US3015704A

US3015704A - Acoustical apparatus

Info

Publication number: US3015704A
Application number: US64630A
Authority: US
Inventors: Behymer Donald James
Original assignee: Maico Electronics Inc
Current assignee: Maico Electronics Inc
Priority date: 1960-10-24
Filing date: 1960-10-24
Publication date: 1962-01-02
Anticipated expiration: 1979-01-02

Description

Jan. 2, 1962 Filed Oct. 24, 1960 D. J. BEHYMER ACOUSTICAL APPARATUS 6 Sheets-Sheet 1 FIG. I

INVENTOR.

D. JAMES BEHYMER ATTORNEY Jan. 2, 1962 D. J. BEHYMER ACOUSTICAL APPARATUS Filed Oct. 24, 1960 FIG. 2

6 Sheets-Sheet 2 FIG. 3

INVENTOR. D. JAMES BEHYMER BYW J VX ATTORNEY Jan. 2, 1962 J. BEHYMER 3,015,704

ACOUSTICAL APPARATUS Filed Oct. 24, 1960 6 Sheets-Sheet 3 INVENTOR. D. JAMES BEHYMER FIG.5 BYWMJUQi/ ATTORNEY Jan. 2, 1962 D. J. BEHYMER 3,015,704

ACOUSTICAL APPARATUS Filed Oqt. 24, 1960 6 Sheets-Sheet 4 v 3? Q 50 a 9 43 P AMP 77 56 1 1:;

FIG. 6

INVENTOR.

D. JAMES BEHYMER ATTORNEY Jan. 2, 1962 Filed Oct. 24, 1960 D. J. BEHYMER ACOUSTICAL APPARATUS 6 Sheets-Sheet 5 FIG. 70'

87 i sci J QI 9a 97 INVENTOR. D. JAMES BEHYMER ATTORNEY United States Patent a corporation of Minnesotn Filed Oct. 24, 1960, Ser. No. 64,630 9 Claims. (Cl. 179-107) This invention relates to the field of acoustical apparatus and more particularly to the field of body worn hearing aids.

One of the problems confronting the hearing aid manufacturer is to devise a hearing aid which will have high gain and high power output for use with people who have a severe hearing loss, but at the same time be able to provide a unit which may have its power output adjusted for hearing losses less severe with a subsequent increase in battery life. This requirement comes about because of the various types of hearing losses that people have which require that a hearing aid have a flexible power output.

Of course another problem which must be eliminated if the hearing aid wearer is to receive the maximum comfort from his unit, is the problem of distortion of the sound waves. The problem of distortion is increased when sounds of a high degree of loudness, or a signal with a large amplitude in the audio range, is mixed with or substantially combined with, signals of smaller amplitude. Unless some means is provided to compensate for the large signals, they may produce discomfort to the hearing aid wearer and may overshadow the lesser sounds which are just as important to the hearing aid wearer. It should also be kept in mind that the large signal amplitudes which are applied to the input of a hearing aid receiver generally produce distortion unless the signal is reduced in magnitude.

One method of reducing distortion in an audio amplifier is to couple the output of the amplifier to the input through a compression circuit. However, in a hearing aid that is capable of having its output changed, it may be seen that any change in the output circuit which will change the output power will also affect the compression circuit. The prior art has generally taught that compression circuits of most audio amplifiers are drivenfrom the output stage and therefore any change in the output also affects a change at the input of the amplifier through the compression circuit.

It is therefore a general object of the present invention to provide an improved hearing aid which is to be worn on the body of the user;

It is another object of the present invention to provide a hearing aid amplifier having at least four stages of amplification in which the last two stages are driven in pushpull relation;

It is a further object of the present invention to provide a hearing aid amplifier which incorporates a compression circuit which couples the driver stage to the input;

It is yet another object of the present invention to provide a compression circuit which will reduce sudden peaks of sound at the input of the circuit and prevent the loud sounds from disturbing the hearing aid wearer;

It is still another object of the present invention to provide a compressor circuit which will keep the output level of the hearing aid at a low level of distortion;

It is another object of this invention to provide an amplifier which has its output readily adaptable to change the power output;

It is another object of the invention to provide an amplifier circuit in which a transformerless push-pull output is used; and

It is a further object of this invention to provide an amplifier with a readily removable power output limiter plug.

