JP5050231B2 - Audio amplifier hum noise canceling device - Google Patents

Description

  The present invention relates to an apparatus for canceling hum noise of an audio amplifier and a low-frequency amplifier, and more particularly to canceling hum noise generated when a direct-heated vacuum tube is used with an AC ignition as an amplification element of an audio amplifier.

In recent years, transistors and integrated circuits (ICs) have been used in audio amplifiers, and digitization techniques have also been adopted. On the other hand, the musicality of vacuum tube audio amplifiers has been supported by many enthusiasts, and its manufacture continues today.
An audio amplifier needs to amplify a signal having a dynamic range close to 100 dB as faithfully as possible, but its maximum output is several tens of watts at most. In such a case, reducing the minimum output level of the amplifier, that is, the level of hum noise is an effective means for widening the dynamic range. Although the required dynamic range depends on the type of performance, the recording method of the audio signal, and the like, in reality, if it is 80 dB or more, it is considered that realistic music reproduction can be performed.

  The hum noise is a continuous noise having a substantially constant amplitude and a relatively low frequency having the same frequency as the frequency of the AC power supply or an integral multiple of the frequency. The hum noise includes harmonics and has a waveform like a distorted sine wave. The main causes of hum noise in audio amplifiers are the electrical and magnetic coupling between components and wiring, the residual ripple component of the smoothing circuit of the DC power supply, and the case of AC ignition of the direct heat vacuum tube amplifier. The fundamental frequency of hum noise is 50 Hz / (or) 60 Hz, but it is 100 Hz / 120 Hz in the case of a residual ripple component and an AC ignition directly heated vacuum tube. As described above, there are many cases where hum noise is generated, but in general, frequency components of 50 Hz / 60 Hz and 100 Hz / 120 Hz may be canceled.

  Here, as a specific example, if the output of the audio amplifier is 10 W and the output is applied to a speaker with an impedance of 8Ω, the voltage between the speaker terminals is 8.9V. If the level of the hum noise of this amplifier is 3 mV, the ratio between the maximum and minimum levels of the amplifier output is about 69 dB, and the dynamic range is insufficient. In this case, if the hum noise level can be reduced to 0.5 mV, the dynamic range becomes about 85 dB, and the music can be reproduced more faithfully.

  On the other hand, a fundamentally important condition for the level of hum noise in audio amplifiers is that hum noise cannot be heard from the sound emitted from the speaker under normal listening conditions, but in the case of an amplifier using an AC ignition directly heated vacuum tube Has a high level, often in the range of several mV to 10 mV. In this case, the hum noise level that is allowed varies strictly depending on the speaker conversion efficiency, listening position, listening room structure, and the like.

  Considering the above two conditions, the hum noise level of the audio amplifier needs to be about 0.5 mV or less between the output terminals of the power amplifier. It should be noted that the dynamic range of human hearing is about 120 dB, and if hum noise is large, cross modulation distortion may occur. Therefore, it is preferable to reduce hum noise as much as possible.

Next, two problems specific to the vacuum tube audio amplifier will be described. The first problem is that hum noise is likely to occur because an anode high-voltage DC power supply circuit and a heater power supply circuit are required.
The second problem is that the direct heat vacuum tube is said to have the best sound quality when the filament is AC-ignited and used as a non-feedback or low-feedback single amplifier. Noise is generated. It occurs for the following two reasons.

  The first cause is electrical. A typical vacuum tube amplifying action is to change the voltage fluctuation between the grid and the cathode into a change in anode current. However, a direct-heated vacuum tube operates as an AC power supply applied to the filament because the filament operates as a cathode. The waveform is mixed in the audio signal, and the fundamental frequency of this component is the same as that of the AC power supply and is 50 Hz / 60 Hz. Since this hum noise appears in opposite phases at both ends of the filament, it can be reduced to some extent by a general hum balancer circuit.

