Dynamic Loudspeaker driving apparatus

Abstract: The dynamic loudspeaker driving apparatus consists of at least a power amplifier (21) and a feedback circuit (41). The power amplifier amplifies an input signal so that the dynamic loudspeaker is driven by the amplified input signal. A detecting circuit (32) is further provided in order to accurately detect a motional voltage (VM) produced at an equivalent motional impedance of dynamic loudspeaker, and the feedback circuit negatively feedbacks the detected motional voltage to the power amplifier so that distortions due to a transient response of a vibration system of dynamic loudspeaker will be eliminated. The amplified input signal is supplied to a first terminal of dynamic loudspeaker and a voltage at a second input terminal of dynamic loudspeaker is supplied to the feedback circuit, and impedance components other than the equivalent motional impedance can be canceled. In addition, it is possible to be further provided with a filter circuit (20) having a frequency response characteristic which can be obtained by electrically simulating a voltage transmission characteristic against the equivalent motional impedance of dynamic loudspeaker. Thus, the input signal (Vi) can be given with a desirable frequency characteristic by the filter circuit and then supplied to the power amplifier.

Description: DYNAMIC LOUDSPEAKER DRIVING APPARATUS The present invention generally relates to a dynamic loudspeaker driving apparatus, and more particularly to a dynamic loudspeaker driving apparatus which can reduce levels of distortions in sound from a dynamic loudspeaker. In general, a feedback circuit is arranged between input and output of a power amplifier provided within an amplifier unit of an audio device. By use of this feedback circuit, it is possible to reduce levels of noises and distortion components included in an output signal of the power amplifier. In addition, the amplifier unit of the audio device may also be provided with a motional feedback circuit (hereinafter, referred to as MFB circuit) which feedbacks a signal corresponding to a vibration of a dynamic loudspeaker so as to reduce a distortion in an operation of the loudspeaker. Theoretically, motional voltage must be applied to a motional impedance of the dynamic loudspeaker, and the MFB circuit negatively feedbacks such motional voltage to the input of the power amplifier. The above-mentioned motional impedance can be represented by ZM of an electrically equivalent circuit of the dynamic loudspeaker (hereinafter, referred simply to as a loudspeaker) shown in Fig. 1. In Fig. 1, Rv designates a dc resistance component of a voice coil, and Lv designates an inductance component of the voice coil. In Fig. 2, a solid line designates voltage Vi supplied to the dynamic loudspeaker, while a short dashes line designates motional voltage VM which is produced at the equivalent motional impedance ZM representative of a vibration system of the dynamic loudspeaker. The operating distortion of the vibration system of the loudspeaker represents a transient response component of the motional voltage VM. When the MFB circuit is provided to the dynamic loudspeaker, the negative feedback quantity must become extremely large at the frequencies in the vicinity of a lowest resonance frequency of the dynamic loudspeaker. Hence, it is avoided to provide too much negative feedback quantity for the MFB circuit. In general, a frequency characteristic of the dynamic loudspeaker provided with the MFB circuit has a tendency that the frequency response characteristic must be easily lowered at low frequencies at which the negative feedback quantity must be concentrated. In order to prevent such frequency response characteristic from being lowered at low frequencies, a compensating low-pass filter circuit (i.e., compensating LPF circuit) is conventionally provided at an input side of the dynamic loudspeaker so that the frequency response characteristic will be raised at the low frequencies. However, it is impossible to obtain a perfect compensating characteristic from such LPF circuit. Fig. 3 shows an example of a conventional dynamic loudspeaker driving apparatus providing the above-mentioned compensating LPF circuit. In Fig. 3, a feedback circuit 2 is connected between input and output sides of a power amplifier 1. In this case, a feedback ratio b of the feedback circuit 2 is set further smaller than one, while a gain of the power amplifier 1 is set further larger than one. Meanwhile, a dynamic speaker 3 and three resistors 4 to 6 constitute a bridge circuit 7. An output signal Es of this bridge circuit 7 diagrammatically corresponds to the motional voltage of the dynamic speaker 3, and such signal Es is detected by a transformer 8. A part of a detection signal outputted from the transformer 8 is feedbacked to the input side of power amplifier 1. In the circuit shown in Fig. 3, the resistors 4 to 6 and the transformer 8 represent the MFB circuit. In addition, a compensating LPF circuit 9 is provided at input side of the power amplifier 1, and lowering of low frequency characteristics of the circuit shown in Fig. 3 is improved and compensated by the MFB circuit. More specifically, the compensating LPF circuit 9 adequately raises a signal level of input signal Vi in the low frequency range, and the lowering of the low frequency characteristics is improved. The MFB circuit