These and other features of this invention will become more apparent from the specification and drawings in which:

FIGURE 1 is a cut-away view of the back of the hearing aid showing the opening for the batteries and a portion of the printed circuits;

FIGURE 2 is a partial section of FIGURE 1 taken from the side which shows the placement of the microphone and the power output limiter plug;

FIGURE 3 is a side view of FIGURE 1 which shows a partial section having the socket for the hearing aid receiver cord exposed;

FIGURE 4 is a drawing of the assembled printed circuit which shows the first two stages of the amplifier;

FIGURE 5 is a drawing of an assembled printed circuit board which contains the last two stages of the amplifier;

FIGURE 6 is a block diagram of the entire hearing aid; and

FIGURE 7a is a schematic diagram of the first two stages of the amplifier circuit; and

FIGURE 7b is a schematic diagram of the last two stages of the hearing aid circuit.

Referring to the drawings, the hearing aid is contained in a housing 10 that includes a pair of terminals 11 and 12 which are connected to the negative and positive terminals respectively of a battery 13. Battery 13 supplies all power necessary for the operation of the hearing aid. A pair of

grounding straps

14 and 15 are provided to make contact with the outer portion of the metal covering which is snap-fastened to housing 10 (but not shown) so that any stray capacitance or interferences will be conducted to an electrical ground 16.

A microphone 17 (FIGURE 6) is connected to a terminal 20 of an input switch 21 which has a contact arm 22 connected to the input of an amplifier 23 and has a second contact 24 connected to a telephone pick-up coil 25. The other ends of microphone 17 and pick-up coil 25 are connected to one terminal of a capacitor 26 and the other terminal is connected to a common terminal of an amplifier 23 through a lead 27. The output of amplifier 23 which is a class A audio amplifier, is applied to a primary winding 30 of a transformer 31. A secondary winding 32 of transformer 31 has one terminal connected to ground 16 and the other end of secondary winding 32 and is further connected to a second class A amplifier 34 at its input through a variable capacitor 33. Amplifier 34- is connected to ground 16 through a common lead 35. A resistor 36 is connected to ground 16 and to the input of amplifier 34 so that the variable capacitor 33 in combination with resistor 36 form part of a tone control. The output of amplifier 34 is applied to the wiper arm 37 of a potentiometer 38. A resistive element 40 of potentiometer 38 has one end connected to ground 16 and the other end connected to the input of a driver amplifier 41. Driver amplifier 41 also has a common lead 42 connected to ground 16. The output from driver amplifier 41 is applied to another driver amplifier stage 4-3 through a connecting lead 44. Amplifier 43 has its common ele ment connected to ground 16 through a common lead 45. The output of amplifier 41 is further applied to the input of one stage 46 of a push-pull amplifier and the output of amplifier 43 is connected to a second stage 47 of the push-pull amplifier. The input of stage 4-6 is connected to the input of stage 47 through a pair of oppositely

poled diodes

50 and 51 that are connected in series. The common junction to both diodes is connected to a common junction formed by a pair of series connected

resistors

48 and 49 that form a voltage divider which is connected between ground 16 and terminal 11. The common element of stage 46 is connected to the common element of stage 47 through a pair of series connected

resistors

52 and 53. The common connection of the two resistors is connected to ground 16 through a lead 54. Terminal 12 of battery 13 is connected to an arm 57 of a switch 56 and a switch terminal 55 is connected to ground 16 and is engaged by arm 57 when power is to be applied to the circuit. When power is to be turned off, switch arm 57 is switched to a terminal 58 which is not connected to any circuit.

The outputs of amplifier stages 46 and 47 are connected to a center tapped receiver 59 where the center tap is connected to terminal 11 of battery 13.

The output of amplifier 41 is connected to a switch arm 60 of a switch 61 through a connecting lead 62. Switch 61 has a pair of

terminals

59 and 63 where terminal 63 is a normally open position of the switch and terminal 59 is connected to a voltage doubler 64 through a lead 65. Voltage doubler 64 is also connected to ground 16, and its output is applied to the terminals of microphone 17 and pick-up coil 25 which are connected to capacitor 26, through a resistor 66. Power is supplied to

amplifiers

23, 34, 41, and 43 through a lead 68 which is connected to battery terminal 11.

