Potentiometers are probably the most visible electrical component in an analog synth. The face of any traditional modular analog synthesizer is covered with potentiometers. Behind every knob or fader is a potentiometer or “pot” for short. Clearly, the majority of controls on modular analog synthesizers are potentiometers. You can literally see their importance in synth design.
All these pots are nothing more than a variable resistor. They come in a variety of resistance values – and a variety of sizes, shapes and even features. Here is a photo of some common potentiometers.
The type of potentiometers often seen in synthesizers look like this:
This pot has a shaft coming out the top and three “leads” or “tabs” on the side. Inside the potentiometer, the two outer leads are connected to a resistive strip. The resistance between these outer leads is the pot’s resistance value.
The shaft is connected to a wiper that rotates around the resistive strip. This wiper is also connected to the center lead. You can measure the resistance between the center lead and an outer lead. When the shaft is rotated all the way to one side, the wiper will be in contact with one of the leads. At that point the resistance between the center lead and the outer lead will be 0. As you turn the shaft the resistance will gradually increase. When the shaft has rotated all the way to the opposite side, the resistance measured will be the pot’s value.
This is the schematic symbol for a potentiometer:
In this symbol, the top and bottom connections represent the outer leads; the center lead is represented by the connection opposite the arrowhead.
Potentiometers are distinguished by what is called “taper”. In building an analog synth you will generally encounter either “linear taper” or “audio taper” (also called “log” or “logarithmic” taper) pots. The resistance of a linear taper pot increases evenly across its entire range. If the first 10% of turning a linear taper pot yields 10 k of resistance, each additional 10% will also yield 10k resistance.
As its name implies, the resistance in a logarithmic taper pot idally follows a logarithmic curve. This results in less resistance when you first start to turn the pot and more resistance as you get towards the middle. Actual logarithmic pots generally do not follow this ideal. Most log taper pots simulate a logarithmic curve by dividing the resistance into two sections. The first section takes about 1/2 to 2/3 of the turn of the pot. This first section has only about 10-20% of the pot’s total resistance. The second section delivers the remaining 80-90%.
Logarithmic taper poteniometers are also called “audio taper”. This is because humans hears a sound’s volume on a logarithmic curve. In synth design, and other audio applications, an audio taper pot would commonly be used in a volume control. An audio taper pot has audio volume rise in a relatively even manner. If a linear taper pot was used, as the volume was turned up, the sound would seem to get very loud at first, and as the pot was turned further, there would appear to be less and less change in the volume.
Finally, lets look at what is probably the most common use of a potentiometer in a synthesizer circuit – as a variable voltage divider. The circuit looks loke this:
We first looked at the voltage divider circuit when we looked at resistors. To configure a potentiometer as a voltage divider, one of the outside leads is connected to ground, and the other is connected to the input voltage (or signal). The wiper (middle lead) is the output. As described in the resitor article, the equation for determining the output of a voltage divider is
V out = V in x (R2/(R1 + R2))
When using a potentiometer as a voltage divider, the valus of R1 and R2 vary as the shaft on the potentiometer rotates. This causes the output voltage to vary from V out = V in to V out = 0 (or ground).