US3895193A

US3895193A - Cross-over network and bias voltage supply for dynamic-electrostatic speaker system

Info

Publication number: US3895193A
Application number: US180131A
Authority: US
Inventors: Lloyd J Bobb
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-04-29
Filing date: 1971-09-13
Publication date: 1975-07-15
Anticipated expiration: 1992-07-15

Abstract

Circuit for use with a speaker system comprising a dynamic speaker and an electrostatic speaker and including a network providing a cross-over point at a frequency of the order of about 500 to 1000 Hz., and further including a voltage multiplier rectifier network delivering bias voltage to the electrostatic speaker, the input for this network being arranged for connection either with a standard 117v 60 Hz. power supply line, or with the secondary of a transformer of which the primary is connected with the source of signal being reproduced by the speaker system.

Description

United States Patent 11 1 ,i1m m 1 11 3,895,193 Bobb July 15, 1975 [5 CROSS-OVER NETWORK AND BIAS 3,345,469 /1967 Rod 179/111 R VOLTAGE SUPPLY FOR 3,358,084 l2/l967 Ashworth l79/l 11 R 3,562,429 2/1971 West 179/111 R DYNAMIC'ELECTROSTATIC SPEAKER 3,632,903 1/1972 Lange 179/111 R SYSTEM Inventor: Lloyd J. Bobb, Glenside, Pa.

Chester C. Pond, Doylestown, Pa. a part interest Filed: Sept. 13, 1971 Appl. No.: 180,131

Related US. Application Data Continuation of Ser. No. 826,972, May 22, 1969, and a continuation-in-part of Ser. No. 820,888, May 1, 1969, Pat. No. 3,654,403, and Ser. No. 104,741, Jan. 7, 1971, which is a continuation of Ser. No. 724,805, April 29, I968, abandoned.

Assignee:

US. Cl. 179/111 R; 179/1 D Int. Cl H04r 19/02 Field of Search 179/111 R, 1 D, 1 AT References Cited UNITED STATES PATENTS 7/1959 Janszen l79/lll R 90 7/l96l Hupert et al l79/l D OTHER PUBLlCATIONS The Radio Amateurs Handbook, th Edition, 1963, p. 239 & 240.

Primary Examinerl(athleen H. Claffy Assistant Examiner-Douglas W. Olms Attorney, Agent, or Firm-Synnestvedt & Lechner [57] ABSTRACT Circuit for use with a speaker system comprising a dynamic speaker and an electrostatic speaker and including a network providing a cross-over point at a frequency of the order of about 500 to 1000 Hz., and further including a voltage multiplier rectifier network delivering bias voltage to the electrostatic speaker, the input for this network being arranged for connection either with a standard 117v Hz. power supply line, or with the secondary of a transformer of which the primary is connected with the source of signal being reproduced by the speaker system.

5 Claims, 4 Drawing Figures PNENTEBJUL 15 ms n K E w 5 km w M 1% m 6 W m 1 I U m CROSS-OVER NETWORK AND BIAS VOLTAGE SUPPLY FOR DYNAMIC-ELECTROSTATIC SPEAKER SYSTEM This is a continuation, of application Ser. No. 826,972, filed May 22, 1969. The present application is also a continuation-impart of my prior application Ser. No. 820,888, filed May 1, l969,and issued Apr. 4, 1972, as US. Pat. No. 3,654,403; and also of application Ser. No. 104,741 filed Jan. 7, 1971, as a continuation of application 724,805, filed Apr. 29, 1968, now abandoned.

This invention relates to audio speaker systems and is particularly concerned with speaker systems incorporating a plurality of speakers adapted to cover different portions of the audio spectrum. In a typical installation the low range speaker comprises a dynamic speaker and the high range speaker comprises an electrostatic speaker. The dynamic speaker is preferably of the well known cone type, having a voice coil, and the electrostatic speaker is preferably of the type disclosed in my copending application Ser. No. 820,888, filed May 1, 1969, and issued Apr. 4, 1972, as US. Pat. No. 3,654,403, most advantageously comprising a pair of acoustically coupled push-pull driven planar electrostatic speakers.

