US3632903A

US3632903A - Electrostatic headphone

Info

Publication number: US3632903A
Application number: US84919A
Authority: US
Inventors: Martin Lange Jr
Original assignee: KOSS ELECTRONICS Inc
Current assignee: KOSS ELECTRONICS Inc
Priority date: 1970-10-28
Filing date: 1970-10-28
Publication date: 1972-01-04
Anticipated expiration: 1989-01-04

Abstract

An electrostatic headphone speaker assembly includes a push-pull electrostatic driver operated off a coupling transformer, with the transformer secondary also supplying a polarizing circuit for the driver diaphragm. For enhanced operation there is a large resistance in the polarizing circuit to provide a long time constant, and the transformer leakage inductance is resonated with the driver input capacitance at 13 kHz.; and for a stereo headphone set with two transformers provided with ground jumpers, an isolation network is provided to decouple the transformer secondaries.

Description

Unite States Patent [72] Inventor Martin Lange, Jr.

River Hillls, Wis. [21] Appl. No. 84,919 [22] Filed Oct. 28, 19711 [45] Patented Jan. 41, 1972 [73] Assignee Koss Electronics, Inc.

Milwaukee, Wis. Continuation 01 application Ser. No. 785,166, Dec. 19, 1968, now abandoned. This application Oct. 28, 19711, Ser. No. 84,919

[54] ELECTROSTATIC HEADPHONE 11 Claims, 2 Drawing Figs.

[52] 11.8. C1 179/111 [51] int. Cl H04! 19/00 [50] Field ofSearch ..179/1.3, 1.3 PS, 111, 111 E; 325/492,494

[56] References Cited UNITED STATES PATENTS 1,622,039 3/1927 Lee 179/111 1,928,503 9/1933 Reisz 179/111 3,084,229 4/1963 Selsted 179/1 1 1 3,417,334 12/1968 Banovic 325/492 FOREIGN PATENTS 1,147,423 5/1966 Great Britain 179/1 1 1 OTHER REFERENCES Lucltett- New Radio Steals Its Power From The Air- Popular Science- Apr. 1958, pp. 108 & 109.

Selsted- The Electrostatic Earphone Journal of the Audio Engineering Society- Apr. 1961, Vol. 9, No. 2, pp. 145- 147 Primary ExaminerRalph D. Blakeslee Anorneys-Allan W. Leiser and Arthur H. Siedel ABSTRACT: An electrostatic headphone speaker assembly includes a push-pull electrostatic driver operated off a coupling transformer, with the transformer secondary also supplying a polarizing circuit for the driver diaphragm. For enhanced operation there is a large resistance in the polarizing circuit to provide a long time constant, and the transformer leakage inductance is resonated with the driver input capacitance at 13 kHz; and for a stereo headphone set with two transformers provided with ground jumpers, an isolation network is provided to decouple the transformer secondaries.

PATENTEB JAN 4 I972 INVENTOR TIN LAN6E,JR.

RIGHT /7 v j? K4 3L MAR ATTORNEY ELECTROSTATIC HEADPHONE BACKGROUND OF THE INVENTION This invention relates to a high fidelity speaker assembly that includes an electrostatic driver as opposed to an usual dynamic speaker, and that is particularly satisfactory for headphones. Headphone units have been widely used in high fidelity equipment, and offer the important advantages, for example, of a controlled listening environment, a high degree of isolation against extraneous noise, and low power requirements which result in an extremely high efficiency and lack of distortion. Similarly, electrostatic drivers have been used in speaker systems, and offer the known advantages among others of extremely smooth response and a very high degree of efficiency.

Heretofore, however, there have been practical difficulties in using electrostatic drivers, particularly in a headphone unit. A prime problem in this regard is providing the necessary high polarizing voltage for the movable diaphragm of the driver. Using commonly accepted techniques, this would require a separate power source which would be highly undesirable for a headphone unit and is also unsatisfactory with standard speakers enclosures.

SUMMARY OF THE INVENTION This invention contemplates a speaker unit incorporating an electrostatic driver in which a polarizing voltage is supplied by a coupling transformer secondary thus eliminating the necessity for a separate power source. It is the general object of the invention to provide an assembly of this type which is extremely efficient and offers pure response while being of minimum size and complexity. Another object is to provide a stereo assembly incorporating two units which provide the foregoing advantages and in which there is special provision to insure stereo separation. The invention is particularly suited for headphones, but is also desirable for standard enclosures.

