US3061675A

US3061675A - Loud speaker improvement

Info

Publication number: US3061675A
Application number: US760036A
Authority: US
Inventors: Macdonald James Ross
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1958-09-09
Filing date: 1958-09-09
Publication date: 1962-10-30
Anticipated expiration: 1979-10-30

Description

Oct. 30, 1962 R MACDONALD 3,061,675

LOUD SPEAKER IMPROVEMENT Filed Sept. 9, 1958 2 Sheets-Sheet 1 INVENTOk Z 4 5 Jameuiws/Vacdma/a MM MfM ATTORNEYS Oct. 30, 1962 Filed Sept 9, 1958 J. R. MACDONALD LOUD SPEAKER IMPROVEMENT 2 Sheets-Sheet 2 ATTORNEYS United States Patent 01 3,661,675 LOUD SPEAKER IMPRQVEMENT James Ross Macdonald, Dallas, Tern, assignor to Terras Instruments incorporated, Dallas, Tera, a corporation of Delaware Filed Sept. 9, 1958, Ser. No. 766,036 SCl-aims. (Cl. 179--ll) The present invention relates to loud speaker systems and more particulmly to a novel loud speaker construction and an associated circuit for developing negative feedback signals indicative of the distortion introduced into audio signals by the speaker as a result of nonlinear movement of its diaphragm in response to audio signals applied thereto.

In recent years there has been an increasing emphasis on high quality reproduction of sound by audio amplifier and speaker systems. The conventional speaker employed in such systems comprises a voice coil which receives the audio signals to be reproduced and a permanent magnetic construction for developing a magnetic held across the voice coil so that when signals are applied thereto the coil is moved axially of itself by a magnitude and in a direction determined by the magnitude and phase of the signal. The voice coil is secured to the central region of a diaphragm of the speaker, which diaphragm has a generally hollow conical shape and which is rigidly secured at its outer peripheral edges to the frame of the speaker. Movement of the voice coil in the field created by the permanent magnet of the speaker produces movement of the diaphragm and thereby produces movement of the air immediately surrounding the diaphragm so as to set up acoustic waves in this region.

Ideally, the diaphragm of a speaker should appear as a completely rigid body so that the entire diaphragm moves as a single unit in response to signals applied to the voice coil. As a practical matter, however, such operation is not achieved and at the lower frequencies considerable harmonic distortion is introduced into the audio waves generated by the speaker. At the higher frequencies, standing Wave patterns are developed across the surface of the diaphragm and further distortion is introduced into the system. This latter phenomenon is commonly referred to as cone or diaphragm breakup. Additional difiiculties arise in the development of loud speakers that are intended to operate linearly over the entire audio frequency band and to date efforts to produce such speakers have not been fully successful.

It is an object of the present invention to provide a novel loud speaker construction and associated circuits tor producing signals which are representative of the distortion introduced into an audio wave by the loud speaker producing the waves which can be fed as negative feedback to the amplifier preceding the speaker.

It is another object of the present invention to provide a novel loud speaker arrangement and associated circuit for producing a push-pull signal indicative of the distortion introduced into an audio Wave by the speaker producing the waves.

It is yet another object of the present invention to provide a novel loud speaker arrangement and associated circuit for producing two negative feedback signals which may be applied to diiferent stages of an amplifier,

to a push-pull input stage of an amplifier or by slight ice modification of the circuit to a single stage of an amplifier.

In accordance with the present invention, a diaphragm of a loud speaker is rendered conductive preferably by metallizing one of its two extensive surfaces. The diaphragm is disposed between two metallic members which are substantially co-extensive with and in close proximity to the diaphragm. The diaphragm and metallic members form two capacitors whose capacities are varied by movement of the diaphragm. An electric circuit is employed to produce signals representative of the movement of the diaphragm and those signals are fed back to the signal source in a negative sense so as to minimize the distortion introduced into the acoustic waves by the speaker. Specifically, the diaphragm is connected to ground and the two metallic members are connected in series through distinct very high impedance resistors to appropriate potentials.

The values of the two resistors are chosen such that the capacitors are substantially isolated from the source of potential at signal frequencies and the only currents flowing in the circuit, once the capacitors are initially charged, is the current required to compensate for capacitor leakage.