One of the major objectives of the invention is to pro vide a cross-over network for use with speaker systems of the kind referred to, in which the response of the dynamic speaker is effectively limited to a frequency rang below a cross-over point which is relatively low as compared with various prior dual speaker arrangements, for instance at a frequency of the order of 500 Hz. This is of advantage in providing highest fidelity of response of the speaker system as a whole for resons which will be explained hereinafter.

Another objective of the invention is to provide a simplified bias voltage supply system for the electrostatic speaker which may be connected either with a standard I 17v 60 Hz. supply line or alternatively with the secondary of a transformer having its primary introduced in the signal circuit. In the latter alternative it is a further objective and advantage of the invention that a separate power source connection for the bias voltage need not be provided for the electrostatic speaker, as has been customary heretofore.

It is also an objective of the invention to provide a voltage multiplier rectifier circuit including current isolating resistors in the input connection, so that, notwithstanding the fact that a biasing voltage of the order of 1000 volts or more is delivered to the electrostatic speaker, the current is restricted to such a low level as to eliminate any danger of harmful electrical shock to any one coming in contact with the equipment.

A further objective of the invention is to provide a cross-over network which is exceedingly simple and inexpensive, and yet highly effective, and which at the same time retains a exceptionally high level of fidelity in the frequency response of both the low and high range speakers.

How the foregoing and other objects and advantages are attained will appear more fully from the accompanying drawings, in which:

FIG. 1 is a schematic view of one embodiment of the cross-over network and bias supply system of the present invention as used in association with a dynamic speaker and with a pair of acoustically coupled electrostatic speakers arranged in accordance with the disclosure of my copending application above identified;

FIG. 2 is an outline somewhat diagrammatic view illustrating certain features of a signal transformer preferably employed to couple the signal circuit with the electrostatic speaker;

FIG. 3 is a view illustrating certain portions of the equipment shown in FIG. 1 and embodying an alternative arrangement for the bias voltage supply system for the electrostatic speaker; and

FIG. 4 is a view illustrating certain portions of a circuit similar to that shown in FIG. 1 but applied to an alternative arrangement in which only a single electrostatic speaker is used.

Referring first to FIG. 1, the dynamic speaker is illustrated diagrammatically as incorporating a cone 5 and a voice coil 6. The electrostatic speaker here shown diagrammatically includes a pair of conductive membranes 77, formed, for example, of a film of resin material carrying a conductive coating at a side thereof. As here shown it is contemplated that the conductive coatings of the two membranes face each other and that the membranes be in closely spaced relation so as to be acoustically coupled. The conductive coatings of the membranes may be connected in common to the conductor 8. The electrostatic speaker arrangement shown in FIG. 1 also includes a series of backing elements or plates 9 for each of the two membranes, most of these plates 9 being interconnected in pairs and the pairs being connected through resistors to the conductors 10 and 11 in the manner shown. Details of this multiple backing plate arrangement, including the electrical interconnections therefor need not be considered herein, but reference may be made to the copending application above identified for such details.

In FIG. 1 the connections 12 and 13 represent the signal source or signal circuit and it will be observed that the voice coil 6 is connected in series with the primary winding 14 of the signal transformer, the coil 6 and winding 14 being connected across the signal circuit. A capacitor 15 is connected across the voice coil 6 and, in combination with the primary 14 of the signal transformer, serves as the crossover network.

The secondary 16 of the signal transformer has its ends connected with the conductors 10 and 11 which are associated with the backing plates of the electrostatic speakers. The signal transformer also has a center tap connection 17 associated with the voltage multiplier rectifier circuit which is enclosed within the dotdash line 18.