Other objects and advantages will appear from the description of a preferred embodiment which follows.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary, somewhat schematic view in cross section of a single headphone formed according to the invention, and

FIG. 2 is a schematic electrical circuit diagram of a stereo headphone set incorporating the unit of FIG:- ll.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. I, the headphone unit shown includes a cup designated generally by the reference numeral 1 which is made of any suitably rigid plastic materials. The cup 1 includes a rear housing 2, a spacer ring 3, and a front plate assembly i which faces the users ear and mounts electrostatic driver elements as will be described. Assembly screws 5 extend rearwardly through the plate 4 and spacer 3 and are received in the housing 2 to connect the three cup elements together. The forward edge of the ring 3 is cut away to define, with the plate 4, a groove 6 which receives the rear or connecting edge of a typical liquid filled ring or cushion 7. The cushion 7 is of course intended to rest against the users head around his ear to form a substantially complete seal against extraneous noise and to cushion vibrations.

The ring 3 is provided with a radially inwardly extending flange 8 to which a rigid baffle 9 formed of any suitable material is attached by means of assembly screws 10 which extend through the plate 4 and through tubular spacers 111. A coupling transformer 12 extends through an opening in the baffle 9 and is connected thereto by means of bolt-spacer assemblies 13. The various other circuit components which will be discussed below are also mounted on the rear of the baffle 9 but have not been shown in FIG. 1 for the sake of simplicity and clarity.

The front plate 4 is split to define a rear portion 14 and a front portion lib, both of which are preferably provided with cross braces to have a generally eight-shaped configuration, as viewed from the front, providing matching upper and lower openings therethrough. Between the portions 114 and 15 are oppositely spaced, rigid front and rear driver plates 16 and a flexible, intermediate diaphragm M. The plates 16 and diaphragm are suitably glued in place and held in spaced relationship by gaskets l8, and together constitute a push-pull electrostatic driver adapted to face the ear of a user. The plates M are formed of a suitably rigid material which is electrically conductive but acoustically transparent-preferably perforated brass. The diaphragm I7 is formed of a suitable polyurethane film which is coated with an electrically conductive material and secured in place under suitable tension. Suitable leads (not shown) lead from the plates 16 and diaphragm 17 so that they can be connected into the circuits that will be described.

A perforated rigid grille 19 is received within the front plate portion openings in front of the front plate 16, and felt layer 20 is in the openings of the rear plate portion 14 behind the rear plate 16. Between the baffle 9 and plate 4 is a layer 21 of a suitable foam material, and a second mass of foam 22 is disposed in the rear of the housing 2, these elements serving to damp and control response in known fashion.

Although relatively unimportant and optional structural details have not been shown or referred to, the foregoing description will enable those skilled in the art to understand one manner in which an electrostatic headphone of the type contemplated herein can be constructed. Various modifications known to the art are possible, however, it being necessary for the purposes of the invention only that there by an electrostatic driver and a coupling transformer within a cup or other enclosure with the driver being adapted to face out of the enclosure.

FIG. 2 is a circuit diagram for a stereophonic headphone or other unit incorporating two units like that shown in FIG. 1, in which the two units would be mounted in usual manner on a suitable headband. With the exception of certain electrical components which will be apparent, the two units are identical, and like reference numerals have been used to identify corresponding elements. There are electrical connections between the two units, and conductors to effect these are of course passed through the headband or simply extend between the units if other enclosures are used.

The primaries of the two coupling transformers 12 are connected to

signal sources

23, 23, which are intended in the preferred embodiment to be the two output channels of a stereophonic amplifier. The secondaries of the two transfor mers 12 are connected to respective sets of driver plates 16 to develop charges thereon which will cause movement of the diaphragm 17 to produce sound, all as is well known in the art. There are ground jumpers 24 across the transformers 12, and the two transformers have a common ground as indicated at 25.

The left-hand unit as seen in FIG. 2 includes a polarizing circuit which supplies both units; and, again, the components and connections forming this circuit will be mounted on the baffle 9 of this unit. The circuit includes voltage tripler means designated generally by the reference numeral 26 which comprises a network made up of three capacitors 27 and three diodes 28, the operation of which is known to those skilled in the art. The tripler network 26 feeds a storage condenser 29, the charge on which is limited by a set of stacked zener diodes 30, a resistor 31 of relatively low resistance value being used as a filter and current limiter for the zener diodes 30. The output of the storage condenser 29 is fed through

lines

32, 32 to the two diaphragms 17 will

resistors

33, 33 of relatively high resistance value being interposed in respective lines.

An indicator line 34 leads from the tripler network 26 through a voltage reducing resistor 35 to indicator lamps 36 which serve to indicate whether the unit is functioning. No indicator lamp 36 is shown in FIG. 1, but it will be appreciated that the lamp will be mounted on and visible through a suitable opening in the cup 1 in known fashion.

An isolation network including a resistor 37 of relatively high resistance value and capacitor 38 are interposed between the right polarizing line 32' and the secondary of the righthand transformer 12. Because of the ground jumpers 24 the two transformers are effectively coupled, and the isolation network serves to isolate and decouple the two transformer secondaries to prevent loss of stereo effect.

