[sdiy] why +/- 15V

[jhaible at debitel.net]

Higher voltage, better SNR, or headroom, depending on your
nominal signal level. For the following, let's just assume
maximum level without clipping, and look at SNR, to
simplify things.

At first glance, 30V (+/-15) are just 3 times as high as 10V.
So with the same noise level, you'd get 20log(3) = 10dB
better SNR from 30V supplies ??

Actually, many standard opamps don't like to go near their
supply rails. Let's say 3V distance to vdd and vss.
So your maximum signal amplitude is 24Vss (30V) vs. 4Vss
(10V supply). That's 20log(6) = 16dB better SNR from the 30V
supply!

Modern rail-to-rail opamps, and generally better low-noise
parts will improove the situation, of course. So some of
this +/-15V idea *is* historic.

I like to be "historic" in most of my own designs. And I
even see the benefits you get from this even older technology
that uses +250V, allthough I rarely use it myself.

JH.

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