Picking Capacitors
by Walter G. Jung and Richard Marsh
Reprinted from Audio Magazine, February and March, 1980
Notes
This is an archived article used for reference purposes. Some content may be outdated or missing.
PICKING CAPACITORS
Walter G. Jung and Richard MarshReprinted from Audio Magazine, February and March, 1980
Table of Contents
Introduction
Capacitor Basics
Signal Path Tests For Capacitor Distortion
Tantalum Capacitor Tests
Ceramic Capacitor Tests
Interpreting Capacitor Performance Data
Measuring Capacitor Impedance Values
Testing Capacitor DA
"Tuning" typically used in audio circuits with quality capacitors
Performance Comparison of Various Dielectrics
Summary
More Specific Recommendations
Acknowledgements
List of Downloadable Figures, Tables, References, and Graphs
From time to time we hear references to distortion and other nonlinear effects produced by passive circuit components, such as capacitors, used in audio circuits. However, only on rare occasion can anything be found in written form which attempts to quantify or otherwise document capacitor problems, particularly as they specifically relate to audio. Yet, distortions are produced by a wide variety of basic capacitor types, and in some cases forms of this distortion are rather easily measurable. Why there hasn't been more written on this topic is truly a good question, as in many instances the audible defects produced by capacitors can easily be the Achilles' heel of a given design. If this were not a truism, why else would there be so many audiophile modifications consisting essentially of capacitor upgrades only? The implications of this will be apparent when this article is fully appreciated.
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While there has been no detailed overview or discussion of these
problems in print, two articles are noteworthy, because they do in fact
address this specific topic. In [1], Dave Hadaway gave a summary of
relative quality rankings for capacitor types. More recently, John Curl
[2] discussed some measured results for two capacitor types. Dick Marsh
[3,4,5] has been specific in cautions against certain types, in several
Audio Amateur letters.
- What we hope to do in this article is cover capacitor basics, means of testing for impedance and distortion, and summarize with some selection criteria which will optimize sound quality. We will begin by discussing some simple (but deceiving, really) distortion tests. A summary of key capacitor performance defining terms is given in the sidebar entitled Capacitor Basics.
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Capacitor Basics A brief review of capacitor fundamental relationships is appropriate to a more complete understanding of the applications-oriented discussions of this article.
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Signal Path Tests For Capacitor Distortion
One of the more frustrating aspects of the distortion problem vis-a-vis
capacitors is that they do not always allow direct quantification as
they typically operate in the signal path. A good example of this very
point will be demonstrated below in the discussion of some THD tests on
tantalum capacitors. By these THD results, one might be led to believe
that tantalum capacitors. By these THD results, one might be led to
believe that tantalum types are adequate when suitably selected.
Nevertheless, they still fail to measure up in auditioning and show
poor electrical quality when measured by other methods even though
they may appear to be operating in a virtually distortionless fashion
by THD tests.
- Two series of THD tests were performed on two types of capacitors, tantalum electrolytics and ceramic discs. These tests seem to be representative, as different capacitors of the same variety produced similar results, and the results here generally correlate with Curl's [2].
Tantalum Capacitor Tests
In the tantalum tests, a circuit was built in the form of a simple
high-pass filter, as shown in Fig. 1. The general test circuit used is
shown in Fig. 1a, and the details of various capacitor connections are
in 1b. The 3-V rms generator and meter is a THD oscillator/analyzer
combination. The general goal of this test is to examine the distortion
sensitivity of the polar tantalum capacitor in handling bipolar a.c.
signals.
-
As the different connections of 1b indicate, there are various ways
that a polarized capacitor such as this can be connected. The circuit
as shown in 1a is a simple a.c.-only circuit with no d.c. polarizing
bias applied to the capacitor in test condition A.
-
For such a mode of operation, a tantalum capacitor will generate
appreciable distortion when the signal conditions are such that there
is appreciable a.c. voltage dropped across it. Or, stated another way,
when its reactance becomes appreciable in relation to that of the load
(here 680 ohms).
-
For condition A, the capacitor is a single 6.8uF unit, and its
reactance equals 680 ohms at about 35 Hz. To generalize, we will talk
in terms of this frequency, which is the corner frequency, fc. As will
be seen, it is a key to understanding this particular pattern of
distortion behavior.
-
THD data were taken on this and the remaining connections, as shown in
Fig. 2. For condition A, it can be seen that distortion is low at
frequencies above about 10 times fc, but rises as fc is approached and
nears 1 percent in level below fc when the capacitor sees a large a.c.
voltage.
-
From these data, it seems somewhat analogous to regard a polarized
tantalum operated thusly as a capacitor shunted by an imperfect diode.
The distortion it produces is even order, which is shown in the
distortion photos in Fig. 3. Since the device's a.c. characteristic is
asymmetrical, it appears that circuit means which tend to minimize the
asymmetry also tend to minimize the distortion produced.
