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Scott Dorsey Scott Dorsey is offline
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Phil is known to have some odd notions about noise, but I am assuming that
we can all agree on the same basic statements:

1. For high impedance, current noise is dominant, whereas for low impedances,
voltage noise is dominant.

2. Microphones are primarily low impedance, with output impedances of
microphones being lower than 500 ohms and input impedances of preamplifiers
being in th 500 ohms to 5k ohm region.

3. NF attempts to combine voltage and current noise, but is normally specified
on the datasheet in a way that is not useful, because the impedance is
seldom at a useful value, and the assumption is made of source/load
impedance matching which is almost never the case for a microphone
interface.

4. All the GkTB equations we were forced to memorize in school are useful at
RF but not useful in a mismatched low frequency world.

Now, Phil has in the past displayed extreme reluctance to believe that 1/f
noise is significant in these systems, and believes that pink Boltzmann noise
is the only issue. It is futile to attempt to demonstrate otherwise to him.

Marshall Leach's article is a very good introduction to the system and
presents basic noise analyses of several transformered and transformerless
microphone preamps:
https://leachlegacy.ece.gatech.edu/p...ransformer.pdf
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."