To describe the operation of the block diagram of FIG- URE 6, it should be kept in mind that all of the amplifiers are operated in class A operation with the exception of the output amplifier which is operated in a push-pull relationship and may vary between class AB and B operation. A more detailed description of what the amplifiers con tain will follow in the specification. An acoustic signal is detected by microphone 17 or some other suitable trans ducer and is applied to amplifier 23. In case the telephone is being used, telephone pick-up coil 25 may be switched into the circuit by switch 21 thereby removing microphone 17 from the circuit so that the external noises surrounding the telephone are not amplified and the sounds will not interfere with the telephone conversation. The voltage is applied to transformer 31 after being amplified by amplifier 23 and the output is applied to the input of amplifier 34 through variable capacitor 33. Some hearing aid wearers may desire to change the range of the frequency being amplified and this can be done by changing the amount of coupling capacitance through capacitor 33. Amplifier 34 has its output applied to potentiometer 38 which is a volume control so that the signal input to amplifier 41 may have its amplitude adjusted for a proper output level by the hearing aid wearer. Amplifier 41 performs two functions in that it applies an output signal to the input of amplifier 43 and applies a signal to amplifier stage 46. Amplifier 43 acts largely as a phase inverter since the signal output of amplifier 43 is approximately the same in magnitude as the output of amplifier 41. The output signal of amplifier 43 is applied to amplifier stage 4-7 of the push-pull power amplifer. Receiver 59 is connected to the output of amplifier stages 46 and 4-7 and may take on several configurations such as a bone conduction receiver or air conduction receiver. Whenever it becomes necessary to reduce the power output of the power output amplifier,

resistances

52 and 53 may be changed so that in cases where the threshold of discomfort for the person wearing the hearing aid is low, the power output may be adjusted to a most comfortable level for the wearer. The operation of

diodes

56 and 51 will be more fully explained later, it being sufficient to say at this point that they are used to reduce the distortion which could appear at the inputs to the power amplifier stages. When the compressor circuit is required, the signal from driver amplifier 41 is applied to a voltage doubler network so that a direct current signal is applied across resistor 66. Thus, any changes in receiver 59 or in

output resistors

52 and 53 will not affect the signal amplitude on the output of amplifier 41, where the compressor circuit derives its driving voltage. Voltage doubler 64 may be a conventional circuit and the direct voltage signal is applied to microphone 17 and telephone pick-up 25 as well as one side of capacitor 26 so thatcapacitor 26 is charged rapidly by the voltage doubler circuit and slowly discharges through the amplifier to supply the proper bias to compress any large input signal.

Referring to the more detailed diagram of FIGURES 7a and 7 b, one terminal of microphone 17 is connected to a terminal 67 of a multiple position switch 70, and the other terminal is connected to capacitor 26. Also connected to capacitor 26 is telephone pick-up coil 25 and the other end of coil 25 is connected to a switch terminal 71 of switch 70. A wiper arm 72 of switch 70 has a make before break contact which is of suflicient length to engage two contacts simultaneously. A normally open terminal 73 is situated electrically between terminals 71 and 67 of switch 70. Another terminal 74, of switch 70 is connected to one terminal of a capacitor 75. A transistor 76, has a base terminal 77, an emitter terminal 80, and a collector terminal 81. Switch arm 72 is connected to base element 77, and collector element 81 is connected to base element 77 through a feedback resistor 82. The other terminal of capacitor 75 is also connected to collector 81 so that when wiper arm 72 engages terminals 67 and 74, another feedback path is formed, the function of capacitor 75 being to cut out the high frequency audio range and boost the low audio frequencies. In normal operation, switch arm 72 engages terminals 67 and 73 and during use with a telephone, switch arm 72 is in the position shown engaging

terminals

71 and 73. A shield 83 is placed around pick-up coil 25 to provide the proper electrical shielding and is connected to ground 16. Also connected to ground 16 is emitter 80. For the purposes of illustration, transistor 76 as well as those to be described later, is of the P-N-P type in which current flow is from the emitter into the serniconductive material of the transistor. An alternating current path to ground is formed by a capacitor 8'4 which is connected to pickup coil 25 and is connected to base 77 and capacitor 26 so that during the use of the telephone pickup, the high frequencies are cut off. The output of transistor 76 is applied to one terminal of primary winding 36 of coupling transformer 31 where the other terminal is connected to terminal 11 of battery 13 through a pair of

series connecting resistors

85 and 86. The transformer 31 may prove to be more advantageous for this application because of difficulty encountered in the past with certain resistance-capacitance coupling networks which had characteristics such that any sudden change in the output signal of transistor 76 caused a diiferentiating action and the audio coupling transformer 31 has been found to eliminate this undesirable characteristic. Secondary winding 32 is connected to ground 16 at one terminal and the other terminal is connected to one side of three

capacitors

87, and 91, and is further connected to ground 16 through a resistor 92. The other side of capacitor 87 is connected to a base element 92, of a transistor 93 and is further connected to a switch arm 94- of a tone control switch 95. The other side of capacitor 96 is connected to a terminal 96 and the other side of capacitor 91 is connected to a terminal 97. Switch also has a normal open terminal 98 so that when switch arm 94 engages terminal 98