-
As John Curl showed [2], a simple parallel connection of like
capacitors, as in B, reduces distortion appreciably. Compared to
condition A, condition B reduces the distortion at fc by a factor of 2
to 3 (Fig. 2).
-
The series back-to-back connection of condition C can reduce
the distortion further, if the two capacitors happen to have
complementing characteristics. The distortion products for
condition C are also shown in Fig. 3 (at fc). However, it appears
this particular connection depends strongly on the match of the
specific units used. Also, unlike the connection of B, the series
connection of C increases the net equivalent series resistance
(ESR), which is usually not desirable as discussed later.
-
If the series connection is so effective, the logical question is,
then, does polarizing bias applied to the junction help further? The
answer is yes, with increasingly better results with more bias, as shown in conditions D, E, and F. However,
even a relatively small bias, such as in D, is very effective,
reducing THD at fc to 0.01 percent. This bias level is 5 V or just in
excess of the greatest signal peak swing. The distortion for test
condition D is shown in Fig. 3 at fc.
- What this series of tests seems to say is that one should carefully control the a.c. signal developed across such a polar capacitor to minimize this distortion. If you use a simple single-capacitor connection with no d.c. bias, it appears that a just derating by a factor of about 10 times will minimize the distortion. In other words, if a given capacitor used for coupling is calculated to have an fc of 10 Hz, making it corner at 1 Hz will minimize the distortion produced by this particular mechanism. However, as alluded above, this is not the whole story, as the discussions later will show.
Ceramic Capacitor Tests
In a second series of tests, the distortion produced by a
common ceramic disc capacitor was studied. Data in the form of
the THD vs. frequency for this test are shown in Fig. 4. The first
circuit used is a simple low-pass (LP) filter, with the capacitor
under test as the shunt C arm. The values chosen for the test
were R = 1K and C = 0.1uF. A 100-V type was used for the
capacitor.
-
As the LP data show, distortion is produced well below the corner
frequency, which in this case is 1800 Hz. The data shown are corrected,
so the THD 100 percent set level follows the LP roll-off. Even as such,
however, the higher harmonics are attenuated, and this data may be a
pessimistic representation. An IM test might show even worse
performance for this LP filter. Figure 5 shows the nature of distortion
in 5a; as can be noted, it is third harmonic. By contrast, a polyester
type inserted into the circuit shows no discernible distortion (5b).
-
By placing the same ceramic capacitor in an HP filter circuit, the
roll-off of harmonics can be circumvented. In this type of use, the
voltage across the capacitor is highest at low frequencies. Thus
nonlinearities will show up as higher harmonics, which are readily
passed by the filter.
-
The data for the HP test show much stronger distortion at the lower
frequencies, where the voltage is highest. We are not sure what should
be interpreted as the common distortion-producing source in these two
tests. One thing seems quite clear, however, and that is the simple
fact that you cannot "work around" the distortion problem in ceramics.
Our feeling is that they should simply be avoided anywhere near an
audio signal path and probably just avoided altogether for audio. For
example, some listening tests have indicated that they can produce
audible distortion when used as supply bypasses, let alone coupling!
- One obvious implication which emerges from the above is that a capacitor is not just a capacitor by any means. Of course, what we've discussed here are only two types of capacitors, and we really ought to make some general recommendations as to desirable types. This leads us more deeply into just what a capacitor is, and how this knowledge relates to audio.
Interpreting Capacitor Performance Data
One of the most important factors needed for a full and effective
understanding of capacitor audio application criteria lies in
interpreting data. Most of us have probably seer examples of
impedance/frequency curves such as the one hypothesized in Fig. B4.
However, considering such curves in the light of real data actually
allows us to separate the men from the boys among capacitors - and also
shows us which ones to use for audio.
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Typical data of this nature for tantalum capacitors are shown in Fig.
6. Regarding this data and recalling the model of a real capacitor
(Fig. B2), we see that under d.c. or low frequency conditions, Rs and L
are negligible, compared to the C and Rp combination. As the frequency
increases, particularly above a few kilohertz, the effects of both Rs
and L increase.
-
In these practical cases shown here (one of which is typical of
good-quality Ta units), it can easily be noted that Xc does not follow
the ideal capacitor -6 dB-per-octave pattern with increasing frequency.
-
Further examination shows that as frequency increases, Xc tends to
decrease, while X, increases in value. This of course means that the
(Xc-Xl)2 term of the Z formula gradually decreases until at some
frequency, the term (Xc<-Xl)2 disappears. Then the observed
impedance is resistive or Z = Rs. This is the so-called series resonant
frequency of the cap, which for tantalum and aluminum electrolytics
will generally fall between 10 kHz and 1 MHz. From this, it should be
apparent that if a capacitor is operated at a higher frequency than
this, it will no longer be a capacitor to the circuit.
