Gavin wrote:

> Hey all,
> I'd like to build a real simple VCF, it doesn't need to track a VCO, just do
> sweeps based on an input control envelope.  I'd like to develop a design
that doesn't use OTA's.  Has anybody seen or worked on anything like this?