Wednesday, September 17, 2014

Capacitors: All Things are NOT Created Equal - Part 4

In Part 3 of this series, you learned what the three basic components of any capacitor were: two plates separated by an insulator (aka dielectric). Part 3's focus was on the insulator of non-electrolytic capacitor and how the material contributes to the overall cost of the final product. In this part, we will look at the plates to see how they also impact the sonic signature and price of this simple device.

Capacitor Components:
Plates (A and B) and Insulator (dielectric)

You may think that a good material choice for capacitor plates would be copper, gold, or silver based on their conductivity (the property a material has to efficiently permit electricity to flow through it) and you would be absolutely correct. However, these three materials also cost significantly more than another, that being aluminum. So the vast majority of capacitors use aluminum as the plate material of choice. But there is an issue with this material: it is difficult to make connections to it, especially when you consider how extremely thin the plate is.

Anyone who knows even a little about welding understands the issues behind creating a solid weld to aluminum. Unlike steel or iron, aluminum requires significantly higher heat since its melting point is much higher than the other three. Plus making joints between different materials is even tougher (say an aluminum plate to a copper wire). Here the copper tends to evaporate before a weld is achieved so crimped (physically mashed connections that do not use heat) are commonly used. 

Another option is to introduce a momentarily high temperature at a small spot to fuse a tiny portion of the two different materials together (called a tack weld). Tack welds, while they do work, only provide a small cross-sectional area for electrical conduction. This means that several tack welds are required in order for the junction to carry any significant amount of current without destroying the weld during normal use.

The other issue with using different materials in making crimped connections in capacitors is that the expansion rates of the two materials is different: one material expands more or less compared to another. Dynamic temperature changes inside of a connection over time cause the materials in that connection to slide back and forth: what was once solid becomes less so. Also, under certain conditions, tack welds can also create diodes at the joints meaning that instead of conducting a sinusoidal signal they conduct only half of that signal. The solution of course is intuitively obvious: use the same material for the leads as the plates or use two materials that are more compatible (weld at roughly the same temperature without evaporating either material in the process). Unfortunately, soldering an aluminum lead to a copper circuit is impossible so we are back to abandoning aluminum as a plate material of choice.

If low cost drives a design, aluminum plates are the best choice to save money. But because of the inherent drawbacks with bonding different materials together, higher-cost similar materials are desired for high-end audio. Remember, the conductivity property of silver is best followed in order by gold and copper. Each of these three materials readily bond together however over time dissimilar metal corrosion will occur at the joints (a slow degradation process that occurs at the molecular level). So again the best solution is to use the same material for the plates and the leads regardless of what material is selected. 

The next best choice would be copper plates and leads knowing that the conductivity losses are acceptable compared to the aluminum alternative. Or build a capacitor out of gold or silver with copper leads knowing that the conductivity will be better and the life expectancy will be less. Since the life expectancy of an average piece of audio gear is well within the time it takes for significant junction degradation to occur, such dissimilar materials are therefore highly desired.

So making a capacitor is more difficult than one initially suspects, especially if low cost is driving your design. Plate, lead, and dielectric material choices all influence not only the cost but also the life expectancy and built-in signal degradation. It's a wonder that they work at all with so much stacked against them. And we haven't even discussed how large values and mass-production issues introduce other problems with this simple design (we'll briefly look at them in Part 5).

Although this is not a thorough exploration of why capacitors sound different, it does give you an idea of why they could potentially sound different. There is a saying that applies to high-end anything (auto racing, photography, audio, etc.): the devil is in the details. It is at this highly detailed level where the differences between the state-of-the-art are and it is why a good quality capacitor costs what it does.

Related articles:
The Vishay 1837 Review and Modification
Bypass Capacitors
Mundorf Supreme Capacitor Review - Part 1
Mundorf Supreme Capacitor Review - Part 2
Capacitors: All Things are NOT Created Equal - Part 0
Capacitors: All Things are NOT Created Equal - Part 1
Capacitors: All Things are NOT Created Equal - Part 2
Capacitors: All Things are NOT Created Equal - Part 3

Yours for higher fidelity,
Philip Rastocny

Skeptics are essential to keep us sane; skeptics do little to keep us inspired. Philip Rastocny, 7-16-2014

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