Loudspeaker Enclosures 71924.1.4.10 Other Networks
Whether the crossover is passive and high level, passive and low level, or active, other
components may be associated with each drive unit:
- DC protection capacitors are connected in series with drive-units to block
steady current fl ow should a DC voltage appear across the box’s, the speaker’s,
or the driver’s input terminals. They are not required for hf (and mf) drivers
with passive crossovers, where the high and “ bandpass ” fi lters already include
the required series capacitor as part of the crossover.
In some designs, more complex, active crowbar circuitry is used, in order to obviate the
need for a series capacitor.
- Zobel and other “ conjugate matching ” networks. Comprising networks of
capacitors and resistors, and less often inductors, these act to smooth out the
impedance variations of the drive-units, either singly or altogether, and as seen
by the preceding crossover and also amplifi er. - Music overdrive protection —in some designs, a light bulb, usually a rugged
12-V type, is connected in series with hf drive units, which in practice require
protection most of all. At worst, the light bulb will be quicker, easier and less
expensive to change than the hf driver or diaphragm. The effect the light bulb
has on sonic quality can be small and sonically benign if the lamp is not visibly
glowing during normal loud passages. “ Auto resetting ” “ thermal trip devices, ”
alias Ptc (positive temperature coeffi cient) thermistors, are also used. These
are usually in the form of a cement-coated disc. At room temperature, they
exhibit a low resistance. When eventually tripped by excess current, the hot
resistance increases rapidly to about 100-fold, and the protected driver’s power
dissipation drops 10,000-fold ox pro-rata. The effects on sonic quality of series
ptc thermistors are as yet questionable.
24.1.4.11 Other Protection
Loudspeakers have also been protected by add-on boxes, containing historic power-
reading circuitry, for example, which crudely opens a relay in line with the speaker or line
level signal if the drive-unit is seen heading toward a burnout.