CAPACITORS: Capacitor Basics for Pedal Builders

Capacitors can be pretty complex things, given that they are passive components.  There are plenty of in-depth discussions and explanations out there, but this is not one of them.  This is intended as a primer for anyone just starting out building, who wants to know a little more about capacitors, but doesn't like looking at equations and doesn't have an interest in physics.

This handy video on youtube also covers capacitors really well in a simple way, but not so much in the context of the guitar.

Capacitors are also one of the most argued-about components in existence when it comes to anything audio-related, or at least in some circles.  Capacitor X should never be used, Capacitor Y will change your life etc.

SO WHAT ARE CAPACITORS ANYWAY?

The capacitors that you will find in guitar effects are made from two conductors, separated by some form of insulator (called a dielectric).  

The conductor can be a thin foil or an electrolyte (guess where the name for electrolytic caps comes from).  

The insulator can be almost anything - including air....  Common insulators include ceramic, glass, plastics (polypropylene, polystyrene, polyester), paper and mica - which also helps explain the names given to different types of capacitors.

Note that the two conductors are separated - there is no physical connection between the two.  Capacitors work by building up an electric field and transfer or store electrical energy via this field. 

Unlike their friend, the resistor, capacitors are more complex in how they work.  Where a resistor has a set resistance, the resistance of a capacitor, which is called inductance, varies greatly depending on frequency - they pass very high frequencies with ease and block very low frequencies.  

Their ability to block low frequencies is why they are used as coupling capacitors.  DC voltage is basically a flat line (hopefully), which is 0Hz.  You can't go any lower than that.   Transformers are an alternative to coupling capacitors, but they are big, heavy and expensive by comparison.

TYPES OF CAPACITORS

There are quite a few different types of capacitors available, with many opinions on the internet regarding which ones sound the best - or that they all sound the same. 

These are some of the most common types of capacitors and where they might be found. 

Electrolytic Capacitors

• power supplies for decoupling and filtering DC ripple

• large values for coupling capacitors (usually once values are over 1 or 2uF)

Ceramic Capacitors

• filtering high-frequency DC ripple

• small value capacitors, usually less than 1nF

Film Capacitors (any type)

• coupling capacitors, usually between 1nF and 1 or 2uF

• filters for AC signals

Note:  Multi-layer ceramic capacitors range from very low to quite high capacitance and are often used for a range of different purposes.

CAPACITOR POLARITY

Some capacitors have polarity - electrolytic and tantalum in particular.  There are electrolytics that are non-polar, which are marked 'NP'

Ignoring polarity on a power rail will result in a small pop, followed by a dead capacitor and a short circuit.

A common question on social media and forums is about replacing a polarised electrolytic caps with a different type of capacitor - the answer is yes you can.  Pretty much the only reason electrolytics are used, is that they are smaller and cheaper than other types for the capacitance required.  A 100uf film capacitor would be gigantic and expensive, so electrolytics are used.

CAPACITORS IN GUITAR EFFECTS CIRCUITS

Typically capacitors will fulfil one of these roles, and sometimes a couple at once.

Coupling Capacitors

Coupling capacitors join different sections of circuits while blocking DC voltage from the previous section.  They carry AC signal - so they are usually in the signal path.

• AC signal passes

• DC voltage blocked

Decoupling Capacitors 

Decoupling capacitors are used in DC power supplies and are not in the signal path.  Decoupling capacitors may also be distributed around the circuit - different sections may have their own decoupling capacitors.

• Reduce DC ripple

• Remove AC noise by providing a path to ground

• Isolate sections of the circuit in terms of DC power supply

• Supply current quickly without effecting other areas of a circuit

A lot of vintage effects do not have decoupling or any kind of filtering, or very minimal amounts if it is present.  They were designed to run on batteries, so this was never really a consideration.  When building vintage circuits, unless you only plan on only using batteries, add modern filtering and polarity protection.  

