Audio device

Abstract

To improve efficiency η of the entire acoustic device including an amplifier over the entire frequency band including a minimum resonance frequency F0.SOLUTION: An acoustic device 10 includes: a class D amplifier 1; and a speaker system SS comprising a voice coil to be driven by the class D amplifier 1. The class D amplifier 1 comprises a current feedback circuit 200. The speaker system SS has a Q value lower than the lower limit value of a proper range. The current feedback circuit 200 increases the acoustic device 10's Q value to a value in the proper range by increasing output resistance of the class D amplifier via feedback of current flowing through the voice coil to an input unit of the class D amplifier 1.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This invention relates to an acoustic device comprising an amplifier and a speaker system.

Background Art

[0002] In a speaker system in which a speaker unit is provided in an enclosure, the efficiency η of the speaker system is an important factor. This efficiency η is the ratio of the output sound pressure of the speaker system to the electrical energy input to the speaker system, and is given by the following equation. η = ρ·BL 2 ·Sd 2 / (2π·M 2 ·Rsp) ……(1)

[0003] Here, ρ is the density of air which is the medium of sound. Also, BL is the driving force of the speaker unit in the speaker system, and is given by the product of the magnetic flux density of the magnetic gap provided in the speaker unit and the coil length of the voice coil disposed in the magnetic gap. Also, Sd is the radius of the diaphragm that outputs sound in conjunction with the voice coil in the speaker unit. Also, M is the mass of the vibration system in the speaker unit. Also, Rsp is the resistance of the voice coil.

[0004] Also, there is a Q value as another factor related to the performance of the speaker system. When the lowest resonance frequency of the speaker unit is F0, the Q value of the speaker system is given by the following equation. Q = 2π·F0·M·Rsp / BL 2 ……(2)

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] According to the above-described formulas (1) and (2), by adjusting parameters such as increasing the mass M, increasing the driving force BL, or decreasing the resistance Rsp, the Q value decreases, but the efficiency η increases. That is, in a speaker system, by adjusting parameters to decrease the Q value, the efficiency η can be improved and the power consumption can be reduced.

[0007] However, the Q value of the speaker system affects the Q value of the entire audio device including the amplifier that drives the speaker unit of the speaker system. And when parameter adjustment is performed to decrease the Q value of the speaker system, although the efficiency η in the mid-high frequency range is improved, near the lowest resonance frequency F0, the sound pressure decreases due to the decrease in the Q value of the entire audio device, resulting in a decrease in the efficiency η instead. Thus, it has been difficult for conventional audio devices to improve the efficiency η of the entire audio device including the amplifier over the entire frequency band including the lowest resonance frequency F0.

[0008] This invention has been made in view of the above-described circumstances, and an object thereof is to improve the efficiency η of the entire audio device including the amplifier over the entire frequency band including the lowest resonance frequency F0.

Means for Solving the Problem

[0009] An audio device according to this invention is an audio device having a class-D amplifier provided with a current feedback circuit and a speaker system provided with a voice coil driven by the class-D amplifier, wherein the speaker system has a Q value lower than an appropriate range of the Q value of the audio device, and the current feedback circuit feeds back the current flowing through the voice coil to an input section of the class-D amplifier to increase the output resistance of the class-D amplifier, thereby increasing the Q value of the audio device to a value within the appropriate range.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0012] FIG. 1 is a circuit diagram showing the configuration of an acoustic device 10 according to an embodiment of the present invention. As shown in FIG. 1, the acoustic device 10 includes a class-D amplifier 1 and a speaker system SS. The class-D amplifier 1 includes a class-D amplification unit 100, a current feedback circuit 200, and a filter current feedback circuit 300. The speaker system SS includes a speaker unit SP driven by the class-D amplifier 1 and an enclosure ENC in which the speaker unit SP is provided. Note that a class-D amplifier having a current feedback circuit and a filter current feedback circuit is disclosed in, for example, Patent Document 1.

