[sdiy] Hex Inverter VCF Idea

[harry]

It looks like it could work... but with a limited tuning range and small
signal handling capability (easily overloaded, probably noisy)

The 'hidden weenie' is the fixed resistors in series. These will set the highest

cutoff frequency... so you want them small. Then you need to suck lots of
current away from the integrators with the FETs.  I suspect you will need a
series
resistor after the FETs as well... sort of a tee attenuator.

In the phaser case... the input impedance of the opamp where the FET shunt is
connected is very high... in the integrator case... it is very low. The phaser
does not need a very large tuning range to be effective. Usually you want the
notches to move
a couple of octaves at most.

You'd wish you could put the FET in series rather than shunt... but you probably
can't with the 4049.

I'd say... try it. If it works you'll be a hero... if not you'll be a guru (of
how/why not to
do it)

H^) harry

adam at hoodmusic.net wrote:

> I found this in a note book that is about 5 or 6 years old, i am not sure
> where i got the idea or what it came from,if anything. I just came accross
> it and found it to be a intresting idea and wanted to see if it was
> something actualy worth working with. I vaugly remember a phaser that used a
> 4049 in this manner so i might have gotten that part of it from there, but i
> really am not sure. All that was in the notebook was what you see at the
> link, something i drew in school.
>
> adam
>
> Scott Gravenhorst wrote:
>
> > Is this your design?
> >
> > adam at hoodmusic.net wrote:
> > >I found this idea for a 4-pole VCF with VC resonance and a VCA on the
> > >output in one of my old note books. It uses 2 hex inverters, one for the
> > >4 stages of the VCF and the VCAs for the resonance loop and output VCA,
> > >and the other hex inverter is used as six FETs to very the resistance of
> > >of each stage of the VCF and the VCAs. It is just a rough drawing, and
> > >is far from a working filter. I wanted to know if it had any chance of
> > >ever working before I started trying stuff ut in the real world.
> > >
> > >http://synth.massiva.com/vcf-4049.jpg
> > >
> > >adam
> > >
> >
> > =========================================================
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> >
> > -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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