It will be appreciated that the voltage tripler network 26 functions to charge the storage condenser 29 which in turn provides a polarizing voltage to the diaphragm 17. With properly designed driver units, there is very little charge leakage, and thus the polarizing circuitry has no significant effect on overall output. Under normal conditions it can be sufficiently supplied by transient peak signals which would not ordinarily appear as output in any event.

Although the values of the several circuit components are variable to some extent, certain parameters have been found to be desirable. With usual headphone elements, a charge of approximately 500 volts on the storage condenser 29 has been found to provide sufficient polarizing voltage. To supply this, it has been found most satisfactory to use a 1:60 transformer which, in conjunction with the tripler network 26 provides a 1:180 step-up ratio. The tripler network 26 is not theoretically necessary and sufficient step-up could be provided by the transformer alone; but this is not feasible as a practical matter because of size and other limitations and the tripler network has been found highly desirable for operation within practical limits.

The value of the

polarizing resistances

33, 33 should be sufficient to provide a long time constant which provides greater linearity and dynamic stability for the diaphragm. In the preferred embodiment, the values of the resistors 33, 33' are approximately 32 megohms; but in any event it is desirable to have sufficient resistance value to provide an RC time constant which is at least four times greater than the time interval of one-half cycle of the lowest frequency to be radiated. Assuming a lowest frequency of about 20 Hz., therefore, the time constant should be at least approximately 0.1 sec.

In a headphone unit of the type shown, the acoustic shunt capacitance of the volume of air enclosed between the driver diaphragm l7 and the users ear presents a problem, and this space must be reduced to an absolute minimum. Even so, however, there is a tendency to have a cutoff in response at approximately 13 kHz. with a headphone of; the general type shown. In the preferred embodiment, any loss in response is circumvented by selecting transformer and driver elements so that the transformer leakage inductive reactance resonates with the driver input capacitance at about 13 kHz. to restore proper level. The actual selection of component characteristics will include many variables, but these and the net effect desired will be understandable and readily attainable by those skilled in the art.

The preferred embodiment shown and described provides a headphone unit and stereo assembly with an extremely high degree of efficiency and uniformity of response. A prime feature in providing a desirable unit is the use of the polarizing circuit which is supplied by the coupling transformer secondary, and additional particularly important features include the provision of the

polarizing resistances

33, 33 to provide a desirable time constant, the isolation network to decouple the transformer secondaries and prevent the loss of stereo effect, and the resonance of the leakage inductance and input capacitance at approximately 13 kHz. Although a preferred embodiment of the invention has been shown and described, however, it will be apparent that modifications in construction and use might be made without departure from the actual spirit of the invention. The invention is not, therefore, intended to be limited by the showing herein or in any other manner except as many specifically be required.

I claim:

l. A headphone speaker unit comprising: an enclosure with a speaker opening; an electrostatic driver having opposite plates and a central, movable diaphragm, said driver being mounted in the enclosure to be adapted to face outwardly through the speaker openings; a coupling transformer mounted in the enclosure that has a primary adapted to be connected to an audio signal source and a secondary connected to the driver plates; and a polarizing circuit leading from the secondary to provide a polarizing voltage for the diaphragm, wherein said polarizing circuit includes a voltage multiplier network having an input connected to said coupling transformer secondary and an output connected to said driver diaphragm.

2. A unit according to claim 1 wherein the polarizing circuit includes a relatively high value resistance element connected between the voltage multiplier output and driver diaphragm to provide a time constant of approximately at least 0. 1 sec.

3. A unit according to claim 1 wherein the voltage multiplier network feeds a storage capacitor which is provided with voltage limiting means designed to limit the charge thereon to a predetermined maximum voltage.

4. A speaker unit as recited in claim 1 wherein the leakage inductive reactance of the coupling transformer secondary and the input capacitance of the driver are turned to resonate at a frequency above approximately 13 kHz. to enhance the high-frequency response of the speaker unit.

5. A stereophonic headphone assembly for use with a stereophonic signal source, said assembly comprising: a pair of headphone units each of which includes a cup, an electrostatic driver that includes opposite plates and a central diaphragm and that is mounted in the cup enclosure to be adapted to face a users ear, and a coupling transformer that has a primary adapted to be connected to an audio signal source and a secondary connected to the driver plates, the primaries of the two transformers being adapted to be connected to respective channels of the stereophonic source, the two transformers being provided with ground jumpers leading to a common ground connection; a polarizing circuit in the cup of one unit that is connected to the transformer secondary therein to generate a polarizing voltage and has output lines leading to the diaphragms of both drivers; and an isolation network that includes a resistance and a capacitance and that is in the cup of the other unit and is interposed between the polarizing circuit output line leading to the diaphragm therein and the transformer secondary thereof.