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Although the discussion thus far has treated only tantalum
electrolytics, this pattern of non-ideal Z vs. frequency behavior is
actually inherent to all real capacitors to some degree. In the better
quality dielectrics, Rs and L are lower and more closely controlled,
and this is reflected in lower losses (DF), due to the lower parasitic
parameters. This will be more apparent as we present data for other
dielectrics.
-
Aluminum electrolytics show a quite similar broadly resonant frequency,
where Z = Rs (or ESR). Data for a wide range of aluminum electrolytics
are summarized in Fig. 7.
-
As can be noted from these data, the resonant frequency is typically
between 10 and 100 kHz. Note, however, that the absolute level of
impedance is much lower in the case of aluminum, due to the
availability of much larger values. Also, many of the larger
electrolytics are designed to handle large ripple currents and thus
have very low values for Rs, as can be noted.
-
If these data are very carefully interpreted, a number of quite useful
points can be drawn from it. Generally speaking, for two capacitors of
similar value, the one with the higher voltage rating will show lower
Rs (and DF, if viewed thusly). This can be seen, for example, between
units A and B, as well as units F and G. And, it can also be seen in
tantalum units (the two specific cases for comparison in Fig. 6 show
this quite well).
-
One might at this point ask what is the disadvantage of a relatively high Rs (or high DF) in a capacitor used in audio.
The answer can be had by regarding the data of Fig. 7 in a
different light.
-
Using the capacitors A and B as illustrative examples, their
actual effective capacitance values were calculated for various
frequencies, using equation 4 to solve for C. The results, plotted in
Fig. 8, clearly show the A unit (the higher Rs unit) to exhibit strong
changes in capacitance with frequency. The B unit improves the
situation relative to A, but still shows substantial capacitance
change.
-
It is not at all hard to imagine how a capacitor whose value
actually changes with frequency might distort an audio signal's
integrity, particularly with regard to phase.
-
If we can visualize the complex frequency relationships of
music passing through a capacitor (it doesn't really), while Z is
simultaneously changing with the complex frequencies of the
music, it is possible to appreciate how it can be relatively easy to
upset the subtle harmonic/fundamental phase and amplitude
relationships. Not only from the capacitance variation standpoint
but also from the inductive behavior region. Used as a coupling
capacitor, the resulting effects of high DF (L or Rs) are image
blurring, and instrument harmonics/overtones are less accurately
reproduced, with a general overall veiling of the sound. Use in a
feedback path further complicates the matter, because we are
using this signal to provide error correction. For example, if we
consider the general transfer relationship for an amplifier where
the gain = Zf . Zin, it is easy to see that variations in Z with
frequency which depart from the ideal will distort the relationship.
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When we talk of film capacitor types, we find that the situation
of less than ideal behavior regarding impedance vs. frequency is
improved greatly. This is simply due to the fact that film dielectrics,
such as polystyrene, polypropylene, polycarbonate, and polyester,
have much lower dielectric losses. This is reflected in lower DF
and Rs, as well as generally much more stable parameters with
respect to frequency and temperature.
-
The impedance vs. frequency characteristics of a number of
film-type capacitors are shown in Fig. 9. In general, it can be noted
that they all show lower minimum impedances (lower Rs), and
sharper dips around resonance. These points underscore the fact
that the resistive losses can be much lower, in many cases below
10 milliohms.
- Film capacitors also appear inductive above their series resonant frequency, due to inevitable parasitic inductance of the winding and/or leads. However, the inductive effects can be minimized, by suitable winding and termination techniques, which can extend the usefulness of a capacitor to substantially higher frequencies. Useful cues to look for in this regard are specified noninductive winding techniques and extended foil-welded-lead attachments.
Measuring Capacitor Impedance Values
Since the above points are so basic to optimum capacitor
selection, it logically follows that most audio experimenters will
want to have the capability to measure the various capacitor
performance parameters. Since few of us have access to the
necessary bridges (and if so, many of them can't measure Rs or
DF), it seems necessary to devise a setup to measure these
parameters. A setup we find most convenient to these purposes is
shown in Fig. 10, and it was actually used to gather all of the data
for Fig. 8.
-
This setup basically measures impedance (Z) by the voltage
divider method, using a sine-wave generator and voltmeter. From
the impedance data, C, Rs, L and DF can be derived. Table I and
the notes describe the details of the procedure, which is written
for either a bench voltmeter or an oscillator-analyzer combination.
You should, of course, take appropriate precautions regarding bias
voltages, polarity, and so forth. Also, be sure to use shielded leads
on the voltmeter and connections direct to the terminals on low Rs
units.
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You will be pleased with how much power this simple little
setup gives you in grading capacitors, particularly electrolytics.
For example, you can use it to quickly weed out poor-quality units,
such as A (a junkbox special). Given a few units of similar value, it is quite easy to select the lowest Rs unit,
such as H versus 1.
