All capacitors work on the same principal, two plates separated by an insulator (dielectric) that blocks DC while allowing AC on one plate to appear on the other plate; audio is a form of AC. In amplifier audio circuits, capacitors are used in various places along the audio path. From a technical point of view, you would think that something as simple as two plates separated by an insulator would all perform the same and sound the same.

Not necessarily. It is not unreasonable to say that some capacitors may perform better than others depending on the materials or manufacturing process used. Rather than depend on statements of performance stated by individuals or component suppliers (after all, they are trying to sell you something), technical testing with comparison of different types of capacitors would be more definitive.

A Texas Instruments Analog Design Journal article, Selecting Capacitors to Minimize Distortion in Audio Applications, written by Zak Kaye, Applications Engineer, provides insight. If interested, the article is available here. Test results presented in the article are applicable to use in audio amplifiers.

The article primarily deals with solid state circuit voltages. Keep in mind that it is the AC performance of a capacitor that is important. The voltage rating of a capacitor depends on the circuit the capacitor is used in. Also, the article considers tiny surface mounted capacitors used on printed circuit boards. Capacitor size is inconsequential, it is the type of capacitor that matters. For vacuum tubes, larger sized capacitors are used, usually with wire leads for point to point wiring.

On the conclusion page, there are two types of capacitors recommended for lowest distortion. The C0G/NP0 (multilayer ceramic) and film (polyethylene or polypropylene) type capacitors. Multilayer ceramic capacitors with C0G/NP0 dielectric plus long wire leads and high voltage ratings are difficult to find.

Capacitors Across Pot Terminals

However, C0G/NP0 multilayer ceramic capacitors with short leads are useful for close terminal connections such as across pot terminals. Polyethylene or polypropylene film capacitors with high voltage ratings and long wire leads are available for normal point to point wiring.
Sample of suppliers:
Antique Electronic Supply
Amplified Parts
Just Radios Capacitor Order Form

In Zak Kaye's article, he mentions that to reduce its distortion, increase the value of a capacitor until its impedance is low enough in the band of interest (for instance, 20Hz to 20,000Hz). It's a matter of calculating the –3 dB cutoff frequency of the capacitor-load impedance to the lowest possible value. For instance, a .47uF capacitor will have a lower –3 dB cutoff frequency than a .1uF capacitor. It would also be wise to voltage rate capacitors with a generous safety factor. For example, a 600 volt rated capacitor in a 400 volt circuit. If on a tight budget, low-cost polyethylene or polypropylene film capacitors are available. With careful circuit design, they should perform as well as any other polyethylene or polypropylene capacitor.

Box & cone speaker systems (cone speakers mounted in an enclosure) are prone to deficiencies that introduce distortion.

The typical cone speaker design is a cone suspended in a frame by a flexible membrane from the outer and inner area of the cone to the frame. In the center of the cone is a coil attached to the cone. The coil is held in position within a gap of a fixed magnet by the cone suspension. The amplifier sends a signal to the speaker coil, producing a magnetic field in the coil that interacts with the fixed magnet. This results in the coil and attached cone moving in and out according to the amplifier signal. The cone moves air molecules, producing sound. Although a speaker of this design is driven by an electrical signal, it is by nature a mechanical design.

A speaker cone must stay rigid. To keep the cone rigid, it may be made out of heavy paper or some other stiff material. At lower volume levels, the speaker cone moves with relative ease. As the volume increases with higher amplifier power, the cone suspension, weight of the cone, and momentum work against cone movement.

Momentum is "the force or energy gained by a moving object." The weight of the cone adds to momentum. At higher volume levels, the speaker cone is rapidly pushed and pulled a significant distance as compared to lower volume levels. At the same time, the suspension, attaching the cone to the frame, is trying to keep the cone in the center of travel.

Ideally, a speaker cone should be able to move without any restrictions. Any force placed upon the cone in opposition to its intended motion in effect alters the cones performance, resulting in distortion. At higher volume levels, the amount of opposition to cone travel increases exponentially as momentum and the pull of cone suspension increase.

