Linear sound amplifier circuit
This is a continuation of application Ser. No. 690,329, filed Dec. 12, 1975, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a linear sound amplifier circuit for increasing the output power of stereophonic tape or record players. 
     2. Description of the Prior Art 
     Many systems are described in previous art or are available in the marketplace to amplify high fidelity signals received from stereophonic tape or record players, but these generally consist either of simple and relatively low output circuits built integrally in the tape or record player mechanism or of needlessly complex circuits that are generally complicated and expensive to manufacture. Additionally, available amplification systems typically do not use a balanced bridge configuration but are provided in a single ended configuration so that there is generally crossover distortion and a signal provided by the bias current is continually present through the circuit mode. There is obviously a need for a Class A linear amplifier using a balanced bridge configuration for more effectively increasing output power from stereo tape or record players, with the output being provided without distortion and with the idle bias currents being cancelled when no signal is being fed through the circuit. 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel linear sound amplifier circuit specifically of the class AB type for increasing the fidelity and output of stero tape or record players with a minimal amount of distortion by use of a balanced bridge amplifier for effectively doubling or quadrupling the output power. 
     It is a feature of the present invention to provide a linear sound amplifier circuit of the class B or AB type for stereo record or tape players. 
     A further feature of the present invention provides a linear sound amplifier circuit which is reliable and efficient in operation. 
     Yet still a further feature of the present invention provides a Class A linear sound amplifier circuit which can be economically produced and which, therefore, may be marketed by the manufacturer at a competitive price. 
     Other features of this invention will be apparent during the course of the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings forming a part of this specification, and in which like reference characters are employed to designate like components throughout the same: 
     FIG. 1 is a schematic diagram of the linear sound amplifier circuit using two complementary pairs of transistors; and 
     FIG. 2 is a schematic diagram of the linear sound amplifier circuit using eight transistors. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings in detail, there is illustrated a preferred form of the linear sound amplifier circuit schematically drawn in accordance with the principles of the present invention and which is designated generally in its entirety by the reference numeral 10 and which is comprised of NPN transistors 11 and 12, PNP transistors 13 and 14, a transformer 15, thermistors 16 and 17, resistors 18, 19, 20, 21, 22, 23, 24, and 25, Zener diodes 26 and 27, and associated connections between the components as will be later described. 
     Typical values proposed for the resistors and thermistors are, but are not limited to, the following: 
     
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       Component     Value                                                
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Resistors 18 and 19  1.5K ohms                                            
Resistors 20 and 21  330 ohms                                             
Resistors 22 and 23  330 ohms                                             
Resistors 24 and 25  1.5K ohms                                            
Thermistors 16 and 17                                                     
                     800 ohms (cold)                                      
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     In operation, the input signal from the stereophonic playing device is applied to transformer 15 at its primary 30, the primary winding 30 of transformer 15 having a resistance of 8 ohms to match the resistance of the input device. The signal is then stepped up at the secondary 31 of transformer 15, the secondary winding 31 having the resistance of approximately 500 ohms, from whence the signal is applied to the bases of each of the complementary pairs of transistors 11, 13 and 12, 14 through the center of the resistive divider network defined by resistors 20 through 23. The Zener diodes 26 and 27 are connected in series across the secondary winding 31 of the transformer 15 to limit the input signal to the value of the breakdown voltages of Zener diodes 26 and 27 and of the base emitter junctions of the transistors 11 to 14. Transformer 15 also functions to isolate the output stage of the preceding amplifier in the stereo player device from the linear sound amplifier circuit 10, and to match the impedance from the preceding stage to the amplifier circuit 10. 
     When a positive signal appears at the secondary 31 of transformer 15 at terminal 32, then a negative signal will simultaneously appear at the secondary 31 of transformer 15 at terminal 33. The positive going signal at the terminal 32 will be transmitted to the base 44 of transistor 11 through resistor 20 and to the base 35 of transistor 13 through resistor 22. A DC bias (+12 to 14 volts) is applied to terminal 36, such as from an automotive battery, to maintain transistor 11 to be idling in its linear region, so that a positive signal at the base 34 of the transistor 11 will drive it further into conduction, causing current from its emitter 37 to increase approximately to the beta of the transistor multiplied by the change in base current, thereby in turn causing a positive going increase in current across the load at terminal 38 for transmission of the signal to subsequent speakers or other resistive load. The positive going input signal at terminal 32 of the secondary 31 of transformer 15 is also transmitted to the base 35 of transistor 13 through resistor 22, this effectively biasing transister 13 into its cutoff region so that no signal is transmitted through it. The negative going input signal at terminal 33 of the secondary 31 of transformer 15 is simultaneously transmitted to the base 39 of transistor 12 through resistor 21 and the base 40 of transistor 14 through resistor 23. A negative going signal at the base 39 of transistor 12 will effectively drive it into cutoff, so that no signal is transmitted therethrough, with a negative going signal at the base 40 of transistor 14 driving it further into conduction (transistor 14 is idling in its linear region due to the DC bias applied at terminal 36). The absolute value of the current from the emitter 41 of transistor 14 will increase by approximately the beta of the transistor multiplied by the change in base current, this causing a negative going increase in current amplification across the load at terminal 42. The combined positive going increase in current at terminal 38 from transistor 11 and the negative increase in current at terminal 42 from the transistor 14 causes a current doubling across the load or a quadrupling in output power. This is derived from the relation, P = I 2  R, for a single ended configuration; and substituting 2I for I in a balanced bridge, then P = (2I) 2  R = 4I 2  R or a fourfold increase in power. 
     The above operation applies for a negative going input signal at terminal 32 of the secondary 31 of transformer 15 and the simultaneous positive going input signal at terminal 33 of the secondary 31 of transformer 15 except that, in this instance, transistors 12 and 13 conduct and transistors 11 and 14 are cut off. 
     As shown is FIG. 2, the effective power output of the linear sound amplifier circuit 10 in FIG. 1 can be doubled by adding transistors 50 through 53 in parallel with transistors 11 through 14 respectively. The preceeding discussion concerning the circuitry operation would apply whether using eight transistors or four transistors, except that the incoming signal would be applied simultaneously to the bases of the two transistors rather than to the base of a single transistor. Since the amplification caused by 2 transistors would be double that of a single transistor, the effective output of the amplifier circuitry would be doubled. 
     There is thus described a novel Class B linear sound amplifier circuit for use in conjunction with stereophonic tape or record players for doubling or quadrupling the output of the stereo player without distortion. The circuitry described can be utilized within the existing cabinetry of stereophonic tape or record players or it can be provided in a separate unit which can be interconnected between an existing player unit and the audio speakers. 
     It is to be understood that the form of this invention as shown and described is to be taken as a preferred example thereof, and that this invention is not to be limited to the exact arrangement of parts described in the description or illustrated in the drawings as changes thereto pertaining to size, shape and arrangement of parts thereof are envisioned within the scope of the invention without departing from the novel concepts of the invention.