Rupert Neve Portico 5012 - Duo Mic Pre-Amp - Manual (Page 7)

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5012 DUO MIC PRE
There are two ways to provide more information about the noise performance of a microphone
amplifier:
A graph can be drawn showing Noise against Resistance. Thermal, or pure Resistor noise is a
straight line whilst Amplifier noise is a curve that depends upon the optimum source resistance for
that particular amplifier. The difference between the two noise measurements in dB is the “Noise
Factor”. (“N.F.” Familiar to R.F. and communications designers.)
Such a graph can be very informative if we substitute impedances of an actual (silent) microphone,
because microphone source impedances are not the same at all frequencies - but that subject is for
another day!
A second method that reveals more than just the E.I.N. combined figure, is to measure the
combined amplifier and resistor noise over a range of resistances right down to zero resistance. At
zero resistance connected to the input (in other words when the input is short-circuited), there is,
theoretically, NO resistor noise so any residual measured noise must ALL be due to the amplifier.
We then calculate the value of a resistor that would produce noise equal to the residual measured
amplifier noise.
So we can also state the amplifier noise performance by equivalence to a resistor having the same
noise. This is a good way because it’s easy to measure and takes account of any resistance in the
amplifier circuit such as Printed Circuit tracks, connector resistance and length of cable between
the amplifier proper and the point at which you have applied an input short circuit. (Little point in
having a superb amplifier noise performance, equivalent to a very low resistance, if we then add a
microphone cable or snake that worsens the noise by several dB!)
THERMAL V MEASURED NOISE
The difference is NOISE FACTOR
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1 2 4 8 16 32 64 128 256 512 1,024
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B
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Input Resistor Ohms
Thermal Noise 5012 Measured