Buying Parts

Capacitors

Almost every stompbox design you see is going to have capacitors. They are tremendously useful little beasts and come in a bewildering array of types, sizes, colors and compositions.

To keep it simple, capacitors:

• Can store power, which is useful in building power supplies or power supply filtering circuits

• Can filter alternating current, which is useful in shaping the frequency response of a circuit

Capacitor Types

When you look in the back of a stompbox, you see a lot of capacitors of different types. Here are the major types you’ll find.

Electrolytic
Electrolytic capacitors are visually distinguished by their “can” form factor. They are commonly used in power supply filtering and decoupling applications. They are usually polarized which means that they have a positive side and a negative side. (See "Non-Polarized Electrolytics" below).
Configurations Axial: There are leads coming out either end of the cap. Typcially mounted parallel to the board. Radial: Both leads come out of one end. Typically mounted vertical to the board. Snap-In: For larger electros, not recommended for DIY stuff because they lack the long leads that make it easy to fit them to a board. SMD: Surface mounted device. Small and pretty much impossible to work with unless you are an industrial soldering robot. Pros and Cons Pros Higher capacitance values in smaller packages   Reasonable price Cons Leakage is higher than most types   Service life: Electros typically don't last near as long as other types   Tolerance: Most passive electronic components have a tolerance rating which denotes how close to the part is to the actual printed value. Tolerance for electrolytics is abysmal, in the 20-40% range, but for stompbox applications, this doesn’t matter. Polarity and Orientation The polarity of the electrolytic capacitor is almost always indicated by a printed band. Additionally, the positive lead is usually longer.   Mystery axial polarized: Occasionally, you may run across a polarized axial capacitor that has a printed band, but no arrow pointing to the lead in question. WTF? Yes, they do exist and are very confusing. The trick I’ve learned is to look for the “dimple ring” on the capacitor end. That is the positive end. Choosing Your Electro There are hundreds of different types of electrolytics. Typically, you are going to make your choice based on value in farads (see "Units of Measure" below) and voltage rating. For safety, it is a good idea to get voltage ratings at least twice your circuit’s input voltage. So if you are building a 9v stompbox, 18v-rated electrolytics would be good. If you expect that your stompbox will see higher voltages (some folks like to run 9v pedals with a 12v adaptor for more headroom) bump them up to 12x2 volts. Other than the capacitance rating and voltage rating, you can find lots of very interesting electro colors. Will that change the sound? Nope, but it will make your board look more interesting. Things To Look Out for Polarity: Most electrolytic capacitors are polarized. Hook ‘em up the wrong way and at best, you’ll block the signal passing through. At worst (for higher voltage applications) they’ll explode. Getting Shocked and Possibly Dying: This is not usually a concern for 9-16vDC stompbox applications, but for high-voltage circuits, especially tube amplifiers, big electros can hold a charge for quite a while. Before you open up anything that plugs into the wall, google capacitor discharging and approach with caution. See "Capacitor Fires and Explosions" below. Radial vs. Axial: To maximize the real estate on a PCB, you’ll almost always want to use radial leads. When you order caps, get the radial ones. If you order Axial by mistake, it isn’t hard to bend the leads so as to mount them in a radial, upright configuration. Non-Polarized Electrolytics To further confound you, electrolytics are made in non-polarized versions. These are rarely used. The only place I've seen them is on either side of the first opamp stage in the Tube Screamer.
Film Caps
Film caps are typically available in ranges from picofarads up to 1mfd or so. They are used in decoupling stages, tone controls and sometimes in power supply filtering. Film caps also come in an almost bewildering array of compositions but you'll find polyester film, metallized polyester film, and propylene to be the most commonly available.  Film caps are non-polarized, and as such,lack orientation markings.
Configurations Axial: There are leads coming out either end of the cap. Typcially mounted parallel to the board. Radial: Both leads come out of one end. Typically mounted vertical to the board. Box:  Compact and easy to work with. Box-configuration caps are sexier than non-box capacitors. SMD: Surface mounted device. Small and pretty much impossible to work with unless you are an industrial soldering robot. Pros and Cons Pros Low leakage and they last a long time Cons Larger values are physically large Choosing Your Film Caps As with electros, your determining factors are capacitance and voltage rating. Beyond that, you can experiment with different compositions to determine if you can "hear" a difference between types. Most anecdotal and pseudo-scientific information regarding tone differences within film caps suggests that you won't hear anything. Things To Look Out for Nothing Really: Film caps are pretty easy to use and fairly forgiving as you man-handle them about the board with solder and pliers and such. Voltage Rating: As with electros, to be safe choose film caps that have double the voltage your circuit will see.
Ceramic Caps
Ceramic caps are typically used for lower capacitance jobs. Values are usually in the picofarad to low nanofarad range. They are ugly looking, and that is about as technical as I'll get on the whole ceramic vs. film caps debate. Most folks cannot discern an audible difference between the two types in common stompbox use, so you'll have to try for yourself. A good rule of thumb is to remember that from an electrical engineering standpoint, film capacitors are generally preferred over ceramics in audio path applications. Ceramics are non-polarized and usually supplied in the radial lead configuration.
Tantalums
These were popular in the eighties in stompbox designs like the Ibanez Tube Screamer and various MXR and DOD designs. The primary benefit of tantalums is that they offer a higher range of capacitance values in package that is physically smaller than electrolytics. Like electrolytics, they are polarized so you'll want to get the direction right. Tantalums are *very* susceptible to polarity inversion. In other words, if you hook one up backwards you might as well throw it away--there is a good chance it is cooked.
What is the Sound of One Tantalum Clapping? Do they sound better? Do they sound different? The answer is a definitive yes. No wait, that's a definitive no. There are many opinions about tants, so I really cannot offer you anything definitive on this subject. I can however, share some of the feedback and comments I've heard and read. Some folks like to replace all electrolytic caps in the signal path with tantalums for a smoother sound.   Some folks hear more "grit" and treble with tantalums. Some hear a smoother sound.   Some folks replace the .022 tantalum in their tube screamers with a poly film part for better sound, others claim the original part is integral to the true tube screamer sound.   Some folks claim tantalums are not as reliable as electrolytics, but this may be mostly due to older composition and packaging types uses in decades past. As always, your mileage will vary. But this is one of the most wonderful areas of stompbox design--there are so many variations, we'll probably never get bored. Try the variations yourself until  you find your ideal sound. Interesting Info about Tantalum (as in the element) from Wikipedia: Tantalum is dark, dense, ductile, very hard, easily fabricated, and highly conductive of heat and electricity. The metal is renowned for its resistance to corrosion by acids; in fact, at temperatures below 150 °C tantalum is almost completely immune to attack by the normally aggressive aqua regia. It can be dissolved with hydrofluoric acid or acidic solutions containing the fluoride ion and sulfur trioxide, as well as with a solution of potassium hydroxide. Tantalum's high melting point of 3017 °C (boiling point 5458 °C) is exceeded only by tungsten and rhenium for metals, and carbon.
Other Cap Types, Special Mojo, and Rare/Esoterics
Tropical Fish Caps These are vintage poly film capacitors that use color codes to denote the capacitance value. Very rare nowadays and expensive too. Some builders like to use them in vintage circuits, especially wah pedals.
The Wima Audio Black Box Audio Cap Rare, elusive and really expensive. I don't have much info on these, but some audiophile people swear by them.
Wet Tantalums Most tantalum caps are of the dry-slug variety. This means that they are composed of dry tantalum powder. Wet-slug tantalums on the other hand use gelled sulfuric acid. For more mojo, I wonder if wet-slug tantalums would be worth trying. Although they are typically used for high temperature and voltage applications, one has to wonder...
Audiophile Parts Again, very expensive, with exotic specs and materials with prices to match.