used in the conventional audio amplifier unit is exclusively used for reducing distortions and noises included in a signal outputted from the power amplifier. However, such MFB circuit is not used for perfectly eliminating distortions due to the transient response of the vibration system of the dynamic loudspeaker at all. In short, the main portion of the conventional dynamic loudspeaker driving apparatus is the negative feedback portion, and the MFB circuit is merely used as an auxiliary circuit of the dynamic loudspeaker driving apparatus. As shown in Fig. 3, the MFB circuit is a detection circuit constituted by the transformer and the bridge circuit consisting of resistors only. Hence, detection voltage detected by this detection circuit is not identical to the motional voltage in a strict sense. In other words, the detection voltage and the motional voltage are different in waveform, peak value and phase. For this reason, it is naturally impossible to provide much negative feedback, and the over-all frequency characteristics must be irregularly varied. Hence, the characteristics which must be given to the compensating LPF circuit must be extremely complicated, so that it is impossible to compensate the frequency characteristic of the dynamic loudspeaker with accuracy. Therefore, the conventional dynamic loudspeaker driving apparatus can only provide the circuit which can adequately raise the output level in the low frequency range. As described heretofore, in the conventional audio amplifier unit, it is impossible to perfectly eliminate the all distortions due to the transient response of the vibration system of the dynamic loudspeaker. Meanwhile, the conventional MFB circuit can use a pressure sensor, a temperature sensor, a microphone or other sensors in order to detect the motional voltage. Instead of using the above-mentioned sensors, a bridge circuit can be used for detecting the motional voltage produced at a voice coil of the loudspeaker, as described before. These techniques are disclosed in a monthly magazine "Radio Technique" published in Japan; October Issue and November Issue in 1984, and February Issue in 1985, for example. However, in the above-mentioned MFB circuit using the sensors, a phase revolution of a detection output of such sensor must be increased, for example. Hence, there must be a limit of a feedback quantity due to an ability of the sensor. If the feedback quantity is set large, the MFB circuit will oscillate by itself. As a result, the conventional MFB circuit is disadvantageous in that a distortion reducing effect of the loudspeaker must become small. On the other hand, the MFB circuit using the bridge circuit is disadvantageous in that the circuit constitution thereof must be complicated. As described heretofore, the conventional dynamic loudspeaker driving apparatus adopting the MFB circuit must detect the motional voltage. For this reason, it is impossible to sufficiently reduce the levels of the distortions of the loudspeaker. It is accordingly a primary object of the present invention to provide a dynamic loudspeaker driving apparatus which can detect the motional voltage with accuracy and then negatively feedback the detected motional voltage by 100% so that the distortions due to the transient response of the vibration system of the dynamic loudspeaker will be perfectly eliminated. There have been developed various kinds of dynamic loudspeaker driving apparatuses. An example of a loudspeaker driving apparatus is given in the document EP-A-0181608, where a power amplifier is disclosed which may be a conventional state-of-the-art power gain block. In the same document, high and low frequency correction signal components are developed by applying a program signal to a reference load having both inductor and capacitor components, or circuits simulating such components which are preferably tuned to cancel and provide zero correction signal from the system at approximately the 400 Hz nominal impedance frequency of most loudspeaker systems. However, a detection of the motional voltage with the highest possible accuracy and, consequently, a perfect correction of the detected motional voltage by means of a negative feedback from the loudspeaker driving apparatus is not known from this document. A similar problem is mentioned in the document "Journal of the Audio Engineering Society", vol. 33, no. 6, June 1985, pages 430-435, US; J.A.M. Catrysse: "On the design of some feedback circuits for loudspeakers" which relates to feedback circuits for loudspeakers driven by a power amplifier. An overall circuit for loudspeaker feedback comprises two Proportional Integral (PI) controllers, namely a mechanical one and an electrical one, which are built with low-noise high-gain operational amplifiers. The tuning of these two PI controllers which is accomplished with the aid of a square wave is first carried out on the current feedback circuit and then on the mechanical feedback circuit. However, each time the controller parameters are tuned to give the well-known step response, an overshoot of about 4% cannot be avoided. Thus, a 100% negative feedback of the detected motional voltage is not achieved and a perfect elimination of the distortions due to the transient response of the vibration system of the loudspeaker is not possible. According to the present invention, there is provided a dynamic loudspeaker driving apparatus comprising (a) an amplifier having a large open-loop-gain for driving a dynamic loudspeaker; and (b) input means for supplying an input signal to an input terminal of the amplifier via a filter circuit which electrically simulates a voltage transmission characteristic against an equivalent motional impedance of the dynamic loudspeaker, characterized by the provision of (c) detecting means for detecting a motinal voltage applied to said equivalent motional impedance of said dynamic loudspeaker; and (d) feedback means for negatively feeding back said motional voltage to said input terminal of said amplifier by a transmission gain. Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown. In the drawings: Fig. 1 is a circuit diagram showing an electrically equivalent circuit of the speaker; Fig. 2 shows waveforms of the input voltage supplied to the loudspeaker and the motional voltage applied to the equivalent motional impedance of the vibration system of the loudspeaker; Fig. 3 is a circuit diagram showing an electric constitution of the conventional dynamic loudspeaker driving apparatus; Fig. 4 is a block diagram showing a basic constitution of an embodiment of the present invention; Figs. 5A and 5B and Figs. 6A to 6C show frequency response characteristics for explaining an operation of the embodiment of Figure 4; Fig. 7 is a circuit diagram showing an electric constitution of the embodiment of Figure 4; Figs. 8A and 8B are circuit diagrams for explaining functions of a bridge detection circuit shown in Fig. 7; Fig. 9 is a circuit diagram showing an essential constitution of the dynamic loudspeaker driving apparatus according to another driving arrangement; Fig. 10 is a circuit diagram showing the driving arrangement of figure 9; Fig. 11 is a circuit diagram showing an embodiment of an essential portion of the driving arrangement of fig. 9; Fig. 12 is a circuit diagram showing a concrete constitution of the arrangement of fig. 9; Fig. 13 is a circuit diagram showing a modified example of the arrangement of figure 9; Figs. 14A to 14C are graphs showing frequency characteristics for explaining an operation of the circuit shown in Fig. 13; Fig. 15 is a circuit diagram showing a further loudspeaker driving arrangement; Fig. 16 is a graph showing frequency characteristics of the arrangement of figure 15; and Fig. 17 is a circuit diagram showing a concrete constitution of the filter circuit 110 of the arrangement of figure 15. Next, description will be given with respect to a preferred embodiment of the present invention in conjunction with Figs. 4 to 8A, wherein like reference characters designate like or corresponding parts throughout the several drawings. First, description will be given with respect to a basic constitution of an embodiment of the present invention. Fig. 4 is a block diagram showing the basic constitution of an embodiment of the present invention. In Fig. 4, the motional voltage VM is applied to the equivalent motional impedance ZM of the vibration system of the dynamic speaker (or dynamic loudspeaker) 23, and such motional voltage VM is directly supplied to an inverting input terminal of power amplifier 21, whereby the motional voltage VM will be negatively feedbacked by 100%. Hence, a system AP consisting of the power amplifier 21 and the dynamic speaker 23 can be considered as an equivalent voltage amplifier having a voltage gain "1" against the motional impedance ZM. In addition, 20 designates a band-pass filter (BPF) circuit which constitutes input means of the first embodiment. The reasons why such BPF circuit 20 is provided will be described as follows. In general, when the constant voltage is applied to the input terminal of the dynamic loudspeaker to thereby drive the dynamic loudspeaker with the constant voltage, it is possible to obtain a flat curve of tone pressure vs frequency characteristic as shown in Fig. 5A. In this case, a relation between the motional voltage VM and the frequency within the dynamic loudspeaker can be shown in Fig. 6A. In Figs. 6A to 6C, each of hatching parts represents actual acoustic energy. On the other hand, when the motional voltage VM is negatively feedbacked by 100%, a curve of tone pressure vs frequency characteristic does not become flat and the tone pressure in the low frequency range must be lowered as shown in Fig. 5B. In this case, the relation between the motional voltage VM and the frequency will be as shown in Fig. 6B, wherein the motional voltage VM directly corresponds to the input signal Vi perfectly and thus the variation of motional voltage VM itself is perfectly suppressed. As a result, the distortions in an operation of the loudspeaker are prevented from being caused. However, in the case where the motional voltage VM is negatively feedbacked by 100% as shown in Fig. 6B, the curve of tone pressure vs frequency characteristic does not become flat as shown in Fig. 5B. For this reason, the waveform of input signal Vi is modified as shown in Fig. 6C by the BPF circuit 20 so that the waveform of motional voltage VM will become equivalent to the waveform in case shown in Fig. 5A. In other words, the BPF circuit 20 provided to the input side of the power amplifier 21 is the circuit which can electrically simulate the voltage transmission characteristics against the motional impedance of the vibration system of the dynamic speaker 23. Due to this BPF circuit 20, the