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If you use an audio oscillator and meter, you will most probably
be limited to upper frequencies below 100 kHz or 200 kHz.
However, you can use the same basic technique with a
wide-range function generator as a sine-wave source and a
high-gain scope as the readout device. This will allow testing of
the smaller value film capacitors, which typically are series resonant at appreciably higher frequencies.
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A somewhat lesser known performance parameter of
capacitors called dielectric absorption (DA) is also a major
contributor to sonic problems. Actually, in spite of the fact that DA
is not generally understood, it may well be more important than DF.
-
This phenomenon is really a reluctance on the part of the
capacitor dielectric to give up the electrons that it has stored within
itself whenever the capacitor is discharged. Then, when the
shorting mechanism is removed, these electrons that remained in
the dielectric will, in time, accumulate on an electrode and cause a
"recovery voltage" gradient to appear across the capacitor
terminals. This has been referred to as a capacitor's "memory" of
what was just previously applied. The recovery voltage, divided by
the initial charging voltage, and expressed as a percent figure, is
called the "percentage dielectric absorption" (% DA).
-
Conversely, there is also a reluctance on the part of the
dielectric to accept all of the energy presented to it with a
uniformity of speed. These factors may be understood by
regarding the capacitor model of Fig. B2. The effect of DA is
represented by the capacitor C2, with a series resistance, RDA
The total capacitance seen externally is C = C1+C2. Variation of the
relative size of C2 and C1, and RDA, represents the equivalent of
real capacitors, with varying degrees of DA. (Note that this model
suggests that the externally perceived effects of DA might be
controllable to some degree by manipulation of the relative
impedances controlling charge and discharge of the real
capacitance. Experimental evidence discussed later tends to
support this contention.)
-
In addition to the "bound" electron phenomenon, a secondary
factor in the magnitude of recovery voltage values is that of "free"
electrons in random movement in the dielectric. These free
electrons take finite time to move from the dielectric to the
electrode, and therefore contribute to this recovery voltage.
-
Dielectric absorption becomes a critical factor in circuits which
are highly dependent upon speed of response. As the a.c. signal
goes to zero (as in a short circuit) the trapped or bound electrons
within the dielectric do not follow as fast. These electrons take a
finite time to move from the dielectric to the electrode. As
capacitors are typically used in audio circuitry, we could translate
these defects into loss of accuracy in reproducing the fine inner
detail of music, as well as the music's dynamic structure.
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It is quite illuminating to consider what effect a phenomenon
such as DA will have on an a.c. signal consisting largely of
transients (such as audio) might have. For example, when an a.c.
voltage is applied, there is a tendency for the dielectric absorption
phenomenon to oppose this change in polarity.
-
When music is the a.c. signal, the sonic degradation is one of
compression or a restriction of the dynamic range. Also, a loss of
detail results, and the sharpness is noticeably dulled. With
dielectric types which have high DA, there is a definite "grundge"
or hashy distortion added to the signal.
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It is quite important to describe the sonic thumbprint that DA
contributes to subjective audio. The effects of DF and DA can be
perceived differently. DF is primarily a contributor to phase and
amplitude modulation DA reduces or compresses dynamic range.
This it does by not returning the energy applied all at once. With
signal applied to a capacitor with DA present, the amplitude is
reduced by the percent DA. When this energy does get returned
(later), it is unrelated to the music and sounds like noise or
"garbage" being added; the noise floor is also raised.
High-frequency and/or transient signals are audibly compressed
the most. Signals that look like tail pulses (a lot of transient music
information is of this nature) are blunted or blurred in their sound.
"Dulling," loss of dynamics, added garbage or hash, and an
inability to hear further into the music have been subjective terms
used to describe the DA effect in capacitors.
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All polardielectrics have relatively high DA; the best examples of
this pattern are tantalum and aluminum electrolytics, which can
have DAs as high as several percent. There is also a general
correlation between dielectric constant and DA, with the high K
dielectric types being worst in terms of DA (we would like to thank
T. Von Kampen of TRW Capacitors for making this point to us). For example, regarding Table Bl, ceramics
and both the Al and Ta oxides have high values for K, and also
show correspondingly high values for DA.
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Glass and mica dielectrics have intermediate values for K, and
also intermediate levels of DA. They are nowhere nearly as bad
as ceramics or the Al and Ta oxides, but neither are they as good
as the films.
-
Interestingly, it should be noted that there is also a general
correlation between low values for DF and low DA, particularly
among the film dielectrics. However, a low DF does not always go
hand in hand with a low DA, and the glass and mica dielectrics are
good examples of this fact. Both of these dielectrics have excellent
properties with regard to DF, and also low capacitive variations
with regard to frequency and temperature. Unfortunately,
however, these excellent properties (which make these types
highly desirable for such applications as resonant circuits and
equalizers) are not realized concurrently with low DA. So, these
types are therefore not ultimately as desirable for
high-performance audio.
