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"Damping Factor" is a measure of amplifier output impedance.

Posted by Al Sekela (A) on November 1, 2004 at 14:57:00

In Reply to: Any "Damping Factor" tweaks? (Not vibe isolation!) posted by dlarkin_dc on November 1, 2004 at 13:28:34:

The power amplifier attempts to deliver an output voltage independent of its load. It fails to do so because there is some voltage drop within the amplifier caused by the current drawn by the load. The simple model for this is a fixed impedance in series with an ideal voltage source. The fixed impedance is called the output impedance.

Since speaker impedances and amplifier load impedance ratings vary, the output impedance of the amplifier by itself is not as useful as a figure of merit called "Damping Factor."

DF = Eno load/(Eno load-Eloaded),

where "Eno load" is the output voltage with the load disconnected, and "Eloaded" is the output voltage with the rated load connected. A very large damping factor number results if the amp has low output impedance such that the loaded output voltage is close to the unloaded output voltage.

Amplifier output impedance relates to the series resistance of the output devices and power supply, and whether or not negative voltage feedback is used. Humble output device complements may be made to appear more robust by use of negative voltage feedback in the design of the amplifier.

One could contemplate tweaking the feedback gain to decrease the output impedance and increase the damping factor, but there is a real risk of making the amp unstable. Not to sound snotty about this, but if you had to ask the question, you are not qualified to modify the circuit.

Keep in mind also that a higher damping factor is not necessarily better. In the 1950s, when hi-fi speakers had monster magnet structures, low damping factors were necessary to avoid overdamping the woofers. There are some high-efficiency speakers today that only give good bass with low-powered amps with low damping factors.

Most speakers these days, however, are designed to work with solid-state amps that have high damping factors as a consequence of the negative voltage feedback necessary to reduce their harmonic distortion. The speaker designers are casual about load impedance variation over the operating frequency range, and these speakers can sound underdamped and colored with amps without high damping factors.

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