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



Mike Rivers wrote:

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.

In what way do you expect measurements not to confirm the equation for thermal noise ?

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.

You won't see quite as much as 6dB because of the preamp's self noise. The better the
preamp, the bigger the difference you'll see. Noise sums as the sqrt of the sum of the
squares.

To measure the preamp's noise*voltage* you can indeed short the input but this won't
establish the input noise *current* which will add to the figure when the preamp sees a
real mic.

Graham



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