Speaker and audio system

Abstract

To provide a speaker using a piezoelectric speaker unit, of which a degree-of-freedom of a design for a miniaturization is high and has a higher sound quality.SOLUTION: A speaker comprises: a first state variable type filter that is a state variable type filter that performs a band division of a sound signal, and includes a high-pass output, a band-pass output, and a low-pass output, of which a cut-off frequency is common; a piezoelectric speaker amplifier into which a signal from the band-pass output is input; a piezoelectric speaker that converts the signal output from the piezoelectric speaker amplifier into a sound by a piezoelectric element; a dynamic speaker amplifier into which the signal from the low-pass output is input; and a dynamic speaker that outputs the signal output from the dynamic speaker amplifier.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a speaker and an audio system having a piezoelectric speaker unit. [Background technology]

[0002] A speaker system using a piezoelectric speaker has been proposed (for example, Patent Document 1). The speaker system in Patent Document 1 discloses a multi-channel speaker system that includes a plurality of sounders each having a different frequency band, a plurality of amplifiers that amplify signals to each sounder, and a plurality of band-pass filters through which signals of the frequency band corresponding to each sounder pass. It is described that sound of a required frequency band is output by the entire plurality of sounders. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 4-331599 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the speaker system of Patent Document 1 requires a large number of sounders and a large number of corresponding devices and parts, and the sound quality is not sufficient.

[0005] SUMMARY OF THE PRESENT DISCLOSURE An object of the present invention is to provide a speaker using a piezoelectric speaker unit that has a high degree of freedom in design, such as in terms of miniaturization, and that produces higher quality sound. [Means for solving the problem]

[0006] The present invention has been made to solve the above problems, and the gist of the invention is as follows.

[0007] (1) A first state variable filter that divides an audio signal into bands, the first state variable filter having a high-pass output, a band-pass output, and a low-pass output that have a common cutoff frequency; a piezoelectric speaker amplifier to which a signal from the bandpass output is input; a piezoelectric speaker that converts a signal output from the piezoelectric speaker amplifier into sound using a piezoelectric element and outputs the sound; a dynamic speaker amplifier to which a signal from the low-pass output is input; a dynamic speaker for outputting the signal output from the dynamic speaker amplifier; A speaker comprising:

[0008] (2) A DC-DC converter that boosts the direct current supplied from the secondary battery as a power source, The speaker according to (1) above, wherein the piezoelectric speaker amplifier operates on direct current boosted by the DC-DC converter.

[0009] (3) The speaker according to (2) above, wherein the DC-DC converter boosts the voltage using a circuit including a semiconductor switch, a control circuit for the semiconductor switch, and a transformer.

[0010] (4) The speaker according to (3) above, wherein the DC-DC converter further uses a voltage doubler rectifier circuit to output positive and negative voltage direct currents.

[0011] (5) The DC-DC converter comprises: a positive output side feedback circuit having a feedback resistor and a phase compensation circuit connected to the positive output; a negative output side feedback circuit having a feedback resistor and a phase compensation circuit connected to the negative output; a current mirror connected to the positive output side feedback circuit and the negative output side feedback circuit, The speaker according to (4) above, characterized in that the positive and negative output voltages are adjusted based on voltages generated by currents from the positive output side feedback circuit and the negative output side feedback circuit.

[0012] (6) The speaker according to (1) above, wherein the piezoelectric speaker amplifier comprises a speaker amplifier IC and a voltage booster circuit that amplifies the voltage of the signal.

[0013] (7) The speaker according to (1) or (6) above, wherein the piezoelectric speaker amplifier amplifies an input signal to a signal with a higher voltage than the dynamic speaker amplifier.

[0014] (8) The speaker according to (1) above, wherein the Q value of the first state variable filter is 0.5.

[0015] (9) The speaker according to (1) above, wherein the amplifier for a dynamic speaker is a class D amplifier.

[0016] (10) The speaker, The amplifier for the piezoelectric speaker includes a first amplifier for the piezoelectric speaker and a second amplifier for the piezoelectric speaker, a three-way speaker including a first piezoelectric speaker to which a signal is input from an amplifier for the first piezoelectric speaker, and a second piezoelectric speaker to which a signal is input from an amplifier for the second piezoelectric speaker, a second state variable filter that divides the bandpass output from the first state variable filter into bands; The speaker described in (1) above, characterized in that a high-pass output signal of the second state variable filter is input to an amplifier for the first piezoelectric speaker, and a low-pass output signal of the second state variable filter is input to an amplifier for the second piezoelectric speaker.

[0017] (11) When the cut-off frequency of the first state variable filter is f1 and the cut-off frequency of the second state variable filter is f2, the speaker according to (10) above is characterized in that f1 < f2.