Capacitors on Schematics

Here's what capacitors look like on schematics:

What about Variable Capacitors?

One of the first questions I had when I started building stompboxes was "I have variable resistors (potentiometers) all over the place. Why don't I have variable capacitors?" The answer is that they are limited to a very small capacitance and are quite expensive too. As such, they are not practical for stompbox usage.

Here's a trick to simulate a variable capacitor, especially useful for tone control applications. Attach two different capacitor values to a potentiometer--moving the wiper then sends more or less of the signal to one of the caps thereby changing the frequency response.

Units of Measure

Capacitance is measured in farads. 1 farad is a really huge value, so it is more convenient to parse the farad into the following units of measure: microfarads, nanofarads and picofarads.

1 microfarad = 1000 nanofarads = 100000 picofarads

Electrolytics and tants are usually rated using microfarads, films are usually in nanofarads and picofarads are usually for ceramics.

Although I often appear to be quite smart, I am actually a bit slow. As such,  the concept of base 10 arithmetic is wildly advanced and causes my head to hurt. So I invariably turn to the awesome online and downloadable calculators from http://www.electronics2000.co.uk for doing unit conversions.

Capacitor Fires and Explosions

Like other components, capacitors can explode, burn, and/or stink when they are voltage-abused. Here are some fun fire and explosion pictures. Note that many capacitors were harmed during these experiments.

Some builders have intimated that tantalum capacitors smell the worst when on fire. This is a very useful piece of engineering knowledge to have.

The Application of Capacitors in Stompboxes

So now we are familiar with the basics of capacitors, How can we use them in stompboxes? In a surprisingly large number of ways actually.

Power Supply Filtering

In the context of stompboxes, power supply is low voltage (1.5-18 volts) direct current. The battery is a pretty ideal power source for stompboxes. As long as the battery isn't dying or depleted, it doesn't fluctuate wildly or introduce DC ripple into the equation. So if you are running solely on battery power, you really don't need to worry much about filtering.

