[sdiy] Shruthi 4PM (was re something else...)

Roman Sowa modular at go2.pl
Mon Dec 2 13:02:21 CET 2013


I can only confirm,
self-oscillation for 2-pole state variable filter with zeners accross 
one cap starts at few hundred Hz, anyway it still sounds nice.
When making it I wasn't after big oscillations at low frequencies, but 
what I can achieve with 2 chained filters, their different responses and 
tuning.

Roman

W dniu 2013-12-02 12:35, Neil Johnson pisze:
> Tom Wiltshire <tom at electricdruid.net> wrote:
>> I don't know whether it's "weird in theory", but there are clear differences in practice.
>
> Putting diodes parallel to one of the integrating capacitors is not a
> sensible route, since the leakage current through the diodes will ruin
> low frequency performance.  Even zener diodes.
>
> In my 3-pole 2164-based VCF I was able to get it to self-oscillate
> from a few Hertz up to about 16kHz with visually nice sine wave at
> constant amplitude (maybe 1% distortion, haven't measured yet).  But
> not with diodes across capacitors.
>
> My next project after the current one (the list is long) is to develop
> the filter further to get the self-oscillation performance an order of
> magnitude better (I'd like to get to below 0.01% distortion).  Diodes
> just won't cut it.
>
> Neil
> --
> http://www.njohnson.co.uk
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