JP3874101B2 - Power amplifier device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a power amplifier device.
[0002]
[Prior art]
When negative feedback is applied to the amplifier, characteristics such as frequency characteristics, distortion characteristics, and S / N are improved compared to when negative feedback is not applied. For this reason, negative feedback is generally applied to audio power amplifiers. FIG. 6 schematically shows negative feedback in the power amplifier. An analog audio signal is supplied to the non-inverting input terminal of the power amplifier 1 and amplified, and the amplified output is supplied to the speaker 2. A part of the amplified output is fed back to the inverting input terminal of the power amplifier 1 through the feedback circuit 3.
[0003]
[Problems to be solved by the invention]
However, if negative feedback is applied to an in-vehicle audio device, that is, a so-called car audio power amplifier, the sound quality deteriorates when the vehicle travels. That is, in FIG. 6, while the vehicle is running, the speaker 2 is always exposed to vibration, but the vibration of the speaker 2 is vibrated by this vibration, and the back electromotive voltage Vv resulting from the vibration is applied to the speaker 2. Produce.
[0004]
The counter electromotive voltage Vv is supplied to the inverting input terminal of the power amplifier 1 through the feedback circuit 3. This is because a voltage having a phase opposite to that of the counter electromotive voltage Vv is supplied to the non-inverting input terminal of the power amplifier 1. Is equivalent to That is, the back electromotive voltage Vv is superimposed on the input audio signal through the feedback circuit 3. Therefore, if negative feedback is applied to the power amplifier of the car audio, the sound quality deteriorates when the vehicle travels.
[0005]
FIG. 7 shows an example of an observed waveform of the counter electromotive voltage Vv generated in the speaker 2 due to vibration when the speaker 2 having a general diameter of 16 cm is used. The magnitude of the counter electromotive voltage Vv is as follows. 1.35 Vp-p, that is, about 500 mVrms. In addition, the back electromotive voltage Vv of this magnitude is generated quite frequently while the vehicle is running.
[0006]
If the impedance of the speaker 2 is 4Ω, the damping factor of the power amplifier 1 is generally about 100. At this time, the output impedance of the power amplifier 1 is about 0.04Ω. Therefore, if the feedback factor β of the feedback circuit 3 is 1/16, the voltage value when the back electromotive voltage Vv generated in the speaker 2 is supplied to the inverting input terminal of the power amplifier 1 is
500mVrms × 0.04Ω / (0.04Ω + 4Ω) × (1/16) = 0.31mV
It becomes.
[0007]
If the gain of the power amplifier 1 is 40 dB (= 100 times), the voltage value when the counter electromotive voltage Vv supplied to the inverting input terminal of the power amplifier 1 is amplified and appears at the output terminal of the amplifier 1 is
0.31 mV x 100 times = 31 mV
It becomes. Therefore, if the amplifier output in a normal vehicle is 0.1 W to 5 W, the output voltage at that time is
(0.1W × 4Ω) ^ 0.5 = 0.63V
...
(5W × 4Ω) ^ 0.5 = 4.47V
Therefore, the S / N of the output signal of the power amplifier 1 is
20log (0.63V / 31mV) = 26dB
...
20log (4.47V / 31mV) = 43dB
It can be seen that the sound quality is significantly affected.
[0008]
Therefore, it is not preferable from the viewpoint of sound quality to apply negative feedback to the on-vehicle power amplifier.
[0009]
However, when negative feedback is not applied to the power amplifier, fluctuations in the power supply voltage appear as output fluctuations. In particular, a so-called class D power amplifier converts an audio signal into a PWM signal, and as shown in FIG. 8 (in this figure, the PWM signal is not modulated for simplicity), the power supply voltage is adjusted by the PWM signal. Since switching is performed and the switching output (shaded portion) is integrated to obtain an audio output, if the power supply voltage fluctuates, the influence appears remarkably.
