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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, let me help.
First, you haven't read about noise theory enough, so you make some mis-assumptions.
First, higher resistances have higher measured Voltage noise, but lower Current noise. As one noise goes up, the other noise goes down. Therefore, with very high resistances like vinyl, you would expect to measure very high voltage, BUT you don't! Why? Because any residual capacitance to ground, shorts out the noise and it actually gets lower as the resistance gets higher. This is why condenser microphones require 100 million to 10 billion ohm parallel source resistors to be quietest. Lower value resistors in the same location would increase the noise of the microphone.
Now back to the noise of your system.
Any potentiometer in an audio system is a potential source of added noise. To understand this, think of a pot as actually two resistors connected in series and tapped between them to make an attenuator. The noisiest place for this attenuator to be set is exactly 6 dB down from full output. (if the source is low impedance and the load is high impedance compared to the pot value). The added noise will then 1/4 the nominal value of the pot eg 50K/4 = 12.5K Putting the pot in the maximum position is the quietest location, but then, why have a pot at all?
It is impossible to improve the noise of the pot by adding more resistance. Matching is not important or effective here.
Actually the best position for your system, as it is today, is to use the 5K pot in the dB as the level control, with the amp pot set on maximum gain. This is because of the value of the pot, 5K, is lower than the 50K value of the amp pot. This could give you several dB improvement, BUT you will then have a relatively high drive impedance (up to 1.5K or so) worst case and this could increase noise pickup on long lines between the xover and the preamp. Shielded wire will probably be necessary.
Alternatively, you could reduce the amp input pot to 5K, just like you were thinking about, and remove the 5K unit from the dB unit. This would give the same loading to the xover electronics and allow low impedance drive to the amp.
Give it a shot, if you wish.
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Follow Ups
- Re: Reducing Noise by Choosing Impedances - john curl 13:44:22 12/30/05 (0)
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