Introduction to Audio Amplifi cation
John Linsley HoodIn the fi eld of audio amplifi ers there has been great interest in techniques for making
small electrical voltages larger ever since mankind fi rst attempted to transmit the human
voice along lengthy telephone cables. This quest received an enormous boost with the
introduction of radio broadcasts and the resulting mass production of domestic radio
receivers intended to operate a loudspeaker output. However, the fi nal result, in the ear
of the listener, although continually improved over the passage of the years, is still a
relatively imperfect imitation of the real-life sounds that the engineer has attempted
to copy. Although most of the shortcomings in this attempt at sonic imitation are not
because of the electronic circuitry and the amplifi ers that have been used, there are still
some differences between them, and there is still some room for improvement.
I believe, very strongly, that the only way by which improvements in these things
can be obtained is by making, analyzing, and recording, for future use, the results of
instrumental tests of as many relevant aspects of the amplifi er electrical performance as
can be devised. Obviously, one must not forget that the fi nal result will be judged in the
ear of the listener so that when all the purely instrumental tests have been completed and
the results judged to be satisfactory, the equipment should also be assessed for sound
quality and the opinions in this context of as many interested parties as possible should be
canvassed.
Listening trials are diffi cult to set up and hard to purge of any inadvertent bias in the way
equipment is chosen or the tests are carried out. Human beings are also notoriously prone
to believe that their preconceived views will prove to be correct. The tests must therefore
be carried out on a double-blind basis, when neither the listening panel nor the persons
selecting one or other of the items under test knows what piece of hardware is being
tested.
CHAPTER 6