Interfacing and Processing 267- Back-to-back zeners to 0 V, working in concert with series current-limiting
resistors (which may already be part of the input stage’s RF fi ltering). Both CM
and DM voltages can be clamped to any available zener voltage. Designers must
allow for quite wide variations with tolerance and temperature, and possible sonic
degradation. Programmable zeners may also be used or zeners may be combined
with BJTs. - Ordinary, fast diodes across the active differential inputs, in concert with series
input resistors in both legs. Protects against DM overdrive only. Internal to some
IC op-amps, for example, NE5534. External diodes with larger junctions may be
used to enhance protection. - Clamping relays. Placed after the series input current limiting resistors, inputs are
shorted to 0 V until power is up on all rails. With suitably rapid action and power
sensing, relays in this confi guration can provide complete protection against both
DM and CM input signals. - Bin^13 describes a method developed at the BBC, using VDRs, zeners, and current
sources, providing input protection to audio balanced line inputs (including power
amps) up to 240 V ac. Alas, sonic quality may be detracted from.
8.6 What Are Process Functions? ...............................................................................
When in use, an audio power amplifi er is always but part of some greater system. In
domestic audiophile and even recording studio systems, it is commonplace for power
amplifi ers to have no gain controls and to be devoid of any processing functions.
However, in professional music PA applications, by contrast, it is the exception to fi nd
power amplifi ers without panel gain controls (really attenuators). This facility turns into a
system processing function when the gain control element becomes remote controllable,
most particularly when all the amplifi ers in a system or grouping are so equipped and also
when the rate of gain control change is fast enough for it to be used dynamically.
8.6.1 Common Gain Control (Panel Attenuator)
The most common, almost universal form of “ gain control ” is passive attenuation, set
usually via a panel knob, with a rotarypot or potentiometer.