In addition to mechanical distortion, speaker systems that employ more than one driver using a crossover filter are subject to artifacts such as phasing between drivers. Although crossovers, typically filtering circuits with inductors and capacitors, may be carefully designed there will be some phasing in the crossover region. Multi-speaker system artifacts are fairly consistent at low or high volume. The distortion produced by mechanical artifacts increases at higher volume levels.

Distortion caused by the mechanics of cone speakers is seldom, if ever, considered. The reason for this writing is to point out that it does not matter if amplifiers in the path to the speaker have a couple percent of distortion, especially at higher volume levels where speaker distortion increases. In practice, at lower volume levels, a decent box and cone speaker system should provide a pleasing sound with perhaps one or two percent distortion. Exactly how much distortion is hard to tell, as speaker manufacturers very seldom state distortion figures (although involved, it is possible to measure speaker distortion). An AI search for the question of which speaker manufacturers provide distortion figures returned the following answer. "Most speaker manufacturers do not provide distortion specifications in their product listings, as it is often considered less favorable compared to the low distortion levels typically associated with electronics."

The bottom line is, if you are using a box and cone speaker system, you will have one or two percent of system distortion regardless of how low of a distortion figure an amplifier system has. Regardless of equipment specifications, sound reproduction preference is a matter of the listener's perception. Beyond the realm of equipment test results, it is an area more appropriately left to the field of psychoacoustics.

Electrostatic speakers deserve a mention. In 1971 I visited a coworker that had a pair of electrostatic speakers. At the time, they were the cleanest speakers I ever heard. From a reliability and budget standpoint, they are pricey and prone to overload failure.

Stereophile article
Pros & Cons of electrostatic speakers

Amplifier damping, used to reduce speaker ringing, is not effective in reducing mechanical speaker distortion.

Source: Crown Audio
Date: Unknown
Loudspeakers have a mind of their own. You send them a signal and they add their own twist to it. They keep on vibrating after the signal has stopped, due to inertia. That's called "ringing" or "time smearing." In other words, the speaker produces sound waves that are not part of the original signal.

Suppose the incoming signal is a "tight" kick drum with a short attack and decay in its signal envelope. When the kick-drum signal stops, the speaker continues to vibrate. The cone bounces back and forth in its suspension. So that nice, snappy kick drum turns into a boomy throb.

Fortunately, a power amplifier can exert control over the loudspeaker and prevent ringing. Damping is the ability of a power amplifier to control loudspeaker motion. It's measured in Damping Factor, which is load impedance divided by amplifier output impedance. Let's explain. If the speaker impedance is 8 ohms, and the amplifier output impedance is 0.01 ohms, the damping factor is 800. That's a simplication.

Since the speaker impedance and amplifier output impedance vary with frequency, so does the damping factor. Also, the impedance of the speaker cable affects damping. Thick cables (with low AWG) allow more damping than thin cables with (high AWG).The lower the amplifier's output impedance, the higher the damping factor, and the tighter the sound is. A damping factor of 1000 or greater is considered high. High damping factor equals tight bass.

How does an amplifier control speaker motion? When the loudspeaker cone vibrates, it acts like a micro-phone, generating a signal from its voice coil. This signal generated by the speaker is called back EMF (back Electro Motive Force). It travels through the speaker cable back into the amplifier output, then returns to the speaker. Since back EMF is in opposite polarity with the speaker's motion, back EMF impedes or damps the speaker's ringing. The smaller the amp's output impedance, the greater is the effect of back EMF on the speaker's motion.

An amplifier with low output impedance does not impede the back EMF, so the back EMF drives the loud-speaker with a relatively strong signal that works against the speaker's motion. When the speaker cone moves out, the back EMF pulls the speaker in, and vice versa. In short, the loudspeaker damps itself through the amplifier output circuitry. The lower the impedance of that output circuitry, the more the back EMF can control the speaker's ringing.

Damping factor varies with frequency. As you might suspect, damping factor is most important at low fre-quencies, say 10 Hz to 400 Hz.