gaussmarkov: diy fx

Potentiometers, or pots, give control over tone, distortion, volume, and many other stompbox characteristics. Despite the huge variety of effects, pots only directly change resistances in a circuit so we can leverage our previous discussion of resistors into a complete explanation of these important components.

Appearance

Pots are resistor voltage dividers in which the component resistances can be varied by rotating the shaft. The sum of the resistances is constant and equal to the value of the pot. Pots appear in such controls as volume (or level), tone, distortion/overdrive, compression, and for blending/mixing. Pots are also often wired to work as variable resistors.

The photo above is taken from the on-line catalog of Small Bear Electronics. This is the pot that I have learned to use most often. It is made by Taiwan Alpha. On the shaft are a washer and a nut for fastening the pot to an enclosure. On the left you may also notice a small tab sticking up from the case. This can prevent the pot twisting when the shaft is rotated. Lots of people break this off with a pair of pliers because they find it more trouble than it is worth. On the front you see 3 solder lugs. These lugs are for a panel mount pot. There are also pots with lugs that go directly into the holes on a PCB.

This is a 16mm pot, which means that the diameter of the case is roughly 16mm. I use this size more than any other. I will talk more about this below in the Ratings section.

There is another kind of pot, called a trim pot or a trimmer, that also appears often in stompbox builds. These are small potentiometers that are mounted on the circuit board and are only accessible inside the stompbox enclosure. They are typically used to set a fixed resistance that cannot be deteremined before the circuit is made. For example, some types of transistors have characteristics that vary so widely that resistors connected to them must be individually adjusted.

Somewhere on the case, on the bottom in this picture, the characteristics of the pot are stamped or printed. The printed resistance, 5KΩ in this case, refers to the fixed resistance across the outer solder lugs of the pot. The letter B refers to the taper (or path of resistance) as the shaft is turned. To explain this, I have to describe the conventional way to hookup a pot. And for that description, it is useful to have some symbols and a schematic.

Symbols

Just as there are two symbols for resistors, there are two symbols for pots. Both are resistors with an arrow pointing into the middle. In addition, you may see numbers or letters on the three terminals and another arrow pointing along the resistor. Sometimes this arrow is accompanied by the letters CW, or CCW, or something similar. Not everyone is clear in their schematics about their hookups for pots.

These schematic symbols follow what appears to be a fairly common, but certainly not universal, convention on Aron’s forum that coincides with familiar ways

1. to put a voltage divider into a schematic and
2. to picture the structure of a pot.

On the left is a schematic for the voltage divider discussed on the resistors: series page. On the right is a diagram for the inside of a pot looking down from above the shaft, the way the pot would be actually used.

The lugs marked 1 and 3 are connected to the ends of a resistive strip and a wiper connected to lug 2 rotates from lug 1 around to lug 3 as the shaft is turned clockwise (CW). When the pot is “turned down” (fully counter-clockwise or CCW), lug 2 is connected directly to lug 1 without any resistive strip between them. On the other hand, when the pot is “turned up” (fully clockwise), lug 2 is connected directly to lug 3 with the complete resistive strip between them.

So, looking at the schematic symbols, the voltage divider schematic, and the pot diagram, the convention is that

• lug 1 is the CCW lug or the bottom lead of R2,
• lug 2 is the wiper or Vr junction
• lug 3 is the CW lug or the top lead of R1, and
• the arrow indicates a clockwise rotation from lug 1 towards lug 3.

If you look at the image of the bottom of the pot above full size, you will see 1, 2, and 3 stamped into the bottom of the case just as described here. Another convention replaces 1 with A, 2 with W, and 3 with B.

If you hook up a volume pot the way the voltage divider is drawn in the schematic, with the voltage source connected to lug 3, ground connected to lug 1, and the output connected to lug 2, then the volume knob works as you would expect. With the knob fully CCW, there will be no output because the output is connected to ground. With the knob fully CW, you will get the maximum output and there will be a smooth transition between these extremes as the knob is turned. The taper of a pot determines how the volume changes as you turn the knob.

To see an example, look at the schematic for the Red Llama clone from the layouts page. On the far left you will see a pot labelled “VOLUME” hooked up with this configuration. If you look at the top of the schematic, you will also see a pot labelled “GAIN” that is hooked up differently. This pot is configured as a variable resistor.

