Damping Factor - in Practice

In Practice

Over a range from less than 1 to up to around 50, damping factor describes the ability to which the amplifier helps to control the oscillation of the speaker, expressing the degree to which the amplifier does not exert a resistance against the current generated by the speaker. A damping factor of 50 is high, indicating that the output impedance of the amplifier is negligible compared to the internal resistance of the speaker and the speaker cabling.

Higher electrical damping of the loudspeaker is not necessarily better. Some loudspeakers sound better with lower electrical damping. A lower damping factor helps to greatly enhance the bass response of the loudspeaker, which is useful if only a single amplifier is used for the entire audio range. Therefore, some amplifiers, in particular vintage amplifiers from the 1950s, 60's and 70's, feature controls for varying the damping factor.

One example of a vintage amplifier with a damping control is the Accuphase E-202, which has a three-position switch described by the following excerpt from its owner's manual:

"Speaker Damping Control enhances characteristic tonal qualities of speakers. The damping factor of solid state amplifiers is generally very large and ideal for damping the speakers. However, some speakers require an amplifier with a low damping factor to reproduce rich, full-bodied sound. The E-202 has a Speaker Damping Control which permits choice of three damping factors and induces maximum potential performance from any speaker. Damping factor with an 8 ohm load becomes more than 50 when this control is set to NORMAL. Likewise, it is 5 at MEDIUM position, and 1 at SOFT position. It enables choosing the speaker sound that one prefers."

By contrast, in modern high fidelity amplification, the trend is to separate the bass signal and amplify it with a dedicated amplifier. Often, amplifiers for bass are integrated with the speaker cabinet, a configuration known as the powered subwoofer. In a topology that includes a dedicated amplifier for bass, the damping factor of the main amplifier is not relevant, and that of the bass amplifier is also irrelevant if that amplifier is integrated with the speaker and cabinet as a unit, since all those components are designed together and optimized for the reproduction of bass.

Damping is also a concern in guitar amplifiers (an application in which distortion is desirable) and low damping can be better. Numerous guitar amplifiers have damping controls, and the trend to include this feature has been increasing since the 1990s. For instance the Marshall Valvestate 8008 rack-mounted stereo amplifier has a switch between "linear" and "Valvestate" mode:

"Linear/Vstate selector. Slide to select linear or Valvestate performance. The Valvestate mode gives extra warm harmonics plus the richness of tone, which is unique to the Valvestate power stage. Linear mode produces a highly defined hi-fi tone that gives a totally different character to the sound and suits certain modern "metal" styles, or PA applications."

This is actually a damping control based on negative current feedback, which is evident from the schematic, where the same switch is labeled as "Output Power Mode: Current/Voltage". The "Valvestate" mode introduces negative current feedback which raises the output impedance, lowers the damping factor, and alters the frequency response, similarly to what occurs in a tube amplifier. (Contrary to the claim in the handbook, this circuit topology has appeared in numerous solid-state guitar amplifiers since the 1970s.)