The voltage multiplier rectifier is of a type known per se, including, for example six diodes as indicated at 19 and six capacitors as indicated at 20. The

input connections

21 and 22 of this network include current isolating impedences which may comprise resistors, inductances or capacitors, preferably resistors such as indicated at 23 and 24 adapted to be connected with the power supply line, for instance the standard 117v Hz. supply line.

It will be noted that the negative output connection 25 of this network is coupled with the center tap connection 17 of the secondary 16 of the signal transformer. The positive output connection 26 of this network is coupled through a resistor 27 with the connector 8 leading to the conductive coatings on the membranes 7. If it is desired to connect more than one speaker covering the same frequency range, this may be done by coupling one or more additional speakers in parallel and connecting them through the resistor 27. In this case it may be desirable to change the value of the resistor 27, for instance from the 2 megohm value referred to herein to l megohm. Where additional electrostatic speakers covering the same frequency range are to be incorporated the backing plates thereof would be connected in parallel with those of the speaker illustrated and fed in parallel from the connectors and 11. In this case some change in the signal transformer may be desirable.

Where it is desired to connect additional electrostatic speakers covering a different frequency range, it may be desirable to do this through separate resistors such as indicated at 27a.

With the circuit arrangements described above, the multiplier rectifier network will deliver a bias voltage of the order of 1000 volts, thereby providing an appropriate bias across the elements of the electrostatic speakers. At the same time the signal voltage will be impressed upon the two electrostatic speakers in pushpull manner, and for reasons fully brought out in my copending application'above identified, this push-pull drive of a pair of acoustically coupled electrostatic speakers provides an exceptionally high fidelity of reproduction and also provides for high efficiency, so that the electrostatic speakers need not be made of excessive size.

More or less than six diodes and six capacitors may be used in the multiplier rectifier circuit, depending upon whether more or less than about 1,000 volts is desired.

The provision of the current isolating

resistors

23 and 24, which may, for example, be of the order of 10k to 100K ohms each, serves to limit the current in the multiplier network and in the connections to the electrostatic speakers to such a low value that danger of harmful electrical shock is eliminated. In a typical network according to the present invention, the diodes 19 should have PIV and current rating adequate to provide the desired bias voltage, in the case of 1,000 volts, the diode identified as 1Nl695 being usable. The capacitors 20 may be of the order of 0.01 to 0.1 mfd., and

of appropriate working voltage. The resistor 27 may 7 suitably be of 2 megohms.

With a signal transformer in which the primary 14 comprises about 34 turns of No. 19 wire and in which the secondary comprises about 2800 turns of No. 37 wire, and with a dynamic speaker having an 8 ohm voice coil (6) and still further with a capacitor 15 of about mfd., an effective cross-over point is provided at a frequency of the order of 500 Hz. With a doubleended, push-pull driven electrostatic speaker system as illustrated, the electrostatic speaker is effective in reproducing frequencies from the upper limit of audibility down to a value approximating the cross-over point of 500 Hz. Moreover, the dynamic speaker, which is limited in its effective reproduction to a frequency of the order of 500 Hz. in the system as described, is not subject to the distortions commonly encountered with dynamic speakers as a result of attempts to reproduce sound running up to frequencies of at least several thousand Hz. Tests have shown that a dynamic speaker, for instance of the well known cone type, functions to very accurately reproduce sound within the frequency range in which the cone can operate as a piston, this range ordinarily being limited at its upper end to about 500 to 1,000 Hz. depending on the cone design.