The second cause is electrical and thermal, and the filament is heated and cooled periodically. That is, the absolute value of the current flowing through the filament is maximized when the voltage of the AC power supply is a positive (+) maximum value and a negative (−) minimum value. At this time, the temperature of the filament is also maximized, and the amount of thermoelectrons emitted from the filament (anode current) is also maximized. The current flowing through the filament becomes the minimum value when the voltage of the AC power supply becomes 0V. The temperature of the filament is independent of the direction of the current, and a similar temperature change appears twice during one cycle (20 ms / 16.7 ms) of the AC power supply. Therefore, the fundamental frequency of hum noise due to the second cause is 100 Hz / 120 Hz which is twice the power supply frequency. The amplitude and waveform of this hum noise depends on the voltage standard of the filament, the filament material, structure, heat capacity, uniformity of temperature distribution, thermal conductivity coefficient between the filament and surrounding objects, and the amplitude and waveform of the AC ignition power supply. Change. In this case, the amplitude of the hum noise increases as the voltage at both ends of the filament increases, and is generally about several mV to 10 mV. Further, as an extreme example, when the heat capacity of the filament is very large, hum noise due to the second cause hardly occurs. Since this noise appears in the same phase at both terminals of the filament, it cannot be removed by the hum balancer circuit.
For this reason, the hum noise generated by two different causes appears in the output of the direct-heated vacuum tube amplifier ignited by alternating current. To cancel the hum noise, Optimal separate processing is required.

  In order to cancel the hum noise, it is necessary to add a cancel signal having the same frequency to the audio signal by controlling the amplitude and phase so that the hum noise is canceled at the output terminal of the audio amplifier. The phase relationship between the waveform of the AC power supply and the fundamental frequency component of the hum noise varies depending on the circuit of the audio amplifier, the component arrangement, and the structure of the filament in the case of a direct heat vacuum tube amplifier. Further, in the case of a stereo signal, the phase is different between the left and right channels of the amplifier. Further, the phase of the cancel signal itself also changes due to the cancel signal injection circuit.

There are three conventional methods for canceling or reducing hum noise in audio amplifiers. It is applied to a vacuum tube audio amplifier.
In the first method, in order to cancel the 120 Hz hum noise, which is twice the frequency of the AC power supply, the AC power supply used to ignite the filament is full-wave rectified to generate a waveform with a fundamental frequency of 120 Hz. Then, it is applied as a cancel signal to a control grid of a direct heat vacuum tube through a variable resistor or the like. In this case, the phase of the cancel signal is adjusted only by selecting the same phase or the opposite phase. In this document, 60 Hz hum noise of the AC power supply frequency is suppressed only by the hum balancer circuit (see Non-Patent Document 1).

  The direct heat vacuum tube amplifier is said to have the best sound quality when AC ignition is performed, but the second method is most commonly used to reduce the hum noise of the direct heat vacuum tube generated by AC ignition. Inevitably, the filament is ignited by direct current. This can reduce only the hum generated in the amplification stage. In this case, hum noise is generated due to the residual ripple component of the direct current power supply for the filament, which can be reduced to some extent by the hum balancer circuit (see Non-Patent Document 2).

  The third method uses hum noise generated by a separate AC ignition direct heat vacuum tube as a cancel signal in order to reduce hum noise (100 Hz / 120 Hz) of the direct heat vacuum tube generated by AC ignition. It is. A DC voltage of several tens of volts or more is applied to the anode of the AC ignition directly heated vacuum tube. In this case, the phase of the cancel signal is adjusted by selecting only the same phase or opposite phase using an inverting amplifier or the like (see Patent Document 1).

