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Phil Allison[_4_] Phil Allison[_4_] is offline
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Scott Dorsey wrote:
-------------------

Phil is known to have some odd notions about noise,


*** Nah, that is not one tiny bit true.


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.


** Yes. JFET inputs have negligible current noise.



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.


** Yes, sources like MC picks are only a few ohms yet is it still possible to make god pre-amps for them without out transformers.


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.


** False. With a mic-pre you simply specify the ratio in dB between the measured (audio band) noise with a 150 to 250ohm load and the calculated Johnson noise.



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.


** False. Johnson noise is dominant in any resistive source situation.


Now, Phil has in the past displayed extreme reluctance to believe that 1/f
noise is significant in these systems,



** And he is dead right too.


and believes that pink Boltzmann noise
is the only issue.



** LOL there is no such animal.

Johnson noise is White - equal energy in equal bandwidths.


It is futile to attempt to demonstrate otherwise to him.



** Naturally, cos that would be impossible.


Oh dear, the old " proof of some nonsense I am claiming is hidden in this massive link somewhere" trick.

No way.



....... Phil