The Synth-DIY Capacitor FAQ

[JBaker]

 I'm really NOT enjoying the text editor in Communicator 4.0.3.
Otherwise, there are some points to be clarified in the 'What about
polcarbonates?' section; otherwise, besides answering most all of my
questions, this FAQ has made me come to terms with the fact that I spent
about $10 on caps I probably won't be using.  C'est la vie.


Q: What's a capacitor, anyway??
A: A capacitor consists of 2 metal plates seperated by an insulator.
More generically, a capacitor consists of two or more conductors
separated by a non-conductor or 'dielectric'.

Q: Plates?? Dinner plates??
A: The first caps were the infamous "plate glass and tin foil" types
used (still today) in high-powered RF circuits, like a Tesla coil. They
consisted of an alternating stack of aluminum foil and a piece of window
glass, about 20cm square.  The odd foils were connected to one
electrode, and the even foils to another like so:

Glass   ============
Foil 1  ******************
Glass   ============
Foil 2  ******************
Glass   ============
Foil 3  ******************
Glass   ============

The first relatively large cap was a jar with a metal coating on the
outside and inside, the official name for which is a 'Leyden Jar'.

Q: How come mine are so small??
A: Technology has demanded ever smaller capacitors with differing
dielectrics for various purposes.  Manufacturers figured out that if you
replace the plates with plastic film, you could roll capacitors like a
pastry to make them smaller.

Q: So why so many types: Mylar, polyester, etc etc.??
A: The types describe the "quality" of the film used. That's why most
caps are grouped as:

 1) electrolytic
 2) film
 3) ceramic
 4) misc (like mica)

Capacitors are referred to by their dielelectric material.  Common types
include plastic, ceramic, tantalum, mica, air, and electrolytes.

Q: Why do electrolytic caps have + and -?
A: Because the insulator used is a liquid (actually a gel), the the
chemisry of it requires one end to be more positive than the other, like
a battery.

Some capacitors are constructed such that they have a greater DC leakage
in one polarity. In the proper DC polarity they offer great amounts of
capacitance in a small volume.

Q: Great, now I'm even more confused!! How do I know which one to use in
a certain appliation??
A: Well, here are some rules to go by, listed by application:

BYPASS CAPS - Use 0.1uf (microfarad = 10^-6) ceramic caps.  Don't use
the "disc" types if you can avoid it; use axial-leaded types.  The
reason is they are coated with epoxy and resist moisture.  Also, they
are much smaller.

AUDIO COUPLING CAPS - Used to AC couple an audio input.  Use 0.47uf
metallized polyester or polyproplyene.  Values less than this will
attenuate bass frequencies.

VCF CAPS - Use polypropylene film.

VCO CAPS - Use polystyrene if you can find them (the one German company
that makes the film will stop selling it this year!!) or use metallized
polypropylene.

POWER SUPPLY CAPS - For the cap between the diodes and the regulator,
use a cap rated to 105 degrees C. They are only slightly more expensive
(pennies) than the 85 deg. C types, but much higher quality.  For the
output caps on 3-terminal regulators, use a 1, 2.2 or 3.3uf tantalum in
parallel with an 0.1uf ceramic.

Following are some guidelines, listed by capacitor type:

CERAMIC CAPS - Low cost capacitors in the range from about .1pf
(picofarad = 10^-12) to 1uf.  Dielectrics can alter values with Temp
(X7R, Z5U) some are Temp stable (COG, NPO), but some vary extremely with
temperature.  Relatively small sizes but they exhibit lossy behavior and
a little waveform distortion.  Good low cost, low fidelity coupling and
power supply bypassing.  Good RF caps.  Low self inductance makes them
good for use as bypass in digital circuits.  Avoid ceramics in the audio
path.

TANTALUM CAPS - Moderate cost capacitors in the range from about .1uf to
470uf. They have polarized dielectrics and fairly small volume wise.
They have a limited voltage capability; usually less than 50 volts.
Good capacitors for supply bypassing but have some leakage current. Not
recommended for decoupling or signal processing.

ELECTROLYTIC CAPS - Moderate cost capacitors in the range from about
.1uf to 2.2f.  They have polarized dielectrics and are used in a wide
variety of voltages.  Some types are optimized for decoupling and audio
signal processing.  Most have some leakage current.  It should also be
noted that electrolytic caps SHOULD NOT be used where the DC potential
across the cap is substantially below the rated working voltage.  Some
people might assume that doing so provides a safety margin. But since
the dielectric is "formed" by the voltage applied across the capacitor,
they will lose capacitance when operated much below their rated voltage.