One of the two capacitors formed by the loud speaker arrangement subsists between the grounded, conductive, diaphragm and one of the metal members and the other capacitor subsists between the diaphragm and the other of the metal members. The movement of the diaphragm increases the capacity of one of these capacitors and decreases the capacity of the other capacitor. Since the charges on the capacitors cannot vary at signal frequencies due to the high impedances of the series resistors, an increase in capacity produces a reduction in voltage across the capacitor in order to satisfy the equation while a decrease in capacity produces an increase in voltage across the capacitor. By connecting the high impedance series resistors to equal positive potentials with respect to ground the signals generated at the junction of the capacitors and resistors are push-pull signals with respect to ground. If one of the resistors is connected to a positive source and one to a negative source both with respect to ground, the voltages at the junction of the capacitors and resistors vary in the same sense with respect to ground. In consequence the system is highly flexible and may be employed to provide negative teedback signals for various stages of an amplifier, a single stage or a push-pull stage thereof.

The signals generated in the circuits as a result of the rnovement of the diaphragm are indicative of the average movement of the incremental elements of the entire surface of the diaphragm and therefore reflect any distortion introduced into the signal generated by the loud speaker as a result of improper movement of the diaphragm. The signals are fed back to the amplifier driving the speaker to modify the signal to the speaker so that signal elements are produced by the speaker which help tocompensate for improper movement of the diaphragm.

It is, therefore, another object of the present invention to provide a novel loud speaker construction in which the diaphragm of the speaker is employed as a movable condenser plate and in which an electrical circuit is employed to generate signals indicative of the movement of the diaphragm.

It is yet another object of the present invention to provide a loud speaker having a metallized diaphragm and two rigid plates which conform in shape to the diaphragm and which are disposed on opposite sides thereof and in close proximity thereto, all to said conductive elements being insulated from one another so that they may serve as distinct capacitor elements in an associated electrical circuit.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a vertical cross-sectional view of a speaker construction suitable as a dual capacitive element;

FIGURE 2 is a schematic circuit diagram of an electrical circuit incorporating the speaker of FIGURE 1 and employed to produce push-pull negative feedback signals; and

FIGURE 3 is another schematic circuit diagram of an electrical circuit incorporating the speaker of FIGURE 1 and employed to produce push-pull negative feedback signals.

Referring specifically to FIGURE 1 of the accompanying drawings, there is illustrated a loud speaker arrangement which may be employed as one element of the apparatus of the present invention. The loud speaker is provided with a conductive diaphragm 1 disposed between two apertured and conductive plates 2 and 3, which conform, in general, to the shape of the diaphragm 1 and are disposed in close proximity thereto but spaced therefrom. The diaphragm 1 and metallic elements 2 and 3 are generally conical in shape and the diaphragm may be fabricated from the conventional materials normally employed for such purposes. However, in accordance with the present invention, this material is metallized on either of its front or rear surface so that the diaphragm may be considered to be an electrically conductive member and is illustrated as such in FIGURE 1. The members 2 and 3 are provided with a plurality of apertures 4 and 5, respectively, so as to reduce'the air resistance to movement of the diaphragm 1. The member 2 may constitute a wire screen having a one-quarter to one-half inch mesh. The apertures 5 in the member 3 should also be quite large and closely spaced and if the member 2 does not constitute a part of the frame, it being such in the embodiment illustrated, it may also be a wire mesh.

The outer peripheral edges of the diaphragm 1 and the members 2 and 3 are supported and maintained in fixed spaced relationship with respect to one another by means of an annular metallic support member 6 to which the diaphragm 1 and elements 2 and 3 are securely bolted. Specifically, the annular ring 6 is disposed against the right-hand peripheral surface of the apertured member 3 as viewed in FIGURE 1 while a non-conductive annulus 7 is disposed in contact with its left-hand face, as viewed in FIGURE 1, so as to space the conductive member 3 from the diaphragm 1. The diaphragm 1 is disposed between the insulating disc 7 and a further annular insulating disc 8 which contacts its left hand outer peripheral edge. Disposed to the left of the insulating disc 8 and in contact therewith is the outer peripheral edge of the apertured metallic member 2. The diaphragm 1, metallic members 2 and 3, the annular frame member 6 and the annular

insulating washers

7 and 8 are provided with a plurality of aligned apertures, only one being illustrated in FIGURE 1, which are adapted to receive the body portion of a bolt 10. The aperture in the annular frame 6 is threaded so that the screw may be threaded thereinto. A fiber or other insulating washer 11 is disposed between the head of the bolt and the apertured conductive member 2 so as to insulate the member 2 from the bolt 10 and therefore from the frame member 6. The apertures in the diaphragm and the conductive member 2 are of sufficient diameter that they do not contact the shank or body of the bolt 10 and if desired a fiber insulating cylinder may be disposed about the body of the bolt It) in the region of the diaphragm 1 and the apertured member 2.

