You may not have thought about it, but when you turn a volume or tone knob on a guitar, or a setting on an effects pedal or amp, some complicated things happen. The little potentiometer that’s hiding behind the knob has some interesting properties. There are already some excellent articles on the web about how pots work, so there’s no point in rehashing that here. I’ll link a good one in at the end. However, one thing you’ll find in most of them is the discussion of taper. This describes a relationship between the mechanical and electrical properties; how much electrical change happens, relative to the mechanical change. For example, you may hear people describe a volume pedal with terms like ‘smooth linear sweep’. In fact, a linear taper is usually the last thing you want in a volume control, because human hearing is not linear. For a volume change to SOUND proportional, the ACTUAL change in terms of voltage, SPL etc should be logarithmic.
Anyway, sometimes you may want to change a pot. Maybe the tone control on your guitar is too sensitive, or not sensitive enough. Maybe your volume pedal seems to make all its change in the last few degrees of travel. Was your amplifier gain fine at one time, but now you are older and deafer it doesn’t seem to work the same? If you read up on potentiometer tapers you’ll find that some may not be exactly what they seem. If we could measure exactly what taper our pots are, then we could compare them and decide what change could be made. So, let’s do that.
You’ll need some things. A function generator, multimeter, and an oscilloscope would be awesome, but we can get by with much less. You will at least need a multimeter, but it doesn’t need to be fancy. A hardware store or low cost Ebay or Amazon one will work fine. Depending on the device, you may be able to take the measurements in circuit or you might need to remove the potentiometer that you want to test by de-soldering or cutting the wires. You’ll have to do it anyway if you are going to replace it. Measure the resistance across the outside two pins of the pot (sometimes marked 1 and 3 or CW and CCW). This is the maximum resistance of the pot. This one is 10K Ohm.
You’ll also need something to generate a fixed voltage. A tone generator works well. If you have a cable tester with a tone generator on it, you can use that, or you can use a computer generated one if you have an audio interface. There are some low-cost tone generators available from the usual sources. It’s helpful to have a simple tone generator for many audio projects. If none of these are available you can use a DC source. A small battery will be fine. Make sure not to use a larger voltage battery than the pot is rated for.
Decide how many samples you want to take and at what points. We are essentially going to be doing digital sampling by hand. For example, if we measured our potentiometer max resistance at 10K Ohm, we could take 11 samples starting from 0 and then at 1 KOhm intervals. Turn the pot until it’s all the way as close to 0 Ohm as it goes. Now move the + of the resistance meter to the center pin on the pot and connect the voltage source to the open pin. Measure the voltage and record it next to the resistance. Then measure the resistance again and turn the pot until it reads about 1 Ohm. Measure the voltage again and record it. Keep doing this until you have rotated the pot all the way to the other end and have all your samples. For example:
0 Ohm – 0V
1K Ohm – 0.1V
2K Ohm – 0.3V
10K Ohm – 1.5V
The number of samples we have taken (11) is analogous to sampling rate. The accuracy with which we record the voltage is analogous to bit depth. The more samples and the more accurately we record the voltages, the more detailed our plot will be.
Once you have all the values, you can graph them out. A spreadsheet program like Excel will work for this, or there are a number of free online graphing tools on the web. I like www.chartgo.com. Plot resistance (rotation) against voltage and if all went well you should get a plot of the taper of your potentiometer.
Now you can tell if everything is how you expect, and if not, how you might change it. For example, if your control seems to change too fast and you measure a linear pot, then try changing it to a log one. If the control appears ‘backwards’ try swapping a log for anti-log or vice-versa. Some low-cost log pots are not true log, but just two different slopes. If you have one of these it will show as two straight lines at different angles on the graph rather than a continuous curve. If you have one of these, and it’s causing problems, then replacing it with a true log part may help.
In the link, there are some instructions on how you can add tapering resistors to a pot to change the taper. If you do this, you can use the plotting technique to visualize the change. Measure the pot first without the resistors (the stock pot, ha ha), then add the tapering resistor and measure again. Plot these as two lines on the graph. With two plots on the one graph, you’ll easily be able to compare them.