[sdiy] OTA performance (was SSM chip reissue)

[Richie Burnett]

> But if the gains match well at 0dB, pretty well at -20dB, and not so well 
> by the time you're down at -80dB, then you'd expect resonance to drop off 
> at lower frequencies.

Or it could do the opposite, depending which of the two integrator gains is 
lowest/highest!  Like in that paper you referenced about varying SVF gain by 
manipulating the integrator gains.

-Richie,

-----Original Message----- 
From: Mattias Rickardsson
Sent: Thursday, April 27, 2017 3:39 PM
To: Tom Wiltshire
Cc: Andre Majorel ; SDIY List
Subject: Re: [sdiy] OTA performance (was SSM chip reissue)

On 25 April 2017 at 13:15, Tom Wiltshire <tom at electricdruid.net> wrote:
>
> You raise an interesting possibility though - it could be that variations 
> in gain matching at large attenuations are one reason why it's harder to 
> make 2164-based SVFs resonate at the low end than at the high end. 
> Previously when this has been discussed, phase lag has been proposed as 
> the likely culprit. But if the gains match well at 0dB, pretty well 
> at -20dB, and not so well by the time you're down at -80dB, then you'd 
> expect resonance to drop off at lower frequencies.
>
> Incidentally, Oberheim managed to make an SVF with CA3080s in the OB-X, 
> and they're not matched at all (unless they matched them?). The filter is 
> quite highly thought of. Ultimately, it's not always about technical 
> perfection.

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? :-)

/mr

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