[sdiy] simple vcf

[Theo]

My Tidal Quad filter bank works with diode bridges.
There are examples of this in old korg designs (770 & MS50).
Two MS50 inspired stages to get 24dB, works ok for me.

I also build state variable filters using the diode bridge method in both 12
and 24 dB flavors.
This also worked ok.

Down side of the diode bridge is the rather high CV feed-through.
Using matched diodes helps.

Before i turned to the diode bridge I experimented with a single sided diode
ladder.
The single sided ladder had excellent AM radio reception,
not a feature I was looking for :(
Still not sure whether the radio thing is a bad habit of all single sided
diode VCFs or if I picked a wacky design.

The Tidal Q-o-Matic has LDR based filters.
More expensive and for some "a bit slow".
But zero CV bleed, low noise and allow high signal levels :)

I know a guy who builds "tank-like" VCF units featuring a switched capacitor
design.
What really amazed me was the wide range he obtained with this method.
His VCF core is little more than a 4066 and a 074.
It really goes from subsonic to supersonic, didn't see this from a OTA VCF
yet.

Theo

From: Gavin <elmystico at earthlink.net>

> 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?