Note: Descriptions are shown in the official language in which they were submitted.
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.. ~ 'L'h-i.s :inverl-L~:ioYI.le~LaL:e~ to :LolA(-lsl?ecl:k(~:K S a YLd
associated c:ircuit:ry and has been c'l.~ ised pa:r~icularly
thou.yh not solely as a loudspeaker system incorpora-~,incJ
an elec-tro mechanical feedbac:k and the circuitry as;o~
ciated therewi-th.
Using conventioY!al techniques it is diP.ficult -to
design a sinyle loudspeaker which will operate satis-
eactorily over .the Pull audio .~-`requency spectr-um. For
hiyh fidelity sound reproduc-tion it is usual -to c'Lesign
loudspeakers to operate over a limite~ frequency 'band,
and to achieve reproduction over -the complete a1ldio
frequency spectrum multi-speaker systems have been used.
Loudspeakers designed to handle bass fre~uencies have
required sophistica-ted mechanical design in order -to
reduce non-linearities in the suspensi.on system and
maynet:ic circuit and furthermcJre they have been re(.luired
to be mounted in carefully designed enclosures of.`ten
haviny a considerable volume and t:herefore occUpyiYly a
consi.der'ab].e spaee in,orcLer to ensll:re p:roper loa-'L:i.ny
and dampin(~.
: In o:rder to avold. these compl:icati.ons it is known
to derive a signal related -to -the.cone movemen-t and -to
use this siynal to provide nega-tive Peedback Por -the
amplifieY driving the loudspeaker.' In -thi.s ~jlay -the
25 ' amplifier substantially cornpensa-tes Por -the lo~AcLspeaker
deficienci.es. ~resen-t feedbaclc arransements have not
been found -to be completely satisfactory in.use often
givi.ny a non-l.inear response and furthermore -théy are
expensive to manuPact-ure and require carePul assembly
and alignmen-t of the components within the loudspealcer.
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It is therefore an object of the present invention to
provide a loudspeaker and driving circuit therefor, which will
obviate or minimise the foregoing disadvantages in a simple yet
effective manner or which will at least provide the public
with a useful choice.
Accordingly the invention consists in a loudspeaker system
comprising a support frame, a first magnet mounted in said frame,
a voice coil operable in a driving magnetic flux field from
said magnet, a diaphragm or cone suspended in said frame and
capable of being driven by said voice coil, a second magnet
means for providing a magnetic flux field substantially
separate from said driving magnetic field, a moving coil
transducer operable by movement of said diaphragm or cone
and incorporating a transducer coil operable in said magnetic
flux field substantially separate from the driving magnetic
flux field, connections coupling the output voltage of said
transducer to an input of an amplifier adapted to drive said
voice coil in a manner such that the coupling between said
transducer and said amplifier does not have any common
electrical path with the coupling between said amplifier and
said voice coil, the output signal from said transducer being
coupled to one end of a first resistor of suitable value and
fed through said resistor to said input of said amplifier,
a first capacitor of suitable value provided connected between
said one end of said first resistor and the earth of the
circuitry associated with said amplifier and a second resistor
and a second capacitor connected in series between the said
inpu~ of said amplifier and earth, the values of said second
resistor and said second capacitor being selected to give a
3a flat frequency response from the amplifier at high frequencies.
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'rO those skilled in the art to which this inven-tion
relates, many changes in construction and widely differing
embodiments and applications of the invention will suggest
themselves without departing from the scope of the invention
as defined in the appended claims. The disclosures and the
description herein are purely illustrative and are not intended
to be in any sense limiting.
ne preferred form of the invention will now be described
with reference to the accompanying drawings in which,
Fig. 1 is a cross-sectional view of a loudspeaker
incorporating feedback elements according to the invention,
Fig. 2 is a block diagram of the amplifier and
associated circuitry required to feed back the signal --
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~lZ05Bal
from the lo-udspeaker sho~/n -ln ~'ig. 1 and
l~ig. 3 is a c:ircuit di~g:rarrl of~ the ac~tuclL c~m-
ponents used in l:he block diayram oP ~:ig. 2.
The use of Peedback -to control the response of
high qual~ty amplifiers and other components in the
sound recording and reproduction chain iS well es-ta-
blished. Ilhis inven-tion provides means for incor-
porating feedback in a loudspeaker system -to con-trol
the electro-mechanical conversion process and can be
exploited in any of the followiny ways.
1. By application to a single loudspeaker designed
as a wide frequency range unit,
2, By applicatlon to the bass driver of a two
(or more) way speaker system~ all speakers being
driven from the same amplifier.