The apertured conductive members 2 and 3 are of relatively rigid construction whereas the diaphragm 1 is fabricated from conventional loud speaker diaphragm materials and therefore, although being stifi, may be caused to move as a body between the members 2 and 3, in response to the electric signals applied thereto as will become apparent subsequently. The members 2 and 3 are apertured so that the diaphragm may expel the body of air between it and one of the members 2 and 3 rather than compress it. If the diaphragm were required to compress this body of air, it would become unduly loaded.

The diaphragm I is provided with a centrally disposed aperture 12, and terminates adjacent this opening in a rearwardly extending cylindrical sleeve 13. The cylindrical sleeve 13 is secured as by cementing to the outer peripheral of a hollow cylindrical collar 14. The inner periphery of the cylindrical collar 14 is secured to a nonconductive, cylindrical, voice coil support .15 about which is wound a voice coil 16. The coil 16 comprises many turns of line wire adapted to receive signal voltages from a suitable amplifier. The left end, as viewed in FIGURE 1, of the hollow cylindrical sleeve 15 is provided with a conductive end wall 17 which conforms to the shape of the apertured member 2 in the central region thereof and which is electrically connected to the diaphragm 1 through the sleeve 14.

The apertured member 3 is also provided with a large centrally disposed aperture 18 of a greater diameter than the outer diameter of the sleeve 13 which extends rearwardly from the diaphragm 1. The metallic apertured member 3 also has a rearwardly directed cylindrical sleeve 19 which terminates in an annular shoulder 20 extending outwardly from the sleeve 19 and perpendicular thereto. The sleeve 19 and shoulder 20 may be a separate member from the plate 3 and may be secured thereto, as by spot welding, etc. The cylinder 14 is provided at its rear edge with an outwardly extending annular shoulder 21 which is disposed rearwardly of the shoulder 20 of the sleeve 19, but is aligned therewith. The annular shoulder 21 serves as a resilient spider 24 for the diaphragm of the speaker and maintains the diaphragm in a predetermined position in the absence of electric signals being applied to the voice coil 16. The predetermined position for the diaphragm is chosen to be precisely half-way between the apertured metallic members 2 and 3. The conductive flanges 20 and 21 are spaced from one another by means of a fiber or other insulating washer 22 and the members 2t), 21 and 22 are maintained in contact with one another by means of bolts 31 which extend through these members and into a threaded aperture in an inwardly directed metallic annulus 23 of the magnetic structure 24 of the speaker. The bolt 31 is insulated from shoulder 20 so that the shoulders 20 and 21 are not electrically connected.

The magnetic structure 24 further comprises a permanent magnet 25 having a forwardly directed portion 26 of reduced diameter disposed within the hollow, cylindrical, voice coil support 15. The magnet 25 terminates at its forward edge in alignment with the forward edge of the inwardly directed annulus 23 of the magnetic structure 24. The permanent magnet 25 is bolted to a base plate 27 by means of a plurality of bolts, such as the bolts 28, and the base member 2'7 is secured, as by a plurality of bolts 2?, to a cylindrical member 30 which is secured to the annulus 23 and extends rearwardly therefrom and perpendicular thereto. The annulus 23, cylindrical member 30 and base plate 27 are all fabricated from magnetic material so that the magnetic flux emanating from magnet 25 is concentrated across the voice coil 16. The spider 24, which is electrically conductive serves as an electric connection between the diaphragm of the magnetic structure 24 in addition to its function as a diaphragm positioning device.