[0018] (12) The amplifier for the piezoelectric speaker is composed of discrete elements, a first current mirror circuit connected to a low voltage power supply, a differential pair connected to an input terminal of an audio signal and connected to the first current mirror circuit, a second current mirror circuit connected to a high voltage power supply and connected to the differential pair via a transistor, a third current mirror circuit connected to the high voltage power supply and connected between the second current mirror circuit and an output terminal of an audio signal, for amplifying the current of the audio signal, and the speaker according to (1) above is characterized by comprising the same.

[0019] (13) The speaker according to (12) above is characterized in that the first current mirror circuit, the second current mirror circuit, and the third current mirror circuit are Wilson type current mirrors.

[0020] (14) A state variable filter for dividing an audio signal into bands, comprising a first state variable filter having a high-pass output, a band-pass output, and a low-pass output with common cut-off frequencies, an amplifier for a piezoelectric speaker to which a signal from the band-pass output is input, an amplifier device having a dynamic speaker amplifier to which a signal from the low-pass output is input, a piezoelectric speaker that converts a signal output from the piezoelectric speaker amplifier into sound by a piezoelectric element and outputs the sound, a speaker having a dynamic speaker that outputs a signal output from the dynamic speaker amplifier, and an audio system characterized by comprising the same.

Advantages of the Invention

[0021] According to the present invention, in a speaker using a piezoelectric speaker unit, it is possible to provide a speaker that has a high degree of freedom in design, such as in terms of miniaturization, and that produces higher quality sound. [Brief description of the drawings]

[0022] [Figure 1] 1 is a block diagram showing a device configuration of a speaker according to a first embodiment. [Diagram 2] FIG. 2 is a diagram illustrating an example of a circuit configuration of a high-voltage analog amplifier. [Diagram 3] FIG. 1 is a diagram illustrating an example of a circuit configuration of a DC-DC converter. [Figure 4] FIG. 1 is a diagram illustrating an example of a circuit configuration of a state variable filter. [Diagram 5] FIG. 11 is a block diagram showing the device configuration of a three-way speaker according to a second embodiment. [Figure 6] FIG. 11 is a diagram showing a circuit configuration of an amplifier for a piezoelectric speaker according to a third embodiment. [Figure 7] FIG. 13 is a block diagram of an audio system according to a fourth embodiment. [Figure 8] FIG. 13 is a diagram illustrating an example of a circuit configuration of a DC-DC converter according to a fifth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] (First embodiment) A speaker 1 according to the present embodiment will be described with reference to the drawings. Fig. 1 is a block diagram showing the configuration of a speaker 1 according to the present embodiment. Arrows between blocks in Fig. 1 indicate the flow of audio signals. Fig. 1 shows a part of the configuration of the speaker 1.

[0024] The speaker 1 is a multi-way speaker (a two-way speaker in this embodiment) that has two speaker units, in which the high-frequency side of an input sound signal (audio input signal) is reproduced by a piezoelectric speaker 2 and the low-frequency side is reproduced by a dynamic speaker 4. The speaker 1 in this embodiment is a so-called active speaker (or powered speaker) that has a built-in amplifier. The speaker 1 in this embodiment will be described as a one-channel monaural speaker.

[0025] The shape of the speaker 1 of this embodiment is not particularly limited, but it can be, for example, a cylindrical speaker system. The cylindrical speaker 1 is composed of, for example, a cylindrical piezoelectric speaker 2 arranged around a cylindrical housing, and a dynamic speaker 4 arranged inside the cylindrical housing, and can output sound in all directions of the speaker. Of course, the shape of the speaker 1 may be a general bookshelf type, or may be another shape.

[0026] The speaker 1 of this embodiment operates using a built-in battery as a power source. The battery may be a secondary battery such as a lithium ion battery. The speaker 1 may also be capable of using an AC power source, or may be a USB power source that receives power via a USB cable. In that case, the speaker 1 may be appropriately equipped with a battery power source, a circuit for switching between the AC power source and the USB power source, and a circuit (such as an AC-DC converter) required to operate an amplifier (described later) on the AC power source.

[0027] Also, the speaker 1 may not have a battery, and may instead be powered from an external source such as an AC power source or a USB power source. For example, in the case of a USB power source, a secondary battery such as a mobile battery can be connected to the speaker 1 via a USB cable to be used as the power source. In the case of a USB power source, it may also be configured to receive power and input audio signals from a PC or the like.

[0028] The speaker 1 receives an audio signal wirelessly or via a wired connection, and converts it into an analog audio input signal using a signal processing circuit as necessary.The audio signal is then split into audio signals of two frequency bands, and the piezoelectric speaker 2 and dynamic speaker 4 units output sounds of the corresponding frequency bands.

[0029] A specific configuration of the speaker 1 of this embodiment will be described below. The speaker 1 includes a piezoelectric speaker 2, a dynamic speaker 4, a state variable filter 6 (first state variable filter), an amplifier 8, an amplifier 10, and the like.