- The film dielectrics, which are non-polar, are a different story with regard to DA and DF. All types listed in Table Bl have relatively low values (3 or less) for K, and good to excellent performance with regard to DF and DA. Among the film dielectrics there can be found a direct correlation between K and DA, and even the relatively worst film dielectric (polyester) has a DA of less than 1 percent. The best of them, Teflon, has a DA on the order of 0.01 or 0.02 percent, while polypropylene and polystyrene are nearly as good.
Testing Capacitor DA
Measurement of the DA of a capacitor is a rather involved
procedure when it is done in accordance with MIL-C-19978D [28].
This standard is widely used and referenced by the capacitor
industry, and unless you test a particular type according to the
MIL-C-19978D format, you are not likely to get comparable results
(even though the relative quality relationship may still hold between
different dielectrics).
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The procedure outlined in this specification calls for a five minute
capacitor charging time, a five-second discharge, then a
one-minute open circuit, after which the recovery voltage is read.
The percentage of DA is defined as the ratio of recovery to
charging voltages, times 100.
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It should be understood that this is quite a stringent test, with
regard to both the capacitor and the instrumentation. It takes an
excellent dielectric to show small recovery voltages after a full
charge, a five-second discharge, and a one-minute open circuit. It also takes some special low-current voltmeter
techniques to read this voltage without introducing serious errors.
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To simulate a MIL-C-19978D type of test, we built the circuit of
Fig. 11, which reads the recovered voltage (Vo) via a bench DVM.
The capacitor being tested (D.U.T.) is charged to 0.6 V. This level
might seem low, but was chosen because it represented a typical
peak signal level, particularly for lower level circuits. (A slightly
higher charging voltage would
make measurements easier and more applicable to higher peak
signal voltages, should this be desired.) A MOSFET input amplifier
is used, the CA3160BT. This is done because only a few pA of
bias current are allowable in the D.U.T. circuit; if the current were
higher the D.U.T. voltage would vary, by being charged by this bias
current, and not be distinguishable from the true DA-produced
voltage. In the circuit here, the 3160 bias current begins to limit the
accuracy of readings below about 0.1 percent DA.
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The test procedure is largely self-explanatory. However, the
precautions listed in the notes should be followed, and we
recommend no deviations from the parts specified if you want
comparable results.
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Two series of tests were run with this setup, as outlined in
Table II. The first test compares four similar value capacitors with
different dielectrics to see the differences in DA. As can be noted,
both aluminum and tantalum electrolytics are very poor, with
tantalum sample being slightly worse than the aluminum. This might
be expected from their relative Ks.
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The metalized polyester unit is far better than either electrolytic,
measuring less than 0.15 percent. This may be quite good for
polyester types, as typical specification data available do not always show comparably low figures. The metalized
polypropylene unit is extremely good in terms of DA with a
measured figure which compares quite well with the
manufacturer's data. The polypropylene foil unit is not quite as
good, but is still excellent.
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For both units 3 and 4 (or any other comparably low percentage
DA type) the particular test conditions chosen are very sensitive to
millivolt or sub-millivolt errors. This is simply because 0.1 percent of
0.6 V is only 600 u V�a voltage easily lost or obscured without very careful construction and
calibration of the setup. Higher charging voltages (say 10 V) would
ease this burden considerably, but we do not as yet know that
such a test level can always be directly correlated to lower levels.
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Test 2 examined a number of higher value aluminum and
tantalum electrolytics. Comparison of units 1 and 2 shows that a
higher voltage rated unit of the same value will tend to have a
lower relative DA. This is an interesting point, as this same consideration for selection criteria is also true with regard to
DF. It means that wherever possible, if you must use an
electrolytic, use the highest practical voltage rating. This applies to
either aluminum or tantalum units. Units like number 1 should be
avoided at all costs!
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Unit three is a 50- u F non-polar aluminum electrolytic of a type
often seen in solid-state audio circuits. As can be noted it has a
somewhat lower DA. Apparently, a back-to-back connection tends
to reduce the DA of a single unit. For example, unit 4, actually a series pair of two units like number 1,
shows less DA than a single. This tends to say that nonpolar units
or non-polar connected conventional electrolytics will be better in
DA relative to a conventional polar cap. However, this difference is
largely academic we feel, since if you want really high-quality
sound, you cannot tolerate more than a small fraction of a percent
DA. Obviously this rules out all but the best of the film dielectrics.
Unit 6 is an example of one of the better quality aluminum
electrolytics (see also Fig. 7).