It is apparent from the above description of the loud speaker apparatus of FIGURE 1 of the accompanying drawings that the conductive diaphragm 1 and the apertured metallic members 2 and 3 are electrically insulated from one another, and that these various members may be separately connected in an appropriate electrical circuit. It is also apparent that the members 2 and 3 are rigidly supported with respect to one another whereas the diaphragm 1 is readily movable with respect to the members 2 and 3. in consequence the diaphragm 1 and the members 2 and 3, respectively, may serve as separate electric capacitors, the capacity of which is varied in accordance with movement of the diaphragm 1 with respect to the members 2 and 3. More particularly, the diaphragm 1 and members 2 and 3 serve as a physical push-pull arrangement since upon movement of the diaphragm 1 toward one of the members the capacity between this member and the diaphragm is increased whereas the same movement causes the diaphragm to move away from the other member and decrease the capacity between the diaphragm and this latter member. The loud speaker apparatus of FIGURE 1 is intended primarily to produce electrical signals which are indicative of the acoustic wavesproduced by the speaker and therefore these signals reflect the distortion introduced into an audio signal in consequence of non-linearity of motion of the diaphragm 1 in response to signals applied to the Voice coil 16.

A circuit which may take advantage of the novel construction of the speaker of FIGURE 1 in order to produce push-pull negative feedback signals to reduce distortion in the speaker, is illustrated in FIGURE 2, of the accompanying drawing. Referring now specifically to FIGURE 2 of the accompanying drawings, those elements of FiGURES 1 and 2 which are common to the two figures bear the same reference numeral. The voice coil 16 of the speaker is adapted to receive signals from an audio amplifier 32 in order to produce motion of the diaphragm 1 between the apertured plates 2 and 3. The diaphragm 1 is connected to a source of reference potential, which for the purposes of illustration is hereinafter referred to as ground, and as a practical matter, the connection would be made to some member of the magnetic structure 24- as illustrated in FIGURE 1. The apertured member 2 is connected to one end of a resistor 33, the other end of which is connected through a source of potential 34 to ground. The apertured member 3 is connected through a resistor 35 and a source of potential 36 to ground. The negative terminals of both batteries are grounded but the circuit would operate in the same manner if the positive terminals of the batteries 34 and 36 were grounded. The sources of potential 34 and 36 are illustrated as batteries in the accompanying drawings, although it is not intended to limit the source of potential to a specific unit since in a practical embodiment of the invention, the sources of voltage will more than likely be derived from rectifier power supplies. The apertured plate 2 is further connected via a lead 37 to a terminal 38 of the amplifier 32 which constitutes one input terminal to a negative feedback circuit of the amplifier. The apertured conductive member 3 is connected via a lead 39 to a second terminal 40 of the negative feedback circuit of the amplifier 32. A third terminal 41 of the negative feedback circuit of the amplifier 32 is connected to ground.

Under steady state conditions; that is, when no input signals are applied to the amplifier 32 and therefore audio signals are not applied to the voice coil 16 of the speaker, the diaphragm 1 is disposed centrally between the apertured conductive members 2 and 3 and voltages are developed between the diaphragm 1 and the members 2 and 3 equal to the voltages of the sources 34 and 36, respectively. More particularly, the diaphragm 1 and the aperture conductive member 2. constitute a first capacitor which is connected in series with the resistor 33 and the source 34. Similarly, the diaphragm 1 and apertured member 3 constitute a capacitor connected in series with the resistor 35 and the battery 36. So long as the diaphragm is not moving, steady state conditions prevail in the circuit and the voltages across the two capacitors are exactly equal to the voltages of the sources 34 and 36, respectively. The

resistors

33 and 35 are of such a large value that they prevent their respective circuits from responding to variations in capacity of the speaker arrangement but permit initial charging of the capacitors and subsequent compensation for leakage of charge from the capacitors.

Upon movement of the diaphragm 1 toward the member 2 for instance, the capacity, of which these elements form the plates, is increased and since the charge stays essentially constant, the voltage across the capacity decreases. At the same time that the diaphragm 1 moved toward the member 2 it moved away from the member 3 anddecreased the capacity between the latter member and the diaphragm. Since the charge on the capacity remains essentially constant and the capacity is decreased, the voltage across the capacitor is increased.

As a result of the above cited operation, the voltage on lead 3'7 decreases with respect to ground and the voltage on lead 39 increases with respect to ground so that the resulting feedback signal to the amplifier 32 is a push-pull signal with respect to ground. The signal appearing on

leads

37 and 39 is applied across the terminals 38 and 4.6 of the negative feedback circuit of the amplifier 32, and constitutes a push-pull, feedback signal which may be applied directly to a push-pull amplifier stage or to two distinct stages of the amplifier if so desired.

