The Synth-DIY Capacitor FAQ
JBaker
scopey at hooked.net
Wed Oct 29 23:19:34 CET 1997
A couple of things:
1) Are we agreed that polystyrene, metallized polypropylene, and polycarbonate
are pretty much all fine for VCO's? Polycarbs have the highest DA, but the
lowest tempco (50ppm;the polystyrene caps in Mouser catalog, p.223, have a
150ppm tempco). All are stable, so what's the consensus??
2) Are there many cases in synth circuits where a trimmer cap would be worthwhile?
3) Thanks G.R. for the snazzy new chart.
thanks,
Jeff
Q: What's a capacitor, anyway??
A: A capacitor consists of 2 metal plates separated 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 dielectric 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 chemistry 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 application??
A: Well, here are some rules to go by, listed by application (note: all values
given are just typical suggestions):
BYPASS CAPS - Bypass cap recommendations may vary from designer to designer.
The famous 0.1uf (microfarad = 10^-6) ceramic is a good value for audio supply
bypassing; 0.01uf may be a little better if you mix higher speed circuits like
logic on the same supplies. Values also depend on load and input impedance.
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. Do not forget to retain a 10uf or greater aluminum or tantalum
electrolytic on circuits as well. They act as local energy storage reserviors
when the DC supply is more than a few inches away.
AUDIO COUPLING CAPS - Used to AC couple an audio input. Use 0.47uf metallized
polyester or polypropylene. Values less than this will attenuate bass frequencies.
VCF CAPS - Use polypropylene film.
VCO CAPS - Use polystyrene if you can find them (try Mouser); the one German
company that makes the film will stop selling it this year!! Metallized
polypropylene, and polycarbonate are also fine. See "What about polycarbonate
caps?" below.
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 1.0, 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 0.1pf (picofarad =
10^-12) to 1uf. Dielectrics can alter values with temperature (X7R, Z5U
types), some are temperature stable (COG, NPO types), but some vary extremely
with temperature. They exhibit lossy behavior and a little waveform
distortion. Good for power supply bypassing and RF applications. 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 0.1uf 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 0.1uf 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. Avoid a dangerous situation by
double-checking the orientation of and the voltage across your electrolytics.
Also, don't place electrolytics near heat sinks.
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 tolerance capacitors between 0.1pf and 0.0033uf
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 0.01pf 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: This type is known for good performance (i.e. stable) over a wide
temperature range. Polycarbs are used in similar applications as polyester
caps. The Illinois Capacitor company makes a line of polycarbonate caps with
temperature coefficient (tempco) of 50 ppm, similar to good metal film
resistors. The primary concern for VCO timing caps is tempco. Polycarbonates
are specified for Electronote VCO's. They are also specified directly on the
schematics for Buchla VCO's.
Q: Are paper caps OK 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 applications.
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
transducers, like microphones. A vibration may induce voltages due to
internal "piezo-electric" effects. 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 and some plastic caps are the worst offenders;
yet another reason to never design them into the audio path!! Some ceramic
dielectrics are worse than others regarding this effect.
Q: Any other pitfalls to avoid?
A: Yep. A high-valued cap (electrolytic, tantalum) is often used to bypass
relatively low frequencies. Such a capacitor, however, may not bypass
expected high frequencies (based on the circuit's RC time constant for an
ideal cap) because the equivalent series resistance (ESR) of these caps
becomes excessive at high frequencies. The remedy is to parallel the
capacitor with a small (0.1uf) high frequency cap.
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: I wish I had a quick reference I could tack on the wall next to my workspace!
A: Umm, yeah, funny you should ask...This data comes from Analog Devices'
"Analoq Dialog" 30-2 from 1996. The data is tabular and concentrates on
dielectric absorption (DA) criteria.
TYPE Typ. DA Advantages Disadvantages
---------- -------------- -------------------- ----------------------
NPO Ceramic <0.1% Small case size DA generally low
Inexpensive Limited to small values
Good stability
Wide range of values
Many vendors
Low inductance
Polystyrene .001 to .02% Inexpensive Damaged by temps >+85C
Low DA available Large case size
Wide range of values High inductance
Good stability
Polypropylene .001 to .02% Inexpensive Damaged by temps >+105C
Low DA available Large case size
Wide range of values High inductance
Teflon .003 to .02% Low DA available Relatively expensive
Good stability Large size
Operational above +125C High inductance
Wide range of values
MOS (on chip) .01% Good DA Limited availability
Small Only small values
Operational above +125C
Low inductance
Polycarbonate .1% Good stability Large size
Low cost DA limits to 8-bit apps
Wide temp range High inductance
Polyester .3 to .5% Moderate stability Large size
(Mylar) Low cost DA limits to 8-bit apps
Wide temp range High inductance
Low inductance(stacked film)
Monolithic >.2% Low inductance Poor stability
Ceramic Wide range of values Poor DA
(High K) High volt coeff.
Mica >.003% Low loss at HF Quite large
Low inductance Low values (<10nf)
Very stable Expensive
Avail. in 1% values or
better
Aluminum High Large values High leakage
Electrolytic High currents Usually polarized
High voltages Poor stability
Small size Inductive
Tantalum High Small size Quite high leakage
Electrolytic Large values Usually polarized
Medium inductance Expensive
Poor stability
Poor accuracy
--------------------------------------------------------------------------------
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) ceramic axials for bypassing
2) metallized polyester or polypropylene for audio coupling
3) polypropylene for all audio paths (VCF's, VCA's, etc.)
4) polystyrene, polycarbonate, or metallized polypropylene for VCO's
5) tantalum and ceramic for voltage regulators
Hope this helps!!
Paul Schreiber, Mark Amundson, Rich Nelson, Jeff Baker, Paul Perry, Eric
Barbour, Grant Richter
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