capacitors

90 and 91 are eliminated from the input circuit to transistor 93. Forming the rest of the resistance-capacitance coupling network, is a resistor 100 which is connected between the base element 92 and ground 16. A feedback resistor 101 is connected between base element 92 and a collector element 102 of transistor 93 and connected parallel with resistor 101, is a capacitor 103, the combination reducing the high frequency response which is outside the audio range of frequencies while at the same time providing a stabilizing action for the operation of transistor 93. An emitter element 104 is connected to ground 16 to complete the amplification circuit. A voltage dropping resistor is connected between collector 162 and the junction of series connected

resistors

85 and 86. Connected to this same junction is a filter capacitor 106 which has its other terminal connected to ground 16. Another capacitor 107 is connected between ground 16 and the other terminal of resistor 85. Resistor 85 and capacitor 107 form a filter network as does resistor 86 and capacitor 106 so that any alternating voltage appearing on the direct voltage line is shunted to ground 16. The output signal which appears on collector 102 is applied to a resistive element 118 of a variable resistor 1 11. Variable resistor 111 has a wiper arm 112 which is connected to one end of resistive element 110 and is further connected to a capacitor 113. Variable resistor 111 is used as a gain control and is set to a predetermined value of gain or to a predetermined voltage which is measured at wiper arm 1 12. The other side of capacitor 113 is connected to wiper arm 37 of volume control 38 and the resistive element 40 is connected to ground 16 on one end and connected to a base element 114 of a transistor 1115 on the other end. An emitter element 116 of transistor 115 is connected to ground 16 and a collector element 117 is connected to terminal 11 of battery 13 through a voltage dropping resistor 120. A feedback resistor 121 is connected between the collector element 117 and base element 114 to provide stabilization. It should also be noted that wiper arm 37 of volume control 38 is ganged to switch arm 57 of power switch 56 through a mechanical linkage 122. Generally,

the switch and potentiometer are combined so that the switch arm and wiper arm are mechanically driven by the same shaft. The output signal from collector element 117 is applied to a resistive element 123 of a variable resistor 124. Variable resistor 124 has a wiper arm '125 which is connected to one side of a capacitor 126. The other side of capacitor 126 is connected to a base element 127 of a transistor 130 and is further connected to ground 16 through a resistor 131. Capacitor 126 and resistor 131 form a coupling network to couple the signal from transistor 115 to transistor 130. An emitter element 132 of transistor 130 is connected to ground 16 and a resistor 133 is connected between base element 127 and a collector element 134. Resistor 133 also acts as a stabilizing feedback element to stabilize the operation of transistor 138. Collector element 134 is connected to terminal 11 of battery 13 through a voltage dropping resistor 135. The main function of transistor 130 is not to amplify an input signal but to rather invert the input signal and therefore the signal input to transistor 130 must be reduced in order that the output signals appearing on

collector elements

156 and 142 are of equal magnitude and this is done by reducing the input signal magnitude to base element 127 by adjusting wiper arm 1250f balance resistor 124.

The output signal from collector element 117 is applied to a base element 136 of a transistor .137 through a series connected resistor 140 and a capacitor 141. Transistor 137 also has a collector element 142 and an emitter element 143.

The output signal from collector element 134 of transistor 130 is applied to a base element 144 of a transistor 145 through a coupling capacitor 146.

Base elements

136 and 144 are connected by oppositely poled and series connected diodes 50-and 51 in such a fashion that current flow is away from base element i136 and away from base element 144. The purpose of

diodes

50 and 51 is to reduce any accumulated charge or capacitors '141 and 146 which would tend to build up during operation. At the junction of series connected

diodes

50 and 51 is a lead 147, which is connected to the common junction of a resistor 150 and wiper arm 151 of a variable resistor 152 The other end of resistor 150 is connected to ground 16 and connected in parallel with resistor 150 is a negative temperature cefficient resistor such as a thermister 153 to compensate for any temperature change in the junction resistance of

transistors

137 and 145. Wiper arm 151 engages a resistive element 154 which has one end connected to terminal 11 of battery 13.