dynamic loudspeaker driving apparatus provided with the MFB circuit can present the flat curve of tone pressure vs frequency characteristic as shown in Fig. 5A. As described heretofore, the embodiment has a circuit constitution provided with the system in which the motional voltage VM is negatively feedbacked by 100% between the power amplifier 21 and the dynamic speaker 23. Due to this system, the embodiment can perfectly eliminate the distortions caused by the transient response of the vibration system of the dynamic speaker 23. In addition, the embodiment simulates the voltage transmission characteristics of the conventional dynamic loudspeaker at the input side of power amplifier 21. Next, description will be given with respect to the embodiment in detail in conjunction with Figs. 7, 8A and 8B. Fig. 7 is a circuit diagram showing an electric constitution of the embodiment. In Fig. 7, a first fixed terminal 11a of variable resistor 11 is connected to a signal input terminal 10 via a resistor 12, while a second fixed terminal 11b thereof is connected to a first terminal of resistor 13. In addition, a slider terminal 11c of variable resistor 11 is connected to an input terminal of amplifier 14. In this case, resistance Ra denotes resistance combined by resistance of resistor 12 and resistance between the terminals 11a and 11c of variable resistor 11, while resistance Rb denotes resistance combined by resistance of resistor 13 and resistance between the terminals 11b and 11c of variable resistor 11. The amplifier 14 is designed to have a voltage gain "+1". An output terminal of amplifier 14 is connected to a first terminal of capacitor 15 (having capacitance C0), while a second terminal of capacitor 15 is connected to a first terminal of resistor 16 (having resistance R0). A second terminal of resistor 16 is grounded via a parallel circuit consisting of a resistor 17 (having resistance R0) and a capacitor 18 (having capacitance C0) and then connected to an input terminal of amplifier 19. This amplifier 19 is designed to have a voltage gain "+3". In addition, an output terminal of amplifier 19 is connected to a second terminal of resistor 13 and then connected to a non-inverting input terminal of amplifier 21a. The BPF circuit 20 is constituted by the amplifiers 14 and 19, the variable resistor 11, the resistors 12, 13, 16 and 17, the capacitors 15 and 18 as described above. Next, description will be given with respect to characteristics of BPF circuit 20. This BPF circuit has a resonance frequency f1 which is determined by the resistances of resistors 16 and 17, the capacitances of capacitors 15 and 18. In short, the resonance frequency f1 is represented by the following formula (1). f1 = 1/2πC0·R0 In addition, a sharpness Q of resonance is represented by the following formula (2). Q = (1+Ra/Rb)/3 By suitably selecting the capacitances of capacitors 15 and 18, the resistances of resistors 16 and 17 in the BPF circuit 20, the resonance frequency f1 of the BPF circuit 20 can be coincided with the lowest resonance frequency f0 of the dynamic speaker 23. By adjusting the variable resistor 11, a frequency bandwidth in resonance characteristics can be arbitrarily varied. In other words, in the case where the resistance Ra is set larger than the resistance Rb by adjusting the variable resistor 11, the value Q becomes large so that a frequency bandwidth of resonance characteristics will become narrow. On the contrary, in the case where the resistance Ra is set smaller than the resistance Rb, the value Q becomes small so that the frequency bandwidth of resonance characteristics will become wide. Accordingly, by using the BPF circuit 20, the resonance characteristics of input signal Vi can be simulated to the voltage transmission characteristics against the motional impedance of dynamic speaker 23 with accuracy. In Fig. 7, the power amplifier 21 is constituted by the voltage amplifier 21a having a large open-loop-gain and a power stage consisting of a NPN type transistor 21b and a PNP type transistor 21c. An output terminal of amplifier 21a is connected to both base terminals of transistors 21b and 21c. Both emitter terminals of transistors 21b and 21c are connected in common to constitute an output terminal of power amplifier 21. The output terminal of power amplifier 21 is connected to a first terminal of dynamic speaker 23, and this output terminal is grounded via a resistor 24 (having resistance of a·Rv; "a" denotes an arbitrary coefficient), a resistor 25 (having resistance of a·Rs/2) and a resistor 26 (having resistance of a·Rs/2) in series. In this case, a capacitor 27 (having capacitance Cv1=Lv/(a·Rs·Rv)) is connected in parallel to a serial circuit consisting of the resistors 25 and 26. In addition, a second terminal of dynamic speaker 23 is grounded via a resistor 31 (having resistance Rs). The dynamic speaker 23 can be electrically represented by an equivalent circuit which is constituted by a serial circuit consisting of a voice coil resistor 28 (having resistance Rv), a voice coil inductance 29 (having inductance Lv) and an equivalent circuit 30 of a mechanical vibration system of dynamic speaker 23. This equivalent circuit 30, i.e., the motional impedance, can be represented by a parallel circuit consisting of a resistor 30a, a capacitor 30b and a coil (inductance) 30c. The above-mentioned dynamic speaker 23, the resistors 24, 25, 26 and 