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Publication numberUS2030573 A
Publication typeGrant
Publication dateFeb 11, 1936
Filing dateApr 30, 1932
Priority dateApr 30, 1932
Publication numberUS 2030573 A, US 2030573A, US-A-2030573, US2030573 A, US2030573A
InventorsLytton Dull Marshall
Original AssigneeDull Radio Lab Inc
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Electric control
US 2030573 A
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Description  (OCR text may contain errors)

Feb. 11, 1936. M, 1 DULL 1 2,030,573

ELECTRIC CONTROL I Filed April 30, 1932 2 Sheets-Sheet 2 awe/whom l7. Z r710 0041,

Wan

Patented Feb. 11, 1936 UNITED STATES PATENT OFFICE ELECTRIC CONTROL Application April so, 1932, Serial No. 608,504

' Claims.

This invention relates to a method of controlling electrical outputs and more particularly to a method of controlling the volume of an electrical reproducer.

The desirability of controlling the volume of sound emitted from a radio reproducer was recognized at an early date and a number of methods of controlling such volume have been developed. These, generally considered, involve the diminution of reproduced sound by modifying the operating conditions of the receiver. A typical method of control, for example, consists of the introduction of a suitable control in the associated equipment to alter the energy amplified at the source, or while passing through such equipment. While prior devices have served to control the volume, this has been done at the expense of the quality ofreproduction,

An object of the present invention is to pro- 30 vide an improved method of electrical control.

Another object is to provide an improved method of controlling the electrical reproduction of sound.

Another object is to provide a method of controlling the volume of reproduced sound while maintaining the full quality of sound to be reproduced.

Yet another object is to improve the effectiveness of a radio receiving system. I A further object is to devise an improved sound reproducer. v

The present invention is based upon a concept quite different from that involved .in prior volume control and is efiectuated in a method which is not only different in its operating principle but which accomplishes results inherently impossituating the reproducer.

With the given concept it will be appreciated that improved results may be achieved. For example, in prior devices, operating upon the prinr ciple of cutting down the electrical input to the reproducer system, as the volume of the reproidentical in character with the energy source, ac-

duced sound was progressively diminished the relative volume of extraneous sound, such as hum, tube noises and so forth, progressively increased. In the present system in which the receiving set is operated at maximum energy out- 5 put, these extraneous sounds, such as tube' noises and hum, are always maintained at a low value relative to the maintained high intensity of the sound signals to be reproduced. Under actual operating conditions the relative value of such 10 extraneous sounds is so low as to be insignificant and indistinguishable in the loud speaker. When the present invention is embodied in a radio receiver, therefore,there is achieved a great reduction in the efiect of foreign energy on the con- 15 trolled output of the desired energy.

In addition to this, utilization of the present improvement achieves a truer response to the sound signals received due to the relatively great energy being used for the reproduction of the 20 sounds than that which is represented in the output, when that output has been controlled to a low value.

Similarly, the present improvement insures an increased sensitivity of the controlled output due to the fact that even feeble impulses received in the system are impressed in their full relative value upon the loud speaker. The present system also insures a great reduction in the persistence of vibration of those parts of. the equipment which are associated with the output arrangement.

The present invention also insures an optimum quality of reproduction. For example, the timbre or color of the musical notes to be reproduced are faithfully secured in the reproducer. Since the present invention insures the operation of the receiving system at its maximum power efficiency, there is secured a. great reduction in alteration of set voltages, under all conditions, resulting in improved selectivity and reduction of first and second power line harmonic modulation of the radio frequency energy, thereby reducing distortion and cross talk. 4

As noted hereinbefore, the present invention is susceptible of embodiment in a number of specific electrical apparatus, but. in order to clearly explain the underlying principles there is shown in the accompanying drawings .a preferred 50 embodiment of it in the reproducer or' loud speaker art.

Fig. 1 is a diagrammatic illustration of the improved system of control applied to a typical dynamic speaker.

Fig. '2 is an illustration of the system utilizing different types of reproducer units.

Fig.3 is a diagrammatic illustration of a modification of the volume control unit.

Fi 4 is a further modification of the device shown in Fig. 3.

Fig. 5 illustrates an additional modification of the resistive type of control shown in Fig. 2.

