The value of a cap affects what it will do The 'right' capacitor simply does not exist. This is a fool's errand, but is often reinforced by others with the same mindset.
The search for 'tone' often involves esoteric capacitors, with some people imagining that if they could just find the 'right' capacitor they will sound like. Guitarists are one group commonly targeted by snake-oil vendors (this may include famous manufacturers!). But where is the data? Who conducted the test? How was it conducted? Was the test ever really conducted at all? Most claims of this nature indicate that there is a hidden agenda, so beware. In some cases you will read things like "listening tests have indicated. even as a supply bypass (yes, it's true - this claim has been made). There are sites that seem to have impeccable credentials, but have managed to create nothing but FUD (fear, uncertainty & doubt) with wild claims of irreparable damage to the signal by using the 'wrong' kind of cap . There are sites on the Net showing that different caps have different properties, and this is often used a 'proof' by many people that the differences are audible. There is no need for 'special' caps in this application, but they still should be metallised film types (not high 'K' ceramics - ever!). In active filters (typically opamp based), the caps generally have very low current (a couple of milliamps at most) and low voltages. Electrolytic capacitors (whether polarised or not) change their value over time, and are simply not suitable for high fidelity systems. It's generally accepted that polypropylene is the optimum dielectric for this (and similar) applications, but for lower powered systems polyester is usually quite alright. These need to be rated accordingly, and although there are bipolar (aka non-polarised) electrolytic caps sold for the purpose, IMO they are suited only for systems where fidelity is not a major concern. This applies for active and passive filters, but caps used in passive loudspeaker crossover networks have to carry high current and often (relatively) high AC voltages as well. Signal capacitors (as opposed to power supply 'storage' caps) work their hardest when used in filter circuits. Dielectric losses (dissipation factor, dielectric absorption) feature heavily, with some fairly outrageous claims made as to the importance of these losses in amplifiers and other audio equipment. In most cases, these debates are centred on coupling caps, which (as noted above) generally have very little voltage across them. In the light of this simple fact, it's very hard to know why such a great deal has been made of the 'sound' of capacitors. There will always be a tiny voltage present, but it's generally small enough to be ignored in an analysis.
With no voltage, there is no stored energy. The AC presented to one side of the cap is coupled through to the other side, and if the cap is large enough (compared to frequency and circuit resistance), it will never have any appreciable voltage across it.
Energy storage is certainly true for caps used in power supplies or to bypass the supply rails of power amps or opamps (for example), but caps that are used for coupling a signal and blocking DC (or simply as a safety measure should DC ever become present) perform no 'energy storage' at all, other than accidentally. It's often said that capacitors provide 'energy storage', but in reality, many used in audio circuits do nothing of the kind. 3.0 Parasitic Inductance in Bypass Applications.1.4.1 - Ceramic Capacitor Acoustic Noise.
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