Resistor 140 and capacitor 141 form a phase shifting network to compensate for the phase shift produced by the additional transistor 130 between

transistors

115 and 145, whereas transistor 137 is driven directly from transistor 115 and maintains a minimum input impedance to base element 136 regardless of the size of

output resistors

52 and 53. The purpose of variable resistor 152 is to set the proper drain for

transistors

137 and 145 or in other words to determine the proper bias so that

transistors

137 and 145 are operated in the class AB to class B range. Transistor 145 has a collector element 155 and an emitter element 156 where emitter element 156 is connected to emitter element 143 through series connected

resistors

52 and 53 which form an output limiter plug 157. Output limiter plug 157 has three terminals, 160, 161, and 162 which mate with-a plug 163 that has terminals 164, 165 and 166 respectively. Terminal 164 is connected to emitter element 156 and terminal 166 is connected to mitter element 143. The common junction of

resistors

52 and 53 is connected to terminal 161 which engages terminal 165 that is connected to ground 16 through lead 54. It will be explained more fully later, but output limiter plug 157 may have different values of resistance so that the output power from transistors 137 and maybe reduced, but in so doing the battery drain is reduced as well. In other words, the output limiter plug does not act like an attenuator as some of the prior art teaches. Receiver 59 is shown as a plug-in unit and has three

terminals

170, 17 1, and 1 72 where terminal 171 is the common terminal. These terminals mate with corresponding terminals of a plug in which terminal 171 is connected to terminal 11 of battery 13 through a lead 173 and terminal is connected to collector element 142 through a lead :173 and terminal v170 is connected to collector element 142 through a lead 174 while terminal 172 is connected to collector element 155 through a lead '175. A feed-back resistor 176 is connected between collector element 155 and wiper arm 112 of gain variable resistor .111 to reduce distortion in the amplifier when the volume is set at a low level.

The output on collector 117 of driver stage 41 is applied to a capacitor 180 which has its other terminal connected to .a resistive element 181 of a variable resistor 182. A wiper arm 183 of variable resistor 182 is connected to switch arm 60 of compressor switch 61 so that the alternating voltage signal may be applied to a voltage doubler network 64 which forms the compression circuit. A series connected pair of

diodes

184 and 185 which are poled in the same direction are connected with a capacitor 186 that has its other terminal connected to ground 16 as does the other terminal of diode 1 84. The junction of series connected

diodes

184 and 185 is connected to switch terminal 59 through lead 65. When the compression circuit is to be operated switch arm 60 engages terminal {59. The junction of diode 185 and capacitor 186 is connected to one terminal of resistor 66 through a connecting lead 190. Resistor 66 hasits other terminal connected to a resistor 191 and is further connected to a capacitor 192. The other end of resistor 191 is connected to the common junction of microphone 17, pickup coil 25, and capacitor 26 while the other side of capacitor 192 is connected to the junction of resistor 8'5 and primary winding 30 through a connectinglead 193.

Variable resistor 182 acts as a signal compressor adjustment so that the proper compression voltage is applied to input amplifier 23. Capacitor 192 serves to produce a feedback filter between the out-put and input of amplifier 23.

As shown in FIGURES 1 through 5, the operating knobs or levers for the various switches and potentiometers and variable resistors are marked with the same reference numeral as the switch arm or wiper arm.

Table I sets forth the values of the various components and elements which form the hearing aid amplifier and these are given by way of illustration only.

TABLE I Description Value Description Resistor 100K ohms.

Capacitor... do

OK 898: CPR-122 2.6 v.

* The values range from to 1000 ohms depending on the output power required.

Operation In normal operation, microphone 17 is connected to base element 77 and if the operator so desires, switch arm 72 may engage both terminals 67 and 74 so that the lower audio frequencies are boosted and the higher audio frequencies are reduced in magnitude because of the filtering action of capacitor 75. Switch arm 57 and wiper arm 37 are set in their operating positions and the volume is adjusted for a comfortable level, the gain control 111 being preset and normally not adjusted by the hearing aid wearer. The compressor switch 61 is operated to the In position where switch arm 60 engages terminal 59. The tone control switch 94, or as shown in FIGURE 1, the response switch, is generally set so that switch arm 94 engages terminal 97 which adds the largest capacitor to the circuit so that the lower frequencies near the low audio range are attenuated. Assuming that normal conversation is taking place,

amplifiers

23 and 34 produce amplified output signals and are operated as class A amplifiers. The output of amplifier 34 is applied to a push-pull

amplifier having stages

41 and 43, also being operated as class A amplifiers. When the positive portion of the audio amplified signal is applied to the base of transistor 145, transistor 145 conducts the signal to receiver 59 through

terminals

172 and 171. On the opposite half cycle of the audio wave, the negative going portion causes transistor 137 to conduct the signal through

receiver terminals

170 and 171.