In order to avoid excessive magnetization of the core of the signal transformer 14-16 at frequencies at or below the cut-off point of the transformer, with consequent tendency to saturate the core and thus cause distortion or non-linearity in the reproduction by the speakers of the system, a special core arrangement is preferably employed for the signal transformer, having a gap in the magnetic circuit. Referring to FIG. 2, it will be seen that the core elements of the signal transformer include both E laminations 28 and I laminations 29, these being shapes customarily employed. However, instead of building up the core by alternately positioning the E and I laminations at opposite sides of the core, so that they interleave, the core is here built up by stacking all of the E laminations upon each other in congruent positions, while the l laminations are similarly stacked directly upon each other. The windings are of course arranged to surround the center leg of the E laminations. In the arrangement of the invention, a non-conductive spacer 30, for instance paper is introduced between the stack of E laminations and the stack of I laminations in order to provide an air gap in the core. Preferably the paper or spacer thickness should provide a gap in the flux path of the order of 0.002 to 0.005 inch. This gap in the flux path reduces the magnetization of the core at frequencies below the cut-off of the transformer and thereby avoids saturation of the core, with consequent non-linearity in the primary winding 14 which is connected in series with the voice coil 6. If such non-linearity or distortion was present in the primary 14 of the signal transformer, this would also adversely influence the linearity of the signal in the secondary 16 of the transformer and thus in the signal as applied to the electrostatic speakers.

With the arrangement as shown, a high degree of linearity and fidelity is provided, while at the same time minimizing the components used and the complexity of the cross-over network which, in effect, is reduced virtually to a single capacitor 15, in combination with the voice coil of the speaker and the primary 14 of the signal transformer.

Although the simplified network, with the transformer having an air gap, achieves various of the objectives above referred to, it nevertheless still supplies some voltage to the dynamic speaker at higher frequencies, but at frequencies upwards appreciably above 500 I-Iz., for instance of the order of about 2,000 Hz., this voltage is not high and, because of the inherent inertia of dynamic speakers designed for low frequency use, the sound generated by the dynamic speaker in consequence is negligible.

The voltage multiplier rectifier circuit provides an exceptionally simple means for attaining an appropriate polarizing or biasing voltage for use in the electrostatic speakers. However, since such a voltage multiplier is line connected, i.e., is directly coupled with the power supply line, in order to avoid danger of electrical shock from the equipment, current isolating

resistors

23 and 24 are introduced. In addition, the isolating

resistors

23 and 24 also protect the voltage multiplier rectifier from drawing excessive current in case of component failure, thereby eliminating fire hazard and damage to other components. In the system of FIG. 1, moreover, it is of importance that the secondary 16 of the signal transformer is electrically isolated from the primary 14 of that transformer thereby avoiding introduction of a high voltage of the multiplier and also of the supply line voltage into the signal circuit 12-13.

The resistor 27 serves the purpose of reducing shock hazard by limiting current to the speaker from the voltage multiplier rectifier, and also protects the speaker membrane from excessive overload conditions and thereby reduces damage to and prevents destruction of the membrane.

In the alternative embodiment illustrated in FIG. 3, provision is made for deriving the biasing voltage for the electrostatic speakers from the signal source 12-13, instead of from a l 17v A.C. supply line. This is accomplished by provision of the circuit enclosed in the dotdash line 31 shown in FIG. 3. This circuit is adapted to be alternatively coupled with the current isolating

resistors

23 and 24 which are provided in the input to the voltage multiplier rectifier, and it will be understood that the remainder of the circuit thereof, not illustrated in FIG. 3, will be the same as in FIG. 1. In the circuit arrangement 31 a voltage step-up transformer is employed, having a primary winding 32 and a secondary winding 33. The primary winding is connected into the signal circuit 12-13, and thus is placed in series with the voice coil 6 and the primary 14 of the signal transformer. The output connections of the secondary winding 33 are coupled with resistors 34 which in turn are bridged by one or more gas discharge voltage regulating devices, for instance, a pair of series connected 60v neon voltage limiters 35, this circuit 31 being adapted to be connected with the

current blocking resistors

23 and 24 in the manner indicated in FIG. 3. Although other kinds of voltage limiting devices may be used, the neon limiters are not only effective but are also of very low cost.

If desired the primary 32 of the transformer 32-33 may be designed to operate in parallel with the signal source.

