[sdiy] SVF stable self oscillation (was OTA performance (was SSM chip reissue))

[Andrew Simper]

Hi David,

Have you tried using the regular resonance limiting diode setup for
SVFs? The trouble with using clipping zeners at the bandpass opamp
buffer is you'll be clipping the bandpass signal (taking current from
the bandpass integrator cap) which is then being sent to the low pass
integrator, which will introduce a frequency dependancy that won't
give you a nice sine wave at all cutoffs, I would expect you to lose
self oscillation as you lower the cutoff.

If you move the clipping to the feedback section then the two
integration stages are back in tune with each other and will give the
same Q no matter the cutoff. The regular way to do this is by
increasing the damping when the bandpass signal louder with a pair of
parallel but opposite direction diodes in parallel to the damping
feedback path.

As an aside the transposed (input mixing) SVF actually uses the output
signal to increase the damping, so you get a more similar sound to an
MS20 type sallen key, which also shapes the output signal to control
resonance.

Cheers,

Andy





On 26 April 2017 at 10:11, David G Dixon <dixon at mail.ubc.ca> wrote:
> 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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