[sdiy] [OT] Analog synths with 2 pole filters

[David G Dixon]

I use a largish resistor for positive feedback in parallel to the damping
path to "goose" the filter into negative damping, and then a couple of
back-to-back zeners in the BP stage's feedback loop.  This gives reasonably
low-distortion sine waves from the LP output (around 0.2% THD). 

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom 
> Wiltshire
> Sent: Friday, February 22, 2013 5:09 AM
> To: rburnett at richieburnett.co.uk
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] [OT] Analog synths with 2 pole filters
> 
> 
> On 22 Feb 2013, at 11:47, rburnett at richieburnett.co.uk wrote:
> 
> >  In the case of the state-variable filter just taking the 
> damping term 
> > down to zero is not enough.  The damping has to actually go 
> slightly 
> > below zero, and this requires increased complication that 
> most filter 
> > designs don't include.  That's my take on it anyway, I hope 
> it helps 
> > the discussion :-)
> 
> Yes, Richie is quite right. Re-reading my post, I made it 
> sound like just removing the damping is enough, but actually 
> you need to tweak the other feedback path to give you more 
> gain. Once you've done that, you've got more feedback than 
> you need, so you need more damping to cancel it. Getting the 
> balance of these two right is the sum total of the work in an 
> SVF design, I reckon. The rest of it is just putting the 
> right bits in the right places, but sorting out the feedback 
> so that it oscillates cleanly over the whole range is the 
> tricky bit for sure.
> 
> Tom
> 
> 
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy