Noise suppression circuit for amplifier circuit

Abstract

To provide a pop noise suppression circuit that suppresses pop noise caused when the power supply to an amplifier circuit is turned off.SOLUTION: A circuit is equipped with a plurality of parallel power supplies connected in parallel having time circuits with different voltage values and different voltage drop rates after an amplifier circuit 2 is powered off, and a photocoupler 3 connected in series to the parallel power supplies and consisting of light emitting means 31 and light receiving means 32 that light up when the voltage value reverses after the power is turned off, and a current signal generated by the conduction of the light receiving means 32 is applied as an input point bias change signal for the amplifying element of the power amplifier circuit 24 that constitutes the amplifier circuit 2.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a noise suppression circuit for an amplifier circuit, and more particularly to a noise suppression circuit for an audio amplifier. This invention relates to a noise suppression circuit for an amplifier circuit, which reduces acoustic noise generated from a speaker due to unwanted oscillations that occur when the amplifier is off. [Background technology]

[0002] In various amplifier circuits, when the power is turned off, the effect of the feedback loop becomes unstable due to a drop in the power supply voltage, causing the amplifier circuit to lose control, resulting in momentary unwanted oscillations that cause acoustic noise, and the unwanted oscillation waveform signal (noise signal) is output to the load, such as speakers, generating unpleasant sounds and unwanted data, or so-called pop noise. This pop noise occurs even when the power is turned on, before the feedback has fully risen. In the following, the unpleasant sounds and the like may be referred to as pop-up noise. In the following explanation, the means for suppressing pop noise when the power is turned off will be mainly explained, but it is also effective in suppressing pop noise when the power is turned on.

[0003] In particular, in audio systems, suppressing pop noise when the power is turned off is particularly important in order to prevent the wonderful afterglow that follows listening to voices, music, etc. from being interrupted by pop noise that occurs when the amplifier is turned off. Note that pop noise is not limited to audio systems, but can also be mixed into measurement results as unnecessary data in various measuring devices, causing effects such as a deterioration in measurement accuracy.

[0004] Even if we focus only on audio systems, conventional technologies for suppressing or preventing this type of pop noise caused by unwanted oscillations often have a configuration that detects the ON / OFF state of a power switch and prevents the pop noise from being transmitted to a speaker using a mechanical relay or an electronic switch (semiconductor switch), as disclosed in Patent Documents 1 and 2. Patent Document 3 discloses a configuration in which a pop noise prevention circuit that detects the operation signal of the power switch and operates is provided in parallel with the speaker, thereby suppressing pop noise caused by unwanted oscillation signals (unwanted signal waveforms). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2013-175822 A [Patent Document 2] JP 2005-538626 A [Patent Document 3] JP 2008-99285 A Summary of the Invention [Problem to be solved by the invention]

[0006] The above-mentioned conventional pop noise prevention means not only requires a large circuit scale for suppressing pop noise, but also has disadvantages such as poor long-term stability and a large mounting area for the above-mentioned mechanical relay circuit. Furthermore, when a semiconductor switch is used, the occurrence of unwanted high-frequency oscillations that are a source of pop noise cannot be completely suppressed due to the parasitic capacitance of the semiconductor, making it difficult to completely prevent pop noise. Furthermore, the occurrence of the above-mentioned unwanted oscillations can cause malfunctions or breakdowns in devices connected to the amplifier circuit, and if the connected devices are headphones or speakers, unpleasant noises will be output. Therefore, it is essential that the amplifier circuit itself does not fundamentally cause unwanted oscillations.

[0007] An object of the present invention is to provide a pop noise suppression circuit for an amplifier circuit which has a simple circuit configuration and which is configured so that signals due to unwanted oscillations that cause pop noise when the power is turned off (including when the power is on) are not transmitted to a load (such as a speaker) and which does not adversely affect other circuit devices connected to the amplifier circuit. [Means for solving the problem]

[0008] In order to achieve the above object, the present invention is configured to forcibly bring the bias current of the power transistor for the output stage of the amplifier circuit closer to 0 (zero) when the power supply voltage begins to drop when the power is turned off, thereby increasing the output impedance and suppressing the transmission of unwanted oscillation signals to a load such as a speaker.

[0009] There are multiple power supplies with different voltages inside an amplifier circuit, and each power supply voltage and the fall time when the power is turned off have different time constants. If there is a high-voltage power supply with a fast fall time and a low-voltage power supply with a slow fall time, the high-voltage power supply and the low-voltage power supply will switch after a certain time constant has elapsed. If the power supply is a negative (-) power supply, this is the time when the rising voltages switch. The following explanation is for the case where the power supply voltage is a positive (+) power supply.

[0010] The present invention takes advantage of this fact and is characterized by a configuration in which the amplifier circuit does not operate during normal operation, but operates when the different voltage drop curves determined by the time constants of high voltage and low voltage switch positions after the power is turned off, thereby suppressing the transmission of unwanted oscillation signals to the load, thereby preventing the generation of so-called pop noise.

[0011] The following is a configuration example that explains the above-mentioned features of the present invention in more detail. For easy understanding, the reference symbols used in the drawings in the embodiments described below are also included. That is, (1) A noise suppression circuit for an amplifier circuit, which processes an input signal 20 and outputs the signal to a load 30, suppresses so-called pop noise (also called pop-up noise) that occurs when an unwanted oscillation signal caused by instability of a negative feedback function of the amplifier circuit 2 that is turned off is transmitted to the load 30, The noise suppression circuit 1 includes a plurality of parallel power supplies connected in parallel and each having a time circuit (such as a time constant circuit) having a different voltage value and a different voltage drop rate (voltage drop speed) after the power supply of the amplifier circuit 2 is turned off, and a photocoupler 3 connected in series to the parallel power supplies. The photocoupler 3 comprises a light emitting means 31 which is turned on when the voltage value reverses after the power is turned off, and a light receiving means 32 which is turned on when it receives the light emitted by the light emitting means 31. The configuration is characterized in that a current signal generated by the conduction of the light receiving means 32 is applied as an input point bias change signal of an amplifier element of a power amplifier circuit 24 constituting the amplifier circuit 2.

[0012] (2) The photocoupler 3 is characterized in that the light emitting means 31 is an LED (Light Emitting Diode) and the light receiving means 32 is a phototransistor (PT).

[0013] (3) The photocoupler 3 is characterized in that the light emitting means 31 is an LED, and the light receiving means is a photodiode (PD).

[0014] (4) The above-mentioned multiple parallel power supplies are characterized in that they utilize a portion of the power supply line that constitutes the amplifier circuit.

[0015] (5) The plurality of parallel power sources are characterized in that they are electrically independent of the power source constituting the amplifier circuit.

[0016] (6) The plurality of parallel power sources are characterized in that they are dual-system power sources with different voltage values.

[0017] In each of the above configurations, the pop noise control circuit 1 does not operate until the voltage applied to the LED, which is the light emitting means 31, becomes higher than its forward voltage Vf. Therefore, it is preferable to use an LED with a low forward voltage.

[0018] The present invention is not limited to the above configurations or the configurations of the examples described in the embodiments below, and it goes without saying that various modifications are possible within the scope of the technical concept of the present invention. Effect of the Invention

[0019] The present invention controls the output of the amplifier circuit by utilizing the decrease in the charge remaining in the power supply capacitor over time after the power supply is turned off and the high and low voltages are reversed. Note that the additional power supply for the noise control circuit may use the voltage line of the amplifier circuit, or may be a power supply electrically independent of the amplifier circuit. When the amplifier circuit is operating normally, the noise suppression circuit according to the present invention is electrically insulated from the amplifier circuit and therefore does not affect the operation of the amplifier circuit. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a block diagram illustrating a first embodiment of a noise suppression circuit for an amplifier circuit according to the present invention. [Diagram 2] FIG. 2 is a circuit diagram showing a portion of the configuration of FIG. 1 in more detail with functional blocks. [Diagram 3] An explanatory diagram of the voltage drop curves of the terminals of the amplifier circuit of the first time constant circuit and the second time constant circuit that make up the time circuit of Figure 2, starting from the time 0 when the power is turned off. [Figure 4] FIG. 10 is an explanatory diagram of a light emitting means and a light receiving means employed in a pop noise suppression signal generating circuit according to a second embodiment of the present invention. [Figure 5A] FIG. 1 is a partial circuit diagram illustrating a specific example of the configuration of an amplifier equipped with a noise suppression circuit according to the present invention; [Figure 5B] FIG. 2 is another partial circuit diagram illustrating a specific example of the configuration of an amplifier equipped with a noise suppression circuit according to the present invention; [Figure 5C] FIG. 1 is a partial circuit diagram illustrating a specific example of a configuration including a noise suppression circuit according to the present invention; [Figure 5D] FIG. 5B is a combined view of FIG. 5A, FIG. 5B, and FIG. 5C for explaining a specific configuration example including a noise suppression circuit according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of a noise suppression circuit for an amplifier circuit according to the present invention will be described in detail with reference to the accompanying drawings. EXAMPLES