In the ordinary receiving set now employed the loud speaker comprises essentially a single movable coil which is conductively connected to the secondary of a coupling transformen. In

such prior sets there is no provisionfor controlling the volume in the reproducer or loud speaker, but such control is obtained by means included in the first stages of the receiving circuit and are operative to vary the energy output of the transformer.

As noted hereinbeiore, the present system operates on a different principle and involves as major features the establishment and maintenance of the effective maximum energy output of the coupling transformer and the opposition to this maximum output of a variable quantum of energy which has the precise electrical characteristics of the output energy. The energy which operates the speaker. therefore, is the residual or diiferential energy.

The different devices illustrated in the draw ings which operate upon the stated principle,

though structurally diflerent are substantially functionally equivalent. These are, however, not to be considered as the exclusive apparatus available for effectuating the underlying principles but are merely illustrative or typical embodiments. Similarly it will be appreciated that while in these structures variations in electrical input are transmuted into sound by means of an air-displacing body, the fundamental ideas may be incorporated in a similar or difierent apparatus to obtain other results. Thus the present system is peculiarly adapted to the orthophonic recording of sound upon a suitable medium. If

such a. result is desired it'may readily be achieved by attaching a cutting instrument to the movable coil or armature in place of the cone of the speaker. All other such ramifications and adaptations are intended to be comprehended within the spirit and scope of the present disclosure.

As shown in the drawings, the -major principles of the invention may be embodied in any desired type of reproducer and may utilize different specific means of control. In Fig. 1 is shown a unit which comprises a. movable coil reproducer l, associated with any desired type of receiving circuit through the coupling transformer 2 and a. variable control unit-3.

The coupling transformer is of special design to more efiectively operate under the conditions of the present improvement. It comprises the primary winding 4, the core 5, and secondary winding 6. This unit should be of a size sufiiciently ample to accommodate the maximumwatt rating of the reproducer. The primary winding i is designed to match the impedance in the associated receiving system. The impediance of the secondary winding 6 matches that tions to the reproducer at one end and to the coupling transformer at the other. However, it will be appreciated that the variable'control element may be made up in-a single unit with the loud speaker. In one typical form, as shown in Fig. l, the control unit may comprise a special variable resistance. This is designed in a novel manner to adapt it to therequirements of the improved unit.

This variable resistance may take the form of a suitable casing 9 in which are mounted the resistors I0 and II. These resistor units should be of suflicient size to efficiently handle the watt rating of the reproducer employed. The resistor I0 is conductively connected through the leads l2 and I3 with the binding post II. The lead I3 is connected at its other end to the binding post IS. The resistor unit H is connected through lead IE to the binding post ll thereby making resistor I0 common to the two voice coils 31 and 38. Therefore the two coils are equally effected by any variation in the resistor Ill.

Suitably mounted on a rotative shaft is the control knob I 8 which carries the conductive arms I9 and'20 and the index or pointer 2|. The arm I9 is adapted to make contact with and slide over resistor unit l0 and isconductively connected through the lead 22 to the'binding post 23. Similarly, the arm 20 which slides over and makes electrical contact with resistor II is connected through lead 24 to binding post 25. The arm 20 is likewise connected through the lead 26 to the terminal binding post 21.

The resistance element II should be of the square-jaw type having atotal resistance of approximately five times the impedance of one coil of the double voice coil assembly of the reprostood that these stated values for the resistance elements will vary considerably depending upon the size and mechanical shape of the reproducer and upon the impedance of the voice coils.

There is also suitably associated with the control knob l8 and pointer 2|. of the volume control unit a dial 28 provided with graduations 29. The dial is divided into degrees, represented by the graduations, and the control units so calibrated that these graduations represent fixed quantities or increments of volume.