[0013] Here, referring to FIG. 2, the characteristics of the speaker system SS connected to the class-D amplifier 1 will be described. In FIG. 2, the horizontal axis represents the frequency of a sine wave with a predetermined amplitude applied to the speaker system SS. Also, the vertical axis represents the sound pressure level output at a position separated from the speaker system SS by a predetermined distance. FIG. 2 shows the frequency characteristics SPL1 of the sound pressure level obtained from the speaker system SS with a driving force BL of 7, the frequency characteristics SPL2 of the sound pressure level obtained from the speaker system SS with a driving force BL of 14, and the frequency characteristics SPL3 of the sound pressure level obtained from the speaker system SS with a driving force BL of 28. As shown in FIG. 2, the sound pressure level in the mid- and high-frequency ranges increases as the driving force BL increases. However, even if the driving force BL is increased, the sound pressure level in the low-frequency range near the minimum resonance frequency F0 = 40 Hz does not increase. This is because when the driving force BL is excessive, the Q value of the speaker system SS decreases, and conversely, the efficiency η of the speaker system SS in the low-frequency range decreases. In the example shown in FIG. 2, as the driving force BL is increased from 7 → 14 → 28, the Q value decreases from 4 → 1 → 0.25.

[0014] In order to increase the efficiency over the entire frequency band, it is preferable to increase the driving force BL. However, if the driving force BL is excessive, the Q value will decrease. Therefore, if no measures are taken, especially in the low-frequency range near the minimum resonance frequency, the sound pressure level output from the speaker system SS will decrease and the bass will become weak.

[0015] Therefore, in this embodiment, the following measures are taken. a. Set the lower limit value of the appropriate range of the Q value of the acoustic device 10 to 0.2. b. Set the driving force BL of the speaker unit SP to a high value so that the Q value of the speaker system SS becomes lower than the lower limit value of the appropriate range of the Q value of the acoustic device 10. c. Control the output resistance of the class-D amplifier 1 by current feedback control performed by the current feedback circuit 200 to increase the Q value of the acoustic device 10 within the appropriate range. Specifically, the Q value of the entire acoustic device 10 including the class-D amplifier 1 is given by the following equation. Q = 2π·F0·M·(Rsp + Ramp) / BL 2 ……(3) In the above formula (3), Ramp is the output resistance of the amplifier that drives the speaker unit. In the present embodiment, the Q value of the speaker system SS alone is given by the above formula (2), and this Q value is lower than the lower limit value of 0.2 of the appropriate range of the Q value of the acoustic device 10. In the present embodiment, the output resistance Ramp of the class D amplifier 1 is controlled by the current feedback control performed by the current feedback circuit 200, and the Q value obtained by the above formula (3) is increased within the appropriate range.

[0016] As in the above-described conventional technology, if only the parameters of the speaker system SS are adjusted, it is difficult to improve the efficiency η of the entire acoustic device 10 over the entire frequency band including the minimum resonance frequency F0. Here, the efficiency η of the entire acoustic device 10 involves not only the power consumption of the speaker system SS but also the power consumption of the output resistance Ramp of the class D amplifier 1. And the output resistance Ramp of the class D amplifier 1, as shown in the above formula (3), helps to increase the Q value of the entire acoustic device 10 to a value within the appropriate range, while being a virtual one resulting from the current feedback control and not generating power consumption itself. Therefore, according to the present embodiment, without causing an increase in power consumption, the Q value of the acoustic device 10 having the speaker system SS with a large driving force BL can be increased to a value within the appropriate range, and the efficiency of the acoustic device 10 can be improved over the entire frequency band including the minimum resonance frequency F0.

[0017] In FIG. 1, the class D amplification unit 100 includes an operational amplifier (or comparator) 110, an output stage 120, a filter 130, and a feedback resistor 140. The operational amplifier 110 is a circuit that forms an input unit for the input signal of the class D amplification unit 100. An audio signal AIN is input to the non-inverting input terminal of the operational amplifier 110 via an input terminal 111.

