pots: log or lin

[scaesic]

I know generally log should be used for volume pots, but i've seen in schems that some drive pots are log and some are lin.

whats the general concensus for: filter pot in front of an RC lpf
                                                drive pot feeding into - of opamp which in turn drives another op-amp with diodes in feedback loop.
                                                output pot being used as potential divider.

i've been thinking: log, log, lin, but i'm not sure.

[R.G.]

First, go read "The Secret Life of Pots" at GEO.

The taper of a pot is a term descriptive of the way resistance varies with pot rotation. In a linear pot, turning it 1 degree near the low end of the pot changes the resistance by the same amoung that 1 degree near the top does.

Audio taper pots were thorught up because if you use a linear pot as a voltage divider for audio volume control, the human ear's logarithmic response to sound levels makes it seem like all of the volume increase is bunched up at the low end. The pot is still being linear, but your ear is not hearing it that way.

To get equal control effect both high and low without all the chang being bunched up at one end, audio taper was invented. An audio taper pot is really an exponential pot - the output voltage when it's used as a voltage divider increases exponentially with rotation, so that it increases only very slowly near the beginning where  your ear is most sensitive to changes in volume, and a lot at the end of rotation where your ear is least sensitive to volume changes, so your eye and hand think that the volume increases linearly with rotation.

Whew. The pot is compensating for YOUR nonlinearities.

You use log when you want something to change exponentially (i.e. change a volume) and linear most other places. Frequency is another of those things that are equal sounding when exponential, so often a log pot will work for filter frequencies. In setups where you want a pot to control the frequency of an oscillator by changing the charging time of a capacitor, you have to make the resistance exponentially bigger as you get lower in frequency. That calls for a reverse log pot.

Best thing to do? Put in all linears, and taper them with resistors if you need taper.