[sdiy] How do i make a simple filter resonant ?

[harry]

Hi Michael:

Filters need to have some gain to get resonant. RC filters will not. LC
filters
will, but it is a bitch to tune them.
There are no simple variable inductors and capacitors... electrically that is.

There are variable capacitance diodes, but these are too small for audio use
(think RF). Likewise, inductors and capacitors have to be mechanically varied.

This HAS been done (some electronic Leslie units, perhaps Hammond organ.)

Usually the feedback is frequency selective... look at the Sallen-Key design
and
you'll see the feedback is through one of the frequency determining
components.
The same is true of the Twin Tee (the early PAiA bandpass filter from the 2700

series is a twin tee, as are most wah pedals that do not use an inductor).

The designs you have tried are some of the most sucessful resonant filters yet
discovered. I'd study how they did the feedback.

Phase shifters are another useful structure for feedback. I've got a board
that will
be released by EFM (soon) that is a six stage FET tuned phaser, with extra
holes
so that ALL kinds of feedback can be applied. I'm using mine with guitar but
it can be a synth unit as well.

H^) harry


Michael Buchstaller wrote:

> Hello friends,
>
> for my modular, i have done several filters. Moog, ARP, and
> other filter clones.
>
> Now, i wanted to do some experiments on my own. Simple RC filters
> seem to work, and an OTA filter straight from the National App notes
> works, too.
> But i want these filters to have resonance so they do not sound that dull.
> What shall i do ? Somewhere i have read to feed back some part of the
> output to the input, but this did not work for me.
> Could some of you experts point me in the right diection, perhaps a
> "resonant filter tutorial" somewhere ?
>
> -Michael Buchstaller