31, the capacitor 27 constitute a bridge circuit 32. Next, description will be given with respect to functions of the bridge circuit 32. The combined resistance of the resistors 24 to 26 within the bridge circuit 32 can be represented by (a·Rv + a·Rs/2 + a·Rs/2). Such combined resistance is set sufficiently larger than another combined resistance (Rv+Rs) of the resistors 28 and 31, and the resistance Rs of resistor 31 is set sufficiently smaller than the resistance Rv of resistor 28. Meanwhile, a condition as described by the following formula (3) is set between the dynamic speaker 23 and the resistors 24, 25, 26 and 31. (a·Rv)/(a·Rs) = Rv/Rs By adequately setting the resistances of resistors as described above, it becomes possible to accurately detect the motional voltage VM between a connection point P4 formed between the resistors 25 and 26 and another connection point P2 formed between the resistor 31 and the second terminal of dynamic speaker 23, which will be described later. Next, the above-mentioned connection point P4 between the resistors 25 and 26 is connected to a non-inverting input terminal of amplifier 34. In addition, the connection point P2 between the dynamic speaker 23 and the resistor 31 is connected to an inverting input terminal of amplifier 34 via a resistor 35 (having resistance r), and this connection point P2 is also connected to a first terminal of resistor 36 (having resistance r). A second terminal of resistor 36 is connected to an output terminal of amplifier 37. This amplifier 37 is designed to have a voltage gain "+1". An input terminal of amplifier 37 is connected to an output terminal of amplifier 34 via a resistor 38 (having resistance b·Rv; "b" denotes an arbitrary coefficient), and this input terminal of amplifier 37 is grounded via a parallel circuit consisting of a resistor 39 (having resistance b·Rs) and a capacitor 40 (having capacitance Cv2=Lv/(b·Rs·Rv)). The bridge circuit 32, the amplifiers 34 and 37, the resistors 35, 36, 38 and 39, and the capacitor 40 constitute a bridge amplifier 41. This bridge amplifier 41 corresponds to detecting means. The output terminal of amplifier 34 is connected to a first terminal of capacitor 42 (having capacitance Cf). A second terminal of capacitor 42 is connected to a first terminal of resistor 43 (having resistance Rf) and also connected to the inverting input terminal of amplifier 21a within the power amplifier 21. A second terminal of resistor 43 is connected to the output terminal of power amplifier 21. The capacitor 42 is used for blocking a direct current, and the resistor 43 is used as a feedback resistor. Next, description will be given with respect to a detecting principle of the motional voltage VM by use of the bridge amplifier 41. First, in the bridge circuit 32 shown in Fig. 8A, the relation between these voltages V0 to V4 can be represented by the following formula (4). In this formula, V0 denotes a voltage supplied from the power amplifier 21, V1 denotes a voltage supplied to the non-inverting input terminal of amplifier 34, V2 denotes a voltage at the connection point P2, V3 denotes a voltage at the output terminal of amplifier 37 and V4 denotes a voltage at the output terminal of amplifier 34. V3 = V4·(b·Rs//Cv2)/(b·Rs//Cv2 + b·Rv) = V4·Rs/(Rs + Rv + jωLv) , wherein Cv2=Lv/(b·Rs·Rv) and "Rs//Cv2" means a combined impedance of parallel circuit consisting of resistance Rs and capacitance Cv. In addition, the following formula (5) can be obtained based on a characteristic of operational amplifier with feedback. V1 = (r·V2 + r·V3)/(r + r) = (V2 + V3)/2 ∴V3 = 2·V1 - V2 Next, the voltages V1 and V2 can be obtained by referring to Fig. 8B as described by the following formulae (6) and (7). 2·V1 = V0·(a·Rs//Cv1)/(a·Rs//Cv1 + a·Rv) = V0·Rs//(Rs + Rv + jωLv) , wherein Cv1=Lv/(a·Rs·Rv). V2 = (V0 - VM)·Rs/(Rs + Rv + jωLv) When the above-mentioned formulae (6) and (7) are put in the formula (5), the following formula (8) can be obtained. V3 = VM Rs/(Rs + Rv + jωLv) Thus, the following formula (9) can be obtained from the formulae (4) and (8). V4 = VM Accordingly, the motional voltage VM of the dynamic speaker 23 can be obtained from the output of amplifier 34 with accuracy. Next, description will be given with respect to the operation of the first embodiment in conjunction with Fig. 7. First, the input signal Vi applied to the signal input terminal 10 is supplied to the BPF circuit 20 wherein the signal level of input signal Vi is raised in the resonance frequency f1. More specifically, a signal (Vi+VM) outputted from the BPF circuit 20 has a frequency bandwidth characteristics which are obtained by simulating the voltage transmission characteristics of the dynamic speaker 23. This signal (Vi+VM) is supplied to the non-inverting input terminal of amplifier 21a within the power amplifier 21 wherein the signal (Vi+VM) is amplified. Then, the amplified signal is supplied to the dynamic speaker 23, whereby the dynamic speaker 23 will be driven. At this time, the motional voltage VM is produced between the both terminals of equivalent circuit 30 of the dynamic speaker 23. Such motional voltage VM is detected by the bridge amplifier 41, and the detected motional voltage VM is supplied to the inverting input terminal of amplifier 21a via the capacitor 42. In short, the motional voltage VM is feedbacked by 100%. Since the motional voltage VM is feedbacked by 100% as