The electro dynamic reproducer l is of special construction in respect of the voice coil assembly. It may comprise a cylindrical case 3|] of suitable magnetic material having an integral wall 3| forming a closure at one end and an annular pole piece 32rat the other end. A central core 33 of soft iron or other suitable magnetic material is attached to the end Wall 3| by any desired means (not shown) The free end of the core, as shots n,

extends into the circular opening or gap in the pole piece 32. A field coil 3 of any approved design encloses the core 33 and is connected by leads (not shown) to a suitable source of unidirectional current, such for example as rectified extension are mounted two voice coils 3! and of the secondary.

overriding of the cone and insuring a more pre- 38. Preferably these coils are made up by convoluti-ng two wires of exact dimensions and exact electrical constants on the revolving coil support. These coils are wound in parallel and the winding operation is so conducted that theparallel wires remain in the same cylindrical plane throughout each layer. Each layer of wire is insulated from the preceding layer and after completion of the winding the entire unit is impregnated with a. suitable embedme'nt compound so that the entire double coil assembly constitutes a solid unit mass. After the winding operation each end of each wire, constituting the double coil, protrudes and these form termini for the attachment of flexible leads 39, 40, 4i, and 42 for the purpose of conductively connecting the coil assembly to the volume control unit. This methodof mounting, therefore, insures a unitary coil assembly comprising two coils of the same electrical constants.

The electrical energy to be reproduced enters the system at the input terminals 43 and 44 entering the primary 4 of the transformer. The corresponding value of induced current then passes by way of the-volume control unit to the voice coils. In this circuit it is, so to speak, divided or dissociated into two components of current, the values of which depend on the position of the control knob. These current components by way of suitable leads are fed to the coils 31 and 38 where they oppose each other. The difference between the components then becomes the available operating current energy for the cone 35.

In the position of the parts shown in Fig. 1 the pointer 2i is at one end of the dial 26. This is the high or maximum volume position. In such an adjustment energy from the secondary 6 enters by way of terminal 23 and lead 22 to conducting arm I8. This arm is at the extreme high end of resistor [land hence the entire resistor is cut out. The maximum current, therefore, traverses lead l2 and terminal 45 and then through the lead 40 to voice coil 38. The circuit of this coil is completed through lead 38, binding post 21, leads 26 and 8, to the other end Tracing the circuit of coil 31- it will be seen that in the position of parts shown in Fig. 1 current entering the volume control mechanism by way of binding post 23 and lead 22 passes directly througharm l9 and lead to binding post 14, since the resistance in is cut out. Current in this line is, therefore, conducted through lead 4| to the second coil 31. On the other side of the coil 31, it will be noted however, the lead 8 from the secondary is connected by lead 24 to the'arm 20. In the maximum value position, current from lead 8 and lead 24 passes through the entire resistance ll. Therefore, in view of the fact-that the resistor l I is in the circuit connecting coil 31 with the secondary of the transformer, the energy coil 31 is at a minimum. In the preferred embodiment this is about 16.6%.

The voice coils 31 and 38 are thus so connected to the energy source that the energy in coil 31 cancels out or nullifies its value from the coil 38. As noted above, even in the extreme high position of the volume control knob, a small quantity of energy (approximately 16.6%) is impressed on coil 31. This is done to increase the faithfulness at the vibration of the cone :5 to the alternations of the input electric energy by operating, so to speak, as an electrical bufler, thus preventing else response to the input.

It will be appreciated that as the knob i8 is turned and the pointer 2i traverses the dial 28 from'the extreme high to the extreme low position, the energy passing through coil 31 approaches in value that passing through coil 38. When the device is adjusted to the extreme low position coils 31 and 38 will contain the same amount of energy and, in view of their cancelling or nullifying effect, there will be no movement of the cone 35. In this extreme low position of the volume control, the resistor I8 is brought into the circuit common to both the coils 31 and 38. This is done for the purpose of maintaining a constant impedance across the terminals 23 and 25.

The mechanical operation of the cone itself will be appreciatedby those skilled in the art. Due to the establishment of a strong magnetic link between the outer. end of pole-piece 33 and the front plate 32, the residual or available alternating energy in coil 38 effects an inward and outward movement of the coil support 36 which, due to its connection, imparts a corresponding movement to the cone. When the source of energy at the input of the transformer is within the audible range, sound will be produced by the movement of the cone.

While the principles of the present invention have been explained as embodied in an electrodynamic speaker, it will be apparent that these may similarly be incorporated in any other desired type of speaker. Examples of such embodiments are shown in Fig. 2 where, for purposes of simplicity, three typical types of reproducers,

-namely electro-dynamic, electro-magnetic and electrostatic, are shown connected to the one input through the terminals 43- and 44. For the purpose of description these three units may be considered either as connected up to the one input or considered as separate units adaptable for use with any improved radio receiving set. The input end of each one of these units including the volume control casings are similar and may take the form of that shown in Fig. 1, each having the elements and parts corresponding to the unit shown in Fig. 1 down to and including the output terminal binding posts l4, l5, l1, and 21. The electro-dynamic unit shown at the bottom of Fig. 2, whether used alone or conjointly with one or more electrical producers, is identical with that shown in Fig. l.