[0022] FIG. 1 is a block diagram for explaining a first embodiment of a noise suppression circuit for an amplifier circuit according to the present invention, and an electronic device to which the noise suppression circuit is applied is an audio system. In Fig. 1, reference numeral 1 denotes a pop noise suppression signal generating circuit, 2 denotes an amplifier circuit, 23 denotes a bias circuit that controls the bias of a power amplifier circuit that constitutes the amplifier circuit, 3 denotes a photocoupler, 31 denotes an LED as a light emitting element, and 32 denotes a phototransistor (PT) as a light receiving element. In the following, the reference numerals are simply indicated as symbols.

[0023] Reference numeral 4 denotes a time circuit (also called a time delay means or a time constant circuit), 41 denotes a first time circuit, and 42 denotes a second time circuit. Reference numeral 20 denotes an input terminal connected to a signal source, and 25 denotes an output terminal for an amplified signal.

[0024] A reproduced audio signal from a recording means or the like that has recorded voice or music, which is the signal source of the amplified signal, or a source signal (electrical signal) such as live voice or music collected on-site, is applied to an input terminal 20. The audio signal or the like applied to the input terminal 20 is amplified as required by the amplifier circuit 2 and is supplied from an output terminal 25 to an output load (such as a speaker).

[0025] This type of amplifier circuit is generally made up of a system organized into functional blocks called a preamplifier, equalizer, main amplifier, etc., but here we will explain the general configuration.

[0026] The amplifier circuit 2 is composed of a signal input circuit that takes in a source signal from an input terminal 20, a voltage amplifier circuit, and a power amplifier circuit. The signal input circuit performs frequency adjustment and line level processing, and the signal that has been voltage amplified to a line level is given a predetermined energy for driving a load by power amplification and transmitted to an output load.

[0027] Fig. 2 is a circuit diagram showing a part of the configuration in Fig. 1 in more detail, with functional blocks. The same reference numerals as in Fig. 1 indicate the same functional parts. In the amplifier circuit 2, which is assumed to be an audio amplifier, a signal source applied to an input terminal 20 is subjected to frequency adjustment processing and the like in a signal input circuit 21. A voltage amplifier circuit 22 amplifies the signal to a line level. Then, the signal voltage-amplified to the line level is energized by power amplification to a predetermined power for driving a load, and is output to an output load (speaker) 25.

[0028] The pop noise suppression signal generating circuit 1 is provided with a first time constant circuit 41 and a second time constant circuit 42 as a time circuit 4 for each of the voltages V1 and V2. <V2、C1R1<C2R2としてある。 The resistor R3 is a resistor for preventing damage to the LED 31 constituting the photocoupler 3, and the diode D1 is a backflow prevention diode.

[0029] When the power switch of the amplifier circuit 2 is turned OFF, the voltage applied to the terminal of the LED 31 of the photocoupler 3 decreases according to a time constant determined by the first time constant circuit 41 and the second time circuit 42 (R (internal resistance value of the power supply)) and C (residual charge of the decoupling capacitor)).

[0030] Figure 3 is an explanatory diagram of the terminal voltage drop curves from the time 0 when the power is turned off for the amplifier circuits of the first time constant circuit and the second time constant circuit that constitute the time circuit of Figure 2. The vertical axis of Figure 3 indicates the terminal voltages V1 and V2, and the horizontal axis indicates the elapsed time T from the time 0 when the power is turned off.

[0031] When positive power supplies are used for the power supplies V1 and V2 of the timing circuit, it is on the positive voltage side of FIG. 3, that is, the upper side of the time T line. When negative power supplies are used for the power supplies V1 and V2 of the timing circuit, it is on the negative power supply side of FIG. 3 (the lower side of the time T line). Here, the case of using a positive power supply will be described. When using a negative power supply, it may be read as the reverse operation of the case of using a positive power supply.

[0032] The two voltages on the upper side of the time T line in FIG. 3 are V1 < V2, and the time constants τ1 and τ2 composed of the capacitance C (the capacitance of the decoupling capacitor) and the resistance R (here, the internal resistance of the power supply) of the two time constant circuits are τ1 (C1R1) < τ2 (C2R2). Therefore, the terminal voltage drop curves of the first time constant circuit and the second time constant circuit cross at time t, and the subsequent terminal voltages are reversed.