Having in mind that the signal circuit 12-13 will carry a fluctuating voltage in accordance with fluctuations of the signal, and further having in mind that the signal voltage may range up to about 25 volts, in the embodiment illustrated, the transformer comprising the primary and

secondary windings

32 and 33 advantageously is wound to provide a step-up ratio of the order of 1:1000. With typical signal voltages, the voltage across the secondary will at the minimum be of the order of 100 volts, although will frequently be much higher than 100 volts. The neon voltage limiters 35, for instance comprising two NE-Z, will provide a voltage limitation appearing at the terminals to be connected with the

resistors

23 and 24, so that the voltage at those terminals will remain at a value of the order of 100 to 120 volts. This voltage will then be multiplied by the multiplier rectifier circuit as described above in connection with FIG. 1, in order to attain a bias voltage of the order of I l ,000 volts, which is appropriate to deliver to the electrostatic speakers.

While the system of FIG. 3 will use a portion of the output power of the power amplifier feeding the signal circuit 12-13, the total amount of power required to establish the desired bias voltage for the electrostatic speakers is of the order of 0.5 milliwatt, neglecting transformer loses, and this drain upon the signal level is insignificant, having in mind that it is common for power amplifiers to deliver upwards of 5 or watts.

In FIG. 4 an alternative connection system is illustrated for use in an installation where only a single electrostatic speaker is employed, instead of the multiple, push-pull driven arrangement of FIG. 1. Here it will be seen that the electrostatic speaker comprises a single membrane 7a and a single set of backing plates 9a, which are coupled to the lead 10a in a manner similar to that illustrated in FIG. 1. Lead 10a is connected to one end of the secondary 16 of the signal transformer through a capacitor 36. The connections from the voltage multiplier rectifier circuit are indicated at 25 and 26, as in FIG. 1, and the positive connection 26 is extended to the head 37 which interconnects one end of the secondary 16 of the signal transformer with the conductive coating on the membrane 7a, a resistor 27 being introduced in the connection 26, as in FIG. 1.

The negative connection 25 of the voltage multiplier rectifier is extended to the lead 10a which in turn is connected with the backing plates 9a of the electrostatic speaker. The capacitor 36 should be of the order of about ten times the capacity of the speaker. Since the capacity of the speaker in a typical construction would be of the order of 0.01 mfd., the capacitor 36 should be of the order of 0.1 mfd. The capacitor 36 is introduced to prevent short circuiting of the DC. biasing voltage.

In the embodiment of FIG. 4, the resistor 27, in addition to the functions described above with relation to FIG. 1, further serves to prevent the bias supply from loading the secondary 16 of the signal transformer.

In the arrangement of both FIGS. 1 and 4 it will be understood that the resistor 27, which may, be of the order of 1 to 10 megohms, may be introduced in either output connection from the voltage multiplier. It should further be understood that the positive output connection of the voltage multiplier need not necessarily be connected with the membranes of the electrostatic speaker or speakers, and the negative output with the backing plates. The negative output connection may be extended to the membranes and the positive connection to the backing plates. Other circuit connections may be adopted, so long as provision is made for the bias voltage to be applied between the membranes and the backing plates of the electrostatic speakers.

In addition to providing a cross-over network of exceptional simplicity, the circuit arrangements of the present invention also achieve the production of an appropriate bias voltage for the electrostatic speaker with parts of very low total cost, and both the cross-over network and the bias voltage supply are coupled with the speakers without impairing the operation of each other and further without danger, of harmful electrical shock notwithstanding the delivery of a bias voltage to the electrostatic speaker as high as 1000 volts or more.

As above indicated the low frequency speaker preferably comprises a dynamic speaker, but certain features of the invention are also applicable to dual or even other multispeaker systems in which electrostatic speakers are even used for both the high and low ends of the frequency range. In such an arrangement, a primary of another signal transformer may be substituted for the voice coil 6 shown in the drawings, with the secondary of the other signal transformer connected to feed another electrostatic speaker adapted for the low frequency range.

