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:
| Symbol | Name | Equivalence |
| µ | micro | 1,000,000µF = 1F |
| n | nano | 1,000nF = 1µF |
| p | pico | 1,000pF = 1nF |
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.
Capacitor Types
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:
-
Electrolytic: Usually for large capacitance values, typically 1µf and above. These are usually polarized, meaning there are positive and a negative leads.
-
Film: The most commonly used types, typically in the range of 1nF to 999nF. These are non-polarized and can go in either way.
-
Ceramic: Used for smaller values, typically from 10pF to 999pF. As with Film capacitors, these are non-polarized.
With these basic types in minds, let's learn a bit
more about each.
Voltage Rating
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
is at least twice the operating voltage of your circuit.
is at least twice the operating voltage of your circuit.
So if you are building a circuit that runs of 9
volts, choose capacitors with ratings of at least 16 volts.
Electrolytic Capacitors
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:
 |
Axial: There are leads coming out either end of the cap. Typically 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, which are designed to be assembled/soldered by automated devices. Not so user-friendly to human solderers. |
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:
-
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 low-voltage 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 Capacitors
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.
 |
Axial: There are leads coming out either end of the cap. Typically 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, which are designed to be assembled/soldered by automated devices. Not so user-friendly to human solderers. |
Ceramic Capacitors
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.
-
Replace place 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. -
Replace the .022 tantalum in your 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.
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.
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: 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
| Capacitor Type |
Typical Value Range |
Schematic Symbol |
Benefits | Drawbacks |
| Electrolytic | >= 1µF |  |
Higher capacitance values in smaller packagesReasonable price | Leakage is higher than most
typesService life: Electros typically
don't last near as long as other types.
This is typically why tube amps need to be re-capped
after a number of years. 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. |
| Film | 1nF - 999nF |  |
Low leakage and they last a long time | Larger values are physically large |
| Ceramic | 1pF - 999pF |  |
Inexpensive | Film caps are usually preferred to ceramic caps where audio performance is the key design factor |
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
frequency attenuation
that happens before the signal hits the main effect circuitry.
that happens before the signal hits the main effect circuitry.
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.
Input and Output Capacitor Values from
Various Classic Stompbox Circuits
|
||
| Circuit | Input Cap | Output Cap |
| Ibanez Tube Screamer | .027uf film | 10uf electrolytic |
| ProCo Rat | 22nf film | 1uf electrolytic |
| Boss DS-1 | .047 film | 1uf electrolytic |
| Dallas Rangemaster | .005uf | .01 uf film |
| Dallas Fuzz Face | 2.2uf electrolytic | .01 uf film |
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.
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.
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.
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:
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