[0033] As a result, a forward voltage Vf is applied to the LED31 constituting the photocoupler 3 in FIG. 2, and it emits light. In response to this light emission, the phototransistor 32 conducts, and the conduction current operates a bias circuit 23 that controls the bias of the signal input point of the power amplification transistor constituting the power amplification circuit 24 of the power amplification circuit 2 of the amplification circuit 2, thereby reducing the gain of the power amplification circuit 24 (approaching 0). Thereby, unnecessary oscillation signals transiently generated when the power supply of the amplification circuit is turned off are amplified and supplied to the output load (such as a speaker), and the output of an unpleasant pop sound (noise) is suppressed.

[0034] Note that since the capacitance error of the electrolytic capacitor constituting the power supply circuit is large, as a technique for further improving the effect of the present invention, the pop noise suppression signal generation circuit according to the present invention is incorporated into both the positive power supply and the negative power supply, and the light receiver sides of the photocouplers are connected in parallel. By doing so, the period during which the function of preventing unnecessary oscillation works becomes longer, and the pop noise suppression effect can be significantly improved.

[0035] In this embodiment, a bias circuit 23 that changes the bias (the drive current value of the amplifying element) is provided at the input point of a power amplifier circuit 24, which is a circuit that performs power amplification. The bias circuit 23 controls the bias current with a signal from the pop noise suppression signal generation circuit 1 to change the operating point of the amplifying element to be close to 0 V, thereby suppressing the power amplification function of the power amplifier circuit.

[0036] Furthermore, in this embodiment, the two power supplies, the high-voltage power supply V2 and the low-voltage power supply V1 of the pop noise suppression signal generation circuit 1, are obtained from the power supply line of the amplifier circuit 2, so there is no need to prepare a separate power supply for the added pop noise suppression signal generation circuit 1, and the circuit size can be minimized.

[0037] With this configuration, the operating point of the bias circuit 23 is changed by a signal (pop noise suppression signal) generated from the photocoupler 3, which lights up when the high and low voltage values ​​of the multiple (two in this embodiment) timing circuits 41, 42 that operate when the power is turned off, and the transmission of unwanted oscillation signals that become pop noise to the load, that is, the speaker, is suppressed. EXAMPLES

[0038] In the above-mentioned embodiment 1, a so-called photocoupler element was used in which the light emitting means 31 and the light receiving means 32 were integrated into one chip. However, separate light emitting means and light receiving means may be used as long as they are capable of electrically insulating the input side and the output side and transmitting signals instantaneously. FIG. 4 is an explanatory diagram of a light emitting means and a light receiving means employed in a second embodiment of the pop noise suppression signal generating circuit according to the present invention, and the configuration other than the light emitting means and the light receiving means is the same as that shown in FIG.

[0039] Figure 4(a) shows an example in which an LED is used as the light emitting means and a phototransistor (PT) is used as the light receiving means, and the two are arranged at an appropriate distance D. Theoretically, there is no limit to the distance D as long as the environment is free from disturbance by external light. In addition, in Fig. 4(b), an LED is used as the light emitting means and a photodiode (PD) is used as the light receiving means. As in Fig. 4(a), the two are placed at an appropriate distance D, and theoretically there is no limit to the distance D as long as the environment is free from disturbance by external light.

[0040] According to this embodiment, only the light emitting means 31 is provided in the pop noise suppression signal generating circuit 1, and the light receiving means 32 is provided on the amplifier circuit 2 side. The light emitting means 31 can be installed at any position without considering the above-mentioned distance D, and the light can be introduced into the light receiving means 32 by a light guiding means such as an optical fiber, or the two can be electrically insulated from each other. EXAMPLES

[0041] In the above-described embodiments, the power supply to the pop noise suppression signal generating circuit 1 is supplied from the amplifier circuit 2 side. In this embodiment, the power supply for the pop noise suppression signal generating circuit 1 is independent from the power supply line constituting the amplifier circuit 2. A description thereof will not be given in particular with reference to the drawings. With this configuration, it is possible to completely isolate the pop noise suppression signal generating circuit 1 not only from the pop noise suppression signal but also electrically from the amplifier circuit 2, and it is also possible to retrofit the pop noise suppression signal generating circuit to an existing amplifier circuit with a simple procedure.