[0010]
And in car audio, all the power of the vehicle is obtained in common from one battery, so the battery voltage is
(1) Voltage fluctuations when the engine is stopped and immediately after the engine is started (2) Audio band noise is generated when an alternator, various motors, etc. are activated. Therefore, when a non-feedback class D power amplifier is mounted on the vehicle, the output is greatly influenced by the items (1) and (2), and the sound quality is greatly deteriorated.
[0011]
FIG. 9 shows the observed waveform of the change in the battery voltage caused by the item (1). The battery voltage increases from 12V to 13.6V before and after the start of the engine. When the engine is stopped, it returns to 12V again. Yes. This voltage fluctuation is a slow fluctuation of several Hz (approximately 2.5 Hz when the engine is started). If there is such a voltage fluctuation, the non-feedback class D power amplifier changes the output by about 2 dB, which causes a problem in hearing.
[0012]
10 and 11 show observed waveforms of noise generated in the battery voltage according to item (2). FIG. 10 shows a waveform when the vehicle is stopped, and FIG. 11 shows a waveform when the vehicle is running. According to this observation result, noise of several kHz is added to the battery voltage, but this noise appears as noise in the audible band of the output in the non-feedback class D power amplifier.
[0013]
For this reason, when a non-feedback class D power amplifier is mounted on a vehicle, the battery voltage is stabilized by a DC / DC converter or the like and then supplied to the class D amplifier. However, in that case, a DC / DC converter having a considerable capacity corresponding to the output of the power amplifier is required, resulting in an increase in the size of the entire device and an increase in price, and a countermeasure for heat generation is also required. End up.
[0014]
In view of the above points, the present invention suppresses the influence of fluctuations in power supply voltage and noise on the power supply voltage on the sound quality without using a DC / DC converter in a non-feedback power amplifier. It is something to try.
[0015]
[Means for Solving the Problems]
In this invention,
A non-feedback power amplifier that is installed in the vehicle and performs class D amplification,
A variable attenuator circuit for controlling the level of the audio signal supplied to the power amplifier ;
A control circuit that controls the volume by controlling the variable attenuator circuit;
A DC line filter to which an output voltage of a battery mounted on the vehicle is supplied;
And a voltage comparison circuit for comparing a reference value the magnitude of the output voltage of the DC line filter,
While supplying the output voltage of the DC line filter to the power amplifier as its power supply voltage,
A power amplifier device configured to supply a comparison output of the voltage comparison circuit as a control signal to the variable attenuator circuit so as to cancel out a level variation of the output signal of the power amplifier caused by a variation in the output voltage of the battery; To do.
Therefore, the influence of the battery voltage fluctuation is canceled out by the level change of the audio signal, and the noise riding on the battery voltage is removed by the DC line filter.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an example of a power amplifier according to the present invention. In this example, the power amplifier 10 is a non-feedback class D amplifier. For this reason, the digital audio signal Din is supplied from the input terminal 11 to the PWM modulation circuit 13 through the variable attenuator circuit 12 for volume adjustment (for level control), and the audio signal Din is converted into a pair of PWM signals DA and DB. The
[0017]
In this case, as shown in FIG. 2, the pulse widths of the PWM signals DA and DB change corresponding to the level indicated by the digital audio signal Din (the instantaneous level when the signal Din is D / A converted; the same applies hereinafter). However, the pulse width of one PWM signal DA is set to the level indicated by the digital audio signal Din, and the pulse width of the other PWM signal DB is set to the two's complement of the level indicated by the digital audio signal Din. It is assumed. Further, the rise time of the PWM signals DA and DB is fixed at the start time of one cycle period Tc of the PWM signals DA and DB, and the fall time changes in accordance with the level indicated by the audio signal Din. The
[0018]
Further, the carrier frequency fc (= 1 / Tc) of the PWM signals DA and DB is, for example, 16 times the sampling frequency fs of the digital audio signal Din, and fs = 48 kHz.
fc = 16fs = 16 × 48kHz = 768kHz
It is said.