[0018] The output stage 120 includes, as switching elements, a transistor 121 inserted between the positive power supply +B and the output terminal 123 of the output stage 120, and a transistor 122 inserted between the negative power supply -B and the output terminal 123 of the output stage 120. In a preferred embodiment, these transistors 121 and 122 are MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). The output stage 120 connects the positive power supply +B to the output terminal 123 by turning on the transistor 121 and turning off the transistor 122, or connects the negative power supply -B to the output terminal 123 by turning off the transistor 121 and turning on the transistor 122, according to the output signal of the operational amplifier 110. Therefore, the waveform of the output signal of the output stage 120 becomes a rectangular wave.

[0019] The filter 130 is a low-pass filter that removes high-frequency components above the audio band from the output signal of the output stage 120 and supplies the remaining frequency components in the audio band to the speaker unit SP. This filter 130 includes an inductor 131 and a capacitor 132. Here, the inductor 131 is inserted between the output terminal 123 of the output stage 120 and one end of the speaker unit SP. Also, one end of the capacitor 132 is connected to the node 133 between the inductor 131 and the speaker unit SP, and the other end is grounded via the current detection resistor 310 of the filter current feedback circuit 300. That is, the capacitor 132 is connected in parallel to the speaker unit SP which is the load. The filter current feedback circuit 300 will be described later.

[0020] The feedback resistor 140 is inserted between the node 133 between the inductor 131 and the speaker unit SP and the inverting input terminal of the operational amplifier 110. This feedback resistor 140 forms a self-excitation feedback loop that feeds back the output voltage to the speaker unit SP to the operational amplifier 110, which is the input section of the class-D amplifier section 100, and causes the class-D amplifier section 100 to self-oscillate. The class-D amplifier section 100 outputs a PWM pulse train that is pulse-width modulated based on the input audio signal AIN while oscillating at a predetermined self-oscillation frequency from the output stage 120. The filter 130 serves to remove high-frequency components above the self-oscillation frequency from this PWM pulse train and supply it to the speaker unit SP. The above is the configuration of the class-D amplifier section 100.

[0021] In the class-D amplifier 1, a current feedback circuit 200 and a filter current feedback circuit 300 are connected to the class-D amplifier section 100.

[0022] The current feedback circuit 200 is a circuit that negatively feeds back the load current flowing through the speaker unit SP, which is the load of the class-D amplifier section 100, to the operational amplifier 110, which is the input section. This current feedback circuit 200 includes a current detection resistor 210, an amplification section 220, and a coupling section 230.

[0023] The current detection resistor 210 is inserted between the terminal on the side opposite to the node 133 in the speaker unit SP and the ground wire. The amplification unit 220 is composed of an operational amplifier 221, and resistors 222 and 223. The inverting input terminal of the operational amplifier 221 is grounded via the resistor 222 and is connected to the output terminal of the operational amplifier 221 via the resistor 223. And the voltage across both ends of the current detection resistor 210 is applied to the non-inverting input terminal of the operational amplifier 221. Therefore, when the resistance value of the resistor 222 is Ra and the resistance value of the resistor 223 is Rb, the amplification unit 220 amplifies and outputs the voltage across both ends of the current detection resistor 210 with a gain of (Ra + Rb) / Ra. In the present embodiment, since the voltage across both ends of the current detection resistor 210 is amplified to a voltage of sufficient magnitude by the amplification unit 220 in this way, the resistance value of the current detection resistor 210 can be made small. The coupling unit 230 is composed of a resistor 231 and a capacitor 232 inserted in series between the output terminal of the amplification unit 220 and the inverting input terminal of the operational amplifier 110. This coupling unit 230 plays a role of adjusting the frequency characteristics of the feedback amount of the negative feedback performed via the current feedback circuit 200.