[0030] The piezoelectric speaker 2 is a speaker unit (driver) using a piezoelectric element (piezoelectric film). The piezoelectric speaker 2 is responsible for outputting the high-frequency sound of the speaker 1. The piezoelectric speaker 2 converts the high-frequency sound signal amplified by the amplifier 8 into sound. The structure and shape of the piezoelectric speaker 2 are not particularly limited, but the piezoelectric element may be formed into an arc shape or processed into a bimorph or monomorph so that the element itself also serves as a diaphragm. A known piezoelectric element may be used as the piezoelectric element constituting the piezoelectric speaker 2. For example, a piezoelectric film using an organic piezoelectric material such as PVDF (polyvinylidene fluoride) may be used. The frequency band of the high-frequency sound signal handled by the piezoelectric speaker 2 of this embodiment is, for example, 800 Hz or higher. The upper limit is not particularly limited, but can be, for example, 20 kHz, which is the upper limit of the audible frequency. However, for the purpose of increasing the sense of realism, it is also possible to reproduce a band exceeding the upper limit of the audible frequency of 20 kHz.

[0031] The amplifier 8 is a piezoelectric speaker amplifier that amplifies an audio signal. The amplifier 8 is a voltage-voltage amplifier. As described later, the amplifier 8 receives an audio signal from the bandpass output 6B of the state variable filter 6. The amplifier 8 is not limited to any amplifier that can drive the piezoelectric speaker 2, but it is preferable to use a high-voltage analog amplifier that operates on a high-voltage DC power supply. The power supplied to the high-voltage analog amplifier is preferably supplied by converting (boosting) the DC from the battery using a boost DC-DC converter (switching regulator). In this case, the amplifier 8 operates on a direct current boosted by the DC-DC converter. In order to obtain sufficient sound pressure, the piezoelectric speaker 2 usually requires a signal with a higher voltage than a dynamic type, and the amplifier 8 amplifies the signal to a signal with a higher voltage than the amplifier 10 described later.

[0032] FIG. 2 shows an example of the circuit configuration of a high-voltage analog amplifier. The high-voltage analog amplifier in FIG. 2 is configured by attaching a voltage booster circuit to a speaker amplifier IC20 that operates at a low voltage. The signal amplified by the speaker amplifier IC20 is further amplified by the voltage booster circuit and output to the piezoelectric speaker 2. The speaker amplifier IC20 is an audio amplifier IC, and for example, an amplifier IC having one operational amplifier for inverting amplification and another operational amplifier that inverts and outputs the output of the inverted amplifier can be used. For example, the LM4819 from Texas Instruments can be used as the speaker amplifier IC20. The 100V power supply is a DC power supply in which the DC output voltage of the built-in battery is boosted by a DC-DC converter. The 5V low-voltage power supply is a power supply obtained by converting the built-in battery power supply by a necessary circuit such as a three-terminal regulator. In this embodiment, as an example, the positive side of the low-voltage power supply is 5V and the negative side is 0V (ground potential), and a 2.5V power supply adjusted to the middle between the two using a circuit such as an operational amplifier is used as the reference potential (virtual ground).

[0033] As described above, the DC-DC converter is a circuit (device) that converts the DC (e.g., 12 V) of the built-in battery into a higher voltage DC (100 V in this embodiment) and outputs it to the amplifier 8. The DC-DC converter includes a circuit that turns on and off at frequencies from about 300 kHz to 2 MHz using a semiconductor switch (MOSFET, etc.), a transformer (or coil), and the like.

[0034] An example of the circuit configuration of a DC-DC converter is shown in Fig. 3. The DC-DC converter in Fig. 3 has a DC-DC converter IC 22 which is a control circuit for a semiconductor switch, a MOSFET 24 which is a semiconductor switch, a transformer 26, and a voltage doubler rectifier circuit 28. The DC-DC converter in Fig. 3 is basically a flyback converter, but the voltage doubler rectifier circuit 28 arranged on the secondary side of the transformer 26 generates DC power supplies with equal positive and negative voltages. Therefore, it is also a hybrid of the flyback and forward types.

[0035] A DC-DC converter IC that can be used in a flyback converter can be used as the DC-DC converter IC 22 in Fig. 3. Also, one labeled for step-up (or boost) use may be used. For example, Nisshinbo Micro Devices' NJM2369 can be used as the DC-DC converter IC 22. Also, the DC-DC converter IC 22 and the MOSFET 24 may be replaced with a DC-DC converter IC with a built-in MOSFET.

[0036] Using a DC-DC converter to boost the power supply has the advantage that the boost circuit and equipment can be made smaller. This is because the frequency of a DC-DC converter is relatively high, allowing the use of a small transformer. Therefore, by combining a DC-DC converter with a high-voltage analog amplifier, it is possible to supply high-voltage DC as the power supply for amplifier 8, and the equipment related to the power supply can be made sufficiently small. The miniaturization of the power supply section ensures sufficient freedom in the design of speaker 1.

