VCAs/OTAs (was: RE: To use CEM or not.)

[Rene Schmitz]

Hello All,

JH wrote:
>I don't know about the 2045. But in attempt to characterize the 3320 and
>it's clean-ness, "OTA based filter" might not be exactly right.
>There *are* OTA based filters that add harmonics to the sound, depending
>on the feedback resistor dividers. The CEM3320 (unlike the CA3080)
>uses linearized gain cells. If memory serves, these are even current-in /
>current-out multipliers, so the "transconductance" in the word "OTA"
>would not fit anyway.

Somehow its still a transconductance amp. The "operational" is
questionable: no differential input. It has a voltage input which
determines the output current, so for me its a 90 deg flipped (not so O)TA.
(The inputs are swapped. And the voltage input has an annoying exponential
distortion ;-)

That leads me to an interesting question: Would it be possible to build a
discrete (four quadrant) OTA where the current input can be positive and
negative? Then one could use the current input for the signal and the
voltage input as the control input. That would be something as in the
datasheet of the SSM2164 or SSM2018. Such structures would be very
interesting for low THD (and SNR) VCAs. I mean we would get rid of the
distortion of the differential pair which delimits the performance in
almost all VCAs I've seen so far. 
(Not that I would find this distortion unpleasant.)

>I don't know what a good word to characterize the type of 3320 filters.
>"Linearized VCA based filter" should cover the character of its sound,
>so maybe that would fit.

I'm thinking more and more that a SSM2164 based filter should sound similar
to the CEM3320. Both haveing current inputs and outputs. The excellent
noise and linearity of the SSM given they will be perhaps even cleaner.

Bye
 René 



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