[0024] In the present embodiment, the current feedback circuit 200 performs constant current drive control for controlling the load current flowing through the speaker unit SP, which is the load of the class D amplification unit 100, to a current value proportional to the voltage value of the audio signal AIN. Further, in this constant current drive control, the current feedback circuit 200 controls the output resistance of the output stage 120 to a resistance value that can increase the Q value of the acoustic device 10 described above to a value within an appropriate range. Specifically, in the above-described formula (3), when the resistance value of the voice coil of the speaker unit SP is Rsp and the appropriate value of the resistance of the output stage 120 for which the Q value of the acoustic device 10 is within the appropriate range is Ramp, the gain of the amplification unit 220 of the current feedback circuit 200 is determined so that the output resistance of the output stage 120 becomes the appropriate value Ramp.

[0025] The filter current feedback circuit 300 is a circuit that negatively feeds back the current flowing through the capacitor 132 of the filter 130 to the operational amplifier 110 which is the input part of the class-D amplifier section 100, and is composed of the current detection resistor 310 and the coupling section 320 described above. Here, the coupling section 320 is composed of a resistor 321 and a capacitor 322 inserted in series between the connection node between the current detection resistor 310 and the capacitor 132 and the inverting input terminal of the operational amplifier 110. This coupling section 320 plays a role of adjusting the frequency characteristics of the feedback amount of the negative feedback performed through the filter current feedback circuit 300. The above is the configuration of the class-D amplifier 1.

[0026] Next, the operation of this embodiment will be described. In the class-D amplifier section 100, the output signal of the output stage 120 is given a phase rotation through the filter 130 and the feedback resistor 140, and is fed back to the operational amplifier 110 which is the input part. As a result, the class-D amplifier section 100 self-oscillates. This class-D amplifier section 100 is designed such that this self-oscillation frequency is sufficiently higher than the frequency band of the input audio signal AIN.

[0027] Here, the output signal of the output stage 120 is a rectangular wave, but in the filter 130, the capacitor 132 performs the first integration of this rectangular wave, so the signal waveform at the node 133 becomes a triangular wave. In the operational amplifier 110, the triangular wave at the node 133 fed back through the feedback resistor 140 is compared with the input audio signal AIN. As a result, a PWM pulse train pulse-width modulated by the input audio signal AIN is output from the operational amplifier 110, and this PWM pulse train is output to the filter 130 through the output stage 120. This PWM pulse train has its high-frequency components removed through the filter 130 and is supplied to the speaker unit SP.

[0028] Specifically, when the voltage value of the input audio signal AIN is 0V, a PWM pulse train with a duty ratio of 50% is output from the output stage 120, and the voltage applied to the speaker unit SP becomes 0V. Also, when the voltage value of the input audio signal AIN changes in the positive direction from 0V, the duty ratio of the PWM pulse train output from the output stage 120 changes from 50% to a maximum of 100%, and the voltage applied to the speaker unit SP changes from 0V to a maximum of +B. On the other hand, when the voltage value of the input audio signal AIN changes in the negative direction from 0V, the duty ratio of the PWM pulse train output from the output stage 120 changes from 50% to a minimum of 0%, and the voltage applied to the speaker unit SP changes from 0V to a minimum of -B. In this way, a signal with a waveform approximated to the input audio signal AIN is applied to the speaker unit SP.

[0029] While the amplification operation by the class-D amplifier section 100 described above is being performed, the current feedback circuit 200 negatively feeds back the load current flowing through the speaker unit SP to the operational amplifier 110 which is the input section of the class-D amplifier section 100, and the filter current feedback circuit 300 negatively feeds back the current flowing through the filter 130 to the same operational amplifier 110. As a result, the following effects are obtained.

