[sdiy] OTA performance (was SSM chip reissue)

[David G Dixon]

Yes, exactly.  A largish resistor between the BP output and the input
ensures robust oscillation at all frequencies.  Without that resistor, just
hitting the "theoretical" level of damping required for oscillation will not
guarantee oscillation.

A couple of zeners across the BP cap ensures relatively clean sine waves.
However, I still haven't figured out how to get the sine waves to have
uniform amplitude at all frequencies.  One would think that the zeners would
ensure this, but they don't.  That's why, when I need really clean sine
waves, I built a four-pole Roland filter and hardwire it for oscillation
(with stage gains of 1.42 for an overall loop gain of just slightly more
than 4).  With back-to-back zeners off of the feedback, the output from the
fourth stage gives very clean (and very stable) sine waves.
 

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Andrew Simper
> Sent: Tuesday, April 25, 2017 6:11 PM
> To: Neil Johnson
> Cc: Andre Majorel; SDIY List
> Subject: Re: [sdiy] OTA performance (was SSM chip reissue)
> 
> It by decent Q you mean Qs that match between voices sure - 
> this is true for any filter topology. If by decent you mean 
> high values of Q then for SVFs in synths you can get around 
> this with positive feedback around the bandpass integrator to 
> push the damping more negative and can easily push the 
> circuit into self oscillation, which makes it just like any 
> other topology where you can increase the feedback past the 
> point of self oscillation with more gain.
> 
> Andy
> 
> On 25 April 2017 at 18:30, Neil Johnson 
> <neil.johnson71 at gmail.com> wrote:
> >
> > Andre Majorel <aym-htnys at teaser.fr> wrote:
> > > How relevant gain matching within the same chip is to 
> synth circuits 
> > > is one of the many things I have no clue about. :-)
> >
> > It's also important in SVFs, where you want good gain 
> matching between 
> > the integrators if you want any decent Q.
> >
> > Neil
> > --
> > http://www.njohnson.co.uk
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