JP 2001-111355 A Steve Bench, “Effects of AC Heating Power Applied to Directly Heated Triodes”, http: // members. aol. com / sbench / humbal / html, 1999 Edited by Radio and Experiment Editing Department, “300B Power Amplifier Masterpiece Selection”, published by Seibundo Shinkosha, January 10, 1998, p. 152-153

However, since the technique disclosed in Non-Patent Document 1 has no function of finely adjusting the phase of the cancel signal, there is a problem that hum noise cannot be canceled sufficiently.
And since a full-wave rectified wave is used as the cancellation signal, even if the fundamental frequency component of the hum noise can be reduced to a certain extent, a large amount of harmonic components and irregular noise components included in the cancellation signal , And flicker of AC power supply (irregular fluctuation of AC power supply voltage of several Hz to several tens Hz) etc. are added to the audio signal, which distorts the waveform of the audio signal and deteriorates its sound quality. There was a problem of reducing the dynamic range. Theoretically, the total harmonic distortion rate of the full-wave rectified wave, which is a cancel signal, is about 23% (−13 dB), and the hum noise level of 120 Hz is assumed when the level of the hum noise is 3.1 mV. Even if the component can be canceled, since the harmonic component of 23% (−13 dB, effective value 0.71 mV) of the cancel signal is added to the audio signal, it can be estimated that the sound quality is deteriorated. Here, the effective value means a root-mean-square value.
Further, the technique of Non-Patent Document 1 has a problem that the original sound quality of the direct heat vacuum tube is deteriorated because the full-wave rectifier circuit is connected to the filament of the direct heat vacuum tube to which the audio signal is applied. .
Furthermore, this technique has a problem that it is difficult to effectively remove the hum noise of the AC power source frequency (60 Hz) because it is suppressed only by the hum balancer circuit.

The technique disclosed in Non-Patent Document 2 inevitably performs DC ignition to reduce the hum noise of the AC ignition direct heat vacuum tube amplifier. However, since the filament of a direct-heat vacuum tube has the functions of both a heater and a cathode and an audio signal is directly applied to it, if a power source for direct current ignition is connected to the filament, There was a problem of deteriorating the sound quality.
In addition, since the filament is dc-ignited, the voltage distribution between the anode and each part of the filament is not uniform, so that when the anode current flows, a constant temperature gradient is generated on the filament and the filament is shortened. There was a problem that there was a possibility of extending the life.

Next, the technique described in Patent Document 1 has a problem that the hum noise cannot be sufficiently canceled because there is no function for finely adjusting the phase of the cancel signal.
Since the cancel signal contains noise, even if the fundamental frequency component of the hum noise can be reduced to a certain extent, the harmonic component, irregular noise component, and flicker of the AC power source included in the cancel signal are reduced. Etc. are added to the audio signal to distort the waveform of the audio signal to deteriorate its sound quality and to reduce the dynamic range of the amplifier.
In the example of measured data, the level of hum noise at 100 Hz when the direct heat vacuum tube DA30 is AC-ignited is about 3.1 mV, and its total harmonic distortion rate is about 8.4% (−22 dB). Even if the fundamental frequency component is canceled, it can be estimated that the harmonic component of 8.4% (−22 dB, effective value 0.26 mV), irregular noise component, and the like are added to the audio signal to deteriorate the sound quality. .
In the technique of the above-mentioned Patent Document 1, a DC power supply of several tens of volts or more is required for the anode of the AC ignition directly heated vacuum tube that generates the cancel signal, so that it is difficult to configure the device as an external type. There was a problem.

  Furthermore, the conventional hum balancer circuit widely used in vacuum tube audio amplifiers can remove only the hum noise generated in the amplification stage to which it is connected. There is a problem that the hum noise generated in the phase cannot be removed.

  The present invention is intended to solve such a problem that the conventional technology has, and by generating a distorted sine wave as a cancel signal, unnecessary frequency components and It is an object of the present invention to provide a canceling apparatus for a plurality of fundamental frequency components and harmonic components of hum noise having any phase of several mV to 10 mV having different generation causes without adding noise.