- While studying the DA problem in tantalum and aluminum electrolytics, we also bench-tested a large number of : various units in a much simpler, unbuffered test setup. The 0 basic procedure was to charge a cap to S V for 10 seconds 0 discharge it (through a 1K) for 10 seconds, then open circuit it, and read the recovery voltage after 30 seconds. With this technique we could grade the various units into relative DA categories. The best would read less than 5 mV (or 0.1 per cent) for this test, the worst over 20 mV (0.4 percent). Obviously, this simple test does not compare directly with the Fig. 11 test results, but it still can grade units relatively. And we would invariably find that lower DA units would sound better in an audio circuit. However, the clincher is that no electrolytic known to us, aluminum or tantalum, sounds like a wire in even so simple an application as a coupling cap. Once you try some of these tests for yourself in a good audio system, one free of masking, you may begin to abhor capacitors and seek means to eliminate them where at all possible and, indeed, where it is possible this is perhaps the most effective method of eliminating these distortions. However, it is not always practical to eliminate capacitors, therefore ways to minimize their degrading of the signal are valuable and will be discussed.
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"Tuning" typically used in audio circuits with quality capacitors Since we would otherwise be endlessly asked "How can I improve my brand X preamp or power amp using the improved capacitor types recommended in this article?" it seems appropriate to make some comments as to the methods which would be typically used. First, readers should understand that we are not equipped to answer individual requests for consultation in these areas. If you cannot translate our general comments into the specific steps appropriate to modify your particular gear, please ask a more technically knowledgeable friend for some help. One should not attempt these changes without some prior experience in electronics and familiarity with the components used. Please be advised that if you choose to do so, you make such changes at your own risk, which is to say we cannot be responsible for any accidental damage you may incur. You should also be aware that the alteration of some equipment may result in invalidating a warranty and may also influence its potential resale value.
Power Amplifiers
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Performance Comparison of Various Dielectrics
At this point we are ready to survey the various capacitor
dielectrics with regard to their parameters relevant to audio This
we will do for all dielectrics mentioned thus far, with the exception
of ceramic and the electrolytics, since for the highest-quality audio these dielectrics should not be used if at all
possible. Where an electrolytic type is a must because of a time
constant or filter criteria, some qualified recommendations can be
made for aluminum types which make them quite useful; this will be
covered at the end.
-
Table 3 is a summary of the various dielectrics most useful for
consideration, with typical specifications listed for each major
performance parameter (left column). These specs are really
ranges, as are typically available from average suppliers, and are
not meant to represent a given type or series in specific terms.
They are, however, broadly representative of what is generally
available. For a given electrical parameter, the dielectric type (or
types) which are outstanding are noted by shaded areas.
-
While Table 3 summarizes comparative data in discrete form,
Figs. 12 through 16 illustrate graphically a selection of these
different characteristics.
-
Dissipation factor of the various dielectrics is usually given at
25� C, but there is always some temperature dependence. Figure
12 shows that polyester is the worst of the films in this regard, but
the better ones show very flat DF change with temperature.
-
Insulation resistance (Rp) has not been strongly addressed in
this discussion, because it is not often a critical parameter in audio
(at least from a distortion point of view). Figure 13 is an excellent
summary of how the dielectrics compare for Rp. As can be noted,
all show decreases in Rp with increasing temperature.
-
While DF is an important parameter for capacitors, it is also
important that DF remain low for different frequencies. However, in
many dielectrics there is substantial frequency dependence
exhibited by DF, as shown in Fig. 14. The better dielectrics in this
regard are parylene, polystyrene, polypropylene, and Teflon (not
shown). A related parameter is capacitance variation with
frequency, shown in Fig. 15. Again, parylene, polystyrene,
polypropylene, and Teflon (not shown) are best in this regard.
These variations are due to the variation in K versus frequency for
the different materials.
-
Film capacitors are generally quite good with regard to
capacitance variation with temperature, as is shown by Fig. 16.
The better a capacitor in this regard, the more stable a tuned circuit
based on it will be when undergoing changes in temperature. Of the films, polyester is the poorest, followed by
polycarbonate. The lowest TC is exhibited by polystyrene.
-
One should view TC minimization with some caution with regard
to audio use, as in certain dielectrics optimization techniques which
minimize TC raise DA. A good example of this factor is the
characteristic "B & C" parylene dielectrics, which have nominally 0
TCs, but a DA several times that of characteristic A parylene,
which has a 0.1 percent DA but a TC of -200 ppM/�C. For audio
use, the A characteristic would be preferred, since you can't
"compensate" for zero DA, whereas for TC you can (where
necessary). One should, incidentally, check for a possible
compromise in DA for any "0 TC" capacitor; they often occur, and
we do not mean to imply the DA compromise is peculiar to
parylene.
-
The remaining parameters of Table lil are not illustrated by
graphical data, but also deserve comment. For example, the
available tolerances and range of values can strongly influence
the selection of a capacitor, aside from the electrical specs.
Generally, very tight tolerances are available in most films, to below 1 percent on special order. The range of values is a
difficulty, though, particularly when large sizes are needed.
