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: Dick Pierce
Date: 1998-2002
In his introduction, Mr. Pierce states that much ballyhoo surrounds the concept of "damping factor." It's been suggested that it accounts for the alleged "dramatic differences" in sound between tube and solid-state amplifiers. Also, many of the claims made, especially for the need for triple-digit damping factors, are not based in any reality, be it theoretical, engineering, or acoustical.

What is damping factor? Simply stated, it is the ratio between the nominal load impedance (typically 8 ohms) and the source impedance of the amplifier. Note that all modern amplifiers (with some rare exceptions) are, essentially, voltage sources whose output impedance is very low. That means their output voltage is independent over a wide range of load impedance.

Mr. Pierce ran an analysis of the effects of amplifier damping factor on the decay time and frequency-dependent response of a closed-box, acoustic suspension loudspeaker system. The results were that any damping factor over 10 resulted in inaudible differences. Calculations suggested that a damping factor in excess of 50 will not provide audible improvements. Also, for audio power amplifiers employing global negative feedback, the source impedance is generally smaller than 0.1 Ω.

Comment:
At 0.1 Ω, with an 8 Ω speaker, the damping factor would be 8 divided by 0.1 = 80. The paper written by Mr. Pierce has a copyright statement concerning distribution of the paper and although a copy is on file, it is not provided here. If you search, you can find more information on Mr. Pierce's paper.