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Old 13th November 2007, 12:43 PM   #1
borges is offline borges  Norway
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Join Date: Dec 2003
Location: Oslo, Norway
Default Log potentiomenter, constant Rin

Is there potentiometer topology for logarithmic-like volume adjustment where the input impedance of the device is always the same?

The reason I'm asking is that I'm working on a passive DAC IVC where the last leg of the lopass filter has a resistor to ground. The resistance influences the filter coefficients, so I'd like to keep it constant.

Replacing this resistor with a volume control pot would save me one gain / impedance conversion stage in the output.

Any ideas?

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Old 13th November 2007, 01:20 PM   #2
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Join Date: Mar 2007
Location: Canandaigua, NY USA
If the pot is on the output of something, and the pick-off point (wiper) is buffered, the pot will have a constant input impedance. That one's fairly obvious. If the something the pot is feeding needs a constant impedance from the source (wiper), you again need a buffer on the wiper, and can set the output impedance of the buffer with a series resistor. There are dual wiper pots that can maintain constant impedance, but those are usually used for level controls in speaker crossovers. Not sure if anybody uses them anymore, and they're not suited to low level higher impedance circuitry. Variable filters like Khron-Hite often used several pots geared together so they'd track, but rigging up something like that isn't practical for a one-off.
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Old 13th November 2007, 01:51 PM   #3
borges is offline borges  Norway
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Join Date: Dec 2003
Location: Oslo, Norway

it's actually the other way around. The impedance seen at the wiper may vary quite a bit since it is a tube grid or an MOS gate.

But the impedance across the pot has to be constant since it is seen by the output impedance (and filter time constant) of the passive IVC filter.

I know I can do this with a linear pot, but I'd like it to be more user friendly and logarithmic.

The topologies I have seen for "logifying" pots involve setting up the source across the pot and having a fixed resistor between the wiper and the common ground. That will present the source with a low impedance at full volume (a little of pot + (a lot of pot || fixed resistor) ) and a higher impedance at low volume (a lot of pot + (a little of pot || fixed resistor) ).

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