6. A headphone unit according to claim 5 wherein the polarizing circuit includes a voltage multiplier network and a relatively high value resistance element in each output line operative in combination with the capacitance of the electrostatic driver to provide a time constant of approximately at least 0.1 sec.

7. A headphone unit according to claim 5 wherein the voltage multiplier network feeds a storage capacitor which is provided with a voltage limiting means designed to limit the charge thereon to a predetermined maximum voltage.

8. A headphone unit according to claim 5 wherein the leakage inductive reactance of each transformer secondary and the input capacitance of its driver are tuned to resonate at a frequency above approximately 13 kHz. to enhance the high-frequency response of the speaker unit.

9. A speaker unit which comprises: an enclosure with a speaker opening; an electrostatic driver having opposite plates and a central movable diaphragm, said driver being mounted in the enclosure to face outwardly through the speaker opening and adapted to have its plates connected to an audio signal source; and a polarizing circuit adapted to be connected to the same audio signal source to receive the audio signal and use it to generate a polarizing voltage for the diaphragm of said driver.

10. A speaker unit as recited in claim 9, wherein said polarizing circuit comprises: a voltage multiplier circuit adapted to be connected to receive the audio signal; a storage capacitor connected in shunt relationship to the output of the source and the speaker unit, said transformer having a primary winding connected to the audio signal source and a single secondary winding connected to the plates of the driver and the voltage multiplier circuit.

PC4050 TED STA'lE PATENT @FHCE l oh unseen Patent No. 39 53 903 Dated January 4, 1972 Inventofls) Martin L ng's, Jro

It is certified that error appears in the shove-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1 line 6, "an" (second occurrence) should be a.

Column 1,, line 24, "speakers" should be speaker Column 2,, line 18,, "circuits" should be circuit Column 2, line 21, M a should be inserted before "felt" Column 2,, line 70 "will" should. be with Column 3, line 4, a should be inserted hefore "capacitor" Column 3, line 37, "32" should be 22 Column 3 line 74, "many" should be may Column 4,, line 25, "turned" should be tuned digned and sealed this 30th. day of May 1972..

(SEAL) Attest:

EDWARD MQFLETCHERMRQ ROBERT GOTTSGHALK Attesting Officer Commissioner of Patents la. J

Claims

1. A headphone speaker unit comprising: an enclosure with a speaker opening; an electrostatic driver having opposite plates and a central, movable diaphragm, said driver being mounted in the enclosure to be adapted to face outwardly through the speaker openings; a coupling transformer mounted in the enclosure that has a primary adapted to be connected to an audio signal source and a secondary connected to the driver plates; and a polarizing circuit leading from the secondary to provide a polarizing voltage for the diaphragm, wherein said polarizing circuit includes a voltage multiplier network having an input connected to said coupling transformer secondary and an output connected to said driver diaphragm.

2. A unit according to claim 1 wherein the polarizing circuit includes a relatively high value resistance element connected between the voltage multiplier output and driver diaphragm to provide a time constant of approximately at least 0.1 sec.

5. A stereophonic headphone assembly for use with a stereophonic signal source, said assembly comprising: a pair of headphone units each of which includes a cup, an electrostatic driver that includes opposite plates and a central diaphragm and that is mounted in the cup enclosure to be adapted to face a user''s ear, and a coupling transformer that has a primary adapted to be connected to an audio signal source and a secondary connected to the driver plates, the primaries of the two transformers being adapted to be connected to respective channels of the stereophonic source, the two transformers being provided with ground jumpers leading to a common ground connection; a polarizing circuit in the cup of one unit that is connected to the transformer secondary therein to generate a polarizing voltage and has output lines leading to the diaphragms of both drivers; and an isolation netwoRk that includes a resistance and a capacitance and that is in the cup of the other unit and is interposed between the polarizing circuit output line leading to the diaphragm therein and the transformer secondary thereof.

9. A speaker unit which comprises: an enclosure with a speaker opening; an electrostatic driver having opposite plates and a central, movable diaphragm, said driver being mounted in the enclosure to face outwardly through the speaker opening and adapted to have its plates connected to an audio signal source; and a polarizing circuit adapted to be connected to the same audio signal source to receive the audio signal and use it to generate a polarizing voltage for the diaphragm of said driver.

10. A speaker unit as recited in claim 9, wherein said polarizing circuit comprises: a voltage multiplier circuit adapted to be connected to receive the audio signal; a storage capacitor connected in shunt relationship to the output of the voltage multiplier circuit, which capacitor is charged to produce the polarizing voltage; and a voltage limiter connected to limit the magnitude of the polarizing voltage.

11. A speaker unit as recited in claim 10, which includes a coupling transformer connected between the audio signal source and the speaker unit, said transformer having a primary winding connected to the audio signal source and a single secondary winding connected to the plates of the driver and the voltage multiplier circuit.