<So ... all three of these signals now go into the +5dB gain black box ampl=
ifier,
and come out as +10dBV signal, -80dBV white box noise and -95dBV black box
noise. In reality the three outputs are summed and don't exist as separate
entities. The two noise sources actually sum as the square root of the sum =
of
their squares, and become
-79.87dBV
So, finally, we end up with a 40dB amplifier with an input of -30dBV and an
output of +10dBV signal and -79.87dBV noise i.e. a SNR of 89.87dB very near=
ly
the full 20dB improvement of the white box vs black box EIN individual figu=
res.
Caveat : Although I said that the gain of an amplifier has no bearing on it=
s
EIN, practically, due to the topology of common input stages, generally spe=
aking
the EIN performance of an amplifier will improve with increasing gain.>
I agree Rob. I think your last comment is important to note. When we hook a=
"white box" external preamp to the mic input, then we also need to reduce =
the gain of the recorder in order to get the same recording level (to preve=
nt clipping). Depending on the specific recorder model, that reduction of t=
he gain will unfortunately further increase it's EIN by a certain amount, w=
hich would finally reduce the effectiveness of the external low noise pream=
p to some extent. So, the EIN of -100 dBV at it's 40 dB gain setting might =
perhaps drop to -90 dBV at the 5 dB gain setting.
To quantify these things, one had to know the EIN of the recorder at that l=
ower gain setting and compare it with the output noise voltage of the white=
box preamp (white box EIN + 35 dB). I guess that this would still work for=
most recorders, but you never know...
Regards,
Raimund
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