In order to solve the above problems, the present invention provides a synchronization signal generating means for generating a cancel signal synchronized with an AC power source causing hum noise, and a cancel signal source in synchronization with the synchronization signal. Cancel signal source generating means for generating a low-distortion sine wave cancellation for removing harmonics, irregular noise, AC power supply flicker, etc. from the cancel signal source in order to make the cancel signal source a low-distortion sine wave a signal generating means, said variable phase shifter means for providing a phase shift amount required to cancel the hum at the output of the low distortion sine wave cancel signal generating means, hum an output of said variable phase shifter means a variable gain adder means for adding to adjust the amplitude necessary to cancel the cancellation signal for injecting the output of the variable gain adder means to an audio amplifier Characterized in that it comprises an injection means.
In the present invention, the cancel signal source generating means includes a frequency doubler circuit, a Wien bridge oscillation circuit, or a PLL frequency synthesizer circuit.
Furthermore, the present invention is characterized in that a low-pass filter and a band-pass filter are provided as the low distortion sine wave cancel signal generating means.
The configuration of the low distortion sine wave canceling signal generating means includes a band pass filter and a low pass filter in order to have an optimum band pass characteristic for making the output of the cancel signal source generating means a low distortion sine wave. Further, a high-pass filter or a band elimination filter may be realized by appropriately connecting them in cascade, and in the simplest case, a single band-pass filter may be used.

As described above, according to the present invention, it is possible to generate a low distortion sine wave cancel signal as a signal for canceling hum noise, and to adjust the phase and amplitude thereof. The apparatus can be easily realized as an external type or a built-in type inside an amplifier by an analog signal processing circuit using a general-purpose operational amplifier that operates stably at a low price.
As a result, the hum noise of about several mV to 10 mV can be canceled to a level of about 0.5 mV or less without adding unnecessary frequency components and irregular noise that deteriorate the sound quality to the audio signal. There is an effect that the dynamic range of the audio amplifier can be widened by several tens dB or more.
In particular, a direct-heat vacuum tube single amplifier (300B, DA30, PX25, 845, etc.) that could not be ignited by alternating current because of a lot of hum noise can be used by igniting alternating current. There is an effect that the original high sound quality of the vacuum tube can be enjoyed.
Further, according to the present invention, hum noise such as a low-frequency amplifier for an electronic measuring device can be canceled, so that the measurement accuracy can be improved.

  Hereinafter, a method and apparatus for canceling hum noise of an audio amplifier according to an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing a method for canceling hum noise of an audio amplifier according to the first embodiment of the present invention. This figure shows a case where the frequency of the cancel signal (hereinafter referred to as cancel frequency) is one type and the audio signal is one channel.
In FIG. 1, the connection terminal 11a is connected to an AC power source that causes hum noise.
The synchronization signal generating means 12 performs processing such as voltage transformation, frequency multiplication, and waveform shaping on the AC power input from the connection terminal 11a.
The cancel signal source generation unit 13 generates a cancel signal source synchronized with the synchronization signal output from the synchronization signal generation unit 12.
The low distortion sine wave cancellation signal generation unit 14 removes harmonics, irregular noise, flicker, and the like from the output of the cancellation signal source generation unit 13 and outputs a low distortion sine wave cancellation signal.
The variable phase shifter means 15 sets a phase shift amount capable of canceling hum noise at the output terminal of the audio amplifier in the cancel signal input from the low distortion sine wave cancel signal generating means 14.
The variable gain adder means 16 sets the amplitude of the input signal from the variable phase shifter means 15 to an amplitude that can cancel hum noise, and outputs it to the cancel signal output terminal 17a.
The cancel signal output terminal 17 a is connected to the cancel signal injection means 18.

Here, an example of a cancel signal injection circuit and an injection method will be described. The cancel signal injection circuit 28 in FIG. 2 operates as the cancel signal injection means 18. The output of the cancel signal output terminal 17a in FIG. 1 is connected to the cancel signal input terminal 28a in FIG. A cancel signal is added to the audio signal input from the connector 28b of the same figure through the resistor R28a connected to the cancel signal input terminal 28a, and is output to the connector 28c. A signal input cord to the audio amplifier is connected to the connector 28c.
Here, the cancel signal is injected into a portion where the amplitude and phase of the cancel signal are not changed at the output terminal of the amplifier even when the gain / volume of the audio amplifier is adjusted. The parts that can be easily injected are an input connector, an output connector, a speaker connector, and the like of the power amplifier. The injection points to the amplification stage inside the audio amplifier are preferably the control grid or cathode of the vacuum tube amplifier, the gate of the FET amplifier, the base of the transistor amplifier, and the signal input terminal of the operational amplifier.
Further, in the case of an audio amplifier having a negative feedback (NFB) circuit, it may be injected into a negative feedback signal input portion or the like.
In general, the cancel signal is injected by inserting a resistor in series. If possible, the value is set to a value more than 10 times the internal impedance of the injected portion, for example, several hundred kΩ to several MΩ. Further, when direct current is applied to the portion to be injected, the direct current component is cut by a capacitor before injection.
Also, the cancel signal injection circuit 28 and the like are configured in a cancel device, an audio amplifier, a dedicated box, and the like depending on the injection location.