If output limiter plug 157 contains

resistors

52 and 53 of the proper value, when volume control 38 is set for a maximum value there will be no discomfort felt by the wearer. However, if the threshold of discomfort is low and near the most comfortable level of power, it may be necessary to change power output limiter plug 157 and insert a plug having resistors 52 and '53 of a higher value. When limiter plugs of higher value resistance are used, the drain on battery 13 is reduced proportionally. During the half cycle when

transistors

145 and 137 are not conducting, current is drawn through

diodes

50 and 51 so that any charge remaining on

capacitors

146 and 141 during a conducting half cycle, is drained off and thereby prevents any buildup of a direct voltage which could cause distortion in the output of the amplifier.

Since the compression circuit is not affected by any change of value of the power limiter plug because it is connected to the preceding driving stage rather than the output stage, its operation is of a highly efficient nature. The alternating signal is taken from the output of driver stage 41 and is applied to the junction of

diodes

184 and 185 where the positive going half cycles are applied to the side of capacitor 186 which is connected to lead 190 and the negative going half cycles pass through diode 184- to charge capacitor 1811 thereby creating a direct voltage which is approximately double in value to the peak value of the applied alternating voltage and this appears across capacitor 186 between ground 16 and lead 190 in the form. of a direct voltage signal. This signal is applied to

resistor

66 and 191 so that capacitor 26 is charged rapidly from the compressor circuit and slowly discharges through microphone 17 or telephone pickup 25 to base element 77 of transistor 76. Thus any large signal which appears on collector 117 of driver stage 41 is effectively reduced at the input to amplifier 23 and the larger the input signal becomes, the more compression takes place. The action of the compressor is to keep the output level of the hearing aid at a low distortion level which greatly aids the comfort of the wearer and increases the intelligibility of the output signal. The use of the compression circuit also makes the volume control adjustment much less critical and will require less adjustment of the volume control when moving between areas having a sound differential.

FIGURE 2 shows the power limiter plug 157 in its operating position and FIGURE 1 shows its access for readily changing the plug for diiferent output requirements.

FIGURES 1 and 3 show more clearly the circuit connections for plug-in receiver 59 which may be connected by a three element cord (not shown). Contacts and 172 also serve to make connection with two

silvered areas

194 and 195 on a printed circuit board 196 which contains the elements of the driver stages and output stages.

Another printed circuit board 197 is shown in FIG- URE 4 with the lower left hand corner out way exposing circuit board 196. Printed circuit board 197 contains the elements of the first two class A amplifiers. Situated at the uppermost part of printed

circuit boards

196 and 197 is a recessed portion which contains a spring 200 that is used to support microphone 17 in a resilient casing 201. The black areas of the figures simulate the silvered or conducting areas on the printed circuit boards and the various controls have been shown where they are accessible on the different circuit boards.

While the preferred embodiment of the invention has been described, this embodiment is illustrative only and I intend to cover all modifications and alternative constructions by the spirit and scope of the appended claims.

I claim as my invention:

1. An amplifier for use in a hearing aid having at lea-st four stages of amplification comprising: a first 9 amplifier having first transistor for amplifying input signals received on the base element thereof; a first transducer connected between the base and emitter elements of said first transistor and being responsive to audio frequency sound waves to provide an output signal for driving said first amplifier; a second amplifier including a tone control and having a second transistor for amplifying input signals received on the base element thereof; said base element being connected to said tone control; a transformer coupling the emitter and collector elements of said first transistor to said tone control and said emitter element of said second transistor; a third amplifier having first and second transistor driver stages to provide amplified output signals on the collector elements thereof; a first resistance-capacitance coupling having a variable resistor for changing the amount of coupling connected between the collector and emitter elements of said second transistor and the base and emitter elements of said first driver stage; a second resistance-capacitance coupling having a variable resistor for changing the amount of coupling connected between the collector and emitter elements of said first driver stage and the base and emitter elements of said second driver stage; a fourth amplifier having first and second transistor power amplifier stages to provide power output signals on the collector elements thereof, the emitter elements being connected through a pair of plug-in series connected resistances where the common junction of said resistances is connected to the emitter elements of said first, second, and third transistors, and the base elements being connected through a pair of oppositely poled series connected diodes; a third resistance-capacitance coupling connecting the collector element of said first driver stage to the base element of said first power amplifier stage; a capacitive coupling connecting the collector element of said second driver stage to the base element of said second power amplifier stage; a direct voltage source having one terminal connected to the emitter elements of said first, second, and third transistors and having the other terminal connected to the collector elements of said first, second and third transistors, a voltage divider connected in parallel with said direct voltage source having a center tap connected to the common junction of said diodes in said fourth amplifier; a second transducer responsive to electrical signals received between said collector elements of said first and second power amplifier stages and said direct voltage terminal connected to the collector elements of said first, second, and third amplifying means for transforming said electrical signals to audio frequency sound waves; and a signal compressor responsive to the output signals on the collector element of said first driver stage and connected to the base element of said first amplifying means so that distortion of the output signals received by said second transducer are substantially reduced.