described above, it is possible to perfectly eliminate the distortions due to the transient response of the vibration system of dynamic speaker 23. In addition, the embodiment simulates the voltage transmission characteristics of dynamic speaker 23 at the input stage thereof. Hence, similar to the conventional apparatus, the embodiment can realize the flat curve of tone pressure vs frequency characteristic. Moreover, the frequency range of the frequency characteristic of the embodiment can be stretched to further lower frequency range by varying the voltage transmission characteristics at the input stage, regardless of the lowest resonance frequency of the frequency characteristic. Fig. 9 is a circuit diagram showing an essential constitution of another dynamic loudspeaker driving apparatus. In Fig. 9, an input terminal 101 applied with an input voltage Vi is connected to an inverting input terminal of an operational amplifier (or a power amplifier) 102 via a resister R1. A non-inverting input terminal of the operational amplifier 102 is grounded, while the output terminal thereof is connected to a connection point between the resistor R1 and the non-inverting input terminal thereof via a resistor R3. In addition, the output terminal of the operational amplifier 102 is grounded via a load 103 (which is a speaker, for example) having an impedance ZL and a resistor Rt in series. A connection point between the load 103 and the resistor Rt is connected to a connection point among the inverting input terminal of the operational amplifier 102, the resistors R1 and R3 via an amplifier (or a servo amplifier) 104 having gain "-A" and the resistor R2 in series. In the above-mentioned constitution, when voltage Vo is applied between both terminals of the load 103, a transmission characteristic represented by "-Vo/Vi" can be obtained from the following formula (10). -Vo/Vi = (R3/R1)·[1/{1+(Rt/ZL)·(1-A·R3/R2)}] Hence, an output impedance (or a drive impedance) Zo can be obtained from the following formula (11). Zo = Rt(1-A·R3/R2) According to the above formula (11), it is possible to set the value of the output impedance Zo to a negative value under a condition where a value of A·R3/R2 is larger than one. Next, description will be given with respect to the arrangement of Fig. 10. This arrangement represents a case where the essential circuit shown in Fig. 9 is applied to an actual speaker driving circuit. In Fig. 10, parts identical to those shown in Fig. 9 will be designated by the same numerals. As shown in Fig. 10, a resistor R2a (having a resistance equal to that of the resistor R2) is used instead of the resistor R3. As the amplifier 104, a servo amplifier consisting of an operational amplifier 105, impedance loads 106 and 107 is used. Further, a dynamic speaker 108 is used instead of the load 103. In Fig. 10, a connection point between the output terminal of the operational amplifier 102 and the resistor R2a is connected to a terminal 108a of the dynamic speaker 108, while another terminal 108b of the dynamic speaker 108 is grounded via the resistor Rt. In addition, the terminal 108b is connected to an inverting input terminal of the operational amplifier 105 via the impedance load 106 (having an impedance Z1), and a non-inverting input terminal of the operational amplifier 105 is grounded. The output terminal of the operational amplifier 105 is connected to a connection point between the inverting input terminal thereof and the impedance load 106 via the impedance load 107 (having an impedance Z2) and also connected to the resistor R2. Meanwhile, in the speaker 108, Rv and Lv respectively designate a dc resistance and an inductance of a voice coil, and a resistor RM, a capacitor CM and a coil LM within a parallel circuit designate respective components of a motional impedance ZM of a drive system of the speaker 108. When relations of R2=R3 and A=Z2/Z1 are respectively put into the formulae (10) and (11), the transmission characteristic (-Vo/Vi) and the output impedance of the second embodiment can be obtained from the following formulae (12) and (13). -Vo/Vi = (R2/R1) [1/{1+(Rt/ZL)·(1-Z2/Z1)}] Zo = Rt(1-Z2/Z1) Next, description will be given with respect to a detailed constitution of the servo amplifier 104 in conjunction with Fig. 11. In order to drive the motional impedance ZM under a constant voltage, the value of the drive impedance Zo is to be set equal to a value of -(Rv+jωLv). When such relation is put into the formula (13), the following relation can be obtained. -(Rv+jωLv) = Rt·(1-Z2/Z1) ∴Z2/Z1 = (Rt·Rv)/Rt + jωLv/Rt Hence, a capacitance of the capacitor C1 and resistances of resistors R4 and R5 can be set as follows. R4 = k1·Rt R5 = k1·(Rt+Rv) C1 = C/k1 where C = Lv/[Rt·(Rt+Rv)] and k1 is set further larger than one. When the circuits shown in Figs. 10 and 11 are combined together, a circuit shown in Fig. 12 can be obtained. In this case, when the condition represented by the formula (14) and a relation of ZL=Rv+jωLv+ZM are put into the formula (12), the following transmission characteristic (-Vo/Vi) of formula (15) can be obtained. -Vo/Vi = R2/R1·[(Rv+jωLv+ZM)/ZM] In addition, when a relation of VM/Vo=ZM/(Rv+jωLv+ZM) is put into this formula (15), the transmission characteristic including the motional impedance ZM can be obtained from the following formula (16). -VM/Vi = R2/R1 Further, an output impedance Rd and a drive impedance