US2870255A

US2870255A - Microphone assembly

Info

Publication number: US2870255A
Application number: US403224A
Authority: US
Inventors: Jr Charles Douglas Jenkins; Jr Harry Ashland Greene
Original assignee: REMLER Co Ltd
Current assignee: REMLER Co Ltd; REMLER COMPANY Ltd
Priority date: 1954-01-11
Filing date: 1954-01-11
Publication date: 1959-01-20
Anticipated expiration: 1976-01-20

Description

4Filed Jan. 11. 1954 Jan. 20, 1959 K c. D. JENKINS, JR., ET Al. 2,370,255

MICROPHONE ASSEMBLY 2 Sheets-Sheet 1 41 Lgf PIE-3 2..

A Tra/WE V5 Jan. 20, 1959 c. D. JENKINS, JR., ET Al. 2,870,255

MICROPHONE ASSEMBLY 2 Sheets-Sheet 2 Filed Jan. ll. 1954 PIE A O 5 m .b

8 IOOO soo,l 4 s e vAUDIO FREQUENCY cHARAcTEmsnc FIE E BATTERY SUPPLY VOLTAGE. (VOLT s) HAPPY A. @,Qf'E/VE, J2.

and treated anthracite coal United States Patent 2,870,255 MICROPHONE ASSEMBLY Application January 11, 1954, Serial No. 403,224

3 claims. (Cl. 179-1) This invention relates generally to microphones used in radio, telephone and public address apparatus.

It is well known that the carbon microphone has had continued application in many fields because of its simplicity and high sensitivity. The high sensitivity is due to the fact that the carbon microphone is actually an amplifier since the amount of electrical output that the diaphragm motion controls is greater than the sound power required to operate the diaphragm. However, the carbon microphone also has numerous disadvantages, the most important of which are the steady background hiss resulting from random changes in the resistance which the carbon granules offer to the direct current .passing through them, the poor frequency response, and

the instability of the carbon button The carbon button is normally made of granules made from selected and mounted against the diaphragm to form a buttonf As a result, at the present time, the carbon micro-phone is used where high sensitivity is a great asset such as in the telephone system, amateur, police and other radio work where a transmission of information rather than good intelligibility of speech is the primary object, but is seldom used in a public address or high quality radio telephone work.

lt is also well known that other types of microphones such as a variable reluctance or a dynamic microphone have frequency response characteristics which are far superior to the frequency response characteristics of the carbon microphone. For instance, in the dynamic microphone the response can be made practically fiat from 40 to 10,000 cycles. However, the primary disadvantage of these other microphones is that they have a low sensitivity in comparison with the carbon microphone.

In general it is the object of the present invention to provide an improved microphone which has the frequency response characteristics of the low sensitivity type of microphone and which has the sensitivity that is at least as great as that of the carbon microphone.

Another object of the invention is to provide an improved device of the above character which will fit into the same space in a handset as is required by a carbon microphone so that the device may be substituted for the carbon microphone with a minimum number of alterations in the handset.

Another object of the invention is to provide a device of the above character which utilizes the same power supply and same voltage leads used by the carbon microphone thereby eliminating the necessity of additional leads.

Additional objects and features of the invention will appear from the following description in which the preferred embodiment has been set forth in detail in conjunction with the accompanying drawings.

Referring to the drawings:

Figure 1 is a view of a handset incorporating one embodiment of the invention. The microphone assembly is shown in an exploded view.

Figure 2 is a view taken along the line 2-2 of Figure l,

and in 2,870,255 Patented Jan. 20, 1959 ICC V2 :Figure 3 is a circuit diagram illustrating a preamplifier circuit which may be used in the invention.

Figure 4 isv a graph comparing the audio frequency response characteristics of a magnetic element, and of the magnetic element having a transistor preamplifier.

Figure 5 is a graph showing the relative gain of a transistor preamplifier with varying supply voltage.

In general the presentinvention makes use of a conventional low sensitivity, wide frequency response, microphone element together with a sub-miniature transistor preamplifier which are both placed in the same housing to replace a carbon microphone assembly.

Referring to Figure l we have shown a handset 10 having a housing 11 formed to provide a handle 12 and space for a receiving unit 13 and a transmitting unit 14. A switching unit 15 to operate the transmitter is mounted on the handle 12. A suitable conductor cable 16 leads from the handset to the equipment with which it is desired to use the handset.

, `The body 18 of the transmitting unit 14 is a part of the variable reluctance microphone element 19 and serves as a mounting for the transistor preamplifier 21. The

body 18 is held in place within the hollow annular por tion 17 in the housing 11 by the retaining ring 22 which is threaded o nto the` annular portion 17. l

The microphone element 19 can be of any convention-al type having a good frequency responseV characteristic such as a variable reluctance microphone. The

conductors

23 and 24 leading from the coil of the microphone element 19 are connected to the

microphone input conductors

26 and 27 of the transistor of preamplifier 21. The

conductors

28 and 29 connect the output of the transistor preamplifier to the cable 16 whereby the output of the microphone and the preamplifier are. fed into a conventional apparatus well known to those skilled in' the radio and telephone arts. However, any conventional means may be used for connecting the output of the preamplifier to the cable 16 such as ring contacts or female and male receptacles.

Any suitable audio frequency amplifier may be used for amplifying the output of the microphone element 19 so that the sensitivity of the microphone element 19v will be the same or greater than that of a carbon microphone. lt is desirable to operate this amplifier from the same voltage supply that was used for the carbon microphone and it is also desirable that the microphone and the amplifier fit within the space previously occupied by the carbon microphone. By doing `this it is possible to substitute our microphone for a carbon microphone with a minimum of effort and expense.