When there are n types of cancel frequencies, n sets of the synchronization signal generation means 12, the cancellation signal source generation means 13, the low distortion sine wave cancellation signal generation means 14 and the variable phase shifter means 15 of FIG. The addition is performed by the n-input variable gain adder means 16. In [Embodiment 3] (FIG. 5), which will be described later, there will be described a case where there are two types of cancel frequencies (n = 2) and a configuration for two channels of left and right (L, R) stereo signals.

(Constitution)
FIG. 3 is a block diagram showing the configuration of a hum noise canceling apparatus for an audio amplifier according to the second embodiment of the present invention. FIG. 3 shows an example suitable for canceling hum noise having a frequency (100/120 Hz) twice the AC power supply frequency, and the audio signal has a configuration for stereo left and right (L, R) two channels. .
In FIG. 3, the first transformer circuit 22 operates as the synchronization signal generating means 12 of FIG. An AC power supply is connected to the connection terminal 21a. The connection terminal 21 a is connected to the input of the first transformer circuit 22.
The frequency doubler circuit 23 operates as cancel signal source generation means 13. The output of the first transformer circuit 22 is connected to the input of the frequency doubler circuit 23.
The first low-pass filter 24 a and the first band-pass filter 24 b operate as the low distortion sine wave cancel signal generating unit 14. The output of the frequency doubler circuit 23 is connected to the input of the first low-pass filter 24a. The output of the first low pass filter 24a is connected to the input of the first band pass filter 24b.
The first phase inversion circuit 25 a and the first phase shifter 25 b operate as the variable phase shifter means 15. The output of the first band pass filter 24b is connected to the input of the first phase inverter circuit 25a. The output of the first phase inverting circuit 25a is connected to the input of the first phase shifter 25b.
The variable gain adder 26 operates as the variable gain adder means 16. The variable gain adder 26 includes a left channel variable gain adder 26L and a right channel variable gain adder 26R. The output of the first phase shifter 25b is connected to the inputs of the variable gain adders 26L and 26R. The outputs of variable gain adders 26L and 26R are connected to cancel signal output terminals 27L and 27R, respectively. In this case, variable gain adders 26L and 26R each have one input.
The cancel signal output terminals 27L and 27R are connected to two cancel signal injection circuits 28, respectively.

(Operation)
Here, the operation in the case of canceling hum noise having a frequency of 100/120 Hz will be described in association with the waveform diagram of FIG. In FIG. 4, the horizontal axis indicates time, and the vertical axis indicates voltage. The phase relationship between waveforms A, B, and C in FIG. 4 is arbitrary.
A waveform A in FIG. 4 is an output waveform of the first transformer circuit 22 in FIG. 3. A waveform A is the same waveform as that of the AC power source input to the connection terminal 21a in FIG. In Japan, the voltage of the commercial AC power supply is 100V and the frequency is 50/60 Hz. The first transformer circuit 22 uses a small power transformer, and its output voltage is about 6V. As an example of measured data, the measured value of the total harmonic distortion factor of waveform A is about 3.3% (-30 dB), and contains a large amount of harmonic distortion components, irregular noise, flicker, etc. of AC power supply. It is. In the prior art, such a waveform is used as a cancel signal for 50/60 Hz hum noise.

