260 Chapter 8
switchable (or programmable or otherwise controllable) so that its action can beremoved
positively when not required.
8.4.5 Indication
A few power amplifi ers have light-emitting diodes (LEDs) (often jointly error indicators)
that indicate subsonic activity or protection shutdown arising from excess subsonic
levels. This kind of protection is most common where the maker is also a speaker maker
or where the amplifi er is closely associated with a particular speaker, as the protection’s
frequency–amplitude envelope that will allow the most low frequency action is very
specifi c to the cabinet and driver used.
Overall, in high performance professional power amplifi er designs benefi ting from
modem knowledge, fi ltration and any HP fi ltering are avoided as far as possible or else
minimized by adaptive circuitry.^9
8.4.6 Hi-End Approach
In “ hi-end ” hi-fi and professional power amplifi ers, high-pass fi ltering is (or should be)
depreciated or at least kept to the bare minimum, for two reasons.
First, all practical HP fi lters progressively delay low frequencies relative to the rest of the
music. Every added HP fi lter pole only adds to this “ signal smearing. ”^10 Simulation in
time and frequency domains shows this.^11
Second, HP fi lters require the use of capacitors. Capacitors that are almost ideal for audio
and not outrageously expensive and bulky are limited in type and values. Capacitors that
arefaradically large enough not to cause substantial “ signal smearing ” are, in practice,
medium-type electrolytics, and not in practice nor in theory anywhere near so optimal for
audio as other dielectric types.
For these reasons, even routine HP fi ltering (alias DC blocking or ac coupling ) may be
absent altogether. Figure 8.7 shows the points where HP fi ltering occurs in the majority of
otherwise direct- and near-direct-coupled power amplifi ers.
8.4.7 Low Approach
In “ consumer-grade ” audio power amplifi ers, HP capacitors are made as small as possible
in value while maintaining what is judged by casual listening or fi rst-order theory to