2. An amplifier for use in a hearing aid having at least four stages of amplification comprising: first amplifying means having control, output, and common elements for amplifying input signals received on said control element; first transducer means connected between said control and common elements and being responsive to audio frequency sound waves to provide an output signal for driving said firstarnplifying means; second amplifying means including tone control means having control, out-' put, and common elements for amplifying input signals received on said control element, said control element being connected to said tone control means; transformer coupling means coupling said common and output elements of said first amplifying means to said tone control means and said common element of said second amplifying means; third amplifying means including first and second driver stages each having control, output, and common elements to provide amplified output signals on said output elements; first resistance-capacitance coupling means having a variable resistor for changing the amount of coupling connected between said output and common elements of said second amplifying means and said control and common elements of said first driver stage; second resistance-capacitance coupling means having a variable resistor for changing the amount of coupling connected between said output and common elements of said first driver stage and said control and common elements of said second driver stage; fourth amplifying means including first and second power amplifier stages each having control, output, and common elements to provide power output signals on said output elements, said common elements being connected through a pair of changeable series connected resistances where the common junction of said resistance is connected to the common elements of said first, second, and third amplifying means, and said control elements being connected through a pair of oppositely poled series connected diodes; third resistancecapacitance coupling means connecting said output element of said first driver stage to said control element of said first power amplifier stage; capacitive coupling means connecting said output element of said second driver stage to said control element of said second power amplifier stage; direct voltage means having one terminal connected to said common element of said first, second, and third amplifying means and having the other terminal connected to said output elements of said first, second, and third amplifying means; voltage divider means connected in parallel with said direct voltage means having a center tap connected to the common junction of said diodes in said fourth amplifying means; second transducer means responsive to electrical signals received between said output elements of said first and second power amplifier stages and said direct voltage terminal connected to said output elements of said first, second, and third amplifying means for transforming said electrical signals to audio frequency sound waves; and signal compression means responsive to the output signals on said output element of said first driver stage and connected to said control element of said first amplifyingmeans so that distortion of the output signals received by said second transducer means are substantially reduced.

3. An amplifier for use in a hearing aid having at least four stages of amplification comprising: first amplifying means operating as a class A amplifier for amplifying signals received at the input thereof; first transducer means responsive to representative of audio frequencies to provide a signal for driving said first amplifying means; second amplifying means operating as a class A amplifier including tone control means for amplifying signals received at the input thereof; transformer coupling means coupling said first amplifying means to said second amplifying means; third amplifying means having first and second driver stages operating as class A amplifiers to provide amplified signals at the output thereof; first resistance-capacitive coupling means coupling said second amplifying means to said first driver stage; second resistance-capacitance coupling means coupling said first driver stage to said second driver stage; fourth amplifying means including first and second power amplifier stages connected for operation substantially as a class B push-pull amplifier to provide power output signals; third resistance-capacitance coupling means connecting the output of said first driver stage to the input of said first power amplifier stage; capacitive coupling means connecting the output of said second driver stage to the input of said second power amplifier stage; direct voltage means connected to said first, second, third,

. and fourth amplifying means; second transducer means tially reduced and independent of the load characteristics of said second transducer.

4. An amplifier for use in a hearing aid to be worn on the body of a person having at least four stages of amplification comprising: first amplifying means operating as a class A amplifier for amplifying signals received at the input thereof; first transducer means responsive to representative of audio frequencies to provide an output signal for driving said first amplifying means; second amplifying means operating as a class A amplifier including tone control means for amplifying signals received at the input thereof; transformer coupling means coupling said first amplifying means to said second amplifying means; third amplifying means having first and second driver stages operating as class A amplifiers to provide amplified signals at the output thereof; first resistance-capacitive coupling means coupling said second amplifying means to said first driver stage; second resistance-capacitive coupling means coupling said first driver stage to said second driver stage; fourth amplifying means including first and second power amplifier stages connected for operation substantially as a class B push-pull amplifier to provide power output signals; third resistance-capacitance coupling means connecting the output of said first driver stage to the input of said first power amplifier stage; capacitive coupling means connecting the output of said second driver stage to the input of said second power amplifier stage; direct voltage means connected to said first, second, third, and fourth amplifying means; second trans ducer means responsive to electrical signals received from said outputs of said first and second power amplifier stages for transforming said electrical signals to audio frequency sound waves; changeable power limiting means responsive to said first and second power amplifier stages for reducing the power output of said stages to a value near the most comfortable level of the wearer of said hearing aid; and signal compression means having a voltage doubler responsive to the output signals of said first driver stage connected to said input of said first amplifying means so that distortion of the output signals received by said second transducer are substantially reduced and independent of the load characteristics of said second transducer.

