[sdiy] Any noise avantage to parallel 2164 VCAs?

[Neil Johnson]

Steve wrote:
>
> Douglas Self wrote something about using opamps in parallel for noise
> reduction, in his small signal audio book. Don't remembner details, and no
> idea how much it's applicable to that VCA chip. A former colleage of mine
> mentioned it, he tried it and it did what it was supposed to. I think the
> outputs were not "tied together", one of them had a small resistor. Not sure
> though.

Op-amps have voltage outputs, so you can't just connect them together
as the individual feedback loops would fight each other.  Doug puts
something like 10R in each output before combining them.

The point is quite simple: for multiple separate devices assuming
uncorrelated noise the signal voltage goes up with +6dB when you
double the number of amplifiers (1 -> 2 -> 4 -> 8 etc), but the noise
only goes up by +3dB, so overall the S/N increases by +3dB.

The 2164 is different in that it has current outputs.  You can combine
the outputs directly, for example if making a multi-channel mixer (see
Figure 25 of the SSM2164 datasheet), or paralleling them for better
SNR (although you cannot parallel the inputs so you still need the
input resistor and compensator RC network on each input).

Neil
-- 
http://www.njohnson.co.uk