High and Low Pass Filters

Capacitors form high or low-pass filters when paired with a resistor or an induction.  Most commonly with resistors in guitar effects pedals.

• Power filtering to reduce DC ripple (a decoupling capacitor can also be part of a low pass filter)

• Equalisation provided by filters (a coupling capacitor can also form a high pass filter)

Coupling capacitors often form high pass filters in circuits, especially if they are placed before pots or bias resistors - basically, any resistance going to ground after the coupling capacitor will form a filter.

In the example below, which just happens to be a Big Muff, the input capacitor (C6) is a coupling capacitor, blocking DC from entering from another source, or leaking out from the DC bias voltage applied to the base of the transistor (Q1).  One of the resistors in the bias network is after the coupling capacitor and goes to ground (R10).  

A 100nf cap with a 100k resistor forms a high pass filter, which works out to be around 16Hz - of course, there are a lot of other things happening to the signal; I'm only using these two components as an example.  

The output capacitor is also a coupling capacitor, which forms another high pass filter with the Sustain pot, which will be variable (pots are variable resistors).  The graph shown is only in relation to the 100n/100k RC filter; it is not the frequency response of the Big Muff booster stage.    

CAPACITOR VALUES

Capacitance is measured in Farads - and the annoying thing about farads is that one farad is a truly gigantic amount of capacitance compared to the actual size of capacitors commonly used, so we end up measuring them in microfarads (uF) which is one-millionth of a farad, nanofarads (nF) which are one billionth, and pico-farads (pF) which is one trillionth.  

This makes the math a bit more interesting with any equation requiring farads.  I generally avoid math where possible by using online calculators.  

So a garden variety capacitor can be listed as 0.01uf, 10nf, or 10,000pf, which causes a bit of confusion when you are just starting out.  

CAPACITOR MARKINGS

Electrolytic capacitors are easy to read, as they always have the capacitance and voltage clearly marked.  Smaller capacitors are a bit harder to read, as there's less space for printing, so manufacturers resort to number codes.

If there are only two digits listed, you can take this value as the capacitors value in pF.  If there are three then you have a multiplier to factor in.

The additional number tells you how many zeros to add to the first two.  So 101 = 100pF, 102 = 1,000pF and so on.   

Get the magnifying glass out for small MLCC caps

CAPACITOR TOLERANCE

Not unlike resistors, capacitors are also built to certain tolerances, which also relates to price.

Further to the number code mentioned above to determine capacitance value, if there's a letter at the end, the letter indicates tolerance.  example:  101J = 100pF 5% tolerance

Common tolerances are;

F    1%

G    2%

H    3%

J    5%        (common for MKT caps)

K    10%

L    15%

M    20%    (common for electrolytic caps)

N    30%

Z    +80%, -20% 

CAPACITOR VOLTAGE RATING

All capacitors have a voltage breakdown rating, which is often printed on the capacitor, sometimes this is in the form of a code.  

Exceed the voltage breakdown rating, and the capacitor will become conductive.  Note the point above about the two conductors in the capacitor being separate, well that's not quite the case when voltages are exceeded.  

As a general rule, the voltage rating of a capacitor should be double what it will encounter in the circuit.  This is usually only an issue with electrolytic capacitors, as they have a low voltage rating when compared to other types of capacitors.

Can you replace a capacitor with one of a larger voltage rating?  Yes, you can.  It makes no difference, as the capacitance is still the same.  

Can you go smaller?  Yes, so long as it still meets the double the expected voltage rule.  example:  replacing a 100v cap with a 50v cap in a 9v circuit is not going to be a problem.

Voltage codes (Max operating voltage);

1A    10V

1H    50V

2A    100V

2T    150V

2D    200V

2E    250V

2G    400V

2J    630V

3A    1000V

STRAY OR PARASITIC CAPACITANCE 

Capacitance can be formed by accident, which is where terms like parasitic or stray capacitance come into play.  Thinking about what a capacitor is - two conductors separated by an insulator (including air), well that's practically every component placed on a board, including the off-board wiring.  

