Who's measured the noise on their RIAA preamp? - Page 3 - diyAudio
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Old 19th February 2004, 07:35 PM   #21
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Join Date: Feb 2002
Location: Belgium
Okay, a try at posting a formula......

Working from the assumption that there are only resistors (or that the capacitors are negligable at the frequencies of interest) Otherwise, replace every R with the corresponding Z(f) and the same for en(f) and in(f).

A single opamp,
Rin+ from positive input to ground
Rf from output to negative input
Rin- from negative input to ground
en noise voltage in V/sqrt(Hz)
in noise current in A/sqrt(Hz)
k Boltzmann constant 1.38*10exp(-23) J/K
T absolute temp in K
B noise bandwidth

let Req be equal to the parallel value of Rin- and Rf

The total noise voltage:

E= sqrt{ [ sqr(en) + sqr(in*Rin+) + sqr(in*Req) + 4*k*T*(Rin+) + 4*k*T*Req ] * B}
And sqr(x)= x*x

You can use this as the noise is independent of the signal, so the result can be added by superposition. Rin+ can be the input resistance of your source in a non-inverting setup, or Rin- can be the sum of your source and one of your gain setting resistors etc...

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Now, where did I put those 1500uF caps?
Whats wrong with 1500uF? I would use 4700uF

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In any case, I still need to put in a regulated voltage reference as opposed to that RC network. That might make the biggest difference of all.
Good idea.
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