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Capacitors, |
Passive devices, on the other hand, are the electronic equivalent of couch potatoes. They just kind of sit there, unchanging in behavior if the stimulus is a constant, or is constantly repeating. Resistors resist, capacitors and inductors store energy, the former as an electric charge and the latter as a magnetic field. Today's "lesson" focuses on Capacitors, and a rather more "High Class" example of them at that. Contrary to what this title suggests, High Speed Capacitors are not actually a bunch of little speed freaks, whizzing electrons around with any kind of gusto that other, more pedestrian, capacitors can't quite muster. Nonetheless, High Speed Capacitors are in fact the ones that look rather too well fed, they are the fatter ones, often with a more glamorous assortment of colors, shapes and of course fatter prices, too. So don't look for them in your Sony Walkman, there won't be any!
"High Speed" implies lithe, slim and purposeful, which as as a group, they appear cosmetically to be anything but. Part of it is the misnomer of the name: A"high-speed" capacitor is not the fastest one in the race, but rather more the most forgetful one in the store! That's right. The more information they forget, the "faster" they are.
Linear, and that other behaviour:
All electronic components, at least according to established theory, have two sets of behaviour. One set is the really obvious common denominator, the behaviour that is readily predicted, linear and repeatable. The other set of behaviour is the one you never hear about, the ones that set the real world expectations up against the theoretical ones. After all, electronics works on theory first and foremost. Implementing an idea turns that theory into reality. The best designers and engineers are the ones that notice the little (and sometimes not so little) idiosyncrasies in component behaviour, and make the necessary choices. Usually, those choices are mainly predicated on price, hence no "high-speed" capacitors in that Walkman.
The other is size. If you were to replace the power supply capacitors in even a small Bryston transistor amplifier with "high-speed" polypropylene types, you'd end up with an amplifier about as large as your home refrigerator that would cost about as much to buy at retail as a Mercedes E-Class automobile! So even if you wanted to do this for the sake of improved performance, you'd have to admit that there are more space and cost-effective ways of improving that amplifier.
| Let's look more closely at capacitor technology. Capacitors consist of two conductive surfaces (lets call them plates), separated by an insulating material (the "dielectric"). To save space, the capacitor structure is wound upon itself into either a cylindrical or ovoid shape. The ovoid types are those that look like little "boxes", and typically have short 5 mm leads exiting the bottom towards each end. These are types made for Printed Circuit Board (PCB) mounting to speed and simplify production-line assembly. Some larger motor-starting capacitors may also come in a steel ovoid case that approaches the size of a cola can. They have "quick-disconnect" terminals that you can either solder to or use an industry-standard "blade" type of connector (usually a "crimp-on" type). |  |
| But most capacitors are cylindrical. And while the "better" capacitors use a plastic film as the dielectric material, others, such as electrolytic types, use a paper soaked liquid paste instead. These electrolytic types are much inferior in performance, but there is a place for them. They work efficiently in low voltage (solid-state) environments, and can pack incredible amounts of capacitance into a relatively very small package. They are therefore (and dread these words), often the most cost-effective types for a given application. And like the Bryston example above, this often precludes the consideration of anything better for pragmatic reasons, and the electrolytic capacitor predominates in all but the highest-end products. |  |
Differences among Electrolytics
Yet even electrolytic capacitors have their differences - their "high-speed" variants. A look through a manufacturers' catalogue shows that some are made better than others, with a premium being charged for those that have "long life" or "Improved ESR" (equivalent series resistance). Some have heavy metal parts, for high current applications. Others use a better gel instead of the usual paste electrolyte to improve things. Some are optimized for small size yet can provide higher discharge currents, such as those for photoflash unit work. Yet these types can trade off one set of desirable parameters by sacrificing other necessary performance aspects.
The Photoflash types mentioned typically are "leaky", requiring current to maintain charge that better types wouldn't need, and while their ability to deliver current is without question, they don't necessarily like the high-inrush currents required in a high-power audio amplifier. And they don't last as long as say, a "Computer Grade" type.
The best capacitors are the plastic film dielectric types, with Teflon being at the top of the pile. Even air or a vacuum can be used as dielectric, and are arguably better yet, but seldom are because of their outrageous size and cost. Air is used for ultra-low value "trim" capacitors, where a tiny value is needed to tweak a circuit parameter for optimum performance. High-frequency work is very value-sensitive, and air types are most often used here for this purpose. Vacuum has the advantage of extremely high insulation resistance, making it the trim capacitor of choice in high-frequency, very high power amplification such as radio and television transmitters. They function reliably at tens of thousands of volts, something that other types can't do. These capacitors can cost thousands of dollars each, and can be very large. Suffice to say that they are really beyond our interest for high-quality audio work.
Dielectric Constant and Absorption
There are other "solid" dielectric types, such as tantalum oxide(a glorified electrolytic whose main advantage is smallest size), ceramic (cheap in low capacitance values, reliable), and silver mica (better than ceramics for high frequency work). They all boast a high "dielectric constant", which means that you need less dielectric material for a given capacitance value. While that means a nice small capacitor, with a low inductance that is needed for high frequency work, there is a price to pay. And in sonic terms, the price is high.
There is a rough correlation between dielectric constant and dielectric absorption (DA). Dielectric absorption is that "memory" aspect we mentioned earlier, the very thing that sorts out the "high speed" capacitors from their lesser brethren. As the dielectric constant increases (good) giving that smaller package, dielectric constants for the materials used tend be correspondingly worse. So the capacitors with the highest "speed" at passing all their retained charge are the ones with the least efficient dielectric constants. In order, they are:
1) Vacuum 2) Air 3) Teflon 4) Polypropylene and polystyrene 5) Polycarbonate
6) Polyester 7) Ceramic, mica, tantalum oxide, aluminum oxide, "electrolytic"
Now you can see why teflon, polypropylene and polystyrene types are so revered. Their DA values, are in the range of 0.01-0.03% typically. For polycarbonate this is increased tenfold, for polyester it doubles yet again. And as bad as that all seems, for the junk lumped into category seven, the DA is in the order of 5%! Ironically, DA doesn't really manifest itself very much with today's (well, let's call it yesterday's) measurement protocols of simple harmonic and twin-tone intermodulation measurements. Yet in the best sounding equipment, the substitution of capacitors for different types can make or break its subjective performance.
Measurements still have a poor track record of correlating with what you can hear, but given the complexity of real-world behaviour in just the parts alone, you may be able to see why I feel that measurements will never be able to adequately express the sonic value of a component. While components generate distortion, and nonlinear behavior is distortion, we don't have any definitive yardstick to use to try to correlate these distortions with their subjective impact on the listening experience. DA is a major source of distortion in high-quality amplification, and practically all high end manufacturers pay heed to the importance of reducing capacitor memory. Some have gone to the length of using high speed polypropylene capacitors exclusively in an attempt to define the state of the art.
Art...
One example is the conrad-johnson ART pre amplifier, a horrendously expensive ($15K USD) line-level preamplifier. In a future essay, I will discuss just how DA manages to distort the audio signal. Suffice to say that a certain amount of linear signal "absorption" probably wouldn't be subjectively devastating, otherwise Mini-Disc and MP3 would be unpalatable. Not that anyone pretends that they offer real fidelity, mind... DA has a "sting" in its' tail, that small percentage of signal/charge absorption is only the retained charge. DA correlates with the degree of time smear, charges absorbed in the dielectric material that pass through...later! And I mean much later, like hundreds of milliseconds if not seconds later! This is the mechanism that does the real signal damage, generating uncorrelated garbage instead of an analog to the musical signal. Stay tuned...
Joseph Rosen
