[sdiy] Filter problems

David G Dixon dixon at mail.ubc.ca
Mon Jan 29 09:29:00 CET 2018

Yes, that's exactly how the Intellijel uVCF is configured as well -- 220pF
caps, and 5.1V zeners across the first integrator cap.  It oscillates well,
it sounds great, so I'm not going to worry about it.

> -----Original Message-----
> From: Neil Johnson [mailto:neil.johnson71 at gmail.com] 
> Sent: Sunday, January 28, 2018 3:56 PM
> To: David G Dixon
> Cc: Tom Wiltshire; SYNTH DIY
> Subject: Re: [sdiy] Filter problems
> 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:
> https://www.dropbox.com/s/acjvwb17nmuok1n/PrismFilter.pdf?dl=0
> 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
> --
> http://www.njohnson.co.uk

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