[sdiy] FatMan Hacking

[harry bissell]

I think you should increase the gain after the
filter..

If you increase the input signal you might start to
overdrive and change the overall waveshape. This
wouldn't happen if the input level stays constant.

I'm using a VCA controlled by a modified Q control CV.
At very low Q levels, I do not increase gain. At
higher
Q levels I increase gain, but then limit at the point
of self-oscillation when the filter volume starts to
rise again.

This is "perceptual" volume. Like a loudness control
on a stereo... it depends on what volume you are
listening at (fletcher-munson curves).

It works "OK" for me. Its much better for my live
performance use, where changing the volume can be
tough...

H^) harry



--- Don Tillman <don at till.com> wrote:
>    From: jhaible at t-online.de
>    Date: Mon, 29 Oct 2001 14:52:38 +0100 (MET)
> 
>    The low end drops on both, transistor and diode
> ladder filters when
>    resonance is increased. But the amount of bass
> drop is bigger on
>    diode filters: This has to do with the different
> phase & amplitude
>    values for the self oscillation point of a
> straight 4pole vs. a
>    5pole filter which approximates the 24dB/Oct
> slope.
> 
> Yes...
> 
>    So in this circuit there is a dual ganged pot,
> one for feedback,
>    and one for the filter input level.
>    To compensate the bass loss ?
>    Yes, but (big surprise!) *only* to the level of a
> straight Moog
>    ladder! It's funny to see this in a circuit
> simulation. Rather than
>    bringing the bass end up to unity gain, they only
> bring it up
>    to the Moog filter's level. 
> 
> I didn't know that.  That's very funny.
> 
> But also very practical.  Without the gain
> compensation, you lose too
> much low end to really describe it as a low pass
> filter.  And with
> full gain compensation there would be noise and
> overload issues.
> 
>   -- Don
> 
> -- 
> Don Tillman
> Palo Alto, California, USA
> don at till.com
> http://www.till.com
> 
> 
> 


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