Amplifier

Abstract

The effect of biased magnetism caused by installing an impedance conversion transformer is eliminated. [Solution] The amplification device (1) includes a power amplifier (3) that outputs a voltage in phase with the input voltage (Vin), a power amplifier (4) that outputs a voltage in opposite phase with the input voltage (Vin), a transformer (5) that has a primary winding (5a) to which the output terminal of power amplifier (3) and the output terminal of power amplifier (4) are connected, and a secondary winding (5b) connected to a speaker (6), and converts the output impedances of power amplifiers (3) and power amplifier (4) into a plurality of different impedances, and electrolytic capacitors (C1, C2) that remove the DC component of the current flowing through the primary winding (5a) of the transformer (5).

Description

【Technical Field】 【0001】 The present invention relates to an amplifier device for driving speakers having different impedances. 【Background Art】 【0002】 As a simple configuration of an audio power amplifier, a single - ended power amplifier is known. Conventionally, in order to drive speakers having different impedances with such a power amplifier, an impedance - converting transformer has been provided at the output stage of the power amplifier. A plurality of taps are provided at positions with different winding ratios on the secondary side of the transformer, and different impedances can be obtained at each tap. 【0003】 Also, for a low - output single - ended power amplifier, as disclosed in Patent Document 1, a BTL (Bridge Tied Load) type power amplifier in which the output ends of two power amplifiers that output voltages of opposite phases are bridge - connected is known as a power amplifier that can obtain high output. 【Prior Art Documents】 【Patent Documents】 【0004】 【Patent Document 1】 Japanese Patent Application Laid - Open No. 2003 - 46345 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0005】 When attempting to drive speakers with different impedances using a BTL-type power amplifier, it is conceivable to install an impedance-changing transformer, as described above, in the output stage of the power amplifier. However, when an impedance-changing transformer is installed in a BTL-type power amplifier as described above, magnetic polarization, which is characteristic of transformers, is likely to occur. Even if the output voltages of the two power amplifiers, which are in opposite phase, are applied to each of the primary input terminals of the transformer, the output voltages of the two are slightly different, causing the output voltage of one of the primary input terminals to become higher and thus biased, resulting in magnetic polarization. 【0006】 When magnetic bias occurs, the primary current of a transformer, which is normally AC, becomes biased towards either the positive or negative terminal, and eventually becomes completely biased towards either the positive or negative terminal, making it comparable to DC. Speakers will be damaged when such DC current flows through them. 【0007】 One aspect of the present invention aims to eliminate the effect of biased magnetism caused by the provision of an impedance conversion transformer. [Means for solving the problem] 【0008】 To solve the above problems, an amplification device according to one aspect of the present invention comprises: a first power amplifier that outputs a voltage in phase with the input voltage; a second power amplifier that outputs a voltage in opposite phase with the input voltage; a transformer having a primary winding to which the output terminal of the first power amplifier and the output terminal of the second power amplifier are connected, and a secondary winding connected to a speaker, which converts the output impedances of the first power amplifier and the second power amplifier into a plurality of different impedances; and a capacitor that removes the DC component of the current flowing through the primary winding of the transformer. [Effects of the Invention] 【0009】 According to one aspect of the present invention, the effect of biased magnetism caused by providing an impedance conversion transformer can be eliminated. [Brief explanation of the drawing] 【0010】 [Figure 1] This is a circuit diagram showing the configuration of an amplifier according to one embodiment of the present invention. [Figure 2] This waveform diagram shows the waveforms of the current flowing through the primary winding of the transformer in the above-mentioned amplifier and the voltage applied across both ends of the primary winding. [Figure 3] This waveform diagram shows the waveforms of the current flowing through the primary winding of the transformer and the voltage applied across both ends of the primary winding in the comparative example amplifier. [Figure 4] This is a circuit diagram showing the configuration of an amplifier according to the first modified example of the above embodiment. [Figure 5] This is a circuit diagram showing the configuration of an amplifier according to a second modified example of the above embodiment. [Modes for carrying out the invention] 【0011】 [Embodiment] One embodiment of the present invention will be described in detail below. 