  A waveform B in FIG. 4 is an output waveform of the frequency doubler circuit 23 in FIG. This is obtained by full-wave rectifying the output of the first transformer circuit 22 with a bridge diode, and its fundamental frequency is 100/120 Hz, and the fundamental frequency of the output of the first transformer circuit 22 is It has been multiplied by 2. Waveform B contains a large amount of even-order harmonics of the full-wave rectified wave in addition to harmonics and noise of the AC power supply. The total harmonic distortion rate (theoretical value) of the waveform B is about 22.5% (−13 dB). In the prior art, such a waveform is used as a cancellation signal for 100/120 Hz hum noise.

A waveform C in FIG. 4 is a waveform after passing through the first low-pass filter 24a and the first band-pass filter 24b in FIG. 3, and is a low distortion sine wave cancellation signal of 100/120 Hz. The first low-pass filter 24a is realized by an RC filter, an active filter using an operational amplifier, or the like as a 1st to 5th order configuration. The cut-off frequency of the first low-pass filter 24a is set to about several tens Hz to several hundreds Hz. Next, the first band pass filter 24b can be easily realized by an active filter using an operational amplifier. The configuration is about the first to second order, and a two-stage amplifier type band-pass filter (DABP: Dual-Amp Band Pass Filter) is used. The Q of the first bandpass filter 24b is set to about 50 to 100, and the center frequency is set to 100/120 Hz by a variable resistor or a semi-fixed resistor.
The first low-pass filter 24a and the first band-pass filter 24b operate as the low distortion sine wave cancellation signal generation means 14, and the configuration thereof is for making the output of the cancellation signal source generation means a low distortion sine wave. In order to have an optimum bandpass characteristic, a lowpass filter, a highpass filter, a band elimination filter, or a bandpass filter may be appropriately combined and cascade-connected. In the simplest case, one bandpass filter But it ’s okay.

A waveform D in FIG. 4 shows a low distortion sine wave of the cancel signal output terminal 27L or 27R after passing through the first phase inverting circuit 25a, the first phase shifter 25b, and the variable gain adder 26 in FIG. This is a cancel signal waveform, and in order to cancel hum noise, the phase of the output (waveform C) of the first bandpass filter 24b is set to about 135 by the first phase inversion circuit 25a and the first phase shifter 25b. In this case, the amplitude is adjusted by the variable gain adder 26.
The first phase inverting circuit 25a gives a phase shift of 0 ° or 180 ° (opposite phase) to the output of the first bandpass filter 24b, and is easy by switching the non-inverting input / inverting input of the operational amplifier. Can be realized. The first phase shifter 25b gives a phase shift amount of about 0 ° to 180 ° to the output of the first phase inverting circuit 25a, and uses an all-pass filter or the like. The phase shift amount of the first phase shifter 25b is set by a variable resistor, a semi-fixed resistor, or the like. An arbitrary amount of phase shift can be given to the cancel signal by the first phase inversion circuit 25a and the first phase shifter 25b.
Here, the functions of the first phase inverting circuit 25a and the first phase shifter 25b can also be realized by cascading all-pass filters in two stages.
The variable gain adder means 16 is realized by using an operational amplifier or the like. The variable gain method can be easily realized by inserting an attenuation circuit such as a variable resistor or a semi-fixed resistor at the input portion of the adder.

  In [Embodiment 2] of FIG. 3, the phases of the cancellation signals of the left and right channels are set to be the same. However, in order to further increase the cancellation effect, the first phase inverting circuit 25a and the first phase shifter are used. Two sets of 25b may be used and connected to the output of the first bandpass filter 24b so that the phases of the left and right channels can be set independently.

