"Mojo" Parts in Guitars, Effects, and Amplifiers Part 2: Capacitors

August 5, 2010

There's a lot of information, good and bad, on the internet about capacitors and their impact on guitar tone. People pay over $50 for paper in oil or "bumblebee" capacitors for their guitars, dismiss "box capacitors" in amplifiers, and in general pay through the nose for "the ultimate tone." At the other end of the spectrum there are the "who cares, just plug in" guys.

So, who's right? One answer is "neither" but the more kind answer is "it depends."

First, let's discuss the kinds of capacitors most commonly found in guitars, effects, and amplifiers.

• Electrolytics – these capacitors are usually used as filter capacitors in amplifiers, or anywhere a value larger than 1µf is required, as they are relatively small for the values and voltages needed. They are usually polarized, though we won't get into that in this article. Drawbacks of electrolytics, as they apply to amplifiers, effects, and guitars, include higher leakage current, a short lifespan (determined by the temperatures the capacitor is exposed to - typically 15 years in an amplifier, usually longer in an effect – electrolytics are almost never used in guitars, save perhaps in some active preamplifiers), and that many players find them "stiff" sounding in an audio signal path (we agree to a certain extent, though it depends on the circuit). Electrolytics are a necessary evil, if you will, because to avoid them entirely requires a great increase in space and cost – amplifiers and effects would have to be huge and would significantly more if they were eliminated entirely.
• Ceramics – these disc shaped capacitors are very common in all electronics, including amplifiers, effects, and guitars. They work well for their intended uses, and for some values in the picofarad range they are the only practical option available. That said, not all ceramic capacitors are equal. Many styles of ceramic capacitors, including those most commonly used in electronics, are severely prone to microphonics, and have a relatively short life span. For that reason, we use multi-layered ceramic capacitors in effects and silver mica ceramic capacitors in amplifiers. We do not use ceramics at all in values where a film cap is available, which brings us to...
• Films – we're lumping in polyester, polystyrene, and polypropylene in together for simplicity (which is guaranteed to give someone fits, but it's accurate enough for the purposes of this article). These capacitors give excellent performance and long life, with the drawback that they are only available in relatively small values but not in tiny ones (about 1µf to .001µf, commonly). Many people think they sound better than electrolytics, and that they sound different from ceramics (some prefer ceramics, others prefer film). As they last longer than either electrolytics or ceramics, tolerate heat well, and are not prone to microphonics, we use them whenever possible, even to the point of running them in parallel to avoid an electrolytic (available space and cost limitations apply).

Capacitors and Voltage

Now, in terms of how different capacitors sound, assuming no defective capacitors (microphonics, etc) the bad news is there is no discernible difference when the voltage is low, say less than 100V. It takes a lot of voltage for the dialectrics in a cap to do anything audible. We do find this in an amplifier circuit, where voltages commonly run from 100-500V, and this can be a big part of the response of an amplifier. But in an effect or a guitar, there is insufficient voltage present to make the type of capacitor play a role in the tone.

Capacitors in Guitars

So are the guitar snobs imagining things? Are they deaf? Are they crazy?

Well, some of them are crazy, and lots of guitarists are deaf. But no, the differences they hear are real. But the tonal differences are due not to the kind of capacitor (film, ceramic, oil-in-unobtanium, whatever), but rather to wide variation in actual capacitance.

Capacitors usually have a +/-5-20% tolerance. This means that a capacitor labeled .022µf (the most common value used for a guitar tone circuit) may measure anywhere between .0176µf and .0264µf and still be considered a ".022µf" cap. And as capacitors drift with age that range gets even wider. There are Sprague "bumblebees" (highly sought after for Les Pauls) that are marked as .022µf but actually measure as high as .04µf.

Most guitarists don't have meters that can measure capacitance, so they understandably just go by what's printed on the capacitor.

So if you have a Les Paul, and compare the stock ceramic ".022µf" cap (that actually reads .021uf) and put in an expensive vintage ".022uf" cap (that actually reads .028uf), yes, yes, yes, you will hear a difference. Will you prefer it? Odds are, after you paid big bucks for it, yes, you'll prefer it – that's just human nature. But if you were to try 10 different vintage capacitors and 10 different modern capacitors, you'd probably express a preference for a particular value of cap, and would be equally pleased with that value in a modern or old cap.