[0019]
Then, one PWM signal DA output from the PWM modulation circuit 12 is supplied to the drive circuit 14A, and as shown in FIG. 3A, a pair of drive pulse voltages + DA, which have the same level and level inversion as the signal DA, -DA is formed, and these pulse voltages + DA and -DA are respectively supplied to the gates of a pair of switching elements, for example, n-channel MOS-FETs (Q11, Q12). The FETs (Q11, Q12) constitute a push-pull circuit 15A. The drain of the FET (Q11) is connected to the power supply line 24, the source is connected to the drain of the FET (Q12), and the FET (Q12). ) Source connected to ground. The power line 24 is supplied with a voltage + VDD from the car battery, as will be described later.
[0020]
The source of the FET (Q11) and the drain of the FET (Q12) are connected to one end of the speaker 17 through an integration low-pass filter 16A having a coil and a capacitor.
[0021]
Further, the other PWM signal DB output from the PWM modulation circuit 11 is configured in the same manner as the PWM signal DA. That is, the PWM signal DB is supplied to the drive circuit 14B, and as shown in FIG. 3B, a pair of drive pulse voltages + DB and -DB which are the same level and level inverted as the signal DB are formed, and these pulse voltages + DB, -DB is supplied to the gates of a pair of n-channel MOS-FETs (Q13, Q14) constituting the push-pull circuit 15B. The source of the FET (Q13) and the drain of the FET (Q14) are connected to the other end of the speaker 17 through an integration low-pass filter 16B having a coil and a capacitor.
[0022]
Therefore, when + DA = “H”, −DA = “L”, the FET (Q11) is turned on, and the FET (Q12) is turned off. Therefore, the voltage at the connection point of the FETs (Q11, Q12) As shown in FIG. 3C, VA becomes a voltage + VDD. Conversely, when + DA = “L”, −DA = “H”, FET (Q11) is turned off, and FET (Q12) is turned on, so VA = 0.
[0023]
Similarly, when + DB = “H”, −DB = “L”, the FET (Q13) is turned on, and the FET (Q14) is turned off, so that the connection point of the FET (Q13, Q14) is The voltage VB becomes a voltage + VDD as shown in FIG. 3D. Conversely, when + DB = “L”, −DB = “H”, the FET (Q13) is turned off, and the FET (Q14) is turned on, so that VB = 0.
[0024]
Then, during the period of VA = + VDD and VB = 0, as shown in FIGS. 1 and 3E, from the connection point of the FETs (Q11, Q12), through the line of the low pass filter 16A → the speaker 17 → the low pass filter 16B. The current i flows through the connection point of the FETs (Q13, Q14).
[0025]
Further, during the period of VA = 0 and VB = + VDD, the connection point of the FET (Q11, Q12) from the connection point of the FET (Q13, Q14) through the line of the low pass filter 16B → the speaker 17 → the low pass filter 16A. In addition, the current i flows in the opposite direction. Further, the current i does not flow during the period of VA = VB = + VDD and the period of VA = VB = 0. That is, the push-pull circuits 15A and 15B constitute a BTL circuit.
[0026]
The period during which the current i flows changes corresponding to the period during which the original PWM signals DA and DB rise, and when the current i flows through the speaker 17, the current i is integrated by the low-pass filters 16A and 16B. Therefore, as a result, the current i flowing through the speaker 17 is an analog current corresponding to the level indicated by the digital audio signal Din, and becomes a power-amplified current. That is, the power-amplified output is supplied to the speaker 17.
[0027]
Thus, the signal line from the terminal 11 to the speaker 17 operates as a non-feedback class D power amplifier.
[0028]
At this time, a control circuit 30 is configured by the microcomputer, and various operation keys 33 are connected to the control circuit 30. When the volume up key or the down key among these operation keys 33 is operated, In the control circuit 30, predetermined volume data is formed, and this volume data is supplied to the variable attenuator circuit 12 as its control signal D30, and the level of the digital audio signal Din passing through the variable attenuator circuit 12 (when D / A conversion is performed). Level) is controlled. Therefore, the volume can be changed by operating the up key or the down key of the operation keys 30.