The operation of the circuit when the diaphragm moves away from the apertured member 2 and toward the apertured member 3 produces the same effect as above except that the voltage on the lead 39 falls while the voltage on the lead 37 rises and therefore the two possible conditions of movement of the diaphragm I produce oppositely phased signals, the phase of which is dictated by the direction of movement of the diaphragm 1 which in turn is dictated by the phase of the signal applied to the voice coil 16. Therefore, the signal on the

leads

37 and 39 is always of such a phase as to constitute a negative feedback signal regardless of the phase of the output signal generated by the amplifier 32. The feedback signal generated by the circuit of FIGURE 2 is a direct function of the movement of the diaphragm 1 regardless of whether the diaphragm moves as a unitary stiif body as at low frequencies or moves as a discontinuous body as when high frequencies are applied to the diaphragm which produce high frequency break-up. Regardless of the manner in which the diaphragm 1 moves, the total change in capacity between the diaphragm and one of the elements 2 and 3 is a function of the movements of each increment of the diaphragm 1 and therefore the signal generated represents the average movement of the diaphragm and includes signal elements indicative of distortion thus introduced.

In the arrangement illustrated in FIGURE 2 of the accompanying drawings, the signals produced are push-pull signals which, as indicated above, may be applied directly to a push-pull stage of the amplifier or may be applied to two distinct stages of the amplifier which have signals that are out of phase applied to the control element of the respective stages. If it is desired to provide feedback signals to two stages of an amplifier having input signals of the same phase applied to their respective input grids, then a provision must be made to produce signals on the

leads

37 and 39 having the same phase with respect to ground.

Referring specifically to FIGURE 3 of the accompanying drawings, there is illustrated an embodiment of the invention in which signals are produced on

leads

37 and 39 that are of the same phase with respect to one another. Those elements of FIGURE 3 which are common to both figures bear the same reference numerals as FIG- URE 2. In this embodiment of the invention a resistor 42 is connected between a terminal 43 which is positive with respect to ground and a terminal 44 which is negative with respect to ground. The end of the resistor 33 re mote from the member 2 is connected to a point on the resistor 42 which is positive with respect to ground whereas the end of the resistor 35 remote from the member 3 is connected to a point on the resistor 42 which is negative with respect to ground to the same extent that the point to which the resistor 33 is connected is positive with respect to ground.

In operation of this embodiment of the invention, and it is assumed that the diagram 1 moves toward the member 2, upon movement of the diaphragm the voltage be

tween members

1 and 2 is decreased and therefore the voltage on the lead 37 decreases with respect to ground. The voltage initially on the lead 39 appearing at lead 3% is negative with respect to ground and upon movement of the diaphragm ll away from the member 3 the voltage across this capacitor increases and therefore the voltage on the lead 39 decreases further with respect to ground. As a result, the voltages on the leads 37 -and 39 both exhibit a decreasing magnitude with respect to ground and therefore the voltage on these leads are the same phase, and the operation of this circuit is the opposite to that of this circuit of FIGURE 2.

The circuit of FIGURE 3 is essentially the same as that of FIGURE 2 except for the use of the voltage divider instead of the batteries 34 and 36. The only modification of the circuit of FIGURE 2 that it is required to produce the circuit of FIGURE 3 is to reverse the battery 36 so that its positive terminal is grounded and the negative terminal is connected to one end of the resistor 35. The voltage dividing resistor 42 of FIGURE 3 is employed merely to illustrate an alternative arrangement to the one utilizing the batteries illustrated in FIGURE 2.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What I claim is:

1. In combination, a loud speaker having a conductive diaphragm, a first, apertured conductive member disposed in a plane parallel to the plane of said diaphragm; said first apertured member being arranged in close proximity to and spaced from one side of said diaphragm; a second, apertured, conductive member disposed in a plane parallel to the plane of said diaphragm; said second, apertured member being arranged in close proximity to and spaced from the other side of said diaphragm; said diaphragm and said apertured members being electrically insulated from one another, means for electrically connecting said first apertured member in series with said diaphragm and a source of voltage; means for electrically connecting said second apertured member in series with said diaphragm and a source of voltage; and means for producing two electric signals each being indicative of the variation in voltage across said diaphragm and said first and second apertured members, respectively.

