Re: Is this a good way to test pre-amp hiss of a digital recorder?

"Mike Rivers" <mrivers@xxxxxxxxxxx> wrote in message
news:1149505487.415223.236580@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
Pooh Bear wrote:

The difference between 200 ohms and 600 ohms is the
square root of their ratios i.e. sqrt 3 = 1.73 or 4.8dB.
Between 150 ohms and 600 sqrt 4 = 2 or 6dB.

I asked for mreasurements, not theory. If I measure the
noise of a preamp with a 150 ohm resistor and a 600 ohm
resistor at the input, can I expect the noise to change
by 6 dB? I can certainly measure that, and I will report
back.


Depends on how much of the noise you measure is due to the interal circuitry
of the preamp, and how much is actually due to thermal noise in the
resistor.

The relevant (but not often given) parameter is called "noise figure". The
noise figure of a preamp is taken from the difference between the actual
measurement of the preamp, and the theoretical noise generated by the
resistor.

Therefore the noise figure of a preamp is partially dependent on the value
of the resistor used to test. A good preamp might have 1-2 dB noise figure
with a 150 ohm resistor. Often, a preamp' snoise figure *improves* with
higher valued test resistors.

If your preamp has a very good noise figure, then the increase in noise with
larger resistors will follow the square root of the resistance of the test
resistor.


.

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