[0030] The impedance of the speaker unit SP changes depending on the driving frequency of the speaker unit SP. Here, when the impedance of the speaker unit SP increases due to a change in the driving frequency of the speaker unit SP and the load current flowing through the speaker unit SP decreases, the feedback signal to the input section of the class-D amplifier section 100 via the current feedback circuit 200 decreases, and the output signal of the class-D amplifier section 100 increases. As a result, the effective voltage applied from the class-D amplifier section 100 to the speaker unit SP increases, increasing the load current flowing through the speaker unit SP. On the other hand, when the impedance of the speaker unit SP decreases due to a change in the driving frequency of the speaker unit SP and the load current flowing through the speaker unit SP increases, the feedback signal to the input section of the class-D amplifier section 100 via the current feedback circuit 200 increases, and the output signal of the class-D amplifier section 100 decreases. As a result, the effective voltage applied from the class-D amplifier section 100 to the speaker unit SP decreases, decreasing the load current flowing through the speaker unit SP. As a result of such negative feedback control, the load current flowing through the speaker unit SP becomes constant regardless of the driving frequency of the speaker unit SP.

[0031] Thus, in this embodiment, by applying negative feedback of the load current flowing through the speaker unit SP to the input section of the class-D amplifier section 100, the output impedance of the class-D amplifier section 100 can be effectively increased, and the load current flowing from the class-D amplifier section 100 to the speaker unit SP can be kept constant. Therefore, in the low frequency range near the lowest resonance frequency at which the impedance of the speaker unit SP increases, a sufficient load current can flow through the speaker unit SP, and high-volume sound reproduction can be realized. Further, in this embodiment, since control is performed to keep the load current flowing through the speaker unit SP constant regardless of the driving frequency of the speaker unit SP, an excessive large current can be prevented from flowing through the speaker unit SP in frequency bands other than the low frequency range near the lowest resonance frequency, and damage to the speaker unit SP can be prevented.

[0032] Also, in the present embodiment, when the substantial voltage supplied to the speaker unit SP as a load is v and the load current flowing through the speaker unit SP is i, the output resistance Ramp of the output stage 120 depends on v / i. In the present embodiment, the current feedback circuit 210 controls the output resistance Ramp of the output stage 120 to an appropriate value that can increase the Q value of the acoustic device 10 to a value within an appropriate range by performing current feedback control. More specifically, the current feedback control unit 210 performs current feedback control so that at the lowest resonance frequency F0 of the speaker system SS, the output resistance Ramp of the output stage 120 becomes an appropriate value that can increase the Q value to a value within an appropriate range.

[0033] Next, referring to FIG. 3, the effects of the present embodiment will be described. FIG. 3 is a circuit diagram showing the configuration of an equivalent circuit of the acoustic device 10 having the speaker system SS. In FIG. 3, the inductor L1, resistor R1, and capacitor C1 connected in parallel constitute the motional impedance Zm of the speaker unit SP. Also, the inductor L2 and capacitor C2 constitute the impedance Zenc of the enclosure in which the speaker unit SP is provided. In the equivalent circuit of the acoustic device 10, for the circuit in which the motional impedance Zm and the enclosure impedance Zenc are connected in parallel, the inductor Lsp, and the resistors Rsp and Ramp are connected in series.

[0034] Here, the inductor Lsp is the inductor of the voice coil of the speaker unit SP. Also, the resistor Rsp is the resistance of the voice coil. And the resistor Ramp is the output resistance of the class-D amplifier 100. The drive voltage Vamp generated by the class-D amplifier 1 is applied to the parallel circuit of the impedances Zm and Zenc through the resistor Ramp, Rsp, and the inductor Lsp.