In View of this a preamplifier using transistors has been found to be very desirable because of the space limitations in a conventional microphone housing and the manner in which a power supply is conventionally derived. Figure 3 shows a suitable amplifier of this type. Referring to this circuit diagram, we have shown transistors at 33 and 34. Both of the transistors are of the p-n-p'diffused'alloy junction type. Transistor 33 is connected with base bias in a grounded emitter configuration whereas transistor 34 is connected with base bias in a grounded collector configuration.

As is well known in the transistor art, the transistor 33 is comprised of a base 36, an emitter 37 and a collector 38, and the transistor 34 is comprised of a base 39, an emitter 41 and a collector 42. The base 36 of the transistor 33 is connected by conductor 46 to the base bias resistor 47 and the D. C. blocking and coup-ling capacitor 48. The capacitor 48 is directly connected to the microphone input conductor 27. Conductor 49 serves to connect the other end of base bias resistor 47 to one end of the collector load resistor 51 and to one end of aardgas the filter capacitor 52 and tol one end of the voltage divider circuit comprised of resistances S3 and S4.

Conductor 56 `is connected to the microphone input conductor 26 and is directly connected tothe emitter 37 of the transistor 33 through conductor 57- and then it is connected to one end of base bias resistor 58. Conductor 56 is also connected to the other end of the lter capacitor 52 and is directly connected to the collector 42 of the transistor 34 through conductor S9. The conductor 56 is also connected to the midpoint between resistors -53 and 54. The collector 38 of the transistor 33 is connected to the lother end of collector load resis tor 51 by conductor 61, The conductor 61 is also connected to one side of the D. C. blocking and coupling capacitor 62. The other side of the capacitor 62 is connected to the other end of base bias resistor 58 and to the'base 39 of the transistor 34. The emitter 41 of the transistor 34 is'connected to the other side of the voltage divider network comprised of

resistors

53 and 54 by conductor 64. One end of the voltage divider network is connected to the positive preamplifier output conductor v2S and the other end of the voltage divider network is connected to the negative preamplifier output conductor 29.

Suitable means is provided in the associated radio or telephone equipment to provide a suitable source of D. C. t-o the

conductors

28 and 29 to energize the amplifier. Any suitable voltage from 6 volts to 32 volts may be used, but in this embodiment the preamplifier has been constructed for an optimum of 2232 volts D. C.

The above circuit components can be mounted in any suitable manner such as on an etched circuit board.

The circuit shown in Figure 3 operates as a conventional audio frequency amplifier circuit using transistors. The output of the microphone is amplified a predetermined amountby the amplitier and then the output of the amplifier is fed into the conventional network of radio or telephone apparatus.

By way of example, we have constructed an audio frequency amplifier as shown in Figure 3 operating .from a 22 to 32 volt D. C. sistor and having values of resistors and capacitors as follows: resistor 47, 330,000 ohms, resistor 51, 10,000 ohms, resistor 58, 47,000 ohms, resistor 53, 820 ohms, resistor 54, 2,200 ohms, capacitor 43, 4 rnfd., capacitor 62. .05 mid., and capacitor 52, l mfd.

This Vcircuit was found to operate very satisfactorily with a variable reluctance type of microphone to give an output which was comparable to that of a carbon microphone with greatly improved frequency response characeristics.

The graphs in Figures 4 and 5 show the characteristics of a typical variable reluctance microphone having a transistor preamplifier incorporated within it. Curve A of Figure 4 shows the audio frequency response characteristics of a variable reluctance type of microphone by itself and curve B shows the audio frequency response characteristics for the same microphone having a transistor preamplifier. The particular transistor preamplisource using CK 721 junction tranterminal to the emitter of said fier has been designed for aircraft use to give a steeply falling characteristic in the lower frequency range. This is accomplished by using a lowvalue of capacitance for capacitor 62.

Figure 5 shows the gain characteristics of the transistor preamplifier disclosed herein under varying supply voltages. It can be seen that the gain is nearly constant in the 22-32 volt range for which the amplifier was designed and that it is usable at much lower voltages which might be encountered in other applications.

We claim:

l. In a microphone assembly for use as a replacement unit for a carbon microphone mounted in a housing having a relatively shallow circular recess with voltage supply and outlet terminals provided in the housing, the microphone assembly comprising a body adapted to fit in said recess, a microphone element having a wide frequency response mounted on one portion of the bottom side of'said body, a transistorized audio frequency amplilier mounted on another portion on the bottom side of said body, said microphone element and said amplifier being of a size to fit within said recess, the amplifier having input and output terminals, the input terminals being connected to the microphone element and the output terminals being adapted to be connected to said supply and outlet terminals, and means adapted to secure the body to the housing.

2. ln a microphone assembly, first and second transistors, each transistor having a collector, an emitter and a base, a source of direct current having positive and negative terminals, circuit means connecting said positive first transistor, circuit means connecting said negative terminal to the collector of said first transistor and to the emitter of said second transistor, circuit means connecting the base of said first transistor to the collector of said second transistor, a

microphone element having a pair of output leads, circuit 'means for connecting one of said output leads to the baseV of said second transistor, and necting the other output lead to the transistor.

3. In a microphone assembly, first and second transistors, each transistor having a collector, an emitter and a base, a source of voltage, circuit means for connecting said first transistor to said source of voltage in a grounded collector configuration with base bias, circuit means for connecting said second 'transistor to said first transistor and said source of voltage in a grounded emitter configuration with base bias, a microphone element having output leads, and means for connecting said output leadsto said second transistor and said source of voltage.

circuit means for conemitter of said second References Cited in the tile of this patent UNITED STATES PATENTS 2,024,705 Rutherford Dec. 17, 1935 2,276,933 Collom Mar. 17, 1942 2,544,027 King Mar. 6, 1951 2,579,162 Veneklasen Dec. 18, 1951