3. By applica-tion to the bass driver of a two
(or more) way sys-tem where the bass driver is
driven from a separate ampliPier to tha-t oP -the
high ~requency speakers.
4. By application to speakers other than the bass
driver and not necessarily o~ the rnoving coil
type.
One particular form of the inven-tion will be
described in which -the feedbaclc is applied to the bass
driver of a -Lwo or more way speaker system, all speakers ~ -
being driven from~the same amplifier as described in
paragraph 2 above. The degree of feedback applied can
be varied by changing the ~ of the electronic
B componen-ts in -the system to any degree cLesired to give
a usePul result.
~z~s~ ~
. A loud.spealcer is px ovided hav-Lny a frame 1 i.n
which are mounted the ma:in loudspeaker magnet 2
arranged -to provide a driviny magnet.ic flux fiéld
across gap 30 in which the voice coil 3 oP -the
loudspeaker is restrained to move. The voice coil
is connec ted to a cone 'or diaphragm 4 suspended
around the periphery by way of a resillent mounting 5
to the ~rame of: the loudspeaker .in the form well lcnown
in loudspeaker construction.
10~ Attached to the voice coil 3 oP -the loudspeaker a ~:~
tube 6 is provided,~ for ~example, formed or rolled of ~ -
laminated paper or ~cardboard~ or any o-ther suitable
material so ~ -that ln use :the~ tube will move relative .
:
to the frame o the loudspeaker: in the same manner ,-
as the voice coil 3. The ~roice 'coil could oP course
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be wound onto one~end of the tube 6. A moving coil
transducer :7 is provided which may for example, be'.
;: arranged so: that the coil 8 is ~tound about the other
end :of -the tube 6 and the transducer magnet g :is
~:~ 2 0 a-t-tached to the rame oP the loudspeaker. For examp:Le,
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'~ " the magnet may be attache~d to the'.Prame by way o.~ amounting bar (not shown ) extendiny across the .eace oP
the speaker .erom the circurnPeren tial mounting f lange 11
or alternatively the magnet~ 9 may be directly mounted~,
25~ to -the magnet ~2 o the volce coil by way of a rod or
; bolt 10 pa~ssing through the magnet 2, the centre of -the
tube 6 and the magnet: 9. q~he bolt is secured by a nut 25.
The transducer màgnet 9~ is arranged to provide a -trans~
ducer magne-tic Plux fleld across gap 31 and is spaced
Prom the main magnet by a spacer 26.
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Connections to the ~oice coil and ~eedba~k co-Ll
are made in the usual manner.
In this manner a loudspealcer is provided with a
transducer 7 which ~y vlrtue of -the fact that the coil ;
of the transducer ls substan-tially rigidly connec-ted
to -the voice coil of the loudsp~aker will faithfully
~; produce a vol-tage propo:rtional to -the veloci-ty of ~he
speaker cone. Because the driving magnetic ~lux field i~
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across gap 30 is separated from the transducer magnetic
~lux field across gap 31 and because the transducer coil 8
is separte~from~the~voice coil 3 (i.e. is not co-axial
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~ with or adjacent to the voice coil),~the signal derived
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from the transducer is substantially free of inter-
ference from the operation of -the voice coil. The
driving magnetic flux field and the -transducer magnetic
flux field may be provided~by the same magne-t ( by
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suitable rearrangement of components) as long as the
fields are kept separate and do not magnetically
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interfere~ith one another. rrhe leng~h o~ the trans-
ducer coil 8 should be su~eeicient to preven~ a non-
; linear response at extremes o~ movement Oe the voice
coil i.e.~ the transducer coil should never move out o~
the transducer magnetic flux eield.
; In use the loudspeaker as described may be connected
into the circuit of an associated driving amplifier as
:~ :
~ shown in block diagram form in Fig. 2.
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The preferred form oP thé invention uses an
amplif-ier 12 havlng differen-tial inpu-ts. The output of
the pre-amplLfier is fed -to one amplifier input and the
amplifier output is fed through -two leads of a four core
-- 6 --
- .~ cable 11 to a conventicnal two-way cross-over networ:lc 13
in the ].oudspealcer enclosu:re . rL~he c:ro~s-over frequenc~
is such that the bass driver 14 recelves all frequenci.es
where it is desired to utilis~ spea1ce.-r f`eedbaclc, and
higher frequencies a.re recei~ed b~ the high ~requency ' '
speaker 15.
