276 Chapter 8
independent of all the major temperature gradients, dependent on drive patterns, siting
and even amplifi er and rack orientation, as a hot gas usually rises upwards relative to the
earth’s surface. This is true even with amplifi ers employing forced venting, when small
signal parts are not in an air path and are left to cool by microconvection, conduction, and
reradiation.
Without such precautions, differences in channel gains of 2 dB have been observed in an
amplifi er employing VCA-controlled gain when driven up to working temperatures. This
is enough to cause howl round or upset spectral balance.
8.6.3.2 Repeatability
Remote gain settings must not drift or have repeatability errors, which can accumulate
to cause more than (say) /–0.15-dB total error. This may seem stringent, yet on top of
an initial tolerance of another /–0.15 dB, it allows a worst case total difference between
speakers of 0.6 dB. Other errors (cable losses, driver mismatches) are of a similar order
and add to the differencing toll so there is no room for complacency. Least is best.
8.6.3.3 Conclusion
M-DACs and relay-resistor-array attenuators have the highest stability against
temperature and time. Other types may prove acceptable with ameliorative engineering.
Setting precision should not be taken for granted.
8.6.4 Compression and Limiting
Compression and limiting (comp-lim) are gain reduction, alias dynamics processing
techniques, that are employed (among other things) to protect speakers, ears, and
amplifi ers from excess, distorted signal levels. In professional, active crossover-based
systems, they are usually embodied within the active crossover. This is the best position
for logistics in traditional large systems, with only one comp-limper band to worry about.
Positioned within the fi lter chain can also be the best location for sonics.
Where power amplifi ers are driven full range or where active crossover fi lter sections are
integral to the power stage, compression and limiting functions may take place within
individual power amplifi ers.
Compression must be used sparingly, as average power dissipation in the drivers will
be increased, potentially part-defeating the object, as speakers may then suffer burnout.