All About Capacitors
In the realm of passive components, capacitors are second only to resistors in ubiquity. They are everywhere, in almost every electronic device you will ever come across. So it's no surprise that capacitors are an integral part of audio circuits in general, and guitar effects specifically.
What makes capacitors so important? Well, they can be used to perform some very important functions:
Units of Measure
As with all passive components, you need to have a basic understanding of units of measure when working with capacitors. Capacitance is measured in Farads, named after English physicist Michael Faraday. A value of 1 Farad is actually quite high, so we use sub measures as follows:
If you are like me, the concept of base 10 arithmetic is wildly advanced and causes your head to hurt. So I invariably turn to the awesome online and downloadable calculators from http://www.electronics2000.co.uk for doing unit conversions.
Although capacitors come in an almost bewildering array of types and sizes, no need to worry. The majority of capacitors in guitar effect designs fall into three types:
With these basic types in minds, let's learn a bit more about each.
One of the most common questions about choosing capacitors is voltage rating. Different capacitors are rated for different voltage ranges. The best rule of thumb is this:
Choose a capacitor with a voltage rating that
So if you are building a circuit that runs of 9 volts, choose capacitors with ratings of at least 16 volts.
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).
Electrolytic capacitors come in several physical configurations:
The polarity of the
electrolytic capacitor is almost always indicated
by a printed band. Additionally, the positive lead
is usually longer.
When working with electrolytic capacitors, here are a few things to keep in mind:
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.
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.
The Great Tantalum Debate
Tantalum capacitors 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.
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.
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.
Capacitors on the Fringe
There are various esoteric or rare capacitor types that pop up from time to time.
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.
Wima Audio Black Box Audio Cap: Rare, elusive and really
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: In the world of DIY audiophile building, a great emphasis is often placed on capacitor performance. As a result, there are a number of manufacturers of high-end (and expensive) capacitors. I'll leave the subjective vs. objective argument to the reader. But it does make sense to point out that guitar effects, especially in stompbox format, are not designed to be audiophile devices.
Which Type Should I Choose?
As with all component types, there are pros and cons for each type. In general, the choice of capacitor type will be made for you, either by the author of the schematic you are using, or by the simple factor of capacitance value. In other words, the schematic will specify electrolytic or film by the symbol used. That makes the choice easy.
But what about when a specific type is not specified, only the value is shown? In general, you look at the value specified, and choose the type appropriate for that value. Other factors may influence your choice of capacitor type, especially in audio circuits. So I've include benefits and drawbacks of each type
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. Here are two approaches:
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 "reservoirs" 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 (C1), 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
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.
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.
Another great example of the effect of cap values on frequency response is Tim Escobedo's LoFoMoFo. Look at the very small values for the input, output and shunting caps (R1, R3 and R2, respectively). These parts conspire to remove pretty much all the bass content of the input signal:
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.
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.
Another way to implement a low pass filter is to used a potentiometer in series with a capacitor to ground. This type of configuration can be spliced into the signal path of a circuit, but it should be noted that there is some signal loss. This is the case with all such passive circuits. Usually, there is a gain stage after a passive tone control to boost the signal lost in the passive section. For example, look at the last transistor stage in the Big Muff Pi circuit: it's function is to make up for the signal loss in the preceding tone control.
Smoothing Diode Clipping
You can add a small-value capacitor in parallel with a diode clipping arrangement to smooth out the high-end of the clipping. This is a somewhat interesting area for experimentation.
Capacitors for Timing
Another common use for capacitors is to control the time interval of a circuit. For example, in a low-frequency oscillator, a capacitor is used in conjunction with a potentiometer to set the frequency. Our first example is a simple LFO based on the 40106 Hex inverting Schmitt trigger. The combination of C1 and VR1 set the frequency:
Next, we have a classic 555 basic monostable oscillator. In this configuration, the frequency is set by a combination of R1 and C1.
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.
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