Basic Electric Guitar Circuits

Potentiometers and Tone Capacitors (by Kurt Prange)

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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.

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. 250k), 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 0, 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 0 and R2 is 250k, then the output voltage will be equal to the input voltage (full volume). If R1 is 250k and R2 is 0, then the output voltage will be zero (no sound).

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.

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.

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.

Kurt Prange (BSEE) is the Sales Engineer for Amplified Parts (www.amplifiedparts.com) 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 tone.

21 comments sorted by best / new / date

    Dart.Target13
    Mobby32 wrote: i should of payed more attention in my electronics class
    you went to an electronics class and you don't know about a pot? its a variable resistor.
    abbydaddy03
    Wow I'm smarter than I thought - bragging on myself. I learned a lot of this stuff in basic electronics, so if you're in school hang in there and get you're degree.
    PSimonR
    jthm_guitarist wrote: 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. Really though? I thought it was the opposite. Definitely not disputing this, but I've read in many other places that for volume you basically have to use an audio taper because we hear logarithmically (decibels). I read a linear taper would decrease the effectiveness of a 'gradual' volume swell or fade. And I have heard of linear pots being used as tone knobs because the varying capacitance is a linear value.
    http://www.ratcliffe.co.za/articles/volu... t.shtml has "Thanks to the logarithmic nature of our loudness perception, we traditionally use log taper pots for volume controls and linear for tone - although it should be noted that some people do like like one or the other. Unfortunately the laws (sorry) of economics and the curse of the MBA, make it cheaper for a manufacturer to use all of one type of pot, some go as far as So if you find your volume comes up dramatically in the first half of pot rotation, and then there is very little change in the last half - you have a linear pot instead of a log. Also, if your tone control seems concentrated in the last 10% of the rotation, you need to replace the log pot with a linear model" Which is correct. The most sensible configuration is volume pot to be log or audio taper and tone to be linear. That's certainly the way all (analogue) amplifier and mixer controls are configured. Apart from that, good article Kurt.
    technoguyx
    Didn't understand crap, but I'm pretty sure that's because I'm not even being taught this stuff at school yet.
    Deified
    Finally, something that applies things from my circuit analysis class that doesn't make me want to shoot myself. Very well written and very informative!
    jthm_guitarist
    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.
    Really though? I thought it was the opposite. Definitely not disputing this, but I've read in many other places that for volume you basically have to use an audio taper because we hear logarithmically (decibels). I read a linear taper would decrease the effectiveness of a 'gradual' volume swell or fade. And I have heard of linear pots being used as tone knobs because the varying capacitance is a linear value.
    jthm_guitarist
    Neat, this is some stuff that every electric guitarist should have a small grasp on, probably too technical for beginners but for someone looking to complete their understanding of the electronics in a guitar this is great.
    JimDawson
    Doesn't resistance have the opposite effect on tone? It just seems to me that if you were adding resistance to an electrical signal it would get clearer and less overdriven. I always thought that when I turn the tone knob up I was decreasing the resistance, giving a stronger signal with a higher resonance peak.
    BIGD2
    Quote: [in the endless pursuit of tone.] The Holy Grail of Guitarist...
    TheDowners
    this is very well written, thank you for this. it's really helpful for the modding ive been planning
    dial-a-death
    abbydaddy03 wrote: Wow I'm smarter than I thought - bragging on myself. I learned a lot of this stuff in basic electronics, so if you're in school hang in there and get you're degree .
    Initial evidence backs your original thoughts. [/grammarnazimode] Good article, I've never really given it any thought before despite having taken electronics at GCSE and further studied it as part of my degree. Might look into a few things now.