[0037] It should be noted that commercial power can also be used as the power source for amplifier 8 by boosting the voltage with a transformer. However, a transformer that can boost the low-frequency AC of commercial power, such as 50 Hz or 60 Hz, will be larger, and the power supply section will be larger than when a DC-DC converter is used as described above.

[0038] Alternatively, a configuration may be used in which an amplifier for dynamic speakers that operates at a low voltage is used as the amplifier 8, and the output signal of the amplifier is boosted by a transformer and input to the piezoelectric speaker 2. However, the piezoelectric speaker 2 needs to be able to reproduce frequencies up to about 800 Hz on the lower side of the frequency band it handles. In that case, as in the case of the power supply transformer described above, a larger transformer is required as the signal transformer, making miniaturization difficult. Also, transformers do not have a high level of performance for processing high-frequency signals such as those handled by the piezoelectric speaker 2, so they are not suitable in terms of sound quality.

[0039] The dynamic speaker 4 is a speaker unit using a voice coil. In this embodiment, the dynamic speaker 4 is responsible for outputting the low-frequency sound of the speaker 1. The dynamic speaker 4 outputs a low-frequency sound signal amplified by an amplifier 10. A commercially available dynamic type speaker unit can be used as the dynamic speaker 4. The frequency band of the low-frequency sound signal handled by the dynamic speaker 4 in this embodiment is, for example, in the range of 100 Hz to 1 kHz.

[0040] The amplifier 10 is a dynamic speaker amplifier that amplifies an audio signal input to the dynamic speaker 4. The amplifier 10 is a voltage-voltage amplifier. As described later, the amplifier 10 receives an audio signal from the low-pass output 6L of the state variable filter 6. The amplifier 10 is not limited to an analog amplifier that can drive the dynamic speaker 4, but it is preferable to use a class D amplifier. If a class D amplifier is used as the amplifier for the dynamic speaker 4, it is possible to reduce the heat generated by the amplifier with higher efficiency than other analog amplifiers, and the circuit is smaller and lighter. Therefore, it is preferable to use a class D amplifier as the amplifier 10. The class D amplifier may be configured using a commercially available class D amplifier IC or the like. The amplifier 10 may be operated by supplying power directly from a built-in battery.

[0041] The state variable filter 6 is a band division filter that divides an input analog audio signal into signals of three frequency bands. FIG. 4 shows an example of the circuit configuration of the state variable filter 6 of this embodiment. The state variable filter 6 is a second-order filter having at least three operational amplifiers, and is a circuit having resistors, capacitors, etc. in addition. The state variable filter 6 has a low-pass output 6L, a band-pass output 6B, and a high-pass output 6H that share a cutoff frequency. The positive and negative power supplies of each operational amplifier can be, for example, the low-voltage power supplies mentioned above (5V and 0V ground).

[0042] In this embodiment, a band-pass output 6B of the state variable filter 6 is input to an amplifier 8 for the piezoelectric speaker 2. In addition, a low-pass output 6L is input to an amplifier 10 for the dynamic speaker 4.

[0043] The piezoelectric element (piezoelectric film) that constitutes the piezoelectric speaker 2 generates distortion proportional to the applied voltage. When driven with a sine wave voltage of a constant amplitude, distortion of a constant amplitude occurs regardless of the frequency. Therefore, the amplitude of the displacement speed of the piezoelectric element increases in proportion to the frequency. In other words, a piezoelectric speaker has a frequency characteristic in which the higher the frequency of the signal, the greater the sound pressure. However, when outputting sound from a speaker, it is preferable to have as flat an output characteristic as possible, which generates a constant sound pressure regardless of the signal frequency. For this reason, it is necessary to introduce an integrator into the circuit that drives the piezoelectric speaker and make a correction so that the signal voltage amplitude becomes smaller as the frequency increases.

[0044] In contrast, in this embodiment, the frequency characteristics of the band-pass output 6B of the state variable filter 6 are the same as the frequency characteristics of the signal obtained by passing the high-pass output 6H signal through an integrator. Therefore, there is no need to use an integrator. Therefore, if the band-pass output 6B is input directly to the amplifier 8 without going through an integrator, it is possible to output high-frequency sound from the piezoelectric speaker 2 with little change in sound pressure due to frequency.

[0045] The Q value, which represents the filter sharpness of the state variable filter 6, is preferably 0.4 or more and 0.8 or less. By setting the Q value in this range, when the low-pass output 6L is input to the low-frequency amplifier 10 and the band-pass output 6B is input to the high-frequency amplifier 8, and the sound pressures emitted from the corresponding speakers are combined, a flatter frequency characteristic is obtained. A Q value of 0.5 within the above range is more preferable because it further flattens the frequency characteristic.

