Note: Descriptions are shown in the official language in which they were submitted.
v ~ r ~ l .t ~ o a ~ m l- l i [ i ~ \ (1 S O ~
rcproduction sy~stem ~or compensation of loudspcaker charactcr-
istics.
~ onvcntional electromagnctic loudspeakers arc typically
ablc to reproduce signals at thc upper and lower ends of the
audio frequency spcctrum, regardcd as extcnding from 20 H~
to 20 kll~., lcss well than signals over the central frequency
range. ~ typical loudspeaker frequency response curve may be,
and desirably is, smooth over the major central part of the
spectrum but drops considerably towards both ends, perhaps
showing an almost complete inability to reproduce signals
at the extreme upper and lower ~requencies. The ob3ect of
the invention is to provide a system for compensating for
such loudspeaker characteristics.
In accordance with the invention, therefore, there
is provided an amplifier and sound reproduction system for
amplifying and reproducing an input signal covering at least
a ma3Or part of the audio-frequency range, the system
comprising a multi-section filter for receiving the input
signal, the multi-section filter being arranged to divide
the input signal into at least three signals each covering
substantially only a respective part of the audio frequency
range, at least three variable output amplifier means
connected to the multi-section filter so that each amplifier
means receives a respective one of the divided signals,
loudspeacker means receiving the outputs of the amplifier
means, and a compensation circuit arranged to boost the
upper and lower frequency portions of the input signal in
inverse relation to the frequency response characteristics
of the loudspeaker means.
~g
1 ;3
rl.e system is perferably positioned between a pre-
amplifier stage and the main ampliying stage of a high-
fAidelity amplification system, which is desirably a modular
system of the kind described in Application 205 694 filed
July 26th 1974 and incorporates appropriate features of the
system therein disclosed. The invention thus also provides an
ampllrier sy.s~em in which the or each lnput sig-lal is tre~ted
successlvely in preamplifier stage, an active or passive fllter
stage, a multi-section filter and a plurality of parallel
amplifier channels handling respective frequency ranges of
the input. The outputs of the amplifier channels can be fed
to respective loudspeakers or groups of loudspeakers or may be
-2a-
:; :` '' '' ' ' : '
`
1(1~1~13
combined and fed to a single loudspeaker or fewer loudspeakers
than the number of channels. In accordance with the invention,
the or each loudspeaker to which the compensated signal is fed
may be provided with a suspension of compressed rubber or
plastics foamed material.
By way of illustrative example only, an amplifier system
embodying theinvention is described below with reference to the
accompanying drawing, in which:
Fig. 1 shows the circuit of the amplifying system in block
schematic form;
Fig. 2 shows the frequency response curve of a loudspeaker
compensation circuit element included in the circuit of Fig. l;
and
Fig. 3 shows the frequency response curve of a typical
electromagnetic loudspeaker.
The circuit illustrated in Fig. 1 can be employed as a
monophonic system but will more usually be one channel of a
stereophonic or quadraphonic system. A signal to be amplified
is derived from an input source for example a tuner or a record
or tape player through appropriate selection means 10 and
applied to a preamplifier 12. The preamplifier output is
taken to an electronic loudspeaker compensation circuit 14 in
accordance with the invention. The circuit 14 has a frequency
responsive curve 15 generally as indicated in Fig. 2. The
corrected signal is fed to a loudspeaker 18 after amplification
in a main amplification stage 16. The loudspeaker response
curve is indicated at 19 in Fig. 3 and it will be clear from
comparison of Figs. 2 and 3 that in the absence of other factors
the effective sound output of the loudspeaker will be
approximately flat over the entire audio frequency range.
1(~41~
The electronic compensation circuit can of course be
arranged to produce an effective sound output which has
another shape than generally level, if this is desired for
example to deal with special acoustic conditions. The circuit 14
may also include means for selective adjustment of its
characteristics in particular its frequency response curve, but
it is preferred, as mentioned above, to divide the compensated
signal provided at the circuit output in a multi-section filter
20 in the amplification stage 16. The filter 20 divides the
compensated signal into two or more signals each covering a part
only of the full frequency range. Each divided signal is
amplified separately in a channel containing any desired adjustment
means, in particular, gain controls, so that the relative
contributions of the frequency ranges to the actual output of
the system can be chosen. Thus, in the embodiment of the
invent~on illustrated, the filter 20 divides the compensated
signal into three, the divided signals being amplified in
respective variable amplifiers 22, 24, 26. Any suitable number
of divided sign~ls may be provided for.
The divided signals after amplification in the amplifiers
22, 24, 26 are recombined in an amplifier 28 and are then fed to
the signal loudspeaker 18 or to a group of loudspeakers. Instead
however, the divided signals can be supplied to respective
loudspeakers or groups of loudspeakers without recombination or
with recombination of less than all the divided signals. One
or more amplifiers can be provided as desired at appropriate
positions in each respective channel.
In the signal supplied to the loudspeaker 18 or other
loudspeaker means, the lower and higher frequencies will normally
be present in a degree to compensate for the deficiencies of the
4.
, .. . .
, , ,
l~tl'~ ~ 3
loudspeaker means in reproducing them. So as to be able to
reproduce these signal components adequately, the or each
loudspeaker unit is preferably provided with a suspension of
- compressed rubber or plastics foam material which allows the
diaphragm or cone to travel through the rather large excursions
necessary to the accurate reproduction of signals down to 20Hz.
In addition, the loudspeaker should have the maximum magnetic
field strength than can conveniently be obtained.
The invention can of course be embodied in other ways than
as specifically described but will be understood to provide a
substantial improvement in the field of high fidelity sound
reproduction.