[0042] Figures 5A, 5B and 5C show circuit examples of an audio amplifier (hereinafter referred to as an amplifier) ​​to which the noise suppression circuit of the amplifier circuit according to the present invention is applied, and Figure 5D shows a combination of Figures 5A, 5B and 5C. This circuit example corresponds to the above-mentioned embodiment 1. A specific circuit to which the second and third embodiments are applied can be realized by modifying the configuration of the photocoupler portion or the power supply portion according to the contents explained in each embodiment.

[0043] 5A and 5B of this amplifier circuit correspond to the portion indicated by the pop noise suppression signal generating circuit 1 and the amplifier circuit 2 in Fig. 2. Fig. 5A shows a specific embodiment of the pop noise suppression signal generating circuit 1, the bias circuit, and the power amplifier circuit 24. FIG. 5C shows an example of the circuit configuration after the secondary winding of the power transformer of the power supply circuit, which supplies the necessary power to the circuits shown in FIGS. 5A and 5B.

[0044] Basically, the signal input circuit 21, voltage amplifier circuit 22, and power amplifier circuit 24 of the audio amplifier use known circuit configurations, and the generation of pop noise is suppressed by providing the output signal of the pop noise suppression signal generation circuit 1 to the bias circuit 23 of the power amplifier circuit 24 shown in FIG. 2 to bring the bias close to 0.

[0045] The bias circuit 23 is a circuit for stably supplying an appropriate idling current to the output power transistor Q, and functions to prevent the characteristics of the signal amplified by the voltage amplifier circuit 22 from changing due to the influence of the load connected to the output terminal 25. In addition, in FIG. 5A, the power supply ON / OFF detection circuit and the function of driving the pop noise suppression signal generation circuit 1 are integrated into one circuit configuration.

[0046] In the above embodiments of the present invention, a CR time constant circuit is used in the timing circuit, but a CL time constant circuit can be used instead. If the same effect as that obtained by increasing the output impedance of the power transistor for the output stage of the amplifier circuit can be obtained, it is also possible to operate the signal input circuit or the voltage amplifier circuit by a light receiving element. Additionally, appropriate delay means may be used in the timing circuit. In addition, since the amplifier circuit starts up in the reverse sequence when the power is turned on compared to when the power is turned off, the present invention is also expected to have the effect of suppressing unstable operation of the amplifier circuit that occurs during the transition when the power is turned on. [Explanation of symbols]

[0047] 1. Pop noise suppression signal generation circuit 2. Amplification circuit 20: Input terminal for connecting to a signal source 21 Signal input circuit 22 Voltage amplifier circuit 23 Bias circuit 24 Power amplifier circuit 25: Amplified signal output terminal 3. Photocoupler 31 Light emitting element 32 Light receiving element 4. Time Circuit 41... First time circuit 42 Second Time Circuit

Claims

1. A noise suppression circuit for an amplifier circuit that constitutes an apparatus that processes an input signal and outputs it to a load, suppressing noise generated by an unwanted oscillation signal generated by destabilization of a negative feedback function of the amplifier circuit when the amplifier circuit is powered off and transmitted to the load, the noise suppression circuit includes a plurality of parallel power supplies connected in parallel, each having a time circuit with a different voltage value and a different voltage drop rate after the power supply of the amplifier circuit is turned off, and a photocoupler connected in series to the parallel power supplies; The photocoupler comprises a light emitting means which lights up when the voltage value reverses after the power is turned off, and a light receiving means which becomes conductive when it receives light emitted from the light emitting means, A noise suppression circuit for an amplifier circuit, characterized in that a current signal generated by the conduction of said light receiving means is applied as an input bias change signal for an amplifier element of a power amplifier circuit constituting said amplifier circuit.

2. 2. The noise suppression circuit for an amplifier circuit according to claim 1, wherein said photocoupler is configured with an LED as said light emitting means and a phototransistor as said light receiving means.

3. 2. The noise suppression circuit for an amplifier circuit according to claim 1, wherein said photocoupler is configured with an LED as said light emitting means and a photodiode as said light receiving means.

4. 4. The noise suppression circuit for an amplifier circuit according to claim 1, wherein the plurality of parallel power sources utilize a part of a power line constituting the amplifier circuit.

5. 4. The noise suppression circuit for an amplifier circuit according to claim 1, wherein the plurality of parallel power supplies are electrically independent from a power supply constituting the amplifier circuit.

6. 4. The noise suppression circuit for an amplifier circuit according to claim 1, wherein the plurality of parallel power supplies are dual-system power supplies having different voltage values.

Images