[sdiy] Ray Wilson Filter question

[synth at oldmail.charlielamm.com]

I may be slow to catch on, but, I think I get this....

--Hang a 1M resistor off of the first OTA stage's - input, in addition to
what's already there.  

--Hang the wiper of a 100K pot off the end of the 1MB resistor I just
added.  On the trimmer, put one end to V++ and the other to V--

--Put a 1K resistor in between the - input of the OTA and ground...right
now, as Ray did it, this signal goes straight to ground. This 1K
additional resistor guarantees that the current added with the mod doesn't
go straight to ground, but instead goes into the - input as desired.

--That way you can crank a bit more or less DC signal into the - input,
getting rid of any offset in the first stage.

Is this what you're saying?

On Sun, 31 Oct 2004, harrybissell wrote:

> you have the 100K / 1K divider on the audio signal. on the other input, add a 1M /
> 1K
> divider, tie the input to the wiper of a trimpot from +15 to -15  (maybe a 100K
> trimpot)
> 
> This lets you introduce a DC offset to correct for the offset of the OTA.  It will
> never be perfect
> but it can be a big improvement, especially if you have fast attack times (thump)
> 
> H^) harry
> 
> synth at oldmail.charlielamm.com wrote:
> 
> > At the risk of sounding dense....I'm having trouble visualizing this
> > 1M/1K mod....Do you mean run a 1M resistor from IC-2B pin 14 to ground, in
> > addition to what's already there?
> >
> > > Adding a 1M resistor to this input (ie 1M / 1K divider) will allow
> > > you to add
> > > a trimming voltage to reduce the DC offset as well !   From my experience, you
> > > need only
> > > add the trimmer to one stage.
> > >
> > > H^) harry
> > >
> > > synth at oldmail.charlielamm.com wrote:
> > >
> > > > I am trying to use this Ray Wilson filter in something I am building:
> > > >
> > > > http://www.musicfromouterspace.com/analogsynth/statevariablefilter.html
> > > >
> > > > I am using +/- 15V instead of +/- 12V.
> > > >
> > > > I am also using mod CV of 0 to 5V--the circuit looks to me like it expects
> > > > 0-10V for a good sounding filter freq sweep?
> > > >
> > > > To make this work, I think I need to change R6 to about 47K for the 0-5V
> > > > CV for freq mod, and R14 to about 180K to accommodate the greater supply
> > > > V's.  I can't think of any other mod necessary.
> > > >
> > > > Can anyone else?  Are my subs OK?
> > > >
> > > > Thanks in advance for any help.  --Charlie Lamm
> > >
>