Tuesday, September 16, 2014

Capacitors: All Things are NOT Created Equal - Part 2

There are many electrical measurements one can make on a capacitor to help you understand its behavior. Manufacturers of capacitors, for example, sometimes publish the ESR (equivalent series resistance) specification that indicates how much loss is incurred by adding this capacitor to the signal path. This ESR loss in a crossover network means that the power that should go to the driver is instead consumed by the capacitor and translated into heat. High-quality low ESR capacitors are typically more expensive to manufacture and are therefore usually not used in budget electronic devices.

By simply adding a capacitor to a crossover network, the output of the driver will be lower than without it. How much lower is a function of the ESR and the rated impedance of the driver. So it appears from this specification that capacitors with low ESR would be a good thing since it would allow the driver to be louder thereby permitting it to reveal more detail. However, not all capacitors are created equal and some with low ESRs sound absolutely terrible. So a combination of factors must contribute to why one capacitor design sounds better than another.

It is common for equipment designers to use DC-blocking capacitors between gain stages. Doing so permits the two gain stages from adding a DC offset that alters the uniformity of performance of the device, so this can be a good thing. But using a capacitor with a high ESR (i.e., a low-cost capacitor) can also alter (degrade) the signal. How much degradation occurs cannot be measured by standard THD and IM measurements (or the value of the added degradation is so low as to be considered insignificant) but its audible effects can be sometimes detected by the ear.

For example, one capacitor type with low ESR uses a mica insulator. Mica is inexpensive and provides excellent performance in high-voltage applications and in the radio-frequency band. However, mica capacitors in the audio band yields sterile and edgy sounds that significantly color the audio signal. Yet, measuring  IM/THD in an audio circuit provides no hint as to why this occurs. This same phenomenon occurs when using non-polar electrolytic capacitors.

So what gives here? What is it about this simple design (two conductors separated by an insulator) that makes such a huge difference in the sound? In Part 3 we will explore other physical properties that can give us a hint as to why this occurs. Until then, know that any capacitor in the signal path or feedback loop of any audio circuit can be viewed with suspicion.

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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