Zd of the motional impedance ZM can be obtained as follows. Rd = -(Rv+jωLv) Zd = 0 Incidentally, as the setting method of circuit constants for setting the drive impedance Zd equal to -(Rv+jωLv) in order to drive the motional impedance ZM under the constant voltage, modified methods other than the method described before can be adopted. For example, impedance loads Z3 and Z4 (not shown) can be used instead of the resistors R2 and R3 in the circuit shown in Fig. 9, and constants of these impedance loads Z3 and Z4 can be set so that the value of the formula (11) will be set equivalent to the drive impedance Zd. As known well, each of the value Q and a lowest resonance frequency f0 has a value due to a resonance characteristic curve of the motional impedance ZM. However, when the speaker 108 is actually driven, there is a problem in that the above resonance characteristic curve (i.e., a variation of the motional impedance ZM) must be effective due to the resistance Rv of the voice coil and the output impedance Rd of the amplifier on the voltage transmission characteristics. In order to solve such problem, the resonance impedance only must be subjected to a voltage drive by an amplifier having no output impedance and infinite power, for example. In this case, the voltage between both terminals of the resonance impedance is not effected by the value Q and the silent resonance frequency f0 but identical to the input voltage. In short, it is not necessary to consider the value Q and the resonance frequency f0 in this case. In addition, all movements of a vibration plate of the actual loudspeaker is translated into an electromotive force between both terminals of the motional impedance ZM. Hence, by driving the motional impedance ZM under the constant voltage, all free movements of the vibration plate of the loudspeaker can be controlled. For this reason, the transient response of the vibration system can not be caused at all, hence, it is possible to eliminate the distortions due to such transient response. As shown by Fig. 12 and formulae (16) to (18), the present invention can drive the motional impedance ZM by zero-ohm (or under the constant voltage). However, the motional impedance ZM becomes extremely low at the resonance frequency (f0). Hence, current supplying ability at driving side is required to be large at this frequency f0. By the way, it is possible to obtain an equivalent circuit as shown in Fig. 13 by simplifying the circuit shown in Fig. 12. In Fig. 13, the input terminal 101 is connected to a connection point P between the motional impedance ZM and the voice coil inductance Lv of the speaker 108 via the resistors R1 and R2 in series, while the terminal 108b is grounded. Meanwhile, an amplifier 109 having a negative output impedance -(Rv+jωLv) is newly provided. The non-inverting input terminal of this amplifier 109 is grounded, while the inverting input terminal thereof is connected to a connection point between the resistors R1 and R2. In general, the whole system of the dynamic speaker 108 including the voice coil resistance Rv and inductance Lv has a tone pressure vs frequency characteristic the curve of which is set to be flat under the constant voltage. However, it is necessary to consider the potentials at the input terminals 108a and 108b and the connection point P of the speaker 108 shown in Fig. 13 in the actual case. In this case, the motional impedance ZM having the frequency characteristic shown by Fig. 14A becomes extremely small at the frequencies other than the lowest resonance frequency f0. Hence, in order to set the voltage between the both terminals of the motional impedance ZM to the constant voltage, a drive current I of the speaker 108 must be decreased in the vicinity of the resonance frequency f0 as shown in Fig. 14B. This drive current I is actually supplied to the speaker 108 via the voice coil resistance Rv, hence, a voltage V must be produced at the terminal 108a. This voltage V becomes extremely large at the frequencies other than the silent resonance frequency f0 as shown in Fig. 14C. For this reason, the amplifier 109 must be saturated soon. The above-mentioned problem can be solved by using a filter circuit as described in Fig. 7. More specifically, this filter circuit has a (frequency response) characteristic which can be obtained by electrically simulating how the loudspeaker input voltage is transmitted in response to the motional impedance. In this case, the input signal voltage Vi is supplied to the speaker 108 via this filter circuit. Fig. 15 shows a diagrammatic circuit diagram of a further loudspeaker driving arrangement which is further provided with the above-mentioned filter circuit. In Fig. 15, 110 designates a filter circuit having a frequency response characteristic which can be obtained by electrically simulating a voltage transmission characteristic of the speaker 108. More specifically, such filter circuit 110 includes a resistance k2·Rv, an inductance k2·Lv and a motional impedance k2·ZM (where k2 denotes an arbitrary constant value). Due to this filter circuit 110, the voltage applied to the motional impedance ZM within the speaker 108 can have the frequency characteristic identical to that of the input voltage Vi in the case where the speaker 108 is driven by the input voltage Vi. For this reason, it can be naturally said that the tone pressure vs frequency characteristic of the