What is a Potentiometer? Potentiometers, or "pots" for short, are used for volume and tone control in electric guitars. They allow us to alter the electrical resistance in a circuit at the turn of a knob. It’s useful to know the fundamental relationship between voltage, current and resistance known as Ohm’s Law when understanding how electric guitar circuits work. The guitar pickups provide the voltage and current source, while the potentiometers provide the resistance. From Ohm’s Law we can see how increasing resistance decreases the flow of current through a circuit, while decreasing the resistance increases the current flow. If two circuit paths are provided from a common voltage source, more current will flow through the path of least resistance.
By Kurt Prange
We can visualize the operation of a potentiometer from the drawing above. Imagine
a resistive track connected from terminal 1 to 3 of the pot. Terminal 2 is connected
to a wiper that sweeps along the resistive track when the potentiometer shaft
is rotated from 0° to 300°. This changes the resistance from terminals
1 to 2 and 2 to 3 simultaneously, while the resistance from terminal 1 to 3
remains the same. As the resistance from terminal 1 to 2 increases, the resistance
from terminal 2 to 3 decreases, and vice-versa.
Tone Control: Variable Resistors & Tone Capacitors
Tone pots are connected using only terminals 1 and 2 for use as a variable resistor
whose resistance increases with a clockwise shaft rotation. The tone pot works
in conjunction with the tone capacitor (“cap”) to serve as an adjustable
high frequency drain for the signal produced by the pickups. The tone pot’s
resistance is the same for all signal frequencies; however, the capacitor has
AC impedance which varies depending on both the signal frequency and the value
of capacitance as shown in the equation below. High frequencies see less impedance
from the same capacitor than low frequencies. The table below shows impedance
calculations for three of the most common tone cap values at a low frequency
(100 Hz) and a high frequency (5 kHz).
When the tone pot is set to its maximum resistance (e.g. 250kO), all of the
frequencies (low and high) have a relatively high path of resistance to ground.
As we reduce the resistance of the tone pot to 0O, the impedance of the capacitor
has more of an impact and we gradually lose more high frequencies to ground
through the tone circuit. If we use a higher value capacitor, we lose more high
frequencies and get a darker, fatter sound than if we use a lower value.
Volume Control: Variable Voltage Dividers
Volume pots are connected using all three terminals in a way that provides a
variable voltage divider for the signal from the pickups. The voltage produced
by the pickups (input voltage) is connected between the volume pot terminals
1 and 3, while the guitar’s output jack (output voltage) is connected between
terminals 1 and 2. From the voltage divider equation below we can see that if
R1 is 0O and R2 is 250kO, then the output voltage will be equal to the input
voltage (full volume). If R1 is 250kO and R2 is 0O, then the output voltage
will be zero (no sound).
The taper of a potentiometer indicates how the output to input voltage ratio
will change with respect to the shaft rotation. The two taper curves below are
examples of the two most common guitar pot tapers as they would be seen on a
manufacturer’s data sheet. The rotational travel refers to turning the
potentiometer shaft clockwise from 0° to 300° as in the previous visual
How do you know when to use an audio or linear taper pot?
It’s really a matter of personal taste when it comes to volume control.
Notice how the rate of change is much more dramatic on the audio taper pot when
traveling back from 100% to 50% rotation. This means that the same amount of
rotation would give you a more intense volume swell effect with an audio taper
than with a linear taper. Using a linear taper volume pot would give you a more
gradual change in volume which might feel like you have more fine control with
which to ease back the volume level.
For tone control, it’s basically standard practice to use an audio taper.
The effect of the tone circuit is not very noticeable until the resistance gets
pretty low and you can get there quicker with an audio taper.
How do you know what value of potentiometer to use?
The actual value of the pot itself does not affect the input to output voltage
ratio, but it does alter the peak frequency of the pickup. If you want a brighter
sound from your pickups, use a pot with a larger total resistance. If you want
a darker sound, use a smaller total resistance. In general, 250K pots are used
with single-coil pickups and 500K pots are used with humbucking pickups.
Potentiometers are used in all types of electronic products so it’s a good
idea to look for potentiometers specifically designed to be used in electric
guitars. If you do a lot of volume swells, you’ll want to make sure the
rotational torque of the shaft feels good to you and most pots designed specifically
for guitar will have taken this into account. When you start looking for guitar
specific pots, you’ll also find specialty pots like push-pull pots, no-load
pots and blend pots which are all great for getting creative and customizing
your guitar once you understand how basic electric guitar circuits work.
About the Author: Kurt Prange (BSEE) is the Sales Engineer for Amplified
Parts in Tempe, Arizona, United States. Kurt began playing guitar at the
age of nine in Kalamazoo, Michigan. He is a guitar DIY’er and tube amp
designer who enjoys helping other musicians along in the endless pursuit of