e Williamson Amplifier
Basic Design Requirements:
Alternative Specifications
R
ECENT
improvements in the
field of commercial sound
recording have made prac-
ticable the reproduction of a
wider range of frequencies than
hitherto. e useful range of
shellac pressings has been ex-
tended from the limited 50-8,000
c/s which, with certain notable
exceptions, has been standard
from 1930 until the present, to a
range of some 20-15,000 c/s. is
increase in the frequency range
has been accompanied by an
overall reduction in distortion and
the absence of peaks, and by the
recording of a larger volume range,
which combine to make possible a
standard of reproduction not pre-
viously attainable from disc re-
cordings. Further improvements,
notably the substitution of low
noise plastic material for the
present shellac composition, are
likely to provide still further
enhanced performance.
e resumption of the television
service with its first class sound
quality, and the possible extension
of u.h.f. high-quality trans
missions, increase the available
sources of high-quality sound.
Full utilization of these record-
ings and transmissions demands
reproducing equipment with a
standard of performance higher
than that which has served in the
past. Extension of the frequency
range, involving the presence of
large-amplitude low-frequency sig
nals, gives greater likelihood of
intermodulation distortion in the
reproducing system, whilst
the
enhanced treble response makes
this
type of distortion more
readily detectable and undesirable.
Reproduction of sound by elec-
trical means involves the ampli-
fication of an electrical waveform
which should be an exact counter-
part of the air pressure waveform
which constitutes the sound. e
purpose of the amplifier is to
produce an exact replica of the
electrical input voltage waveform
at a power level suitable for the
operation of the loudspeaker.
is in turn reconverts the elec-
trical waveform into a corres-
ponding sound pressure waveform,
which in an ideal system would
be a replica of the original.
e performance of an amplifier
intended to reproduce a given
waveform is usually stated in
terms of its ability to reproduce
accurately the frequency com-
ponents of a mythical Fourier
analysis of the waveform. While
this method is convenient and
indeed corresponds to the manner
in which the mechanism of the
ear analyses sound pressure wave-
forms into component frequencies
and thereby transmits intelligence
to the brain, the fact that the
function of the system is to repro-
duce a waveform and not a band
of frequencies should not be
neglected. Sounds of a transient
nature having identical frequency
contents may yet be very different
in character, the discrepancy being
in the phase relationship of the
component
frequencies.
e requirements of such an
amplifier may be listed as :-
(I) Negligible non-linear dis-
tortion up to the maximum rated
output. (e term "non-linear
distortion" includes the produc-
tion of undesired harmonic fre-
quencies and the intermodulation
of component frequencies of the
sound wave.)
is requires that
the dynamic output/input char-
acteristic be linear within close
limits up to maximum output at all
frequencies within the audible
range.
(2) (a) Linear frequency re-
sponse within the audible fre-
quency spectrum of 10-20,000 c/s.
(b) Constant power handling
capacity for negligible non-linear
distortion at any frequency within
the audible frequency spectrum.
is requirement is less strin-
gent at the high-frequency end of
the spectrum, but should the
maximum power output/frequency
response at either end of the
7
spectrum (but especially, at the
low-frequency end) be substan-
tially less than that at medium
frequencies
,
filters must be
arranged to reduce the level of
these frequencies before they reach
the amplifier as otherwise severe
intermodulation will occur. is
is especially noticeable during the
reprodudction
of an organ on
incorrectly designed equipment
where pedal notes of the order of
16-20 c/s cause bad distortion,
even though they may be in
audible in the sound output
.
(3) Negligible phase shift with
in
the audible range. Although
the phase
relationship between
the component frequencies of a
complex steady-state sound does
not appear to affect the audible
quality of the sound, the same is
not true of sounds of a transient
nature, the quality of which may
be profoundly altered by disturb-
ance of the phase relationship
between component frequencies.
(4) Good transient
response. In
addition to low phase and fre-
quency distortion, other factors
which are essential for the accu-
rate
reproduction of transient
wave-forms are the elimination of
changes in effective gain due to
current and voltage
in any
stages, the utmost care in the
design of iron-cored components,
and
the reduction of the number
of such components to a minimum.
Changes in effective gain during
“low-frequency” transients occur
in amplifiers with output stages
of the self-biased Class AB type,
causing serious distortion which
is not revealed by steady-state
measurements.
e transient
causes the current in the output
stage to rise, and this is followed,
at a rate determined by the time
constant of the biasing network,
by a rise in bias voltage which
alters the effective gain of the
(5) Low output resistance.
is requirement is concerned
with the attainment of good
amplifier
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