Compounding this confusing issue in a guitar tone circuit is the fact that every potentiometer has a different resistance value and a different taper – 500K Audio Taper pots are not all the same. As many guitarists change out potentiometers at the same time they change out capacitors, all of a sudden "it sounds so much better on 7 than it did before." Etc.

We recommend film capacitors for guitar tone circuits, as they sound great and are not prone to microphonics. Depending on the circuit layout, we might recommend a Panasonic "chiclet" cap that would go straight from a pot lug to the ground on the pot casing or a Sozo, Mallory, or "Orange Drop" cap rated from 200-400V for circuits where the cap must support its own weight, as in a Les Paul. The 200-400V rating is not because of the voltage in the circuit, but because capacitors with these values have thick enough leads to bear their own weight and to provide a secure mechanical connection before soldering. We use axial Sozos and Mallory 150s in guitars we work on.

Capacitors in Effects

While the voltages in effects are literally thousands of times higher than the voltages in passive guitar circuits, they are still too low for capacitors to affect the tone. The same arguments above apply to effects as well.

We avoid electrolytics in pedals as much as possible, so as to prevent having to replace a bunch of them in 15-20 years. We only use multi-layered ceramics, so as to prevent microphonics (terrible in an overdrive or distortion pedal).

Capacitors in Amplifiers

Now we have some serious voltages to play with, so now we can hear those capacitors, right?

Well, yes, but not always for the reasons most people think. And some popular beliefs out there are just kind of, well, wrong...

First, let's discuss the three main functions capacitors play in an amplifier:

• Filter capacitors – technically, some of these are reservoir capacitors and others are pure filter capacitors, but for the purposes of this article, these capacitors smooth the DC power supply to the amplifier, lowering noise. The mains filter capacitors (reservoir capacitors) also play an important role in the function of the output transformer, but that's outside the scope of this article. These capacitors are out of the signal path.
• Coupling capacitors- these capacitors both block DC from entering the subsequent stage of the circuit and limit which frequencies are passed to the subsequent stage. These capacitors have significant voltage across them and are in the audio signal path. These capacitors are typically film or ceramic, depending on the value. Again, we use silver micas to avoid microphonics, and in an amplifier, because they handle heat better than standard ceramics.
• Tone capacitors – used in resistor-capacitor filter (RF filter) circuits, found in bypass capacitors on the cathodes of preamplifier tubes (setting the frequency above which gain is applied), the tone stack (some of these capacitors are typically also serving as coupling capacitors), presence controls, and the "Cut" circuit found on the AC30 and Matchless and similar amplifiers.

Some misconceptions about capacitors in amplifiers:

• Filter capacitors - people often look down on modern amplifiers with small radial electrolytics, equating large multisection "can" capacitors, as found in Marshalls, with quality. Multisection capacitors are actually kind of horrible, prone to leakage, noise, and poor performance at high frequencies. Somewhere between when we walked on the Moon and the first Apple computer, capacitor design was immensely improved. Modern radial capacitors are smaller, quieter, and more accurate than the big multi-section can capacitors. Granted, there are some bad ones on the market, but if you see radial capacitors in an amp, that is no bad thing. We use Nichicon and Panasonic capacitors in our original amplifier designs, and happily use them in the modern AC30s we modify and service.
• Coupling and tone capacitors – again, people often look down on modern amplifiers with "box capacitors" (radial film capacitors). Yes, there are some bad sounding box capacitors out there, just as there are bad sounding axial film capacitors. It's not the layout but the quality. Coupling capacitors have sufficient voltage on them that the difference in material can be heard, as can differences in quality. In amplifiers we prefer to use the axial Sozos and Mallorys to the radial Spragues and Wimas due both to differences in sound and to ease of future service/replacement. There are others who quite successfully make great sounding amplifiers using Orange Drops, red box Wima films, and other high quality capacitors.

Closing Thought

Tone, from your pick all the way to your speakers, is a cumulative process. There is no "magic capacitor" (or anything else) you can put in your guitar or your amp that will make you sound better by itself. Rather it's a whole lot of little subtle things that either add up to preserve and enhance your tone, or that serve as tonal obstacles. We have to sweat the details here, with an eye on the big picture, but don't let any one aspect of tone build up in your mind. Our advice - don't spend $50 on a capacitor for the tone control in your guitar - that money is much better spent on a plastic pickguard "exactly like they made them in the late summer of 1958..."