Thefundamental principles of the invention may be embodied in any approved electro-magnetic speaker. Such device is shown in Fig. 2 which comprises a balanced armature unit. This is provided with a. fixed magnet within which is mounted an armature 45 connected through links 48 and 41 with a cone 48. Associated with the armature are the two coils 31 and 38, identical sections of each coil being mounted on opposite sides of the center of oscillation of the armature. As in the construction shown in Fig. 1, the coil 31 is connected through suitable leads to the terminals l4 and I1 and the coils 38 are connected to the terminals 15 and 21. These coils are so associated with the armature that they operate in ture 45. When this type of structure is associated ersy' with an electro-magnetic speaker, the improved results hereinbefore described are achieved.

Similarly, the improvements herein considered. are equally applicable to an electrostatic speaker. This improvement may be incorporated in an electrostatic speaker when used alone or conjointly with an electro-magnetic and/or electrodynamic reproducer as shown. In an installation wherein the present improvements are included in an electrostatic speaker, a slightly different mechanical structure is of course employed. Terminals I4 and I1 are connected through the leads 50 and 5I to the primary 52 of a transformer T1. The terminals I5 and 21 are similarly connected through the leads 53 and 54 to the primary 55 of transformer T2. The transformers T1 and T2 therefore in a sense correspond to the voice coils 3'1 and 38 respectively of the electrodynamic and electromagnetic units. The induced currents from the transformer T1 and T2 are taken off from the secondaries 56 and 57 respectively, which are suitably connected to the condenser plates 58 and 59. These plates 56 and 59 constitute the fixed plates of the condenser and are positioned on opposite sides of the movable plate 60. These plates 58 and 59 are shown conventionally as comprising the typical lattice structure employed in electrostatic speakers so as to impart impulses to the flexible plate 60 at spaced points over its entire area.

It will be seen from this description that the elements 52, 56, and 58 functionally correspond to the coil 3'! shown in the other device, while elements 55, 51, and 59 functionally correspond to the voice coil 38. These are the functional equivalents because in operation of the electrostatic speaker they subserve the same purpose. By varying the control knob the current passing to the primary and hence the induced'current set up in the respective secondary circuits, may be varied and the electrostatic effects in the condenser units are hence precisely and proportionately changed. .In these circumstances the ef- I fects set up in the plate 58 balance out or nullify its value from the plate 59 and the differential or residual energy value in 59 constitutes the operating force for the displaceable element 69.

While these three types of speakers have been shown as connected up to a single input, it will be appreciated that any combination of two or more of these may be employed and that, if desired, the control of two or more of these different reproducers may be achieved by the one volume control knob by suitable circuit arrangements.

As intimated hereinbefore, the invention in its broader aspects comprises the concept of controlling the operation of the loud speaker by utilizing as the operative force of the speaker electrical energy which is the resultant of two opposing energies of identical electrical characteristics. While the objects of the invention have been described as obtainable by using a. system of variable resistances, it will be appreciated that types of variable resistances, other than-that shown in Figs. 1 and 2, may be employed. Furthermore, other electrical characteristics may be varied to achieve the desired results. Thus, in Fig.5 is shown a resistive type of variable element which differs somewhat from the typ s shown in Figs. 1 and 2. In'this unit the coupling transformer may be substantially similar to that previously described comprising the primary 4, core 5 and secondary 6. This unit, like the units shown in Figs. 1 and 2, includes also the resistors I I) and II and the conducting arms I9 and 2B. To maintain a constant impedance, across the terminals 23 and 25, a different circuit arrangement is utilized. One end of the secondary 6 is connected through the binding post 23 and. lead 22 to the conducting arm I9. This arm is connected through the lead I2 to the binding post I4. The other end of the secondary 6 is connected through the binding post 25 and lead to conducting arm 29. This arm is connected through the resistor ii and lead I6 to binding post I5. Binding post 23 is conductively connected to binding post 21,

' and binding post 25 is conductively connected to binding post I'I.

