John Linsley Hood. Valve and Transistor Audio Amplifiers. 1997. ISBN 0 7506 3356 5.
PREFACE
An old friend of mine was in the habit of remarking that an engineer is someone who
can do for five pounds what any damn fool can do for fifty. Needless to say, he was
an engineer. I suppose, in a sense, that this is a fairly good description of the major
aims of engineering practice; that the designer should seek elegant, efficient and cost-
effective ways of achieving a clearly specified objective. Only when satisfactory ways
of doing this cannot be found is there a need to seek more elaborate or costly ways to
get this result.
In the field of audio amplifiers there has been a great interest in techniques for making
small electrical voltages larger ever since mankind first 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 final
result, in the ear of the listener, though continually improved over the passage of the
years, is still a relatively imperfect imitation of the real-life sounds which the engineer
has attempted to copy. Although most of the shortcomings in this attempt at sonic
imitation are not due to the electronic circuitry and the amplifiers which have been
used, there are still some differences between them, and there is still some room for
improvement.
In this book I have looked at the audio amplifier designs which have been developed
over the past fifty years, in the hope that the information may be of interest to the user
or would-be designer, and I have tried to explore both the residual problems in this
field, and the ways by which these may be lessened.
I believe, very strongly, that the only way by which improvements in these things can
be obtained is by making, and analysing, and recording for future use, the results of
instrumental tests of as many relevant aspects of the amplifier electrical performance
as can be devised. Obviously, one must not forget that the final 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 difficult 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, know
what piece of hardware is being tested.
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