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Basic Electric Guitar Circuits 2: Pots & Tone Capacitors

Antique Electronic Supply - based in Tempe, AZ.

Part 2: Potentiometers and Tone Capacitors

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.
Tech Corner Image - Pots Drawing 1
Tech Corner Image - Pots Drawing 2
Tech Corner Image - Pots Drawing 2
Tech Corner Image - Pots Drawing 3

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).
Tech Corner Image - Pots Drawing 4
.022 μF 100 Hz 72.3 kΩ
.022 μF 5 kHz 1.45 kΩ
.047 μF 100 Hz 33.9 kΩ
.047 μF 5 kHz 677 Ω
.10 μF 100 Hz 15.9 kΩ
.10 μF 5 kHz 318 Ω
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).
Tech Corner Image - Pots Drawing 5
Vin = 60mV, R1 = 125k, R2 = 125k
Vout = Vin (R1 / (R1 + R2)) = 60mV (125k / (125k + 125k)) = 60 mV (1/2) = 30 mV

Potentiometer Taper

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 representation drawing.
Tech Corner Image - Pots Drawing 6



How do you know what value of potentiometer to use?


Specialized Pots

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.