[sdiy] What would be THE best, most versatile VCF available on DIY community ?

Lanterman, Aaron lanterma at ece.gatech.edu
Tue May 29 05:48:17 CEST 2012

On May 28, 2012, at 1:19 PM, Andrew Simper wrote:

> How about the Elka Synthex Multimode Filter (LP/BP/HP) as outlined
> here: http://www.electricdruid.net/index.php?page=info.cem3320
> It can be made from any ota core and provides "stronger" high pass and
> band pass responses than the Xpander style summing of output taps.
> I've got some IR3109s and some BA662s I'm going to give this a go
> with.

The Synthex filter is unique.

The way it forms a two-pole filter with variable "Q" is to cascade two usual first-order LP filters (for instance) and then feed the output back to the input. I can't seem to find my notes on it now, but I remember working out the transfer function. Changing the feedback, IIRC, doesn't give you a Q (as typically defined for a 2nd-order filter) that varies independently of the "fc" (usually called the "cutoff"), as in the case of a State Variable Filter or a Sallen-Key. The fc also changes. So there's a subtle difference in how it responds.

One thing to be careful of when looking at the Synthex schematics and thinking of adopting it is that the CEM3320 variable gain elements are not typical OTA cells. They're these weird things that I haven't seen elsewhere, and have some biasing requirements that confused the daylights out of me when I first looked at the schematic.

- Aaron

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