724 Chapter 24
impedances is a major failing with power amplifi ers having low or nil global negative
feedback, and the outcome is a tonal anomaly as gross as 5 dB. This may not be all bad,
but it will certainly be arbitrary.
24.2.2.2 Energy Control
In part, a voltage source is required to drive speakers, because loudspeakers store, as well
as convert, energy. The fundamental resonance is the place (in the frequency domain)
where this is most true. Some of the stored energy “ kicks back ” and needs to be damped
quickly (dissipated) to avoid transient distortion or “ smearing. ” The same reactive effects
may also demand surprisingly high peak currents from the amplifi er at other times, when
driven by music signals.
In both cases, the answer is a high current sourcing and sinking capability. Both of these
features are implied but neither are guaranteed by a low source impedance. The overall
requirement is a “ current-capable-enough voltage source. ” So far, most power amplifi ers
throughout history have aimed to be this, but some have come closer than others.
24.2.2.3 Damping Factor?
The majority of high-performance amplifi ers are solid state and employ global (overall)
negative feedback, not least for the unit-to-unit consistency it offers over the wild
(e.g., / 50%) tolerances of semiconductor parts. One effect of high global NFB (in
conventional topologies) is to make the output source impedance ( Z 0 ) very low, potentially
100 times lower than the speaker impedance at the amplifi er’s output terminals. For
example, if the amplifi er’s output impedance is 40 Wohms, then the nominal damping
factor with an 8-ohm speaker will be 200, that is, 40 milli ohms (0.04) is l/200th of 8 Q.
This “ damping factor ” is essential for the accurate control of most speakers.
Yet describing an amplifi er’s ability to damp a loudspeaker with a single number (called
“ damping factor ” ) is doubtful. This is true even in active systems where there is no
passive crossover with their own energy storage effects, complicating especially dynamic
behavior.
Figure 24.10 again takes a sine-swept impedance of an 8-ohm, 15 driver in a nominal box
to show how “ static ” speaker damping varies. Impedance is 70 ohms at resonance but 5.6
ohms at 450 Hz. The bottom part of Figure 24.10 plots the output impedance of a power
amplifi er that has high negative feedback, and thus the source impedance looking up (or
into) it is very low (6 milliohms at 100 Hz), although increasing monotonically above