Variable Resistor

Pots can also be wired as variable resistors. This is often the case for gain or overdrive pots. Also, a pot is sometimes used in place of a fixed resistor when the exact resistance needed is not known in advance. In such a case, a much smaller pot called a trimpot is often used. Common sizes are 1/4″ and 3/8″ square which fit neatly on a pcb where they are set once, or at least infrequently.

To use a trimpot as a variable resistor, simply short lugs 1 and 2 or lugs 2 and 3, and treat lugs 1 and 3 as the leads of the resistor. The resistive track between the wiper and the shorted lug is irrelevent because there is a wire with no resistance connecting the two lugs.

If you short lugs 1 and 2, then the resistance decreases as you rotate the shaft CW. Conversely, if you short lugs 2 and 3, then the resistance increases as you rotate the shaft CW. To see an example, look at the schematic for the BSIAB2 on the layouts page. At the top of the schematic, slightly to the right, is a 100K pot called “TRIM” serving as a trimpot. You will also see a volume pot labelled “LEVEL” with the volume hook up described above. That pot has the value designation 100K-A. The letter A denotes the suggested taper, which is the next topic.

Taper

So, given that we are going to hook up our pot with lug 1 to ground, lug 2 to output, and lug 3 to input, let’s talk about pot taper. Suppose that the resistive strip inside the pot is uniform in such a way that the resistance along the strip from the wiper to a lug is proportional to the length of the strip between them. If we call the resistance along the entire length of the strip R_p and if we say the wiper (lug 2) is x of the distance along the strip, then the resistance between lug 1 and lug 2 is

and the resistance between lug 2 and lug 3 is

Using these resistance values, the voltage divider forumula tells us the relationship between V_IN and V_OUT:

In words, the percentage attenuation equals the percentage of pot rotation. There is a one-for-one linear relationship. Such a pot has a linear taper and usually, but not always, this is designated with the letter B, as on the bottom of the pot case pictured above.

A linear taper is not ideal for volume control. It turns out that we hear a linear taper as increasing volume too quickly early in the pot rotation and very little in the last half of the rotation. So there is another taper, called audio or log, designed to work better for a volume application. The resistance increases slowly at first and then accelerates. This new pattern can be created by making the resistance strip actually taper over its length. The audio or log taper is usually (but not always) designated by the letter A.

Generally, trimpots have a linear taper. I suppose this is because a linear taper is cheaper to make than other tapers and trimpots are typically used for setting a resistance once, after a circuit has been built, for fine tuning.

If you are not sure what type of pot you have, then follow these helpful instructions from R. G. Keen.

Values

Pots do not come in as many values are fixed resistors because pots provide a range of resistances themselves. A glance at the value of the linear pots available at Small Bear Electronics probably tells the story at most sources: 1K, 2K, 5K, 10K, 25K, 50K, 100K, 250K, 500K, 1M, and 2M. Trimpots start at lower resistances because they are used as substitutes for fixed resistors: 100, 200, 500, 1K, ….

Ratings

Like resistors, pots are rated by power in watts. This is a value that should not be exceeded. Linear 24mm Taiwan Alpha pots have a 0.5W rating, while other tapers are 0.25W. Linear 16mm Taiwan Alpha pots have a 0.25W rating, while other tapers are 0.06W. General good practice is not to go below 0.25W, but lots of people are using 16mm audio taper pots without reporting problems. As I mentioned in resistors: limiting current, one should figure out the actual power requirements and then follow R. G. Keen’s advice to double the power requirements to get an approximate rating for your components.

Further Information

R. G. Keen has written an excellent introduction in The Secret Life of Pots. He even shows how to disassemble a Taiwan Alpha pot and gives pictures of the guts, including the wiper and the resistive strip. R.G. also has some interesting historical notes.

Another useful source is Potentiometers (Beginners’ Guide to Pots) by Rod Elliott of Elliott Sound Products (ESP).

And here’s an additional note about the audio taper described above. First, I have not found a formula on the web for the audio taper so I have used one based on a rule of thumb described many places on the web: to get half the perceived loudness use one-tenth the voltage. The function with this property is

where x is the fraction of pot rotation. This agrees loosely with another rule of thumb: the resistance across lugs 1 and 2 is approximately one-tenth the total resistance of an audio pot when the shaft is turned to the half-way point.

Second, audio pots do not actually have tapers with such smooth acceleration. Look at a data sheet for a Taiwan Alpha audio pot and you will see the figure on the right.