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Sat Jan 1 12:51:01 EST 2011

SVF experiments

Been playing a bit more.  Some conclusions:

  - Limiting diodes in parallel with the integrator capacitors works
    well to keep the self-oscillation in check.  Leaving saturation to
    the opamp doesn't seem to be a good idea.

  - Don't use LM324 for audio.  The crossover distortion is quite
    severe, and it doesn't really pay to bias each stage into class A.

  - The feedback through the 2 integrating sections requires an
    inverting amplifier.  The gain of this section is essentially
    free, and can be used to shift the poles.  Higher gain gives
    higher frequencies.

    This is useful for picking lower capacitor values in case the
    input resistor for the integrators has a low values.

  - Negative Q.  Adding a slight amount of positive feedback after the
    first integrator allows for self osc over the full frequency
    range.  With the straightforward (positive) Q I don't get to
    self-osc in the lower frequencies.

    The inverter is unit gain with 100k / 100k, so I've added a 1M
    negative feedback resistor adding 1/10 to the [0,1] range provided
    by the voltage divider negative feedback.

  - HF Oscillations: I have about 1MHz oscillation going on.  ( It's
    gone now, might be breadboard fluke. )

  - The self-osc is nice though.  Especially how large low frequency
    inputs "squash" a higher frequency oscillation.  That aphex twin
    sound.


It's fun to play with, but the really interesting thing about filters
is when they are time varying (and nonlinear).



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