[0029]
However, with the above configuration alone, if the voltage + VDD of the power supply line 24 varies or noise is present, the sound quality of the audio output supplied to the speaker 17 will deteriorate as described above.
[0030]
Therefore, the present invention is further configured as follows. That is, the car battery 21 is connected to the power supply line 24 through the current line of the DC line filter 23 having the switch 22 that is turned on and off in conjunction with the ignition key and the coil L23 and the capacitor C23. Is supplied as a power supply voltage + VDD.
[0031]
Further, the magnitude of the power supply voltage + VDD of the power supply line 24 is detected, and the attenuation amount of the variable attenuator circuit 12 is controlled according to the detection result. In FIG. 1, the control circuit 30 detects the magnitude of the power supply voltage + VDD. That is, the power supply voltage + VDD of the power supply line 24 is supplied to the A / D converter circuit 31 built in the control circuit 30 and A / D converted into the digital data D31, and this data D31 is supplied to the voltage comparison circuit 32. The The voltage comparison circuit 32 is equivalently realized by a microcomputer constituting the control circuit 30 executing a predetermined routine.
[0032]
In the voltage comparison circuit 32, the data D31 of the power supply voltage + VDD is compared with a predetermined reference value DREF corresponding to the reference voltage, and the comparison output is supplied to the variable attenuator circuit 12 as a part of the control signal D30. The attenuation of the circuit 12 is controlled with a polarity that increases as the power supply voltage + VDD increases.
[0033]
According to such a configuration, even if noise due to the item (2) is added to the output voltage of the battery 21, the noise is removed by the DC line filter 23 and then supplied to the push-pull circuits 15A and 15B. The For example, if the DC line filter 23 has a cut-off frequency of 100 Hz and a cut-off characteristic of −40 dB / dec, when the output voltage of the battery 21 has noise with a frequency of, for example, 1 kHz, the level is − It can be attenuated by 43dB.
[0034]
4 and 5 show observed waveforms of the power supply voltage + VDD supplied to the push-pull circuits 15A and 15B. FIG. 4 shows a waveform when the vehicle is stopped and FIG. 5 shows a waveform when the vehicle is running. These waveforms correspond to the waveforms of FIGS. 10 and 11, but no noise due to the item (2) is observed.
[0035]
Therefore, noise caused by the item {circle around (2)} is not included in the amplified output supplied to the speaker 17, so that deterioration of sound quality caused by the item {circle around (2)} can be suppressed.
[0036]
Further, as the power supply voltage + VDD supplied to the push-pull circuits 15A and 15B increases, as is apparent from FIG. 8, the signal voltage supplied to the speaker 17 should increase, but the power amplifier 10 of FIG. When the power supply voltage + VDD becomes higher than the voltage value corresponding to the reference value DREF, the comparison output of the voltage comparison circuit 32 increases and the attenuation of the variable attenuator circuit 12 increases, so that the signal supplied to the speaker 17 is increased. The voltage becomes smaller, thereby canceling the increase in signal voltage due to the increase in power supply voltage + VDD.
[0037]
Conversely, when the power supply voltage + VDD is lowered, the signal voltage supplied to the speaker 17 should be reduced. At this time, the comparison output of the voltage comparison circuit 32 is reduced and the attenuation of the variable attenuator circuit 12 is reduced. As a result, the signal voltage supplied to the speaker 17 increases, thereby canceling the decrease in the signal voltage due to the increase in the power supply voltage + VDD.
[0038]
Therefore, even if the power supply voltage + VDD supplied to the push-pull circuits 15A and 15B fluctuates due to the item (1), this is offset by the change in the attenuation of the variable attenuator circuit 12, and the signal voltage supplied to the speaker 17 The size is kept constant. At this time, since the fluctuation of the power supply voltage + VDD is very slow as several Hz as described above, the signal level in the variable attenuator circuit 12 can be controlled without delay.