5. A hearing aid amplifier for use in a hearing aid to be worn on the body of a person having at least four stages of amplification com-prising: a first class A amplifier for amplifying signals received at the input thereof; a microphone connected to said class A amplifier and responsive to audio frequency sound waves to provide an output signal for driving said first amplifier; a second class A amplifier for amplifying signals received at the input thereof; a transformer for coupling said first amplifier to said second amplifier; a third class A amplifier having first and second driver stages of amplification where the output of said first driver stage is connected to the input of said second driver stage through a variable impedance device adjusted to reduce the input signal magnitude to said second driver stage so that the output of said second driver stage is equal to the output of said first driver stage; a passive network coupling said second arn plifier to said first driver stage; a power amplifier operated in push-pull relation having first and second stages of amplification; where the input of said first stage is connected to the output of said first driver stage through a phase shifting network to compensate for any phase shift developed by said second driver stage, and where the input of said second stage is connected to the output of said second driver stage through a reactance element, said inputs of said first and second stages of said power amplifier connected by a pair of oppositely poled series connected diodes and the common junction of said series connected diodes connected to a center tap of a voltage divider for setting the operating bias of said first and second stages; a voltage source connected to said first, second, and third class A amplifiers and to said power amplifier; and a three terminal center tapped receiver having provide an output signal for driving said stages; a class A amplifier having first and second driver stages of am plification where the output of said first driver stage is connected to the input of said second driver stage through a variable impedance device adjusted to reduce the input signal magnitude to said second driver stage so that the output of said second driver stage is equal to the output of said first driver stage; a passive network coupling the output of said plurality of class A amplifier stages to said first driver stage; a power amplifier operated in push-pull relation having first and second stages of amplification,

where the input of said first stage is connected to the output of said first driver stage through a phase shifting network to compensate for any phase shift developed by said second driver stage, and where the input of said second stage is connected to the output of said second driver stage through a reactance element, said inputs of said first and second stages of said power amplifier connected by a pair of oppositely poled series connected diodes and the common junction of said series connected diodes connected to a center tap of a voltage divider for setting the operating bias of said first and second stages; a voltage source connected to said first, second, and third class A amplifiers and to said power amplifier; and a three terminal center tapped second transducer having said center tap connected to said voltage source and the other terminals connected to the outputs of said power amplifier first and second stages.

7. A hearing aid amplifier for use in a hearing aid to be worn on the body of a person having a plurality of amplification stages comprising: a plurality of first amplifier stages having an input and an output for amplifying signals; a first transducer connected to said input of said plurality of first amplifier stages responsive to audio frequency waves to provide an output signal for driving said stages; an amplifier having first and second driver stages where the output of said first driver stage is connected to the input of said second driver stage through a variable impedance; a passive network coupling the output of said plurality of first amplifier stages to said first driver stage; a push-pull amplifier having first and second stages of amplification where the input of said first stage is connected to the output of said first driver stage, and where the input of said second stage is connected to the output of said second driver stage; a voltage divider connected to the input of the said first and second stages of said push-pull amplifier for setting the operating bias thereof; a voltage source connected to said first amplifier stages, said first and second driver stages, and said push-pull amplifier; and a second transducer connected to said voltage source and to the outputs of said first and second pushpull stages.

8. -An amplifier for use in a hearing aid to be worn on the body of a person comprising: first amplifying means for amplifying signals representative of audio frequencies and having an input and output; first transducer means responsive to sound waves to produce signals representative thereof connected to said input of said first amplifying means; second amplifying means having a pair of driver stages, each delivering output signals; connecting means connecting said first amplifying means to said pair of driver stages of said second amplifying means; third amplifying means having first and second power amplifier stages connected in push-pull relation to provide 13 power output signals; means connecting the output of one driver stage to the input of said first power amplifier stage and the output of the other driver stage to the input of said second power amplifier stage; voltage means contnected to said first, second, and third amplifier means; second transducer means responsive to said output signals from said first and second power amplifier stages to produce audio frequency sound waves; and signal compres- 14 sion means connected between one of said driver stages and said input of said first amplifier means.

9. The invention as set forth in claim 8 including; variable resistance means connected between said first and second power amplifier stages and said voltage means for reducing the power output of said stages to a predetermined level.

No references cited.