【0012】 <Configuration of the amplification device> Figure 1 is a circuit diagram showing the configuration of the amplifier 1 according to this embodiment. 【0013】 The amplification device 1 is a BTL-type device described above, which amplifies the input voltage Vin as an audio signal using two power amplifiers 3 and 4 whose output terminals are bridged, and drives the speaker 6 with the amplified voltage. The amplification device 1 comprises an input amplifier 2, a power amplifier 3 (first power amplifier), a power amplifier 4 (second power amplifier), and a transformer 5. The amplification device 1 also includes electrolytic capacitor C1 (first polarized capacitor) and electrolytic capacitor C2 (second polarized capacitor) as capacitors. 【0014】 The input amplifier 2 and the power amplifiers 3 and 4 are constituted by a power amplifier IC (Integrated Circuit). The power supply specification of the power amplifier IC is, for example, a single power supply. Therefore, the power amplifier IC operates with a positive power supply voltage. For this reason, the input amplifier 2 and the power amplifiers 3 and 4 output a positive voltage. 【0015】 The input amplifier 2 is provided as a buffer amplifier. The input amplifier 2 outputs the positive voltage Vin+ in the input voltage Vin having positive and negative polarities. Also, the input amplifier 2 outputs an inverted positive voltage Vin- obtained by inverting the polarity of the negative voltage in the input voltage Vin. 【0016】 The power amplifier 3 amplifies the positive voltage Vin+ which is a voltage in phase with the input voltage Vin with a predetermined gain. The power amplifier 4 amplifies the inverted positive voltage Vin- which is a voltage opposite in phase to the input voltage Vin with the same gain as that of the power amplifier 3. 【0017】 The output terminal of the power amplifier 3 is connected to a terminal T1 which is one end of the primary winding 5a of a transformer 5 described later via an electrolytic capacitor C1. The output terminal of the power amplifier 4 is connected to a terminal T2 which is the other end of the primary winding 5a via an electrolytic capacitor C2. 【0018】 Note that the power amplifier IC described above may not include the input amplifier 2. In such a power amplifier IC, the power amplifier 3 amplifies and outputs the positive voltage in the input voltage Vin with a predetermined gain, and the power amplifier 4 amplifies and outputs the negative voltage in the input voltage Vin with the same gain as that of the power amplifier 3 after inverting the amplification. 【0019】 The transformer 5 has an impedance conversion function for converting the output impedances of the power amplifier 3 and the power amplifier 4 into a plurality of different impedances. The transformer 5 has a primary winding 5a and a secondary winding 5b. 【0020】 The primary winding 5a has terminals T1 and T2. As described above, the negative terminal of the electrolytic capacitor C1 is connected to the terminal T1 of the primary winding 5a, and the negative terminal of the electrolytic capacitor C2 is connected to the terminal T2 of the primary winding 5a. 【0021】 The secondary winding 5b has a tap TP0 and a plurality of taps TP1 to TP3 whose winding ratios with respect to the primary winding 5a are different. One end of the negative side in the secondary winding 5b is connected to the tap TP0. One end of the positive side in the secondary winding 5b is connected to the tap TP3. The impedances of the taps TP1 to TP3 are, for example, 2Ω, 4Ω, and 8Ω respectively. The impedances of the taps TP1 to TP3 are not limited to this example and may be other impedances. 【0022】 The amplification device 1 includes speaker connection terminals ST0 to ST3. The speaker connection terminals ST0 to ST3 are respectively connected to the above-mentioned taps TP0 to TP3. The negative terminal of the speaker 6 is connected to the speaker connection terminal ST0. The positive terminal of the speaker 6 is connected to any one of the speaker connection terminals ST1 to ST3. 