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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:
As I understand your post, your DB Systems device uses a 5Kohm pot across the output. This is not the output impedance, as you recognize, but reducing the pot setting below maximum does increase the output impedance of the driving stage.There are two different types of noise that can be affected by the output and input resistances. Thermal noise is generated by any resistor, so increasing the total resistance of the output circuit will increase thermal noise.
However, the larger problem is likely to be noise induced in the interconnect cable. This noise includes electrostatic and electromagnetic induction. A higher amplifier input resistance will increase the voltage developed by magnetic induction of current in the circuit comprising the crossover's output (the driving stage output impedance), the cable, and the amplifier input. This noise can be minimized by reducing the amplifier input resistance to a value as low as is compatible with good operation of the driving circuit, as you also recognize.
So far, so good. A larger value safety resistor in parallel with the output stage of your crossover should not contribute noise to the circuit, as it is shunted by the relatively low effective output impedance of the output stage. The output resistance, which determines the thermal noise, is effectively the output impedance of the driver. If this were not true, it would be possible to build a perpetual motion machine by adding a large number of very high-value resistors in parallel.
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Follow Ups
- Your proposal should work. - Al Sekela 11:21:23 12/30/05 (2)
- Re: Your proposal should work. - Charles Peterson 15:51:50 12/31/05 (1)
- Shorted versus open input. - Al Sekela 08:30:03 01/01/06 (0)
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