Photo by Andy Ellis
Like it or not, there’s a deeply symbiotic relationship between your guitar’s pickups and its controls when you’re using passive pickups. That’s true even when your knobs are turned all the way up. It pays to understand how these components shape (or rather, complete) the sound of your pickups. Remember, the artisans who painstakingly designed your favorite passive pickups did so with pots, caps, and some guitar cable as part of the equation.
There’s a good chance your guitars still contain their original potentiometers and capacitors, and they may be perfect for you and your music. On the other hand, a bit of experimentation just may transform your guitars in ways that better suit your style and tastes. It’s not an expensive process, either—a good potentiometer costs about as much as a set of strings.
But experimenting blindly probably isn’t the best approach. This article explains how the parts in question affect your tone, and offers suggestions to try in pursuit of your ideal sound.
What We’re Not Discussing
We aren’t going to parse over different capacitor materials—only capacitor values. The internet is overrun with opinions on capacitor types, x-rays exposing tiny caps hidden inside bumblebee shells, and rants insisting that it’s impossible to play a convincing rendition of “Red House” without NOS caps dipped in quinoa oil and bong resin. (Don’t listen to that guy.) For demo purposes, I’ve used Russian paper-in-oil capacitors. If you prefer a different material, use caps of that type. But the comments here about relative capacitor values apply, regardless of cap material.
We’re also not discussing potentiometer types or tapers. (“Taper” refers not to a pot’s overall value, but the way the value is distributed across the shaft’s sweep.) Almost all guitar pots are audio taper (as opposed to linear or reverse-logarithmic), though some companies customize the sweep. Our focus here is how pots affect tone, even when they’re on 10.
The Fog of Tone
Much like a foggy day reduces visibility, or grease on the lens blurs your photo, resistance and capacitance filter your pickups. Actually, they go beyond that: In a way, they reach backward and shape the way the pickups resonate. Sometimes it’s in a good way, and sometimes not.
There are a tremendous number of variables that make your pickups sound the way they do—magnets, alloys, wire type and amount, and more. But there’s one universal thing that’s truly and directly impacted by your pots and caps: the pickup’s resonant peak.
Every passive pickup has an inherent resonant peak. It’s usually the pickup’s strongest frequency, since that’s where the coils resonate the most. In general, a single-coil’s peak is higher (more trebly) than a humbucker’s. When comparing otherwise equal pickups, a weaker, more underwound pickup’s peak is higher than a hotter, more overwound version. (As a pickup maker, I realize the vast chasm of detail I’m omitting for now, so bear with me, pickup superfans.)
Although a resonance sweep doesn’t truly represent a pickup’s overall frequency response, it’s a measurable response curve, and it looks something like Fig. 1.
The slope from the bass is usually gradual, and then it peaks sharply, falling off fairly quickly after the peak. This is why a relatively low resonant peak generally coincides with a great deal of high treble loss. That age-old pickup maker’s adage—“I can make it a little hotter, but you’ll lose clarity”—is true at face value. At first glance, the curve looks a little like a wah-pedal sweep, with a big boost at the peak frequency. (A pickup’s resonant peak is not as pronounced as on a wah, but sometimes it helps to think of it like that.)Take a Peak
If you take nothing else away from this article, know that the resonant peak frequency is generally the most identifiable component of your pickup’s tone. It’s what makes your pickup sound like your pickup, and not the next hotter or weaker model in the catalog. But your pickup probably wasn’t designed to be heard straight into the amp without any pots in line. The peak in the image above is very sharp. Here’s what that same pickup looks like with potentiometers attached to it (Fig. 2).
Notice how the resonant peak is flattened. It hasn’t gotten bassier—you’ve just decreased the intensity of that resonant peak, which causes the sound to be a bit more homogenized. As we continue to increase the loading, passive pickups begin to sound more and more like each other as the characteristics that differentiate them are decreased.
Since most passive pickups resonate somewhere in the high midrange and treble frequencies, we’re often taught that lower-value pots “warm up” the sound and higher values brighten it, though not in the same way as a treble knob on an amp or EQ. (So don’t listen to the troublemaker who says “just turn the treble knob up/down on your amp.”)
How Capacitors Contribute
Unlike potentiometers, tone capacitors lower the frequency of the pickup’s resonant peak, as opposed to softening the existing peak frequency. It’s easy to explain tone capacitors as simply “rolling off treble.” An electrical engineer would say the capacitors combine with the resistance of the potentiometer to form a low-pass filter (that is, a filter that cuts treble).
Since the capacitors interact with the pickup’s coils, they lower the resonant peak frequency as you roll back the tone knob. Have you ever lowered your tone control to zero and felt like your guitar just got louder? How can that be if you’re taking frequencies away? Because the resonant peak frequency just descended into a more audible—and more energy-producing—range.The higher the capacitor value, the lower the frequency. A larger-value cap creates a more muffled, bassy sound when rolled all the way off (Fig. 3). A smaller-value cap doesn’t lower the peak as much, and therefore leaves more treble intact. As you roll the tone knob down, your pickup “sees” more resistance and a gradual introduction of capacitance. Part of what you hear is the flattening of the pickup’s peak, much like when you roll the volume control down. The more you roll back the tone pot, the more you hear the full effect of the capacitance running to ground.