726 Chapter 24
to, after it has traversed a given speaker cable and passed through an ideal 10,000- μ F
series capacitor, as fi tted commonly in many professional cabinets for belt ’ n ’ braces DC
fault protection. The rise at 1 kHz is due to cable resistance, while cable inductance and
series capacitance cause the high- and low-end rises, respectively, above 100 milliohms.
We can easily read off static damping against frequency: at 30 Hz, it’s about l00. At
midfrequencies, it’s about 50, and again about 100 at 10 kHz. However, instantaneous
“ dynamic ” impedance may dip four times lower, while the DC resistance portion of
the speaker impedance increases after hard drive, recovering over tens to thousands of
milliseconds, depending on whether the drive-unit is a tweeter or a 24 shaker.
Even with high NFB, an amplifi er’s output impedance will be higher with fewer output
transistors, less global feedback, junction heating (if the transistors doing the muscle
work are MOS-FETs), and more resistive or inductive (longer/thinner) cabling. Reducing
the series DC protection capacitor value so it becomes a passive crossover fi lter will
considerably increase source impedance, even in the pass band. The ESR (losses) of
any series capacitors and inductors will also increase source impedance, with small, but
complex nested variations with drive, temperature, use patterns, and aging. The outcome
is that the three curves—and the difference between the upper two that is the map of
damping factor—writhe unpredictably.
Full reality is still more complex, as all loudspeakers comprise a number of complex
energy storage/release/exchange sections, some interacting with the room space, and each
with the others. The conclusion is that damping factor has more dimensions than one
number can convey.
24.2.2.4 Design Interaction
While high-performance loudspeakers are being designed and optimized, and certainly
before they are fi nalized for production, in-depth listening is a prerequisite. This means
that amplifi ers are required to drive them while they are being tested and optimized in the
design process. Many drive-unit and speaker manufacturers are limited (or limit
themselves) to using just one amplifi er make to test their designs and production. The
situation is rarely publicized.
It follows that many loudspeakers are inevitably looking for that one kind of amplifi er
that was used when they were “ voiced ” and “ tweaked ” by their designer(s). With no
less potential for habitual patterns, listeners are looking for the amp that interacts with a