The signal ~from the transducer 7 is L`ed back to the
ampli~ier~through the remainlng leads o.e 'the four core
: cable. Frequencies above approximately~1cHz are 11tèred , ,1a ou-t.of tl1e feedback slgnal by a capaci-tor 16 which orms
: a -time constant with the inherent resistance of -thej~ .
feedback coil 8. The feedback signal is then fed through
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resistor 17:as~shown to~the ampl~ifier feedback pointO
In this manner the feedback vol-tage from thé -transducer 8
~ is fed into the.amplifier as parallel feedback to control
the output from~-the ampli.Pier and thus ensure that the ~'
; loudspea1cer diaphragm veloci~y closely follows -the input
signal -to the amplifier.
The~ampli.eier output is also fed back vla resistor
' and capacitor 20 to the ~eed'baclc point so I,hat the
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: ampli.eier ope:rates ln the conventlonal manner at high
~requencies where the L`eed'back signal is not applicable.
llhe series resi~tor and capacitor 18 are required to
~: ~ ensure a::flat frequency response output at high: ~;
Prequencles. :
' one of the leads from the feedback transducer 8 is
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earthed at point:19 in the:amplifier. This lead canno-t
be used b~ the s'ame lead that earths the loudspeaker
voice CDil since high currents'are present in the voice
.coil earth lead and potenti.als developed across the lead
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~z~
._ ~ will interfere with the feedback siyn~
The sys-tem describe.d above does not allow for the
characterlstic variation of radlation resistance wi-th
erequency below a frequency determined by the loud-
: ~5 spealcer diaphragm area. Thls charac-teristic requires
. that below this critical erequency the dlaphragm
velocity should vary in inverse p-roportion to the
frequency if:a flat acoustic Prequency respanse is to be
maintained. The applica-~ion o feedback arcund the loud- ,~
spealcer -takes advantaye of the inverse relationship
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which is already:provided by the loudspeaker resonance~
Feedbaclc~ac-ts to damp this resonance, and in fact the
correct degree of feedbaclc is applied when there i5 just
enough damping to ensure a Çlat frequency relationship
between voltage input to the power amplifier and the
. diaphragm~veloclty~over the resion of varying radiation
resistance. ~:
The lnverse relationship required -to counteract
-the varying radiation resistance rnust be restored
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~ 20 ~ elect~onically. This may be done by adding an acti.ve
; ~ilter 21 :in the pre-amplifier. The filter response
fa:l.ls at 6d:B pe:r octave between 20Hz and the p.revi.ously ..
:: mentione~d frequency determined by loudspeaker area. .
bove this.frequency the filter has a flat response.
2~5 The system;as descrlbed above has been found -to be .
particularly effective in providing a clear un-coloured
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: sound over a wide range of~frequencies even with -the
: use of a relativèly:simple and cheap conventional loud-
: speaker~basis in a small enclosure. It is a feature of
the sys-tem tha-t the voice coil and feedback coil are
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- ' electro maynetically inclependen-t mlni,mis:ing incLu.ct~ve
coupllng between t.he .~eedback and voic~ co.il sys tems
and allowiYly residual hi~h-frequency signals in the
feedback to be simply fi.ltered out,
. 5 The advantages to ~e gained from using'the system are
~ that a flat acoustic frequency response can be extended
below the~speakerrs bass resonance even in a small
speaker enclosure oP 'infinite ba~fle' ~esign. ~ low ;,
cost bass~speaker can be used to provide sound quality ' '
~: 10 , exceediny that provided by most conventional domestic :
sound reproduction~systems. Improvement in quality is
brought abou-t~through the increased damping of:the
. speaker diaphragm as a result o~ feedback. ~ con- -'
ventional high quality speaker can be improved by the
15~ a.dditional damping~ provided b~ the eedbaclc system and
conven-tiorlal loudspealcer ma~nufac-turing techniques can
be used in the production of speakers adap-ted ~or this
:, feedbaclc syStem. This enables a high ~i.cl.elity sound system
to be manu~actured at a very eheap and com~etit:ive eost.
~ Modi~i.catlons required to the ampli~ier des:ign are
also si.mple and lo~ ln cost.
'rrhe feedback signal deriv,ed ~rom the tra~sdu.cer
i is proportional to -the velocity of the spealcer diaphragm
which is the parameter of motion direcl.~.y related to .
acoustic power (power lS proportional to velocity
' squared).
' The feedbaclc system used is also particularly
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: suitable for use in multi-spealcer installations where it
is only clesired to provide feedbaclc control over the
~ 3~ bass driver. In a multl-speaker installation -the
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l~h~5~30
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Peedback dampirlg is res~xicted -to a ~acl:or; of~ to allow
the hi~h Prequency units to be properly c~r.iverl withou~
signiPic~ant interPerence ~rom the Peecl'~fack sys tem.
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