I claim:

1. For use in combination with a signal circuit, a dynamic speaker having a voice coil, an electrostatic speaker having a conductive membrane and having a conductive backing plate cooperating with said membrane, and further with a standard 117 v. alternating current source adapted to be used in providing a bias voltage for the electrostatic speaker: a step-up signal transformer having a primary winding connected in the signal circuit in series with said voice coil, a solid state voltage multiplier rectifier having a pair of input connections line connected with said current source each through a separate current isolating resistor and having one output terminal connected with the membrane, the signal transformer having a secondary winding connected between the backing plate of the electrostatic speaker and the other output terminal of the voltage multiplier rectifier.

2. Equipment as defined in claim 1 in which said signal transformer comprises E and 1 core structures each comprising a plurality of laminations, with the primary and secondary windings surrounding the central leg of the E laminations, at least some portions of the E and l laminations being spaced in fixed relation to each other to provide a constant gap in the flux path.

3. For use in combination with a signal circuit, a low frequency speaker, an electrostatic high frequency speaker having a conductive membrane and having a conductive backing plate cooperating with said membrane, and further with a standard 117 v. alternating current source adapted to be used in providing a bias voltage for the electrostatic speaker: connections for delivering signal voltage to the low frequency speaker,

a step-up signal transformer having a primary winding connected in the signal circuit, a solid state voltage multiplier rectifier having a pair of input connections line connected with said current source each through a separate current isolating resistor and having one output terminal connected with the membrane, the signal transformer having a secondary winding connected between the backing plate of the electrostatic speaker and the other output terminal of the voltage multiplier rectifier.

4. For use in combination with a signal circuit, an electrostatic speaker having conductive membrane and cooperating backing plate elements, and further with a standard 117 v. alternating current source adapted to be used in providing a bias voltage for the speaker: a solid state voltage multiplier rectifier providing said bias voltage and having output terminals connected to deliver the bias voltage to the membrane and backing plate elements, a step-up signal transformer having a primary'winding connected in the signal circuit and having a secondary winding connected with the membrane and backing plate elements but conductively isolated from the signal circuit, the voltage multiplier rectifier having input connections line connected with said alternating current source, and a separate current isolating resistor in each of said input connections.

5. Equipment as defined in claim 4 and further including a current limiting resistor in the connection of the solid state voltage multiplier rectifier'with at least one of the speaker elements.

Claims

2. Equipment as defined in claim 1 in which said signal transformer comprises E and I core structures each comprising a plurality of laminations, with the primary and secondary windings surrounding the central leg of the E laminations, at least some portions of the E and I laminations being spaced in fixed relation to each other to provide a constant gap in the flux path.

3. For use in combination with a signal circuit, a low frequency speaker, an electrostatic high frequency speaker having a conductive membrane and having a conductive backing plate cooperating With said membrane, and further with a standard 117 v. alternating current source adapted to be used in providing a bias voltage for the electrostatic speaker: connections for delivering signal voltage to the low frequency speaker, a step-up signal transformer having a primary winding connected in the signal circuit, a solid state voltage multiplier rectifier having a pair of input connections line connected with said current source each through a separate current isolating resistor and having one output terminal connected with the membrane, the signal transformer having a secondary winding connected between the backing plate of the electrostatic speaker and the other output terminal of the voltage multiplier rectifier.

4. For use in combination with a signal circuit, an electrostatic speaker having conductive membrane and cooperating backing plate elements, and further with a standard 117 v. alternating current source adapted to be used in providing a bias voltage for the speaker: a solid state voltage multiplier rectifier providing said bias voltage and having output terminals connected to deliver the bias voltage to the membrane and backing plate elements, a step-up signal transformer having a primary winding connected in the signal circuit and having a secondary winding connected with the membrane and backing plate elements but conductively isolated from the signal circuit, the voltage multiplier rectifier having input connections line connected with said alternating current source, and a separate current isolating resistor in each of said input connections.

5. Equipment as defined in claim 4 and further including a current limiting resistor in the connection of the solid state voltage multiplier rectifier with at least one of the speaker elements.