Let’s face it, this is the internet and there are plenty of guides out there as to what pots (potentiometers) are, and why one is better than the other, etc.  The problem is, these are all spread out and it leaves you searching all over the place for the information you wanted.  This in-depth blog post aims to give you all the information you’ll need to make a decision in one place.

For those who don't want to read this massive post, there's a tl;dr; (too long, didn't read) at the bottom which summarize it all. For everyone else...

Firstly, what is a potentiometer?  A potentiometer is a variable resistor, which actually acts as a voltage divider.  Now for those of you who aren’t familiar with circuit design, what that means is that by adjusting the resistance, the output voltage changes.  Voltage is related to current, and when the output voltage changes, portion of the input current will be sent to the ground (electrical ground) and the rest to the output.  That’s why you see articles that talk about draining off and such, they are referring to the portion of current that gets directed to the ground.

 Fender 500K Split Shaft Potentiometer

 

This is a Fender 500K split shaft potentiometer.

The way this works is that there is a resistive material set up with a “wiper” that touches it. One end of the resistive material leads to the ground, and the other to the output.   When you turn the knob on your bass or guitar, the wiper moves, and the resistance value for the output changes, changing the output voltage.   

One of the laws of electrical engineering known as Kirchoff’s first law, states that the sum of currents flowing into a node, must equal the sum of the currents flowing out of that node.  Without going into too much detail, what that means is that if the node is located between the two resistors, then the sum of the current going to each resistor, must equal the input current from the pickups.

Another law relating to electricity is Ohm’s law (and consequently, the unit for resistance is the Ω (Ohm)) which states that the Voltage must equal to the current multiplied by the resistance, simply put Ohm's Law .  I mentioned earlier that Kirchoff’s first law is applied to currents, which are denoted by I in the above equation, and we can therefore imply that the R is for resistance, and the V for voltage, and that they are all related.  The other rating that potentiometers use is that of Power, or P.  Power is equal to the current (I) squared multiplied by the resistance (R), that is to say Power as related to Current and Resistance.  Power is rated in Watts (W).  By doing a little algebra on these equations, we can see that the Voltage is related to the Power by the formula   .  In general, Guitar & Bass pots have a 0.5W rating.

So we briefly covered what a potentiometer is and how it works, and now it’s time to get a little more into the types of potentiometers.  There are 3 types that are generally used in guitars/basses; linear, logarithmic (also known as Audio), and inverse (or anti) logarithmic.  We’ll look at linear and logarithmic (inverse logarithmic is just the opposite of logarithmic, and from what I read is used for left-handed guitars but I’ve never personally checked)

As the name implies, a linear potentiometer will increase resistance linearly (or in a straight line) as the knob is turned.  This means that if the knob were to go from 1-10, each 10th of a turn would go up (or down) one.  This may seem like a simple approach, and it is in a lot of ways, but it also has drawbacks when it comes to music applications.

The largest problem with linear potentiometers in a guitar or bass is that the human ear doesn’t hear sound in a linear fashion.  The reason we measure noise in Decibels (dB) is that the human ear actually hears in a logarithmic fashion.  Now, this doesn’t mean that you CAN’T use linear potentiometers to control your volume, it just means that when you turn your volume knob, a half turn will not increase/decrease the sound you hear by half. 

So what would happen if you use a linear pot for your instrument? Well, an example for that would be if we used a 100Ω resistor (this is just an example, don’t use a 100Ω on your instrument), and the knob were to go from zero to ten, then for each step you turned the knob from no volume to full volume, the resistance would decrease by 10Ω.  So from 0->1, it would drop from 100Ω to 90Ω, from 1-2> it would go from 90Ω to 80Ω, etc.  In this case, it would be varying constantly by 10Ω for every step, but the volume that the human ear would hear would not seem to be increasing at the same rate, at the beginning it would seem like massive jumps, but by the end it would be hard to tell that the volume changed.

Now onto logarithmic potentiometers!

A logarithmic potentiometer (also called an audio potentiometer) varies the resistance in a quasi-logarithmic pattern. A logarithm is just the inverse of an exponent (so logarithmic is just the inverse of exponential).  There’s an article at http://sound.westhost.com/pots.htm which explains what “commercial log” is, but it basically means that cheaper logarithmic potentiometers won’t match a log curve perfectly, but rather use two or three linear segments that will be close enough that most people won’t notice.  You may, and so we’d recommend a higher end potentiometer if that’s something you want to avoid.

