Audio Amplifiers
The real output impedance (Zsource) of a power amplifier is usually less than 0.1 Ω, but this is rarely specified. Instead it is "hidden" within the damping factor parameter, which is:
Solving for Zsource,
gives the small source impedance (output impedance) of the power amplifier. This can be calculated from the Zload of the loudspeaker (typically 2, 4, or 8 ohms) and the given value of the damping factor.
Generally in audio and hifi, the input impedance of components is several times (technically, more than 10) the output impedance of the device connected to them. This is called impedance bridging or voltage bridging.
In this case, Zload>> Zsource, DF > 10
In video, RF, and other systems, impedances of inputs and outputs are the same. This is called impedance matching or a matched connection.
In this case, Zsource = Zload, DF = 1/1 = 1 .
The actual output impedance for most devices is not the same as the rated output impedance. A power amplifier may have a rated impedance of 8 ohms, but the actual output impedance will vary depending on circuit conditions. The rated output impedance is simply that impedance into which the amplifier can deliver its maximum amount of power without failing.
Read more about this topic: Output Impedance