Boxoffice-April.07.1958

signed transformers cause considerable distortion
of output signal. Likewise, incorrectly
adjusted sound head components
(sound lens out of focus, etc.*. Poor circuit
design can cause poor reproduction of
the original input signal to the speaker
system. Condensers and resistors that have
aged should be replaced to bring the amplifier
up to the manufacturers' standard for
good operation. Corroded connections and
dirty interior of amplifier also contribute
to pwor sound output.
We know that vei-y thorough and complete
shielding of amplifier input circuits,
to reduce noise pickup, is used in building
theatre amplifiers. Modern tube designs
permit use of alternating current for tube
heater and filament operation without the
introduction of excessive hum into the
amplifier circuits, particularly pre-amplifiers
and high gain stages. Good circuit
design also contributes to eliminating possible
hum. The push-pull operation of vacuum
tubes in the output stage helps to reduce
both noise level and distortion, but
the final stage tubes must be "matched"
for good operation.
RELIABLE
EQUIPMENT
Next to good quality performance, there
is probably no requirement of theatre amplifiers
more severe than that of reliability.
This brings out the importance of good
maintenance and high quality sound systems.
Large financial losses may be incurred
very quickly if some piece of equipment
fails to work when the house is full of
people, and this is especially true of power
and pre-amplifiers. Low-cost amplifiers,
such as some types of public address amplifiers,
will not give the service nor high
quality sound reproduction so necessary in
today's modem theatres.
We would like to point out that the relatively
small additional first cost of sound
systems having amplifiers with high-grade
components, liberal design safety factors
and rugged construction, is an investment
of the soundest kind for the theatre
owner. Modern theatre amplifiers insure
practically trouble-free operation, if given
proper maintenance as has been outlined
in articles in this department, for a period
of many years. But like any other mechanical
or electronic devices, good main.tenance
is an absolute necessity if one expects
good operation.
EQUALIZE FREQUENCY RESPONSE
The trend, for the past several years, in
modern theatre amplifiers is toward making
the amplifier circuits perform the
necessary frequency response equalization.
Only a few additional components In the
amplifier are necessary in order to obtain
good frequency response to secure excellent
sound reproduction. The response characteristic
is adjustable via the warping arrangement
in modem amplifiers, which
may be exactly equalized to suit the acoustical
qualities of the auditorium. A crossover
network for the speaker system is also
used to obtain the desired high quality
sound reproduction for voice and music.
Many of the older types of amplifiers
Good Frequency Response
Is
Most Important
Good quality sound reproduction depends
upon the frequency response of
any theatre sound system. We know
that all modern theatre amplifiers have
a range of frequencies over which they
are designed to operate efficiently;
above and below this range the signal
output drops off rapidly. But let us
qualify this statement by stating the
modern amplifier can be adjusted to
I
deliver certain frequencies r'highs" or
"
"lows for best reproduction for certain
types of auditoriums.
If an amplifier cannot amplify all the
frequencies of the human voice by equal
amount, loss of voice quality will result,
which is undesirable. If not adjusted
correctly for reproduction of the desired
frequencies for voice and music,
particularly voice, the distortion may be
so great that voice message cannot be
understood. You should learn how to
measure the frequency response of your
amplifier, and learn how good it actually
is. Corrections can, in most cases,
be made to overcome any disturbing distortion
in the circuit components.
had strictly a flat response for all types of
auditoriums. That is, all of the various frequencies
making up the complex signal
were amplified equally. However, what is
desired by the engineer, to meet various
acoustical requirements, is an overall system
characteristic which will result in the
best sound quality for each particular situation.
Strictly a flat response is not always
suitable for every theatre auditorium. Moreover,
appropriate filter networks are necessary
and must be carefully designed to obtain
the desired adjustment of the system
to meet various auditorium conditions. In
a future article on amplifier circuits, we
will give this subject full treatment in nontechnical
terms as possible. For now, the
febove discussion on frequency response
should suffice.
In many installations, we find a flat response
from 50 to about 3,000-4.000 cycles:
but, you may like more bass, or less bass
and more high frequencies, all depending
on the shape, celling, seating and acoustical
Consistenf
Maximum
Light
Greater
Economy
at
condition of your particular situation.
Generally, from 3,000 or 4,000. the frequency
rolls off at an increasing rate
thereafter to be about 15 db down at 8,000
cycles. With good quality recordings this
characteristic gives reproduced music of
excellent tonal quality and balance, and
very pleasing and natural reproduction of
speech and singing. Again, adjustment of
the circuit for best results will depend upon
the auditorium and its acoustical qualities
in all cases.
Remember, the power level at the output
of the .sound head is very low. A highquality
theatre amplifier must therefore
have a relatively large amount of gain in
order to have enough power to drive the
stage speakers. Keep in mind, that not all
of the amplifier gain is normally used,
since it is good practice to have at least 20
db gain available to take care of prints
with very low recording. Therefore, good
P.E. cells, vacuum tubes and other components
must be in good condition in order
to keep this gain sufficient. Normally the
setting for good recordings is around 9 to
12 points on the volume control, but it may
be necessai-y to use more gain and then
raise it two or three points on the volume
control.
NEED RESERVE
POWER
We always recommend an amplifier with
not less than 15 watts, or more, for any
average theatre. Drive-in theatres will
generally require quite a sizeable output,
but this will depend on the number of
speakers. It is better to have reserve power
than to drive yoiu- amplifier wide open.
The quality of sound reproduction will be
better and it will keep down unwanted
noise in the system.
Please read this carefully: The amplification,
or gain per stage, depends upon the
type of tube used, upon the circuit arrangement
and value of components, and several
other factors: but these are the main
ones. Moreover, gain is frequently deliberately
sacrificed in the design of theatre
amplifiers in order to secure some more desirable
quality, such as reduced overall distortion,
or a needed variation in the response
of the amplifier to input signal
voltages of differing frequencies: or an improvement
in the ratio of the output signal
current, that representing noise or hum
Continued on following page
DIAMOND
PROJECTION
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BOXOFFICE :: AprU 7, 1958 47