SRS Labs SR560 manual Noise Contours, Calibration And Repair

CALIBRATION AND REPAIR

The other two fuses are in-line with the batteries. These fuses will blow if the rear panel ±12 VDC supplies are shorted, or if the unit sources or draws excessive current to or from the batteries.

NOISE CONTOURS

The noise contours (shown upper right) plot the noise figure as a function of source impedance and frequency. Noise Figure (NF) is defined as:

•NF = 20 log (Output noise / (Gain X Source Thermal Noise)

Alow noise figure means that the output noise is dominated by the thermal (Johnson) noise of the source. A high noise figure indicates that the amplifier's output noise is dominated by the amplifier's own noise, which is much larger than the thermal noise of the source.

amplifier's "1/f" noise is large relative to the thermal noise of the source.

The NF gets worse for large source impedances and high frequencies because the signal is attenuated (hence the gain reduced) by the shunting capacitance of the input.

Under no circumstances will adding source resistance reduce the amplifier's output noise! While this does improve the NF, it does so by making the source so noisy that the amplifier is quiet in comparison.

The NF gets worse for low source resistances because the source's thermal noise gets very small, while the amplifier's input voltage noise stays relatively constant.

The NF gets worse for low frequencies and low source resistances because the

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