[sdiy] OTA performance (was SSM chip reissue)

[David G Dixon]

Another thing to consider (possibly) is that when the gain is turned way
down in a linearized 2164, the current into the VC pin increases and the
chip can heat up significantly.  This could affect performance.  I always
limit current into the VC pins by putting a resistor on the output of the
linearizing opamp.  Otherwise, the VCA gets hot when it is turned off. 

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Neil Johnson
> Sent: Thursday, April 27, 2017 7:51 AM
> To: Mattias Rickardsson
> Cc: SDIY List
> Subject: Re: [sdiy] OTA performance (was SSM chip reissue)
> 
> Mattias Rickardsson wrote:
> > Interesting. I've had some filters sometimes lacking 
> resonance in the 
> > low end, and the increaswingly unmatched 2164s could be a 
> reason. But 
> > wouldn't the capacitors need to be matched just as good? AFAIK only 
> > the RC constant determine the pole frequencies, so if one 
> of the caps 
> > are 5 % off, then the filter could perform worse if the 
> gain cells are 
> > perfectly matched - compared to having them 5 % off in the other 
> > direction.
> >
> > How well do you specify the tolerance of your SVF capacitors? :-)
> 
> A 5% tolerance is about 0.4 dB. At high filter frequencies, 
> where the gain of the 2164 cells is close to 0 dB, the error 
> is less than 0.1 dB if you believe the datasheet.  At 40 dB 
> down the errors are creeping up (0.24dB is about 3%), and 
> down at the low frequency end of things the gain cells are 
> probably quite a way off (lets say 60dB attenuation for a 
> 1000:1 range).  And as George Hearn found out in his initial 
> experiments with the Irwin 2164 linearisation circuit the 
> gain response at very low gain (high attenuation) is far from ideal:
> 
> George wrote:
> > There is also a not so nice side effect that the gain 
> response becomes 
> > non-linear below a certain level. In most applications this is not 
> > significant as it is at very low gain.
> 
> Nice to have some meaty technical discussions here again!
> 
> Neil
> --
> http://www.njohnson.co.uk
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