[sdiy] Any noise avantage to parallel 2164 VCAs?

electronicpresskit electronicpresskit at gmail.com
Mon Jun 18 12:07:47 CEST 2018


I’m inspired to make more inaccurate analogies…the resulting conversation is great!  :)

epk

> On Jun 18, 2018, at 10:10 AM, Mattias Rickardsson <mr at analogue.org> wrote:
> 
> electronicpresskit wrote:
> > I think it’s more accurate to think of the 2164 gain cells as a voltage controlled resistor rather than an opamp stage.
> 
> Schwarz Raphael wrote:
> > Thank you ! Now I understand why there is no resistor between the 2164 cell and the following op-amp. The 2164 cell is acting as the input resistor, the gain itself is in fact the work of the op-amp.
> 
> To paraphrase Neil: No. :-)
> 
> The input resistor still exists as the Rin before the VCA. See the VCA as a magic multiplication factor of the current that runs through the resistor and to the op-amp. (It's not 100 % correct perhaps, but it's a simple and working model of understanding the construction.)
> 
> Seeing the VCA as the input resistor gives lots of faulty implications. The outer world doesn't experience any other resistance than Rin, regardless of the VCA gain setting. And Rin sits at a virtual ground at the VCA input, just as it would on the op-amp input if the VCA wouldn't be in the circuit.
> 
> See the VCA as something that boosts what the input resistor *does*, not what the input resistor *is*.
> 

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