[sdiy] Filter problems

Tom Wiltshire tom at electricdruid.net
Sun Jan 28 18:53:11 CET 2018


I *thought* I’d tried this, but I go around and around with this SVF design, so maybe it’s time to bring out that breadboard again and have another play with it.

Thanks Richie.

Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 28 Jan 2018, at 16:12, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> 
> Tom, put two antiparallel diodes in the feedback path of the damping signal of the SVF that is taken from the output of the first integrator, like shown in this schematic here from...  erm... your website Tom! :-)
> 
> http://www.electricdruid.com/images/Jupiter6Filter.png
> 
> The antiparallel diodes need to go in the "forward" path of the damping signal as shown, not to ground like you would do to implement clamping in a typical 4-pole cascade with global feedback.  Because the SVF "resonance control" is a damping control, you actually need to implement "anti clipping" (or a dead-band) in the damping feedback signal to stabilise the self-oscillation.  Instead of limiting the feedback amplitude when the signal gets too large in amplitude, you actually need to feed back *more* damping signal when the amplitude starts to get too high.  The anti-parallel diodes wired like that in the damping feedback path don't conduct for small signals, but once the BP output signal from the SVF gets sufficiently large to forward bias them, they will begin to conduct and increase the damping in the filter in an attempt to tame the self oscillation.
> 
> -Richie,
> 
> 
> -----Original Message----- From: Tom Wiltshire
> Sent: Sunday, January 28, 2018 2:50 PM
> To: Neil Johnson
> Cc: SYNTH DIY
> Subject: Re: [sdiy] Filter problems
> 
> You’d recommend putting them in the feedback path instead, Neil?
> 
> It does make more sense in many ways (since what you’re trying to do is limit the resonance). But the problem is that VC-resonance control is really a “damping” control, so it’s not as simple as just moving them to the Res VCA’s op-amp. Would be nice if it was that simple.
> 
> Tom
> 
> ==================
>      Electric Druid
> Synth & Stompbox DIY
> ==================
> 
>> On 27 Jan 2018, at 23:34, Neil Johnson <neil.johnson71 at gmail.com> wrote:
>> 
>> Also I wouldn't recommend putting the zeners across the integrator capacitor.
>> 
>> Neil
> 
> 
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