【0023】 The positive terminal of the electrolytic capacitor C1 is connected to the output terminal of the power amplifier 3. The positive terminal of the electrolytic capacitor C2 is connected to the output terminal of the power amplifier 4. The electrolytic capacitors C1 and C2 are connected to the primary winding 5a so as to have opposite polarities with respect to the terminals T1 and T2 of the primary winding 5a. The electrolytic capacitors C1 and C2 have the function of removing the DC component of the current flowing through the primary winding 5a of the transformer 5. 【0024】 〈Operation of the amplification device〉 The operation of the amplification device 1 configured as described above will be described. 【0025】 Figure 2 is a waveform diagram showing the waveforms of the current flowing through the primary winding 5a of the transformer 5 in amplifier 1 and the voltage applied across the primary winding 5a. Figure 3 is a waveform diagram showing the waveforms of the current flowing through the primary winding of the transformer and the voltage applied across the primary winding in the amplifier of the comparative example. 【0026】 Input amplifier 2 outputs a positive voltage Vin+ and an inverted positive voltage Vin- based on the input voltage Vin. The positive voltage Vin+ is input to power amplifier 3, and the inverted positive voltage Vin- is input to power amplifier 4. 【0027】 Power amplifier 3 amplifies and outputs the positive voltage Vin+. Power amplifier 4 amplifies and outputs the inverted positive voltage Vin-. The positive voltage Vin+ is applied to terminal T1 on the primary winding 5a of transformer 5 via electrolytic capacitor C1. The inverted positive voltage Vin- is applied to terminal T2 on the primary winding 5a via electrolytic capacitor C2. 【0028】 As a result, an AC voltage, which is the difference between the positive voltage Vin+ and the inverting positive voltage Vin-, i.e., (positive voltage Vin+) - (inverting positive voltage Vin-), is applied to the primary winding 5a. This causes a voltage with positive and negative polarity to appear in the secondary winding 5b and be applied to the speaker 6. 【0029】 Furthermore, the impedances of taps TP1 to T3 in the secondary winding 5b are all different. As a result, the positive terminal of the speaker 6 with the corresponding impedance is connected to one of the taps TP1 to T3. Therefore, the amplifier 1 can drive speakers 6 with impedances of 2Ω, 4Ω, and 8Ω. 【0030】 <Effects of amplification devices> As shown by the solid line in Figure 2, the voltage applied to the primary winding 5a maintains an AC waveform that fluctuates around 0V. Furthermore, even if the current flowing through the primary winding 5a tends to be biased towards the positive or negative side due to magnetic bias, the DC component of the current is removed by electrolytic capacitors C1 and C2. 【0031】 As a result, the current, like the voltage mentioned above, can maintain an AC waveform that fluctuates around 0A, as shown by the dashed line in Figure 2. In this way, electrolytic capacitors C1 and C2 can prevent the current on the primary side of transformer 5 from becoming DC due to magnetization bias in transformer 5. 【0032】 In contrast, the comparative example amplifier, although not shown in the diagram, has a configuration in which electrolytic capacitors C1 and C2 are omitted from amplifier 1. That is, in this amplifier, the output terminal of power amplifier 3 is directly connected to terminal T1 of the primary winding 5a, and the output terminal of power amplifier 4 is directly connected to terminal T2 of the primary winding 5a. 【0033】 In such an amplifier, if a magnetic bias occurs in the transformer 5, the current flowing through the primary winding 5a will gradually shift towards the positive terminal, as shown by the dashed line in Figure 3, and eventually become completely biased towards the positive terminal. In this state, the overcurrent protection circuit of the amplifier will activate, causing the input amplifier 2 or power amplifiers 3 and 4 to stop operating. As a result, no current will flow through the primary winding 5a. Also, the voltage applied to the primary winding 5a, shown by the solid line in Figure 3, will drop to 0V. 【0034】 Here, a single-ended amplifier is, for example, a configuration that has a single power amplifier as the amplification circuit. Furthermore, this amplifier is equipped with a DC blocking capacitor in the output stage to prevent DC voltage from being applied to the speaker, thereby removing the DC component from the output voltage. As a result, the amplitude of the output voltage of this amplifier becomes less than or equal to the power supply voltage. Consequently, the maximum peak value of the output voltage becomes less than or equal to half the power supply voltage of the power amplifier IC. 【0035】 In contrast, the maximum peak value of the output voltage of the BTL-type amplifier 1 is approximately the same as the power supply voltage (positive voltage) of the power amplifier IC. Therefore, amplifier 1 can output an output voltage approximately twice that of the single-ended amplifier described above. 