-
Most film capacitors are readily available, many off the shelf, in
sizes up to 1uF. Above 1 IJ F they become very hard to obtain,
and almost non-existent for some dielectrics, such as polystyrene.
-
In the larger values, any film capacitor will be quite large,
relatively speaking. So, to make use of the excellent electrical
properties and ultimate sound quality, we must be prepared to
accommodate a largish capacitor when 1 u F or more is needed. A
factor which can help minimize the final size is the metalized
dielectric. Most film capacitors (except polystyrene) are available
in metalized types, as opposed to the foil-wound variety. The
metalized dielectric uses a very thin metalized layer for the
electrode and thus conserves space. A danger area of metalized
caps is the lead attachment, which is tricky. If it is not done adequately, a high (or worse, intermittent)
Rs can occur. As a result, metalized caps will usually show
somewhat higher levels of DF than a foil unit. However, they can
still be of excellent quality, and the best advice here is simply to
thoroughly check a given type before use.
-
The final "parameter" of Table 3 is the relative cost of the
various dielectrics. A statement that is unquestionably true here is
that you do get what you pay for the "super dielectric" films will
cost you more money for a given value. For example, a
small-quantity of price for a 5pF polypropylene will be on the
order of $8.00, whereas a 5-1uF aluminum electrolytic will cost
about 20 cents.
- This kind of comparison is a sobering one, and the authors would be foolish to think it will not scare many off. However we should not attempt to kid ourselves that "cheaper is better," as it simply is not if you want the best quality. As time progresses and more become aware of the advantages of these excellent capacitors, we hope volume usage will help their price reduction. Where it is inevitable that a cap be used, we should be prepared to pay more for the quality unit necessary. If this seems like a harsh, unrelenting statement, the final summation should give you better perspective for why we feel the true audiophile must be prepared to bite the bullet with regard to capacitors.
Summary
If we have done a good job on this article, a glance at Table III
and considerations of the distortion discussions should allow the
reader to easily select a good capacitor. For reasons of practicality
and other rationalizations, there are the inevitable trade-offs.
However, here is the way we see it.
-
Up to values of about 10,000 pF, polystyrene is the best
all around choice, as it has reasonable size and is readily available
in many sizes, with tight tolerances available. Above 10,000 pF,
and up to 0.1 uF, it still can be used but is much harder to obtain.
-
Above, 0.1 uF polypropylene (or metalized polypropylene) is
the dielectric of choice, as it has nearly the same relative qualities
of DF and DA as polystyrene. Tight tolerances are available (but
will be special order), and you can get capacitors up to 10 u F or
more.
-
Teflon may well be the best dielectric of all for audio, but is
produced in limited volume and is generally not practical. Parylene
is an excellent dielectric also, but limited in electrical size ( 1 uF or
less) and not widely available. Polycarbonate is perhaps the next
best all-around choice behind these and is generally available in a
wide range of values.
-
Polyester types are the most widely available for all the films
and are already widely used in many audio circuits. There is no
doubt that this is due to the generally low cost of these capacitors,
but convenience and low cost should not be primary selection
criteria to a critical audiophile. Polyester capacitors can be readily
heard in good systems, with defects similar to those described for
tantalum but, of course, reduced in magnitude.
-
In our opinion, polyester capacitors should be very carefully
applied in an audiophile's system, and any system using them in
the signal path may potentially benefit by the substitution of (equal
value, voltage and tolerance) polypropylenes or polycarbonates.
We have done this ourselves on different items of equipment, tube
and transistor, with always the same result, a stunning upgrade
in sound quality. Further, we have observed others do similar
things, either completely independently or at our direction, with the
same type of results.
- It is not surprising to us that this type of reaction occurs, since one single polyester or electrolytic (or other polar type) can be heard, and a typical update to an old preamp or amp might replace a dozen or more! If you did nothing more than take an old (stock) Dynaco PA5 preamp and change the capacitors to polypropylenes, you can be literally astounded at the results. All of this is available at moderate cost to anyone who can solder, and you need not send your amp off to the specialty audio shop either! (Capacitor sources are listed in the appendices.)
More Specific Recommendations
Beyond the above described substitutions (which are basically
of a one-to-one variety), we'd also like to show how to use
aluminum electrolytics effectively, so as to minimize their sound
degradation.
-
In Fig. 17 are shown two types of connections which might
need to employ higher capacity value, 100 p F or more. The trick is
to select a low RS electrolytic, such as one of the two specifically
mentioned for CA (see Fig 7 again). Either of them may be
considered overkill from a time constant standpoint, as 50 uF may
be all that is necessary. But, the high value and voltage listed will
minimize RS, and the relatively high voltage also minimizes DA. At
the higher frequencies where the electrolytic becomes inductive,
the film shunt carries the signal and minimizes the audible
degradation.