2. In combination, a loudspeaker including a frame; a conductive diaphragm having its outer peripheral edge secured to said frame; first and second, apertured, con ductive members having approximately the same shape as said conductive diaphragm; said first and second members subsisting in planes approximately parallel to the plane of said diaphragm and being disposed on opposite sides of said diaphragm in close proximity thereto and spaced therefrom; said diaphragm and said apertured members being electrically insulated fromone another; means for electrically connecting said first apertured member in series with said diaphragm and a source of voltage; means for electrically connecting said second apertured member in series with said diaphragm and a source of voltage; and means for producing two electric signals, each being indicative of the variation in voltage across said diaphragm and said first and second apertured members, respectively.

3. In combination, a loudspeaker having a conductive diaphragm; a first, apertured, conductive member disposed in a plane parallel to the plane of said diaphragm; said first apertured member being arranged in close proximity to and spaced from one side of said diaphragm; a second, apertured, conductive member disposed in a plane parallel to the plane of said diaphragm; said second apertured member being arranged in close proximity to and spaced from the other side of said diaphragm; said diaphragm and said apertured members being electrically insulated from one another; a pair of impedances, means for connectin said first conductive diaphragm in a first series circuit including said diaphragm, said first apertured member, one of said impedances and a source of voltage, means for connecting said diaphragm in a second series circuit including said diaphragm, said second apertured member, the other of said impedances, and a source of voltage and means responsive to the alternating voltages across each of said members and said diaphragm, said impedances being of such a high value as to substantially prevent currents from flowing at audio frequencies.

4. In combination; an amplifier having an input circuit, an output circuit, and a push-pull, negative feedback circuit having two input terminals; a loudspeaker having a conductive diaphragm; a first, apertured, conductive member disposed in a plane parallel to the plane of said diaphragm, said first apertured member being arranged in close proximity to and spaced from one side of said diaphragm, a second, apertured, conductive member disposed in a plane parallel to the plane of said diaphragm, said second apertured member being arranged in close proximity to and spaced from the other side of said diaphragm, said diaphragm and said apertured members being electrically insulated from one another; a pair of impedances, means for connecting said conductive diaphragm in a first series circuit including said diaphragm, said first apertured member, one of said impedances and a source of voltage, means for connecting said diaphragm in a second series circuit including said diaphragm, said second apertured member, the other of said impedances, and a source of voltage and means for connecting said first and second apertured members to different ones of said input terminals of said negative feedback circuit, said impedances being of such high values that said circuits are substantially nonresponsive to signals at audio frequencies.

5. In combination; a loudspeaker including a frame; a conductive diaphragm having its outer peripheral edge secured to said frame; first and second, apertured, conductive members having approximately the same shape as said conductive diaphragm; said first and second members subsisting in planes approximately parallel to the plane of said diaphragm and being disposed on opposite sides of said diaphragm in close proximity thereto and spaced therefrom; said diaphragm being connected to a point of reference potential; said diaphragm and said apertured members being electrically insulated from one another; a pair of impedances, means for connecting said conductive diaphragm in a first series circuit including said diaphragm, said first apertured member, one of said impedances and a source of voltage, means for connecting said diaphragm in a second series circuit including said diaphragm, said second apertured member, the other of said impedances, and first source of voltage, the end of said source of voltage remote from said one of said impedances being connected to said point of reference potential, means for 9 10 connecting said diaphragm in a second series circuit in References Cited in the file of this patent eluding said diaphragm, said second apertured member, UNITED STATES PATENTS the other of said lmpedances and a second source of voltage having its end remote from said other of said im- 3? Harry 30, 1945 pedances connected to said point of reference potential; 5 2,400,993 PS? y 1946 and means responsive to the variation in voltage between 2,683,861 Vlerhng et a1 1 3 1954 said point of reference potential and said first and second 2,755,343 Levy y 17, 1956 members, respectively, said impedances being of such high 2,857,461 Brodie t. 21, 1958 values that said circuits are substantially nonresponsive to 2,864,899 Parker Dec. 16, 1958 signals at audio frequencies. 10 2,896,025 Janszen July 21, 195.