【0036】 Furthermore, the amplifier 1 is equipped with electrolytic capacitors C1 and C2. This allows for a large capacitance to be obtained despite the small size, and enables the use of inexpensive capacitors. 【0037】 <Variations> Next, a first modified example of this embodiment will be described. Figure 4 is a circuit diagram showing the configuration of the amplifier 1A according to the first modified example of this embodiment. 【0038】 As shown in Figure 4, amplifier 1A, like amplifier 1, includes an input amplifier 2, power amplifiers 3 and 4, a transformer 5, and electrolytic capacitors C1 and C2. Amplifier 1A also includes a diode D1 (first diode) and a diode D2 (second diode). 【0039】 Diode D1 is connected in parallel with electrolytic capacitor C1 and applies only a positive voltage to electrolytic capacitor C1. Specifically, the cathode of diode D1 is connected to the positive terminal of electrolytic capacitor C1 (output terminal of power amplifier 3), and the anode is connected to the negative terminal of electrolytic capacitor C1 (terminal T1 of primary winding 5a). Diode D2's cathode is connected to the positive terminal of electrolytic capacitor C2 (output terminal of power amplifier 4), and the anode is connected to the negative terminal of electrolytic capacitor C2 (terminal T2 of primary winding 5a). 【0040】 In the amplification device 1, the waveforms of the output voltages of the two power amplifiers 3 and 4, which are applied to terminals T1 and T2 of the primary winding 5a of the transformer 5, may differ slightly from the waveform of the input voltage Vin. When such a voltage difference occurs, a negative polarity voltage is generated in the primary winding 5a. 【0041】 Diodes D1 and D2 prevent such negative polarity voltages from being applied to electrolytic capacitors C1 and C2. In other words, diodes D1 and D2 do not apply a reverse voltage greater than the forward voltage to electrolytic capacitors C1 and C2. This prevents problems such as deterioration and decrease in capacitance from occurring in electrolytic capacitors C1 and C2. 【0042】 Next, a second modified example of this embodiment will be described. Figure 5 is a circuit diagram showing the configuration of the amplifier 1B according to the second modified example of this embodiment. 【0043】 As shown in Figure 5, amplifier 1B, like amplifier 1, includes an input amplifier 2, power amplifiers 3 and 4, and a transformer 5. Amplifier 1B includes capacitor C instead of electrolytic capacitors C1 and C2. 【0044】 Capacitor C is a bipolar capacitor, with one terminal connected to the output terminal of power amplifier 3 and the other terminal connected to terminal T1 of the primary winding 5a. Examples of capacitor C include ceramic capacitors and film capacitors. However, for the capacitor used in the amplification device 1B that amplifies the audio signal, a film capacitor with low distortion is more preferable. 【0045】 In amplifier 1B, since capacitor C has bipolarity, input amplifier 2 does not need to output a positive voltage Vin+ and an inverted positive voltage Vin-, as input amplifier 2 in amplifier 1 does. Specifically, input amplifier 2 only needs to output the input voltage Vin to power amplifier 3 and the voltage with the polarity of the input voltage Vin inverted to power amplifier 4. Also, as mentioned above, if input amplifier 2 is omitted, power amplifier 3 amplifies the input voltage Vin with a predetermined gain, and power amplifier 4 inverts and amplifies the input voltage Vin with the same gain as power amplifier 3. 【0046】 Furthermore, the power supply specifications for the power amplifier IC that constitutes the amplification section of the amplification device 1B are not limited to a single power supply; a positive and negative power supply may also be used. 【0047】 The amplifier 1B, configured as described above, has a single capacitor C, thus reducing the number of components compared to amplifier 1. However, capacitor C, which is made of film, becomes larger and more expensive as its capacitance increases. Therefore, amplifier 1B tends to be larger and more expensive than amplifier 1. Thus, from the perspective of miniaturization and cost reduction, amplifier 1 is superior to amplifier 1B. 【0048】 〔summary〕 An amplification device according to embodiment 1 of the present invention comprises: a first power amplifier that outputs a voltage in phase with the input voltage; a second power amplifier that outputs a voltage in opposite phase with the input voltage; a transformer having a primary winding to which the output terminal of the first power amplifier and the output terminal of the second power amplifier are connected, and a secondary winding connected to a speaker, which converts the output impedances of the first power amplifier and the second power amplifier into a plurality of different impedances; and a capacitor that removes the DC component of the current flowing through the primary winding of the transformer. 