[0046] The cutoff frequency common to the outputs of the state variable filter 6 can be set appropriately according to the frequency bands covered by the piezoelectric speaker 2 and the dynamic speaker 4 used. The cutoff frequency can be set to about 800 Hz, for example. The cutoff frequency can be set appropriately by the values ​​of resistors R6 and R7 in FIG. 4.

[0047] Other configurations of the speaker 1 will now be described. The speaker 1 can acquire and play back audio signals from a smartphone, tablet, PC, etc., by performing wireless communication such as Bluetooth or Wi-Fi. In this case, the speaker 1 may include an antenna for wireless communication, a demodulation circuit for converting the acquired signal into an analog audio signal, a D / A converter, and other necessary circuits (for example, a Bluetooth module).

[0048] The speaker 1 can also acquire and play back audio signals via a wired connection. The type of wired connection is not particularly limited as long as it can acquire audio signals, and may be digital input, analog input, USB input, etc. When inputting to the speaker 1 via a wired connection, it is sufficient to provide a corresponding input terminal and a signal processing circuit required according to the input signal. For example, in the case of digital input, it is sufficient to provide a D / A converter, etc.

[0049] Furthermore, the speaker 1 may be provided with necessary circuits, such as a terminal for inserting a charging cable and a charging IC, as appropriate, in order to charge the built-in battery.

[0050] According to the speaker 1 of the present embodiment described above, an optimal speaker can be configured using the piezoelectric speaker 2 as the speaker unit on the high frequency band side and the dynamic speaker 4 as the unit on the low frequency band side. The piezoelectric speaker 2 has a lighter diaphragm than a dynamic type, has excellent frequency characteristics in the high frequency range, and has a higher ability to reproduce consonants in voice and percussion in music. Therefore, the speaker 1 of the present embodiment can provide a speaker with excellent reproduction capabilities in the high frequency range. In addition, the amplifier section of the piezoelectric speaker 2 can be miniaturized, providing a speaker with high design freedom. For example, the speaker 1 is also suitable as a compact, high-quality portable active speaker.

[0051] It should be noted that the circuit configurations of the various components shown in this embodiment are merely examples, and may be replaced with other circuit configurations capable of achieving similar functions.

[0052] Furthermore, the speaker 1 of this embodiment has been described as a monaural speaker system, but two or more speakers 1 may be used to provide stereo output.

[0053] Second embodiment While the first embodiment has been described as a two-way speaker, a three-way speaker 100 will be described as a modified example in this embodiment. Fig. 5 shows a block diagram showing an example of the device configuration of the three-way speaker 100. The speaker 100 of this embodiment further splits the signal of the band-pass output 6B, which is the high-frequency output of the state variable filter 6, into audio signals of two frequency bands by another state variable filter 60, and outputs each signal by two piezoelectric speakers.

[0054] The speaker 100 further includes a state variable filter 60 in addition to the state variable filter 6. In the following description, the state variable filter 6 will be referred to as the first state variable filter 6, and the state variable filter 60 will be referred to as the second state variable filter 60. The speaker 100 has an amplifier 8H (an amplifier for a first piezoelectric speaker) and an amplifier 8L (an amplifier for a second piezoelectric speaker) instead of the amplifier 8. Also, the speaker 100 has a piezoelectric speaker 2H (a first piezoelectric speaker) and a piezoelectric speaker 2L (a second piezoelectric speaker) instead of the piezoelectric speaker 2. The other configurations are basically the same as those of the speaker 1 of the first embodiment. Descriptions of the common configurations will be omitted.

[0055] The second state variable filter 60 is a band division filter having the same circuit configuration as the first state variable filter 6. A band pass output 6B of the first state variable filter 6 is input to the second state variable filter 60. A high pass output of the second state variable filter 60 is input to an amplifier 8H, and a low pass output is input to an amplifier 8L.

[0056] When the cut-off frequency of the first state variable filter 6 is f1 and the cut-off frequency of the second state variable filter 60 is f2, the cut-off frequencies are set such that f1 < f2. Although f2 is not particularly limited, it can be set to an appropriate frequency within the range of, for example, 2 kHz or more and 20 kHz or less while satisfying the relationship f1 < f2.

[0057] The amplifier 8H and the piezoelectric speaker 2H output a high-frequency side signal obtained by dividing the signal from the band-pass output 6B of the first state variable filter 6 by the second state variable filter 60. The amplifier 8L and the piezoelectric speaker 2L output a low-frequency side signal obtained by dividing the signal from the band-pass output 6B by the second state variable filter 60. The amplifiers 8H and 8L have the same configuration as the amplifier 8 in the first embodiment and can operate with direct current boosted by a DC-DC converter. The piezoelectric speakers 2H and 2L have the same configuration as the piezoelectric speaker 2 in the first embodiment. For example, in the case of a cylindrical speaker, a cylindrical piezoelectric speaker 2H can be arranged on the upper side of the cylindrical housing, and a cylindrical piezoelectric speaker 2L can be arranged on the lower side thereof. Note that, as these amplifiers and piezoelectric speakers, devices and circuits with different configurations may be appropriately adopted according to conditions such as sound quality, shape, and size.