By referring to Fig. 5 it will be seen that, as the pointer 2| is moved toward the low end of scale 28, resistance is added to the circuit between binding posts 23 and I4, and at the same time resistance is decreased in the circuit between binding posts 25 and I5..

If the above movement of pointer 2I is carried far enough, the energy available at. binding posts I4 and I! will exactly equal that available at binding posts I5 and 21. Since these two latter pairs of binding posts are destined to be conductively connected tocoils 31 and 38 of the reproducer, the output of the reproducer under the above adjustment will be at a minimum.

The terminals 23 and 25 are maintained at a constant impedance at all times, due to the fact that the addition of-resistance in one circuit 'ments may be employed, typical examples of which are shown in Figs. 3 and 4. In Fig. 3 there is shown one type of inductive form of variable element. In this unit a coupling transformer comprising the primary 4, core 5 and secondary 6 may be similar to that previously described. The secondary of the coupling transformer is connected through the binding posts 23 and 25, leads 22 and 24 to the conductive arms I9 and 20 respectively. These arms, instead of traversing a. resistance element as in the units previously described, 'are adapted to cooperate with the phase changing transformers 62 and 63, which are exactly alike. The transformer 62 includes the primary winding 64 and the secondary 65. The primary is divided into seven portions which are brought out to the taps 66. As'will be noted on the drawings, these taps -are adapted to cooperate with the rotating arm 20 so as to progressively cutout an increasing section of the I primary. The general design is such that the impedance of any seven consecutive portions of the circuit comprising the connected primaries 64 and. 6'! of the transformers 62 and 63 and spanned by the conductive arms I9 and 20, will equal the impedance of the secondary 6 of the coupling transformer.

Similarly, the transformer 63 includes the plural winding primary 61 and the secondary 68. The sections of the primary winding 61 are connected to the taps 69 which latter are adapted to make electrical contact with the traversing arm I9. The secondary winding of the phase changing transformer 63 is connected through suitable 31 progressively increases, thus progressively di-,

minishing the residual or effective operating energy in coil 38. As in the device previously described, the output of the radio receiving set is operated at maximum energy and the improved results flowing from such an operation inhere in this inductive type of volume control. In this type'of control it will be noted that the low position is at the center of the dial and the high position may be at either end.

Other modifications of the inductive type of control may be employed, a typical additional *illustrative embodiment being shown in Fig. 4.

This device includes the primary 4 of the input coupling transformer and a plurality of secondaries indicated at 10, 1|, and 12. These secondaries are exactly alike and of the same electrical constants. It is of particular importance that the secondary winding 1| be designed to match the load connected across its terminal binding posts 15 and 21 which, as noted hereinbefore, is the voice coil 38. The phase changing secondaries 10 and 12 arethe functional equivalents of the secondaries 64 and 61 in Fig. 3 and operate in an equivalent manner. The sections of secondary 10 are brought out to the taps 13 which are adapted to be contacted by the conductive arm 18. Similarly the sections of the secondary 12 are brought out to taps 14,. with which taps the arm 20 makes contact.

In the operation of this device as in the others previouslydescribed, the circuit spanned by the rotating arms l9 and 20 will match the load connected across the terminals l1 and I4, which will be the coil 31. As in the other structures, the binding posts I1 and H are connectedthrough suitable leads to the voice coil 31, while the binding posts 15 and 21 are conductively connected to the voice coil 38. As the knob 18 is rotated so as to adjust the volume control from the high to the low position, the value of current flowing by way of the terminals l4 and I1 through the voice coil 31 progressively increases and neutralizes its corresponding energy value in the voice coil 38.

These specific embodiments of the invention have been described in detail in an explanatory but not in a limiting sense. It will be appreciated that other and difierent'speciflc structures which functionsimilarly for obtaining the volume control may be utilized, and all such functional equivalents are intended to be comprehended herein.

It will now be seen thatthe present improvements achieve many novel results. Such an installation insures a great reduction in the eifect of hum, tube noises and foreign, or extraneous energies generally. The system also insures a truer response to the received signals due to the fact that actually greater energy is being used for reproduction than that represented in the output of the reproducer, when it is controlled to any value lower than maximum. An additional accomplishment achieved by the present system is the very great diminution of the persistence of vibrations of the diaphragm and associated parts of the reproducer, due to their mechanical inertia. Such inertia is represented in older reproducers by distortion; for such inertia causes an electrical current to be generated in the voice coil assembly, which current is the electrical equivalent of the mechanical distortion of the parts. In the present system, however, due to the reverse phase connections of the two coils comprising the voice coil assembly, such generated current is, in effect, short circuited, thus largely obviating the mechanical distortion. Hence the vibrations of the diaphragm :of the present reproducer more accurately and faithfully correspond to the electrical voicecurrents.

