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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Reducing Noise by Choosing Impedances posted by Charles Peterson on December 29, 2005 at 17:15:59:
Charles,the noise from a shunt resistor does decrease as the resistor value is increased as in your example of having an almost infinite shunt resistor. If the resistor was placed in series with the output then the noise voltage due to that resistor would add directly to the signal and would increase as the resistor value was increased (noise voltage power is proportional to R). In the shunt case, however, it is easier to think of the equivalent noise current of the resistor that will flow in the signal path. Noise current power is inversely proportional to the resistor value so increasing R gives less current noise.
When thinking about the impact of noise at different places in any signal processing chain it is important to consider the signal to noise ratio at those points. Usually the signal is amplified along the chain and input noise is amplified also. Each block will add some noise of its own but if that is much lower than the noise at the input then it is of little consequence. This is well known to radio engineers as Friis' formula - the noise performance of the first block in the chain is most important so that it adds least to the inherent noise of the signal source. At the output of that first block the signal and noise levels are much higher so the noise performance of subsequent processing blocks in less important (though the overload requirement goes up as the signal is getting bigger).
If you have the power amp pot always on full and adjust the 5k pot then I would expect you to get little benefit from the change - as you're still basically using a 5k pot. Removing the 5k pot from the x-over shouldn't hurt it and you can add a shunt resistor if you like - a higher value will not give more noise.
Regards
13DoW
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Follow Ups
- Re: Reducing Noise by Choosing Impedances - 13th Duke of Wymbourne 14:25:19 12/30/05 (1)
- Re: Reducing Noise by Choosing Impedances - Charles Peterson 18:46:21 12/30/05 (0)
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