[0058] According to the speaker 100 of the present embodiment, it is possible to provide a speaker with a compact configuration and better high-frequency sound quality compared to the case where all speaker units are dynamic type 3-way.

[0059] Also, for example, since the piezoelectric speaker 2L can cover up to the upper limit of the audible frequency band (20 kHz), the piezoelectric speaker 2H, which is responsible for the highest frequency range, can be used as a super tweeter that reproduces a frequency band above the audible frequency band. In the case of a super tweeter using a voice coil, the sound is emitted in a beam shape, which has the property of being too directional. In contrast, when the piezoelectric speaker 2H is used as a super tweeter, sound waves can be emitted evenly in the normal direction of the piezoelectric film. Therefore, it is suitable as a super tweeter in an omnidirectional speaker such as a cylindrical one. In addition, piezoelectric speakers are lighter than types that use voice coils, and are suitable for reproducing high-frequency bands above the audible range, so they have the advantage of being able to reproduce high-pitched sounds with higher sound quality.

[0060] As another embodiment of the three-way speaker, the low-pass output 6L of the first embodiment may be divided into bands by another state variable filter, and the signals of each frequency band may be output by two dynamic speakers. That is, the high-frequency side (band-pass output 6B) may be handled by one piezoelectric speaker, and the lower-frequency side (low-pass output 6L) may be handled by two dynamic speakers. In this case, the cut-off frequency (f2') of the second state variable filter that divides the low-pass output 6L is lower than the cut-off frequency (f1) of the first state variable filter (f2'). <f1)。

[0061] (Third embodiment) In this embodiment, the amplifier for the piezoelectric speaker 2 has a different configuration from the amplifier 8 of the first embodiment shown in Fig. 2. The configuration other than the amplifier for the piezoelectric speaker 2 is the same as in the first embodiment, so a description thereof will be omitted.

[0062] FIG. 6 shows the circuit configuration of an amplifier 80 for the piezoelectric speaker 2 of this embodiment. FIG. 6(a) shows a specific circuit configuration of the amplifier 80, and FIG. 6(b) is a conceptual diagram of the amplifier 80 as a single operational amplifier. The amplifier 80 is an amplifier configured with discrete elements without using a speaker amplifier IC. +LV and -LV are positive and negative low voltage power supplies, and for example, the +5V and 0V power supplies of the low voltage power supply shown in the first embodiment can be used. +HV and -HV are high voltage power supplies, and for example, the +100V and -100V outputs obtained from the DC-DC converter of the first embodiment can be used.

[0063] The amplifier 80 is an operational amplifier with a top-bottom symmetrical structure that operates at a high voltage as a whole. The amplifier 80 has six current mirror circuits 81 to 86. In this embodiment, the current mirror circuits 81 to 86 are Wilson-type current mirrors. The current mirror circuits 81 and 82 are connected to a low-voltage power supply, and the current mirror circuits 83 to 86 are connected to a high-voltage power supply.

[0064] Current mirrors 81 and 82 (first current mirror circuit) in the low-voltage section on the left side of Fig. 6 supply constant current to two differential pairs of transistors Q10 and Q11, and transistors Q12 and Q13. By using a current mirror circuit, it is possible to prevent the current from fluctuating when an audio signal is input to the non-inverting input (+IN).

[0065] The current signal output from the differential pair connected to the audio signal input terminals (+IN, -IN) is guided to current mirror circuits 83, 84 (second current mirror circuit) on the left side of the high voltage section. The differential pair and current mirror circuits 83, 84 are connected via transistors Q17 and Q18. Transistors Q17 and Q18 are high-voltage transistors, and by cascode-connecting them, the differential pair itself can use transistors with low voltage resistance. This allows the differential pair to use low-noise transistors and also reduces costs.

[0066] Current mirror circuits 83 and 84 on the upper and lower left sides of the high voltage section fold back the respective current signals, and these current outputs are combined via transistors Q25 and Q28 to form a voltage amplification stage. The higher the output impedance of the current mirror, the higher the voltage gain. The Wilson-type current mirror circuit used in this embodiment has a higher output impedance, which allows for higher voltage gain and more reliable signal amplification. In addition, by using high-voltage transistors for transistors Q21 and Q24, low-voltage, low-noise transistors can be used for transistors Q19 and Q20 and transistors Q22 and Q23.