As noted hereinbefore, a salient improved teav 'ture of the present system is the high sensitivity of the reproducer to even feeble impulses in the source of energy.

The constant impedance load characteristic of the present system also permits a wide ramification of possible uses. Thus it readily permits the use of two or more reproducers on the same amplifier line without however having one materially affect the other. Such a combination finds peculiar utility in a talkie reproducer system in which it is sometimes desirable to adjust the reproducer system to accommodate different acoustic conditions in particular theaters.

It will be appreciated that the principles described herein are not limited to theparticu'lar structures shown. Thus, while reproducers have been shown and described which comprise a double coil, it is obvious that the improved control unit including the double coil may be associated within reproducers which are themselves provided with a coil or coils which perform useful functions, such for example, as neutralized coils of fixed value which are utilized to counteract the effect of induced currents in the telephone or voice coil emanating from the field coil, such efiects being caused by the varying amplitude characteristics of the uni-directional currents in this field coil that may exist under certain operating conditions.

Similarly also it will be realized that the present system is not inherently or necessarily limited for use in a reproducer of the kind described. Such system on the contrary may be utilized where similar problems are encountered or'comparable results are sought. This system, as noted hereinbefore, may be utilized in the electrical reproduction oi. sound. So also, as typifying its ramifications and scope, the improvement may be utilized for reducing hunting and other causes of alteration in the speed of electric motors. A familiar example demanding constancy of speed and phase is the driving motor of a television disc. So also the system is available generally for the amplification of electrical energies;

for the stabilization of the frequency of alternating current generators, and so forth. It is also obvious that my invention may be applied to electrical instruments, particularly those intended for measurements of alternating or rapidly fluctuating currents. Therefore, while'preferred embodiments have been described, these are given merely for the purpose of illustrating the principles of the invention and not for limiting them to any particular apparatus or use beyond such limitations as are clearly imposed by and a reproducer circuit which comprises operat ing the receiving circuit at the maximum eflicient energy output, dividing such output energy into two respectively opposing components, feeding such components to two opposing coils associated with the cone of the reproducer and simultaneously varying each component to increase or decrease the differential energy and utilizing such differential energy as the effective operating energy of the reproducer.

2. An electrical reproducer circuit comprising a transformer, the primary of which is adapted to be connected with a receiving circuit, a reproducer in the receiving circuit, a secondary for the transformer, means to divide the current from the secondary into two components, said means comprising two coils, each connected to the secondary of the transformer, the respective values of the energy in each of which may be varied but the sum of which remains substantially equal, means to impress such currents upon the reproducer in such a manner that one component directly opposes the other and the difference between the said components constitutes the effective operating current for the reproducer.

3. An electro-dynamic speaker unit comprisspeaker, and means to respectively inversely vary 5 the value of the energy in the said coils by adjusting the said resistors.

4. An electrodynamic speaker unit comprising a. coupling transformer, an electrodynamic speaker, two coils associated with the diaphragm 10 of the speaker in such a manner that the energy in one opposes that in the other, means to connect the coils to the transformer and means to vary the energy in the coils.

5. In an electromagnetic sound reproducer 15 comprising a 'core and a magnet spaced from the core to form an air gap, a coil unit mounted within the gap, said coil unit comprising at least two oppositely acting coils, means to connect the coils to the same source of energy and means to vary -20 the quantity of energy passing from the source to one of such coils without diminishing the energy at the said source.

M. LYTTON DULL.

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US2573122 *May 11, 1949Oct 30, 1951Zenith Radio CorpDual speaker control
US5153914 *Dec 28, 1990Oct 6, 1992Kabushiki Kaisha SeidenkoSound equipment system
Classifications
U.S. Classification455/232.1, 381/109
International ClassificationH03G1/00, H03G1/02
Cooperative ClassificationH03G1/02
European ClassificationH03G1/02