[sdiy] SVF resonance overdrive problem

[Harry Bissell Jr]

The most common SVF configuration has a resonant
peak that exceeds the passband amplitude (the peak
has MUCH higher gain). This is normal.

There is a way of injecting the input through the
resonance path, which limits the peak amplitude to
the same value. The passband amplitude goes down. This
is how the Moog ladder filter behaves.

Another way is to introduce a "Q limiter" into the
resonance feedback path.  I use back to back zeners
to increase the resonance feedback (lower Q) when the
signal amplitude gets too high. (I use 5.1V zeners).

Typically, low frequency harmonics are stronger and
tend to overload the filter. As the filter sweeps
down,
the zeners conduct and reduce the filter resonance.

This does NOT sound transparent... but it does stop
the
overload. I used the technique in my Hex Guitar
processor (Muffy III) because I cannot fidget with
signal levels while playing live. The "Q limit" saves
the day at high resonance levels.

H^) harry



--- Karl Ekdahl <elektrodwarf at yahoo.se> wrote:

> Hi list, 
> 
> i've built a OTA-based SVF whose resonance distorts
> heavily if i have to high input-volume. The problem
> is
> that "to high" is something like >20mVpp so that
> won't
> do. Can anyone give me a hint on what's wrong, is
> this
> a common problem with SVFs?
> 
> Karl
> 
> 
> 	
> 	
> 		
>
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