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

[Theo]

Hi Michael,

Your on the right track, feed back is the way to go.
Important are the phase and amount of feed back.
If it doesn't work try changing the phase 180 degrees (= invert signal).
Some amount of damping or limiting of the feed back "may" be needed to
prevent the filter from driving the signal right in to extreme clipping (no
more sound).

The next description is rather rude, but is should give you an idea how
things work.

Filters cause frequency dependent phase shift, around the cutoff frequency
there is a 180 degree phase shift compared to the pass band.
(he folks, I said it was a rude description!)
Feed back from output to input causes the part of the feedback that is IN
PHASE with the input signal to be amplified.
So to get resonance the frequencies around the cutoff in the feedback path
must have the same phase as the input signal.
However at the same time the frequencies in the feed back path that are not
in phase are damped.
Typically this causes the low frequencies to be "killed" when you turn up
the resonance (assuming the feed back is taken from a LP output).
When there is a BP output the feed back can be taken from there, so there is
no low frequency loss.

Best take a look some examples, there are plenty out there.

Hope this helps,
Theo
------------------
----- Original Message -----
From: Michael Buchstaller <buchi at takeonetech.de>
To: <synth-diy at node12b53.a2000.nl>
Sent: Sunday, March 18, 2001 1:29 PM
Subject: [sdiy] How do i make a simple filter resonant ?


> 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