-
Figure 17A is used where the capacitor will always see a
defined polarity and can thus be correctly polarized. Figure 17B
uses two of the specified types to form a low RS, IOW DA
non-polar electrolytic. For the film cap CB, use a polypropylene if at
all possible; if not, use a polyester. In either case, a smaller
polypropylene shunt Cc helps even further. Optionally, an even
smaller polypropylene or polystyrene (in addition) in the range 0.01
to 0.1 pF may be useful in some circumstances.
-
Figure 17C illustrates how the composite capacitors of 17A or
17B would be best applied as a coupling capacitor (d.c. blocking)
within an actual circuit typical of such use. The load resistance
which the capacitor must feed into is comprised of R, (which may
be the input resistance of, say, a power amp) plus the local
bleeder resistance, RB The net load resistance will be RB.and R,,
added in parallel fashion.
-
For two reasons, this impedance should be minimized. First, and
most obvious, a low impedance is necessary to bleed off any d.c.
leakage of the large electrolytics (which can for certain conditions
be on the order of 1 pA or more). Selecting a load resistance of
10K or less will, for example, reduce the leakage-induced d.c.
offset at the output to 10 mV or less for a leakage of 1 pA.
-
The second reason is to minimize the audible effects of
whatever DA may exist in the capacitors actually used for CA and
CB. A low load (and source) resistance presented to a coupling
capacitor tends to minimize sonic deterioration.
-
In a single blind listening test using such various capacitor
dielectric types as mica, polyester, tantalum, and polypropylene, it
was found that a simple coupling capacitor can degrade sound
quality quite strongly if the load impedance is high. In this test R,
was 50K and RB was varied from infinity down to 1K, and the
source impedance for C was 1K.
-
A tantalum capacitor (Table II, Test 2, sample 1) feeding the 50K
load distorted the sound very strongly, with severe hashy sound
and loss of detail. However, the same capacitor under 1K load
conditions improved in sound quality appreciably (it did not become
transparent, but it did improve). The other dielectrics mentioned
followed similar patterns: Poor performance into the higher
impedance, improvement in clarity with the lower impedance.
However, even the best dielectric on hand in a usable size (5-1JF
polypropylenes) sounded much better into a lower impedance load.
-
Of course, one cannot lower load resistance arbitrarily from this
general viewpoint, as low-frequency response will suffer sooner
or later. But the evidence of these tests and also the general
pattern of bench tests for DA (which show reduced recovery
voltage for low R,) indicate that it is worthwhile to lower load
resistance (within allowable bounds) to minimize DA effects. This
factor can very logically explain points of disagreement on
whether or not capacitors really do sound bad, as it tends to say
they sound bad (within a given dielectric type) to the degree that
the DA is allowed to manifest itself. Minimizing load resistance
tends to optimize the circuit in terms of suppressing the DA.
General procedural guidelines for "tuning" typical audio circuits
with capacitor improvements are described in the sidebar.
-
Interestingly enough, there is very strong indication to us that in
many situations the power supply electrolytics also need the same
careful attention as do signal path units. The general rule of
selection is the same: Use a low RS unit and bypass it with a film
(such as in Fig. 17A). While the degree to which this problem may
be apparent is surely related to the circuit topology, it is certainly
worth consideration in all instances of amplifier circuits, tubes or
transistors.
-
For those readers unfamiliar with the "sound" of capacitors or
this general subject area, much of the above might sound like mad
ravings to some degree or another. We'd like to leave some
implication of what we feel the magnitude of this problem really is.
- After we had gone through all of the above exercises and exorcised our complete system of unnecessary or poor-quality capacitors, the total degree of improvement was greater than any other improvement measure ever employed. With no capacitors (or clean capacitors), you begin to hear the music in a new light, one which is much more like the sound of the real thing. In fact, you will be able to differentiate subtleties you never before even realized existed. Your system simply becomes a new system, in terms of resolution and definition. The "solid-state sound" we've all heard discussed may be largely due to lousy electrolytics�which by and large never got used in the signal path in the tube days.
Acknowledgements
The authors would like to acknowledge private discussions with
John Curl and J. Peter Moncrieff on the subject of capacitors in
audio circuits and how they might influence subjective testing. W.J.
would like to acknowledge the contributions of Dave White on
capacitor problems, and thank him for participating in listening
tests. We also found the discussions by Dow [32] to be particularly
useful in relating the phenomenon of DA to audio circuit behavior.
- In addition, we would like to thank those manufacturers who have allowed use of their data in this article. Technical information as well as price and delivery data can be obtained by writing manufacturers direct (see appendix), mentioning this article.
List of Downloadable Figures, Tables, References, and Graphs
Figures
Figure B1, B2, Table B1
Figure B3, B4
Figure B5
Figure B6
Figure 1
Figure 2, 3
Figure 4
Figure 5
Figure 6, 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16
Figure 17