Power supplies, like the ubiquitous unregulated black wall warts on the other hand aren't so ideal. If you are sure that your stompbox design will only ever see external voltage as supplied by a nicely regulated and filtered AC adaptor, then you don't need to design in filtering. But in the real world, such assurances are not available. You have to assume that at some point you (or the person you build stompboxes for) will plug in a cheap nasty Szechuan special and noise and nastiness will result.

Of course, it is interesting to note that many stompbox schematics will include no filtering at all, and for the majority of uses, that is actually ok. Filtering really becomes an issue when your circuit is presented with a crappy power supply or fluctuating "crazy Ivan" mains voltage.

A wall wart uses a transformer to step down the mains voltage to a pedal friendly 9-11 volts or so (for a 9v adaptor) and then converts AC into DC using a 4-diode bridge rectifier. The rectifier flips all the waveform swings of the AC voltage but still results in some "ripple" in the DC waveform. Ripple equates to noise in your circuit. The simplest way to get rid of this ripple is to tack a largish-value electrolytic cap from the power supply to ground to smooth things out. For most stompbox designs, this works just great. Let's look at an example.

Here we simply add a 100uf polarized electrolytic from the power supply line to ground to reduce ripple:

Now let's look at a more interesting power supply design, this one from the ProCo Rat distortion pedal. The circuit requires both a 9v and a bias voltage of 4.5 volts. In this schematic, you'll see the standard large value cap to ground (C1). But also there is a 47nf filter cap (C2) to ground. This is added because electrolytic capacitors do not handle higher frequencies well. Since noise in the form of radio frequency interference (RFI) and other high frequency content may be coming in through the power supply, the C2 part ensures it will be dumped to ground.

Finally, there is an additional electrolytic on the bias voltage (C3) which smoothes out the bias supply.

A parting note on caps in power supplies. For amplifier circuits, you'll see big electrolytic cans in the power supply section that you don't see in stompboxes. These act as "resevoirs" of current to handle short spikes in power demands from the amplifier and to smooth out the available pool of current.

The Input and Output Caps

Almost every stompbox design has these two caps. As we talk about these, keep in mind the following schematic of the Electro Harmonix LPB booster (I'm using this one because it has input and output caps and is about as simple a circuit as you can find.)

The input cap, if you haven't already guessed, is attached at or very near the input. The purpose of the input cap is to form a high-pass filter, in conjunction with a resistor (here the R2 part). It also acts to stabilize the rest of the circuit from the input which is usually a guitar, bouzouki, or another pedal. The key point here is:

The value of the input cap directly controls any frequency attenuation
that happens before the signal hits the main effect circuitry.

Let's say you are building a treble-booster--you would want to attenuate any low frequency content before it hit the amplifier circuit. So you would put in a lower value input cap to accomplish this. The Dallas Rangemaster, perhaps the most famous of all treble boosters, has an incredibly small .005 uf cap. Alternatively, let's say you want the majority of the useful frequency content to be passed through--in this case you would use a larger value cap, say 100nf-1uf. A rule of thumb is that a 1uf capacitor, input or output, will allow all guitar frequencies to pass through.

Now on to the output cap. In our schematic above, that's the C2 value. The output cap serves two purposes. First, like the input cap, it can serve as part of an RC network to attenuate or pass certain frequencies. If you want the full frequency range, a value from 100nf to 1uf can be used. The output cap also serves to remove any direct current from the signal. Remember that our stompbox designs almost all run on direct current--we want to be sure none if it escapes from the output jack, so an electrolytic cap will do the job nicely.

Filters

Capacitors are the one of the key ingredients in filter circuits. Like the input and output caps we talked about above, filters typically rely on capacitors to form Resistor-Capacitor (RC) networks. Here are some interesting and simple ways to use capacitors to shape the frequency response in a stompbox.

Variable Low-Pass Filter

Here we use a small value cap (500pf up to 50nf is a good range for experimentation) wired in the signal path of a circuit. If the pot's wiper is at the full open position (no resistance) the signal will bypass the cap and go straight through. But as the resistance is increased, more signal will pass through the cap which will attenuate higher frequencies.

Fake Variable Capacitor

You'll recall that true variable capacitors are not practical for stompbox use. But you can easily use a potentiometer and two different value caps to approximate the same thing: <pending>

A Thousand and One Thanks

None of this would have made sense to me, nor would most of my questions have ever been answered with the the dilligent and patient help from the folks at diystompboxes.com and all the emails from readers. Special thanks to Jack Orman for information on the Rat power supply, R.G. Keen for his vast collection of writings and answers, and Barcode80, MartyMart, Idlechatterbox, edster, Rmanen , calculating_infinity, pjwhite , rockgardenlove , Fret Wire, and blanik  for answers, ideas and feedback.