Tone Controls 101

Getting a Handle on Guitar Tone Controls

In Progress

We've all been there before. We see two knobs on our gorgeous guitar and try them out. The first one is the volume knob, as natural as can be. It makes things quieter or louder. Then there is the tone knob. It does...something. It can make things sound muddy at one end or like it's doing nothing at all at the other (spoiler, that last part is true!). So what exactly is going on with the tone control?

The traditional tone control in a guitar is a simple low-pass filter. A low pass filter does exactly as the name implies: it passes low frequencies while attenuating high frequencies. This is accomplished by using a resistor and capacitor wired in the diagram below. However, to make the control more useful, the resistor is replaced by a potentiometer wired as a variable, shown in the second diagram.

So what does this all mean? Well, the signal is going through the resistor, which resists all frequencies equally, but then the signal can go to ground through the capacitor. The capacitor stores charge in it, which means it keeps DC voltage in it. However, as the frequency goes up, it presents an impedance that decreases. Since the other side of the cap goes to ground, the higher frequencies go to ground, only allowing the lower frequencies through. Now, remember that resistor we all but glossed over? There is an interaction between the resistor and the capacitor that determines what frequency the capacitor starts getting rid of (or shunting) voltage. This frequency is called the cutoff frequency (or corner frequency) and is represented in this equation:

You will notice that if we make either the resistance or capacitor larger, the cutoff frequency will go down. Look at our tone control circuit and notice how, as you turn the potentiometer, you are increasing the frequency. This lowers the point where we start shunting signal, which makes the tone warmer and, yes, possibly even muddy.

Now that we understand the basic physics of what is going on, let's talk about what this means in guitars. Common questions abound about what capacitor or tone pot value should be used, or whether they should put in switchable caps and the like. Let's talk about some of these questions and look at some data surrounding these things.

One of the first things that people notice when they start digging in to tone controls is that the capacitor value differs between single coil and humbucker pickups. Typically, a single coil will have a higher capacitor value and a humbucker will have a lower value, usually .047 uF and .022 uF, respectively. The single coil's higher value causes the corner frequency to be lower for any given pot position. This, in turn, rolls off more high frequency content, which can be useful because single coils can be very bright. A humbucker, on the other hand, is naturally warmer, so retaining more of the high frequency content is a wise idea, thus the lower value capacitor.

Tone pots are also commonly debated. Note that the tone pot value and cap value only matter when the tone is not all the way up. With the tone control all the way up, the capacitor is essentially removed from the system and the signal remains unchanged. However, once you start rolling down the tone control, the pot value will affect the corner frequency just like the capacitor does. The plot below shows how the corner frequency changes through a potentiometers rotation. The resistance is expressed as a percentage of the pot value, so it is agnostic to whether the pot is linear or log (which only governs how quickly a turn gets you to a set resistance). Notice how at low resistance, the corner frequency is very high. This means that almost the entirety of the guitar signal will be passed.

Once a pot value has been chosen, the rotation of the potentiometer creates a filter that rolls off at 6 dB per octave. This is known as a single-pole filter. In layman's terms, a filter pole is where the response of the filter goes to infinity. We don't see that in our plots, because we only deal with magnitude, but in a plot that involves the complex phase-magnitude space (or S-plane, as it is commonly called), there will be a single frequency where the complex response goes to infinity. A two-pole filter as two such locations and tends to roll off at 12 dB per octave. A three-pole rolls off at 18 dB per octave...you get the point.

The plot below shows a 250k potentiometer with a .047 uF capacitor: the traditional single coil setup. Notice how at 1% of the pot's resistance (almost all the way turned up), the signal doesn't get touched until very high in frequency. As we roll it down, it starts to attenuate lower and lower frequencies, resulting in less high frequency energy, which makes it sound warmer (yes, or muddier).

So we've seen what happens if we change the resistance in the filter using the pot. What if we change the capacitor value? The plot below shows us our 50% resistance value for a variety of capacitor values. Notice how some of the curves look similar to what we had just by changing the pot resistance? It's true, the pot and cap value work together to determine the response, just as our equation above suggests.

I get it, you're saying to yourself "Plots are cool, but you can't hear plots." Well, this is true. The ToneFiend blog over at Seymour Duncan has a great page with some sound clips of what different tone caps sound like. You can find that here.

So in the end, does the value of a cap or pot make much difference? It can, but those differences are in the extreme low end. If you never roll your tone control all the way off, changing the cap value isn't going to matter too much, unless there is that one little sweet spot you wish you could dial in, but you just can't quite get there. If that isn't your issue and you don't want your guitar to sound muddier, the cap value isn't going to matter much. So don't stress too much about it, pick something reasonable and rock out!

Oh, as a quick note, standard caps are a +/-20% tolerance, so two caps marked the same may be as much as 40% off from one another!