"Don Pearce"
Ian Bell

Even if that were correct it is immaterial in most situations where the
gain
is not set to maximum (and that is the ONLY place a mic amp achieves an
equivalent input figure this good). As gain is reduced, output noise
begins
to dominate and the old Neve mixers still beat most 'modern' designs in
that respect.

Put simply, equivalent input noise is only part of the story.



It is, as you say, important where the gain is high. But the rest is
plain wrong. If you want to include the gain setting in the noise
performance it is done by R = Rs + 1/G, where Rs is the source
resistance, G is the gain and R is the resulting effective source
resistance.


** A completely wrong formula.

Where ever did it come from ?

One of this fool's dusty old books on RF theory ?


The result is that even at unity gain, the source
resistance is only increased by 1 ohm - barely changed from 150 ohms.


** Good grief !!

Has this cretin no idea how a common mode gain control operates ????


Typically, residual noise at the OUTPUT goes almost in hand with gain for
settings between 60dB and 40 dB. Further gain reduction has a rapidly
diminishing effect on the residual noise, it may drop to 0.02mV in a good
design at gains of 20 dB and under.

So, the ein goes like this

- 128dBu at 60 dB gain,

- 125dBu at 40 dB gain

- 112dBu at 20 dB gain.

The equivalent output noise ratios rel 0 dBu are therefo

68 dB, 88 dB and 92 dB.

A preamp can usually output 22dBu, so the maximum possible ratios a

90 dB, 110 dB and 114 dB.



** The SSM 2017 is a typical high performance mic preamp in IC form - ie not
an op-amp.

http://ezphysics.nchu.edu.tw/prophys...et/SSM2017.pdf

Note how the ein figures vary with gain setting.




........ Phil