Log Pots For All!
Audio circuits often use logarithmic and anti-log/reverse log potentiometers. Frequency and volume controls come to mind. Those kinds of tapers (especially the reverse log) are hard to find around here, and they aren’t even real log pots! They usually are a combination of two linear pots. See this excellent article by ESP here for more info. In this same article, the author suggests the use of a linear pot with a loading resistor in order to approximate a log pot (more specifically, a volume pot). With this in mind, I set out to design a simple tool to help me calculate the adequate values. And while we’re at it, let’s do anti-log pots also! (if you just want to see the app already, here it is)
The log approximation is as described in the article. It is usually used as a volume control of sorts, so I tied one end to ground. In this way it is basically a resistive divider and, ignoring possible loadings to this circuit, the output is some fraction of the input. This curve could be presented as a ratio between 0 and 1 (in this configuration the actual total resistance of the pot only matters when relating to what impedances come before and after it). For the sake of clarity, I decided to multiply the ratio by the total resistance. As a reference, the aforementioned article uses a ratio in DB to represent the Vout/Vin relation.
The anti-log approximation is an actual variable resistor, and the results are really good!
While doing research for this project, I came across this great post by Ben Holmes that presents a model for logarithmic pots. Not trying to reinvent the wheel, I used his derivation of the model and chose the 50% rotation travel to represent 15% and 85% of the resistances for the log and anti-log pots respectively.
The live version can be used here. You can also check the source code on GitHub.