[0067] The potential of the output (OUT) of the amplifier 80 is almost the same as the emitter potential of the transistors Q25 and Q28, and by applying negative feedback from OUT to -IN, it operates as a negative feedback amplifier circuit. The current mirror circuits 85 and 86 (third current mirror circuits) on the right side of the high voltage section are arranged between the output terminal (OUT) and the current mirror circuits 83 and 84, and are used to amplify the current of the signal. Here too, by using high-voltage transistors for the transistors Q27 and Q30, it is possible to use low-voltage, low-noise transistors for the remaining transistors Q25, Q26, Q28, and Q29. Since the transistors Q27 and Q30 generate the most heat, it is preferable to use high-power transistors. Note that the transistors Q27 and Q30 may be substituted with MOSFETs. Also, instead of using a high-voltage, high-power PNP transistor, which has become difficult to obtain in recent years, an inverted Darlington connection circuit may be used for Q30.

[0068] The amplifier 80 of this embodiment as described above makes it easier to power up the audio signal than the amplifier 8, and is suitable as an amplifier for the piezoelectric speaker 2 that requires a high-voltage audio signal. In addition, since low-noise transistors can be arranged in the circuit of the amplifier 80, it is possible to obtain the effect of further suppressing the white noise of the entire amplifier.

[0069] (Fourth embodiment) This embodiment is an embodiment in which a circuit portion such as an amplifier is separate from a speaker in an audio system 200. Fig. 7 shows a block diagram of the audio system 200 of this embodiment.

[0070] An audio system 200 of this embodiment includes a speaker 201 having a piezoelectric speaker 2 and a dynamic speaker 4, and an amplifier device 202 having a state variable filter 6, a piezoelectric speaker amplifier 8, and a dynamic speaker amplifier 10. The speaker 201 is a so-called passive type speaker that does not include a circuit part such as an amplifier, and is basically composed of a speaker unit of a piezoelectric speaker 2 and a dynamic speaker 4. The amplifier device 202 is a device in which the circuit parts other than the speaker unit included in the speaker 1 of the first embodiment are housed in a housing separate from the speaker 201. The circuit parts are specifically the state variable filter 6, the amplifiers 8 and 10, a DC-DC converter, a power supply unit, etc., and have the same configuration as in the first embodiment.

[0071] According to this embodiment, the circuit portion such as the amplifier is separate from the speaker 201 as the amplifier device 202, but the speaker 201 portion has a higher degree of freedom in design, and speakers of various forms and shapes can be formed. Furthermore, since the amplifier device 202 has the same circuit configuration as the first embodiment, it is capable of outputting a high-voltage signal to the piezoelectric speaker 2, while keeping the size of the power supply portion and the like down. Therefore, the audio system 200 of this embodiment can also provide an audio device that has excellent high-frequency reproduction capabilities and a high degree of freedom in design.

[0072] As the amplifier 8 in the amplifier device 202, the amplifier 80 shown in the third embodiment may be used.

[0073] Fifth embodiment This embodiment is an embodiment in which the DC-DC converter shown in Fig. 3 in the first embodiment has a different configuration. Fig. 8 shows an example of the circuit configuration of the DC-DC converter of this embodiment. As shown in the first embodiment, the DC-DC converter converts the DC of the built-in battery (+15V as an example in Fig. 8) into a high-voltage DC to be supplied to an amplifier 8 for a piezoelectric speaker.

[0074] The DC-DC converter in Fig. 8, like Fig. 3, has a voltage doubler rectifier circuit 28 that combines two voltage doublers on the secondary side of the transformer 26, and outputs equal positive and negative voltages (+225V and -225V in Fig. 8). The DC-DC converter IC 29 uses a type that incorporates a semiconductor switch MOSFET, and for example, the NJW4132US from Nisshinbo Micro Devices Inc. can be used.

[0075] The load of the DC-DC converter is an audio amplifier, and taking into consideration the fact that the currents required for the positive and negative power supplies may differ, the DC-DC converter has a feedback circuit designed to make the average value of the positive and negative power supplies a predetermined voltage. Specifically, the feedback circuit has a positive output side feedback circuit 30 connected to the positive output, a negative output side feedback circuit 31 connected to the negative output, and a current mirror 32 connected to both.

[0076] The positive output feedback circuit 30 has a feedback resistor 30a (3.9 MΩ) and a phase compensation circuit 30b (1 pF, 4.7 kΩ). The negative output feedback circuit 31 similarly has a feedback resistor 31a (3.9 MΩ) and a phase compensation circuit 31b (1 pF, 4.7 kΩ).

[0077] These circuits convert the current absorbed by the negative power supply into a source current, combine it with the current coming from the positive power supply, and pass it through resistor 33 of 8.7kΩ (3.6k+5.1k) to generate a predetermined comparison voltage (about 1V in the example of Figure 8). This voltage is compared with the reference voltage inside IC29 (1.0V in the case of the above IC), so that the average value of the positive and negative power supplies can be controlled to a predetermined voltage, and the positive and negative output voltages can be accurately maintained at their respective predetermined values. The common-base-connected transistor 32a of current mirror 32 can reduce the difference between the voltage across feedback resistor 31a of the negative output and the voltage across feedback resistor 30a of the positive output.

