Power Supply Design 145would have, for reasons of circuit protection, a fi xed maximum current output. While this
would mean that the peak power and the rms power ratings would be the same, it also
meant that there would be no reserve of power for sudden high-level signal demands—a
penalty that the tonal purists were prepared to accept as a simple fact of life.
However, times change and hi-fi equipment has become easier to accommodate, less
expensive in relative terms, and much more widely available. Also, there has been a
considerable growth in the purchasing power of those within the relatively youthful
age bracket, most of whose musical interests lie in the various forms of pop music—
preferably performed at high signal levels—and it is for this large and relatively affl uent
group that most of the hi-fi magazines seek to cater.
The ways in which these popular musical preferences infl uence the design of audio
amplifi ers and their power supplies relate, in large measure, to the peak short-term output
current that is available since one of the major instruments in any pop ensemble will be
a string bass, whose sonic impact and attack will depend on the ability of the amplifi er
and power supply to drive large amounts of current into the LS load, and it must do this
without causing any signifi cant increase in the ripple on the DC supply lines or any loss
of amplifi er performance due to this cause.
A further important feature for the average listener to a typical pop ensemble is the
performance of the lead vocalist, commonly a woman, the clarity of whose lyric must
not be impaired by the high background signal level generated by the rest of the group.
Indeed, with much pop music, with electronically enhanced instruments, the sound
of the vocalist, although also enhanced electronically, is the nearest the listener will
get to a recognizable reference sound. This clarity of the vocal line demands both low
intermodulation distortion levels and a complete absence of peak-level clipping.
The designer of an amplifi er that is intended to appeal to the pop music market must
therefore ensure that the equipment can provide very large short-duration bursts of power;
that the power supply line ripple level, at high output powers, must not cause problems
to the amplifi er; and that, when the amplifi er is driven into overload, it copes gracefully
with this condition. The use of large amounts of NFB, which causes hard clipping on
overload, is thought to be undesirable. Similarly, the effects of electronic (i.e., fast acting)
output transistor current limiting circuitry (used very widely in earlier transistor audio
amplifi ers) would be quite unacceptable for most pop music applications so alternative
approaches, mainly based on more robust output transistors, must be used instead.