Veroboard is just strips of conductors running directly next to each other separated by an insulator.  It's basically a big flat capacitor with really low capacitance.  I've read reports that there are about 13pf between tracks, on a 300m length board, which works out to about .2pf per hole.   That's pretty small - most multimeters can't even measure that low.   

Components also have internal capacitance, which you will see listed on transistor datasheets.  Again, it's usually pretty small.

So the question is just one of does it matter?  Is the capacitance enough to cause problems?

WHAT IS LEAKAGE & DOES IT MATTER?

Leakage is not a physical substance leaking out of a capacitor (although that's also bad), it usually refers to DC leakage in a coupling capacitor.

Capacitor leakage can cause the dreaded switch pop, which is partially the reason why a resistor is often paired with an electrolytic capacitor on inputs/outputs.  The resistor provides a path to ground for DC to drain.  

All electrolytic capacitors leak to some degree.

SHOULD I SPEND MORE ON FANCY CAPACITORS?

If it makes you happy, yes.   Just don't spend too much...  if it's marketed as audio grade you will probably pay too much, if it says audiophile, you're about to be massively ripped off.  

Also worth noting is that many vintage pedals were built with really cheap parts.  You don't hear of anyone pulling caps out of their vintage pedals to replace them with new audiophile versions (at least, I hope they aren't).

 

WHAT ABOUT VINTAGE OR NOS CAPACITORS?

Will it sound better?  Probably not, but they can look cool, especially if using carbon composition resistors.  Given a choice between grey vs red with stripes...   I prefer the latter.  I have a range of different NOS caps, largely based on looks.  I also have a large stock of regular MKT and greenies that I happily use.  

Just don't buy vintage or NOS electrolytics, as they can actually dry out or physically leak over time.  Paper in oil can also have issues.

Again, don't pay too much for them.  It makes me sad to see people paying excessive amounts of cash for vintage ceramic capacitors to "upgrade" their new Strat or buying bumblebees for their Les Paul copy to get that magic tone.  For a period-correct restoration of a vintage instrument, well fair enough, I'd probably do the same.  Dropping a $50 cap in your Squier? That's maybe not the best use of your money.

CAN YOU HEAR A DIFFERENCE?

So this is where things might get more controversial - people make all sorts of claims about capacitors and their sound.   Different types of capacitors do have different properties, which might affect the sound in different circumstances, but is a guitar pedal one of those circumstances, and is there really much difference anyway?

Things to consider;

• Capacitors tolerance - tolerance can vary quite a bit, up to 20%.  So even changing two capacitors of the same type might actually result in a slightly different sound.  As an example, in a filter it will change the roll-off frequency, resulting in an audible difference.

• Listening conditions - to make a proper comparison, listing conditions need to be identical, as does the sound source.  example:  next time you are playing the guitar, just turn your head away from the amp a little.  Sounds different right?   Sit in a different spot - sounds a little different right?   See where I'm going...   

• Confirmation bias - so you've just dropped a bit of cash on fancy caps.  You've read that they sound better, and someone on youtube was raving about them.  When they finally arrive in the post, and you get a chance to try them out, you actively try to hear something different about them.  Subconsciously you may hear what you want to hear.   This is why double or triple-blind tests are used in scientific studies.  It removes unintended bias from results.   

• What is the capacitor actually doing - is the cap actually in the signal path?  No, you can't hear an electrolytic cap in the power supply.   Should you put a good cap there?  Sure, you want a good quality cap that will last and perform well.

• Have you ever bought an album only to return it after discovering cheap ceramic capacitors were used in the vintage Big Muff used on a track?  Yeah, probably not.

And if you want to hear a bit more about capacitance, inductance and impedance, all relative to guitars - Kingsley explains a great deal in this video on That Pedal Show.