The unit of Decibels (dB) we mentioned earlier is actually the unit for logarithmic functions.  It represents a ratio of two measurements, in this case; the output voltage versus the input voltage. The formula for decibels in this case would look like

Logarithmic Output in Decibels

Where P is power, V is voltage, and the o and i stand for Output and input respectively.  For the third step there, there is a rule when you have the logarithm of a ratio with an exponent, then the inner exponent is multiplied to the front of the Log, so in this case, 10->10*2-> 20.

All that to say, that when a decibel goes up/down by 10 dB, the effect it has on the volume we perceive is double/half.

So why is it good to use logarithmic potentiometers?  Well, we know that each time you turn the knob a fraction of the way, the resistance is either increased or decreased and that when the resistance decreases, the output gain actually increases (less resistance -> increased gain), and vice versa. Therefore, we can see that a having the resistance remain high as you move up from zero (no volume) and gradually decreasing at the beginning before spiking gives us the benefit that at low levels of volume, we have more control (more wiggle room if you would).  We covered that it takes an increase of 10dB to double the volume and so we can look at that 10db as a 10/1 decrease in the resistance (or a 10/1 increase in the output) which requires a higher ratio of resistance changed per fraction of the knob turn (since the amount the knob can turn is fixed).

Hopefully that made it a little easier to understand why we tend to use logarithmic potentiometers for our instruments.

Wondering what the capacitor is for in all this? That depends on if you’re talking about the Tone or the Volume potentiometer. 

First, the volume; the capacitor in this case is what smoothens out the signal as you adjust the knob.  Essentially, as the resistance drops, the signal would start to lose more and more currentThere are a few wiring configurations that can be used when setting this up, but in general it is what is referred to as the “Treble Bleed Mod”.  The basic goal of this mod is so that when you’re adjusting your volume, you don’t start to lose all your high tones, instead the capacitor acts as an extra high pass filter, and allows those higher tones to just bypass the volume pot all together.  This is accomplished by wiring the capacitor (and possibly, a resistor) between the live input (non-ground) and the output of the potentiometer.  More information, complete with images, can be found here: http://www.ratcliffe.co.za/articles/volumepot2.shtml

 

For the Tone knob, the capacitor is used to form what is known as a low pass filter.  A low pass filter does exactly what it sounds like, it allows lower frequencies to pass through, while trimming the higher frequencies.  The formula for calculating a low pass filter is Cutoff Frequency - Low Pass Filter where Cutoff Frequency Symbol is the cut off frequency, R is the resistance, and C is the capacitance (in farads).  We know that our potentiometer will vary the resistance in the equation, therefore, as we adjust our tone knob, the resistance will decrease from maximum (at closed), to almost zero at fully open, and the cutoff frequency (or the frequency after which higher frequencies will not pass) will start to increase, allowing more highs to pass through to the output.

Now that we’ve covered all the ground work, all that’s left is for you to try it out yourself and figure out what sound is right for you!

tl;dr;

A potentiometer is a variable resistor which allows you to adjust the resistance either linearly or logarithmically.

Logarithmic potentiometers are more inline with human hearing, and as such tend to be better for volume control.

A higher value resistance doesn't mean better, it simply is used to adjust the curve of your frequency response, generally higher resistance= more high tones, which might be too high for your style.

A capacitor is used either to adjust the cutoff frequency (Tone pot), or to allow higher frequencies to pass a volume pot at low volume.

There was a guy named Ohm who discovered a law of electricity.

 

This article was put together using a variety of sources, as well as my own knowledge of electronics (degree in Computer Engineering with specialization in Hardware, although for the last few years I’ve been doing strictly software).

 

Alan Ratcliffe: http://www.ratcliffe.co.za/articles/volumepot2.shtml

John Spina: http://www.projectguitar.com/tut/potm.htm (shows you some neat tricks for customizing your pot’s values outside the standard ones)

Rod Elliott: http://sound.westhost.com/pots.htm

What are Log Taper Potentiometers: http://www.learningaboutelectronics.com/Articles/What-are-log-taper-potentiometers

Confusion in Power Gain and Voltage Gain in decibels: https://www.physicsforums.com/threads/confusion-in-power-gain-and-voltage-gain-in-decibels.637346/

Brian Rosengrant & Bill Kostik: http://www.sensorsmag.com/sensors/position-presence-proximity/potentiometers-a-proven-position-sensing-solution-every-engi-9038