[sdiy] OTA performance (was SSM chip reissue)

[David G Dixon]

Very interesting.  I hadn't thought of that.  I have never really liked
using the zeners in this application, but it is way easier than trying to do
some other form of amplitude control, like a JFET on the feedback, etc.


> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Tom Wiltshire
> Sent: Wednesday, April 26, 2017 2:51 AM
> To: Roman Sowa
> Cc: 'Andre Majorel'; synth-diy at synth-diy.org
> Subject: Re: [sdiy] OTA performance (was SSM chip reissue)
> 
> I've got some evidence this is true. I built an 2164 SVF and 
> swapped the 30/33K input resistors for 15K, thereby doubling 
> the currents. This improved the situation somewhat, but it's 
> only a octave's worth of improvement. But running the filter 
> "hot" helps.
> 
> On 26 Apr 2017, at 09:06, Roman Sowa <modular at go2.pl> wrote:
> 
> > At low cut off frequency, when integrator currents are 
> really small, Zeners are damping the oscillation, because 
> most of the current is eaten up by the zener, not the cap - 
> at that small current their voltage is way much smaller than, 
> say, at 1mA, the top of VCF frequency.
> > 
> > Roman
> > 
> > W dniu 2017-04-26 o 04:11, David G Dixon pisze:
> >> 
> >> 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
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