speaker 108 must have the flat curve. In addition, the input voltage of the amplifier 109 must be extremely low at the frequencies except for the frequencies in the vicinity of the resonance frequency f0 of the motional impedance ZM. Further, as described before, even if the circuit gain of the amplifier 109 becomes large at the frequencies other than the resonance frequency f0, the output voltage of the amplifier 109 can not become so large. Next, description will be given with respect to a concrete embodiment of the filter circuit 110 in conjunction with Figs. 16 and 17. This filter circuit 110 must have a frequency response characteristic F which is similar to that of the speaker 108 as shown by a short dashes line in Fig. 16. In order to realize such frequency response characteristic F, this characteristic F is divided into a band-pass characteristic G1 and high-pass characteristics G2 to G4. By electrically simulating these divided characteristics, the circuit as shown in Fig. 17 can be constituted. In Fig. 16, f1 to f3 designate respective cut-off frequencies of the above-mentioned high-pass characteristics G2 to G4. In Fig. 17, 111 and 112 respectively designate input and output buffers; an amplifier 113, a resistor R (having a resistance of 470 kilo-ohm) and a capacitor C (having a capacitance of 0.0056 micro-farad) etc. constitute a band-pass filter having the band-pass characteristic G1; resistors r0 (having a resistance of 10 kilo-ohm), r1 (having a resistance of 22 kilo-ohm) and r2 (having a resistance of 68 kilo-ohm) and capacitors Ca (having a capacitance of 0.016 micro-farad), Cb (having a capacitance of 0.01 micro-farad) and Cc (having a capacitance of 0.08 micro-farad) etc. constitute a circuit realizing the high-pass characteristics G2 to G4. As other circuit elements, resistors Ry (having a resistance of 6.8 kilo-ohm), Rx (having a resistance of 1 kilo-ohm), r3 (having a resistance of 1 kilo-ohm) and 2·r3 (having a resistance of 2 kilo-ohm) are provided. In addition, the band-pass characteristic G1 has a time constant T1=R·C; the high-pass characteristic G2 has a time constant T2=(r0+r1+r2)·Ca; the high-pass characteristic G3 has a time constant T3=(r0+r1)·Cb; and the high-pass characteristic G4 has a time constant T4=Ca·r0. Further, as shown in Fig. 16, the high-pass characteristics G2 to G4 have respective responses of r0/(r0+r1+r2), r0/(r0+r1) and r0/r0=1. As described heretofore, the present invention is constituted so that the impedance components other than the equivalent motional impedance of the dynamic loudspeaker can be canceled. Hence, it becomes unnecessary to consider the value Q and the lowest resonance frequency f0. In addition, it becomes possible to eliminate a low-frequency tone reproducing limitation due to the resonance frequency f0. On the other hand, the present invention is provided with the filter circuit and driving means. Such filter circuit has the frequency response characteristic which can be obtained by electrically simulating the voltage transmission characteristic against the equivalent motional impedance of the dynamic loudspeaker, so that this filter circuit can give a desirable frequency compensating characteristic to the input signal. The driving means has a negative output impedance which cancels the impedance components other than the equivalent motional impedance. This driving means drives the dynamic loudspeaker by the input signal which is supplied thereto via the above filter circuit. Hence, it is possible to arbitrarily raise the level of the low-frequency characteristic in principle by setting the level of the low-frequency characteristic large when setting the characteristics of the filter circuit. Accordingly, it is possible to reproduce ultra-low-frequency tones by use of a small-size speaker. Figures 4 - 8B and the accompanying description describe a preferred embodiment of the present invention. This invention may be practiced or embodied in other ways. Therefore, the preferred embodiment described herein is illustrative and not restrictive, the scope of the invention being indicated by the appended claims and all variations which come within the meaning of the claims are intended to be embraced therein.

Claims: A dynamic loudspeaker driving apparatus comprising: (a) an amplifier (21) having a large open-loop-gain for driving a dynamic loudspeaker (23); and (b) input means (20) for supplying an input signal (Vi) to an input terminal of said amplifier (21) via a filter circuit which electrically simulates a voltage transmission characteristic against an equivalent motional impedance (ZM) of said dynamic loudspeaker, characterized by the provision of: (c) detecting means (32) for detecting a motional voltage (VM) applied to said equivalent motional impedance of said dynamic loudspeaker; and (d) feedback means (34 to 40) for negatively feeding back said motional voltage to said input terminal of said amplifier by a transmission gain "1". A dynamic loudspeaker driving apparatus according to claim 1 wherein said detecting means is a bridge circuit (32) consisting of four impedance portions, one of which is an impedance of said dynamic loudspeaker including said equivalent motional impedance.

Inventors: YOKOYAMA KENJI YAMAHA CORPORAT, YOKOYAMA, KENJI YAMAHA CORPORATION
Applicants: YAMAHA CORP, YAMAHA CORPORATION
Application year: 1988
Main classification: H04R3
IPC codes: H04R3