[0035] As shown in the above formula (3), if the output resistance Ramp of the class-D amplifier 100 is increased, the Q value of the audio device 10 can be increased. Here, if it is assumed that the amplifier driving the speaker unit SP is a normal analog amplifier, the Q value can be increased by increasing the output resistance of the analog amplifier. However, on the other hand, there is a problem that the power consumption of the analog amplifier increases. However, in the present embodiment, the class-D amplifier 100 drives the speaker unit SP. Since the output stage 120 of this class-D amplifier 100 only takes the state where one of the transistors 121 and 122 is ON and the other is OFF or the state where one is OFF and the other is ON, the power consumption is small. And in the class-D amplifier 100, by the current feedback control performed by the current feedback circuit 200, the output resistance Ramp of the output stage 120 is controlled to a resistance value that can increase the Q value to a value within an appropriate range. For this reason, according to the present embodiment, without causing an increase in the power consumption of the class-D amplifier 100, the Q value can be increased within an appropriate range, and the efficiency of the audio device 10 can be increased over the entire frequency band.

[0036] <Other Embodiments> As described above, one embodiment of the present invention has been described. However, other embodiments are also conceivable for the present invention. For example, in the above embodiment, the self-oscillation type class-D amplifier 100 is connected to the speaker system SS, but a separate-oscillation type class-D amplifier may be connected. FIG. 4 is a circuit diagram showing the configuration of an audio device 10A which is another embodiment of the present invention. In this figure, the same reference numerals are given to the respective parts corresponding to the parts shown in FIG. 1 above, and the description thereof is omitted.

[0037] In this audio device 10A, the self-oscillation type class-D amplifier 1 in the above embodiment is replaced by a separate-oscillation type class-D amplifier 1A. In the class-D amplification section 100A of this class-D amplifier 1A, the operational amplifier 110 in the class-D amplification section 100 of the above embodiment is replaced by an error integrator 150 and a comparator 160.

[0038] The error integrator 150 consists of an operational amplifier 151 and an integrating capacitor 152 inserted between the output terminal and the inverting input terminal of this operational amplifier 151. Here, an input audio signal AIN is supplied from the input terminal 111 to the non-inverting input terminal of the operational amplifier 151. Also, an output voltage is fed back from the speaker unit SP to the inverting input terminal of the operational amplifier 151 via the feedback resistor 140, a load current flowing through the speaker unit SP is fed back via the current feedback circuit 200, and a current flowing through the filter 130 is fed back via the filter current feedback circuit 300. The error integrator 150 integrates the error between the input audio signal AIN and the feedback signals fed back via these respective feedback circuits, and outputs an integrated value signal.

[0039] A periodic carrier signal having a frequency sufficiently higher than the frequency band of the input audio signal AIN is supplied from a carrier signal generation circuit (not shown) to the inverting input terminal of the comparator 160. In the illustrated example, this carrier signal is a triangular wave signal, but a sawtooth wave signal may be used as the carrier signal. The comparator 160 outputs a PWM pulse train pulse-width modulated by the integrated value signal to the output stage 120 by comparing the integrated value signal output by the error integrator 150 with this carrier signal. The functions of the output stage 120 and the filter 130 are the same as those in the above-described embodiment. Also in this audio device 10A, the same effects as those in the above-described embodiment can be obtained.

Description of Reference Numerals

[0040] 10, 10A... audio device, 1, 1A... class-D amplifier, 100, 100A... class-D amplification section, 111... input terminal, 110, 151, 221... operational amplifier, 120... output stage, 121, 122... transistor, 130... filter, 200... current feedback circuit, 210, 310... current detection resistor, 220... amplification section, 230... coupling section, 300... filter current feedback circuit, 200... current feedback circuit, 150... error integrator, 160... comparator, SP... speaker unit, ENC, SS... speaker system, 150... error integrator.

Claims

1. A D - class amplifier having a current feedback circuit, and A speaker system having a voice coil driven by the D - class amplifier An audio device comprising: The speaker system has a Q - value lower than the appropriate range of the Q - value of the audio device, The current feedback circuit feeds back the current flowing through the voice coil to the input section of the D - class amplifier to increase the output resistance of the D - class amplifier, thereby increasing the Q - value of the audio device to a value within the appropriate range Audio device.

2. The lower limit value of the appropriate range is 0.2 The audio device according to Claim 1.

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