【0049】 In the above configuration, the output terminals of the first power amplifier and the output terminals of the second power amplifier are bridge-connected via the primary winding of the transformer, thereby forming a BTL-type power amplifier. This allows for higher output compared to a single-ended power amplifier. Furthermore, the transformer allows for the connection of speakers with different impedances to the secondary side of the transformer. In addition, the capacitor removes the DC component in the current flowing through the primary winding of the transformer due to magnetic bias in the transformer. This prevents the current from becoming DC. 【0050】 In the amplification device according to embodiment 2 of the present invention, in embodiment 1, the capacitor may be a first polarized capacitor provided between the output terminal of the first power amplifier and one end of the primary winding, and a second polarized capacitor provided between the output terminal of the second power amplifier and the other end of the primary winding. 【0051】 In the above configuration, polarized capacitors (first polarized capacitor and second polarized capacitor) are used as capacitors. For example, widely available electrolytic capacitors can be used as polarized capacitors. This makes it possible to obtain a small size and high capacity while keeping costs low. 【0052】 The amplification device according to embodiment 3 of the present invention may further include, in embodiment 2, a first diode connected in parallel with the first polarized capacitor and applying only a positive voltage to the first polarized capacitor, and a second diode connected in parallel with the second polarized capacitor and applying only a positive voltage to the second polarized capacitor. 【0053】 In an amplification device, a negative polarity voltage may be generated when the output voltages from the first power amplifier and the second power amplifier are unbalanced. In the above configuration, the first and second diodes can prevent such a negative polarity voltage from being applied to either the first polarized capacitor or the second polarized capacitor. This prevents deterioration, capacitance reduction, and other malfunctions from occurring in the first and second polarized capacitors. 【0054】 [Additional Notes] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Furthermore, embodiments obtained by appropriately combining the technical means disclosed in each embodiment are also included within the technical scope of the present invention. [Explanation of Symbols] 【0055】 1,1A,1B Amplifier 3. Power Amplifier (First Power Amplifier) 4. Power Amplifier (Second Power Amplifier) 5 transformers 5a Primary winding 5b Secondary winding 6 speakers C Capacitor C1 Electrolytic capacitor (capacitor, first polarized capacitor) C2 Electrolytic capacitor (capacitor, second polarized capacitor) D1 Diode (First Diode) D2 Diode (Second Diode) T1, T2 terminals Vin Input Voltage

Claims

[Claim 1] A first power amplifier that outputs a voltage in phase with the input voltage, A second power amplifier that outputs a voltage in the opposite phase to the input voltage, A transformer having a primary winding to which the output terminal of the first power amplifier and the output terminal of the second power amplifier are connected, and a secondary winding to which a speaker is connected, which converts the output impedances of the first power amplifier and the second power amplifier into a plurality of different impedances, An amplification device comprising a capacitor that removes the DC component of the current flowing through the primary winding of the transformer. [Claim 2] The amplification device according to claim 1, wherein the capacitors are a first polarized capacitor provided between the output terminal of the first power amplifier and one end of the primary winding, and a second polarized capacitor provided between the output terminal of the second power amplifier and the other end of the primary winding. [Claim 3] A first diode connected in parallel with the first polarized capacitor, which applies only a positive voltage to the first polarized capacitor, The amplification device according to claim 2, further comprising a second diode connected in parallel with the second polarized capacitor and applying only a positive voltage to the second polarized capacitor.

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