I realize this topic has come up before, but could not find the answer in any earlier thread.
For a tone control to adjust an active speaker for room placement, I need to incorporate a potentiometer. In order to minimize noise and feed the next stage with a low source impedance, I want to limit the resistance of the pot to 2.5K. This obviously drives currents up. I simmed the maximum current to pass through the pot to be 10 mA @ 3 V. To dissipate 30mW in a closed pot will heat up things, so tempco obviously is of importance.
If this were a stationary voltage divider, I would no doubt choose fixed resistors with a low tempco and known low excess noise. However, because it has to be variable, I am stuck with using a pot and this is where there seem to be two options:
Cermet, with 150 ppm/C, or conductive plastic, with 1000 ppm/C. Both are lousy, but the figure for conductive plastic almost takes it into thermistor territory.
Given these figures, it would seem to be a no brainer to go for cermet, but I have no clue as to the excess noise generated by the two technologies.
It would be much appreciated if someone could fill me in.
For a tone control to adjust an active speaker for room placement, I need to incorporate a potentiometer. In order to minimize noise and feed the next stage with a low source impedance, I want to limit the resistance of the pot to 2.5K. This obviously drives currents up. I simmed the maximum current to pass through the pot to be 10 mA @ 3 V. To dissipate 30mW in a closed pot will heat up things, so tempco obviously is of importance.
If this were a stationary voltage divider, I would no doubt choose fixed resistors with a low tempco and known low excess noise. However, because it has to be variable, I am stuck with using a pot and this is where there seem to be two options:
Cermet, with 150 ppm/C, or conductive plastic, with 1000 ppm/C. Both are lousy, but the figure for conductive plastic almost takes it into thermistor territory.
Given these figures, it would seem to be a no brainer to go for cermet, but I have no clue as to the excess noise generated by the two technologies.
It would be much appreciated if someone could fill me in.
Well, not really. It's not quite the schematic you might think of.
R80 and R81 are two legs of a pot. Current through R80 is shown in this sim. I can change it for a fixed resistor though, also better for other reasons, so that should take care of that part of the problem.
Still interested to know if measurements exist of excess current in cermet vs conductive plastic, just for the sake of making things better.
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