[0078] According to the present embodiment described above, the positive and negative output voltages of the DC-DC converter are more accurately maintained at predetermined values, making the DC-DC converter suitable for supplying high-voltage DC to the amplifier 8 of the piezoelectric speaker 2. The amplifier connected to the DC-DC converter of this embodiment may be the amplifier 8 of the first embodiment or the amplifier 80 shown in the third embodiment. [Explanation of symbols]

[0079] 1, 100 speakers 2, 2H, 2L Piezoelectric Speaker 4 Dynamic Speakers 6, 60 State variable filter 8, 8H, 8L, 10, 80 Amps 200 Audio System

Claims

1. A first state variable filter that divides an audio signal into bands, the first state variable filter having a high-pass output, a band-pass output, and a low-pass output that have a common cutoff frequency; a piezoelectric speaker amplifier to which a signal from the bandpass output is input; a piezoelectric speaker that converts a signal output from the piezoelectric speaker amplifier into sound using a piezoelectric element and outputs the sound; a dynamic speaker amplifier to which a signal from the low-pass output is input; a dynamic speaker for outputting the signal output from the dynamic speaker amplifier; A speaker comprising:

2. A DC-DC converter that boosts the direct current supplied from the secondary battery as a power source, 2. The speaker according to claim 1, wherein the piezoelectric speaker amplifier operates on a direct current boosted by the DC-DC converter.

3. 3. The speaker according to claim 2, wherein the DC-DC converter boosts the voltage by a circuit including a semiconductor switch, a control circuit for the semiconductor switch, and a transformer.

4. 4. The speaker according to claim 3, wherein the DC-DC converter further uses a voltage doubler rectifier circuit to output positive and negative voltage direct currents.

5. The DC-DC converter comprises: a positive output side feedback circuit having a feedback resistor and a phase compensation circuit connected to the positive output; a negative output side feedback circuit having a feedback resistor and a phase compensation circuit connected to the negative output; a current mirror connected to the positive output side feedback circuit and the negative output side feedback circuit, 5. The speaker according to claim 4, wherein the positive and negative output voltages are adjusted based on a voltage generated by a current from the positive output side feedback circuit and the negative output side feedback circuit.

6. 2. The speaker according to claim 1, wherein the piezoelectric speaker amplifier comprises a speaker amplifier IC and a voltage booster circuit for amplifying a voltage of a signal.

7. 7. The speaker according to claim 1, wherein the piezoelectric speaker amplifier amplifies an input signal to a signal having a higher voltage than the dynamic speaker amplifier.

8. 2. The speaker according to claim 1, wherein the Q value of the first state variable filter is 0.

5.

9. 2. The speaker according to claim 1, wherein the dynamic speaker amplifier is a class D amplifier.

10. The speaker is The piezoelectric speaker amplifier includes a first piezoelectric speaker amplifier and a second piezoelectric speaker amplifier, a three-way speaker including a first piezoelectric speaker to which a signal is input from the first piezoelectric speaker amplifier and a second piezoelectric speaker to which a signal is input from the second piezoelectric speaker amplifier, a second state variable filter that divides the bandpass output from the first state variable filter into bands; 2. The speaker according to claim 1, wherein a high-pass output signal of the second state variable filter is input to an amplifier for the first piezoelectric speaker, and a low-pass output signal of the second state variable filter is input to an amplifier for the second piezoelectric speaker.

11. The cutoff frequency of the first state variable filter is f １ and the cutoff frequency of the second state variable filter is f ２ In this case, f １ <f ２ 11. The speaker according to claim 10,

12. The piezoelectric speaker amplifier is composed of discrete elements, a first current mirror circuit connected to a low voltage power supply; a differential pair connected to an input terminal of an audio signal and connected to the first current mirror circuit; a second current mirror circuit connected to a high voltage power supply and connected to the differential pair via a transistor; 2. The speaker according to claim 1, further comprising: a third current mirror circuit connected to the high voltage power supply and between the second current mirror circuit and an output terminal for an audio signal, for amplifying a current of the audio signal.

13. 13. The speaker according to claim 12, wherein the first current mirror circuit, the second current mirror circuit, and the third current mirror circuit are Wilson-type current mirrors.

14. A first state variable filter that divides an audio signal into bands, the first state variable filter having a high-pass output, a band-pass output, and a low-pass output that have a common cutoff frequency; a piezoelectric speaker amplifier to which a signal from the bandpass output is input; an amplifier for a dynamic speaker to which a signal from the low-pass output is input; a piezoelectric speaker that converts a signal output from the piezoelectric speaker amplifier into sound using a piezoelectric element and outputs the sound; a dynamic speaker for outputting the signal output from the dynamic speaker amplifier; An audio system comprising:

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