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<div>Neil:</div>
<div>Oh, the 2164 has current outputs, I wasn't aware. Thanks for the explanations :)<br/>
I have some ols SSM ICs somewhere, funnily years ago when I bought them, as well as some old stock SSM VCF chips when they were reasonably priced from chinese sellers, I somehow never dared to actually use them, and used the easily bought "everywhere" LM13700 for "everything", since I, the newbie, didn't want to fry those kinda chips in my first lessons involving magic smoke. (like when in the CAD prog I flipped the opamp, but not the power block. I still have the foto of the magic smoke escape hatch on that opamp.) Now you can buy all those clones, and even some remade parts again. Maybe I should start using those nicer parts now ;) Who knows for how long this will go, though.<br/>
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<div style="margin:0 0 10px 0;"><b>Von:</b> "Neil Johnson" <neil.johnson71@gmail.com><br/>
<b>Betreff:</b> Re: [sdiy] Any noise avantage to parallel 2164 VCAs?</div>
<div name="quoted-content">Steve wrote:<br/>
><br/>
> Douglas Self wrote something about using opamps in parallel for noise<br/>
> reduction, in his small signal audio book. Don't remembner details, and no<br/>
> idea how much it's applicable to that VCA chip. A former colleage of mine<br/>
> mentioned it, he tried it and it did what it was supposed to. I think the<br/>
> outputs were not "tied together", one of them had a small resistor. Not sure<br/>
> though.<br/>
<br/>
Op-amps have voltage outputs, so you can't just connect them together<br/>
as the individual feedback loops would fight each other. Doug puts<br/>
something like 10R in each output before combining them.<br/>
<br/>
The point is quite simple: for multiple separate devices assuming<br/>
uncorrelated noise the signal voltage goes up with +6dB when you<br/>
double the number of amplifiers (1 -> 2 -> 4 -> 8 etc), but the noise<br/>
only goes up by +3dB, so overall the S/N increases by +3dB.<br/>
<br/>
The 2164 is different in that it has current outputs. You can combine<br/>
the outputs directly, for example if making a multi-channel mixer (see<br/>
Figure 25 of the SSM2164 datasheet), or paralleling them for better<br/>
SNR (although you cannot parallel the inputs so you still need the<br/>
input resistor and compensator RC network on each input).<br/>
<br/>
Neil<br/>
--<br/>
<a href="http://www.njohnson.co.uk" target="_blank">http://www.njohnson.co.uk</a></div>
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