Therefore, even if the power supply voltage + VDD varies according to the item (1), the output of the speaker 17 does not vary.
[0039]
Thus, according to the power amplifier 10 described above, although there is no feedback, even if voltage fluctuation or noise caused by the items (1) and (2) occurs in the output voltage of the battery 21, it is supplied to the speaker 17. Therefore, the output signal voltage does not fluctuate and noise does not get on, so that it is possible to obtain a sound output with excellent sound quality. Further, since the power amplifier 10 is non-feedback, even if the speaker 17 vibrates due to running of the vehicle, there is no deterioration in sound quality.
[0040]
In addition, no special hardware is required for this purpose, and a stabilized power supply circuit such as a DC / DC converter circuit is required to stabilize the power supply voltage + VDD supplied to the push-pull circuits 15A and 15B. Therefore, an increase in cost can be minimized.
[0041]
Incidentally, Te above smell, PWM signal Din may also be, eg, PNM signal.
[0042]
[List of abbreviations used in this specification]
A / D: Analog to Digital
BTL: Balanced TransformerLess
D / A: Digital to Analog
DC / DC: Direct Current to Direct Current
FET: Field Effect Transistor
MOS-FET: Metal Oxide Semiconductor type FET
PNM: Pulse Number Modulation
PWM: Pulse Width Modulation
S / N: Signal to Noise ratio
[0043]
【The invention's effect】
According to the present invention, the sound quality of the non-feedback power amplifier is not impaired, and even the non-feedback power amplifier is not affected by the back electromotive voltage generated in the speaker due to car vibration, Deterioration of sound quality can be prevented.
[0044]
In addition, no special hardware is required for this purpose, and a stabilized power supply circuit such as a DC / DC converter circuit is not required to stabilize the power supply voltage. It can be minimized, and space can be saved.
[Brief description of the drawings]
FIG. 1 is a system diagram showing an embodiment of the present invention.
FIG. 2 is a waveform diagram for explaining the present invention.
FIG. 3 is a waveform diagram for explaining the present invention.
FIG. 4 is a diagram of observed waveforms for explaining an application result of the present invention.
FIG. 5 is a diagram of observed waveforms for explaining the application result of the present invention.
FIG. 6 is a connection diagram for explaining the present invention.
FIG. 7 is a diagram of observed waveforms for explaining the present invention.
FIG. 8 is a waveform diagram for explaining the present invention.
FIG. 9 is a diagram of observed waveforms for explaining the present invention.
FIG. 10 is a diagram of observed waveforms for explaining the present invention.
FIG. 11 is a diagram of observed waveforms for explaining the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Power amplifier, 11 ... Input terminal, 12 ... Variable attenuator circuit, 13 ... PWM modulation circuit, 14A and 14B ... Drive circuit, 15A and 15B ... Push-pull circuit, 16A and 16B ... Low pass filter, 17 ... Speaker, 21 ... Car battery, 23 ... DC line filter, 30 ... control circuit, 31 ... A / D converter circuit, 32 ... comparison circuit, 33 ... operation key

Claims (1)

A non-feedback power amplifier that is installed in the vehicle and performs class D amplification,
A variable attenuator circuit for controlling the level of the audio signal supplied to the power amplifier ;
A control circuit that controls the volume by controlling the variable attenuator circuit;
A DC line filter to which an output voltage of a battery mounted on the vehicle is supplied;
And a voltage comparison circuit for comparing a reference value the magnitude of the output voltage of the DC line filter,
While supplying the output voltage of the DC line filter to the power amplifier as its power supply voltage,
A power amplifier device in which a comparison output of the voltage comparison circuit is supplied as a control signal to the variable attenuator circuit so as to cancel out a level fluctuation of an output signal of the power amplifier caused by a fluctuation of an output voltage of the battery.

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