# [sdiy] Filter problems

Roman modular at go2.pl
Mon Jan 29 12:23:47 CET 2018

```Also the zeners, especially those with lower Zener voltage, are not brick-wall devices. At low current the voltage drop across them will be much smaller than the value printed on them, so it will steal the current which would otherwise go to integrator cap, resulting in smaller amplitude. So they will damp oscillation amplitude more and more at lower frequencies. Such filter will oscillate at very low frequency, but with small amplitude, and goes full span at about 500Hz.  I've put those diodes in my filter only because it was super easy and I loved to use zeneres everywhere at that time.  Here's where you probably saw that first: sowa.synth.net  sowa.synth.net   and just for a reason to use the word "amplitude" 5 times in one post here's once again - amplitude.   Roman   Dnia 29 stycznia 2018 01:01 Neil Johnson <  neil.johnson71 at gmail.com > napisał(a):  I wrote:  You certainly don't want them anywhere near the integrator  capacitors otherwise you'll end up with the poles not aligned  due to the diode capacitance slugging one of the integrators.   David G Dixon wrote:  We put zeners across the first integrator cap in our SVF design, and it  works great.  Gives very stable oscillation at the critical damping point,  and doesn't color the filter's sound very much in filtering mode.  The only  issue is that the self-oscillatory sine waves do not have a constant  amplitude with changing frequency, but this is a pretty minor problem.  Cascaded-stage filters oscillate in a much simpler fashion in my experience.   Zener diodes have a measurable capacitance of about 100pF.  Two in  series gives around 50pF.  If we consider the schematic that Tim  posted:   www.dropbox.com www.dropbox.com   then the first integrator has C = 270pF (220pF capacitor plus 50pF in  parallel from the two series-connected zener diodes) and the second  has C = 220pF.  It will sound slightly different from a filter that keeps the two  integrators more closely aligned (subject to capacitor tolerance).   Neil  --  www.njohnson.co.uk www.njohnson.co.uk  ______________________________  Synth-diy mailing list   Synth-diy at synth-diy.org  synth-diy.org synth-diy.org
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