This is why voltage ranges are relatively small: 6.3, 10, 16, 25, 35,
50, etc.  Pick the voltage rating the next step higher than the _peak_
voltage across the cap.

If you exceed the voltage rating for an electrolytic or let it get too
hot it can EXPLODE!!  Great care should be taken to ensure that this
does not happen, as capacitors contain some pretty toxic stuff.
Double-checking that the electrolytic is oriented the correct way in the
circuit and measuring the voltage across it with a meter are two ways to
avoid a dangerous situation.

PLASTIC CAPS - (a.k.a. Mylar, polyester, polycarbonate, polypropylene,
polystyrene, and 'metallized' versions.)  These caps are stacked or
wrapped dielectric construction and may resonate at very high
frequencies.  Usually excellent for decoupling and bypassing of audio
signals.  Some types are better for audio than others (e.g.
polypropylene).  Values between 470pf and 10uf usually. Fairly moderate
to large size volumes.

MICA CAPS - They are tight tolerence capacitors between 0.1pf and
.0033uf typically used for RF coupling or bypass applications.  Fairly
expensive but useful for high frequencies. Fairly large physical size
for their values.

AIR CAPS - Used for small values from .01pf to 100pf and usually
variable capacitance types for RF tuning. Relatively large for their
values and prone to dust leakages.

Q: What about Mylar?
A: Mylar, also called polyester, is to be AVOIDED!!  Mylar is popular
for ONE REASON: PRICE. They are not suitable for serious audio work.
Mylar was the first film cap available (about 1953) and most
'old-timers' associate film caps to a Mylar cap.

Q: Why?
A: They have 2 problems: over time, they 'age'.  They change their
capacitance. The second problem is Mylar isn't that good of a film: it
is "leaky" and the charge bleeds off. That's why in VCO's and S/H's a
Mylar cap is the WORST choice you can make.

Q: What about polycarbonate?
A: No good either. It was to be a low cost Mylar cap! (<--??) Polycarbs
are used in similar applications as polyester caps.  Some specsmanship
is needed for voltage range, tempco, tolerence, etc.(??)

Q: Are paper caps good to use?
A: Paper capacitors predate Mylar by about 60 years.  They are prone to
leaking and other bad age-related effects, and are therefore not
recommended.  In case you want to experiment with "bad" capacitors,
there are new paper-dielectric capacitors being made for the more
extreme audiophiles. Audio Note is the major brand.  Also,
Sprague/Vishay still manufactures the old phenolic-impregnated-paper
'Vitamin Q' types.  All very expensive, and very leaky.  Definitely not
recommended for time constants in VCOs or for temperature-sensitive
uses.

Q: What about oil-filled caps??
A: Oil-filled capacitors still used in high-voltage AC applications.
Some audiophiles build DIY tube amplifiers using oil caps to filter the
high-voltage plate supply; users report that oil-filled caps have some
major advantages over electrolytics, sound quality being one of them.

Q: Do teflon caps really exist?
A: Yes. Made by 2 companies at present. Used in very specialized
military and industrial applications, and in overpriced audiophile tube
preamps.

Q: Are there any strange capacitor-related effects I should be aware of?

A: All caps, particularly DC-blocking caps in the audio path, can become
microphones.  With a polarizing voltage across them, capacitors can be
sensitive to vibration.  A knock on your synth's case, for example, may
be heard through your amplifier.  One fix is to put a glop of RTV
adhesive over the capacitor.  Ceramics are the worst offenders; yet
another reason to never design them into the audio path!!

Q: Where can I get more information?
A: If you care about this stuff you need the analog designers bible,
"the Art of Electronics" by Horowitz and Hill, Cambridge University
Press, readily available everywhere and pirated into most Asian and East
European languages.

Q: Where can I get good caps?
A: In the US, try Digikey (http://www.digikey.com has an online
searchable index). Other sources include Allied, Newark, and Mouser
(http://www.mouser.com).  Both Digi-Key and Mouser'll send ya a free
catalog, requestable at their website.  Delivery is quite prompt.

Q: What are the best types to use in DIY synth projects?
A: In conclusion:

1) 0.1uf ceramic axials for bypassing
2) polypropylene for all audio paths
3) polystyrene for VCOs if you can find them, metallized polypropylene
if not

Hope this helps!!

Paul Schreiber, Mark Amundson, Rich Nelson, Jeff Baker, Paul Perry, Eric
Barbour