In [Embodiment 2] in FIG. 3, a cancel signal having a frequency (for example, 200/240 Hz or 400/480 Hz) that is an even multiple of the frequency of the AC power supply can be generated. That is, the output of the frequency doubling circuit 23 is a full-wave rectified waveform (waveform B) and contains a large amount of even-order harmonics of 100/120 Hz, so that the cutoff frequency of the first low-pass filter 24a, and By matching the center frequency of the first band pass filter 24b with that, a cancel signal of that frequency can be generated. Further, when a cancel signal having an odd multiple of the AC power supply frequency (for example, 150/180 Hz or 250/300 Hz) is generated, the amplitude limiting is performed instead of the frequency doubler circuit 23 serving as the cancel signal source generating means 13. A square wave is obtained from the output of the first transformer circuit 22 using a circuit (limiter), and the cutoff frequency of the first low-pass filter 24a and the center frequency of the first band-pass filter 24b are set accordingly. Just do it.
Here, instead of the first phase inverter circuit 25a, the connection of the output of the transformer used in the first transformer circuit 22 may be switched and inverted by a two-pole double-throw switch or the like. When the variable range of the cancel signal phase can be limited, a semi-fixed phase shifter or the like may be used instead of the first phase inverting circuit 25a.

(Measurement results of performance)
Here, an example of the measurement result of the operating condition and performance of the apparatus of [Example 2] of the present invention will be described. A 50 Hz AC power supply was connected to the connection terminal 21a in FIG. The output of the frequency doubler circuit 23 (waveform B in FIG. 4) is a full-wave rectified waveform with a fundamental frequency of 100 Hz, and the total harmonic distortion rate (theoretical value) is about 22% (−13 dB). A primary RC filter was used as the first low-pass filter 24a. A two-stage amplifier type bandpass filter (Q = 67) was used as the first bandpass filter 24b.
The low distortion sine wave cancellation signal of 100 Hz is the output (waveform C) of the first bandpass filter 24b and the output (waveform D) of the variable gain adder 26, and the total harmonic distortion rate (actually measured value) is It becomes about 0.050% (−66 dB), and harmonics, irregular noise, flicker, etc. are removed from the output (waveform B) of the frequency doubler circuit 23, and a low distortion sine wave cancel signal can be obtained. It was.

(Configuration and operation)
FIG. 5 is a block diagram showing the configuration of a hum noise canceling apparatus for an audio amplifier according to the third embodiment of the present invention. FIG. 5 shows an embodiment suitable for simultaneously canceling the AC power frequency (50/60 Hz) and twice the hum noise (100/120 Hz). The audio signal is stereo left and right (L, R). ) Configuration for 2 channels. The configuration for canceling the hum noise with the frequency of 100/120 Hz shown in the upper part (reference numerals 21a to 26) of FIG. 5 is the same as [Example 2] of FIG.
Next, the configuration and operation for canceling hum noise with a frequency of 50/60 Hz shown in the lower part of FIG. 5 (reference numerals 32 to 35b) will be described. Here, the configurations and operations of the lower reference numerals 34a to 35b in FIG. 5 are the same as those in the upper reference numerals 24a to 25b in FIG. 3 or FIG.
In FIG. 5, the second transformer circuit 32 operates as the synchronizing signal generator 12 and the cancel signal source generator 13 and transforms the AC power supply voltage input from the connection terminal 21a to several volts. The waveform of the output of the second transformer circuit 32 is as shown by the waveform A in FIG. 4, and its fundamental frequency is 50/60 Hz.
Second low-pass filter 34a, and the second bandpass filter 34b operates as a low-distortion sine wave canceling signal generating means 14. The output of the second transformer circuit 32 is connected to the input of the second low-pass filter 34a. The output of the second low pass filter 34a is connected to the input of the second band pass filter 34b. The center frequency of the second band pass filter 34b is set to 50/60 Hz.
Next, the second phase inverting circuit 35 a and the second phase shifter 35 b operate as the variable phase shifter means 15. The output of the second band pass filter 34b is connected to the input of the second phase inverting circuit 35a. The output of the second phase inverting circuit 35a is connected to the input of the second phase shifter 35b. The output of the second phase shifter 35b is connected to the variable gain adder means 26L and 26R.
Here, instead of the second phase inverting circuit 35a, the connection of the output of the transformer used in the second transformer circuit 32 may be switched and inverted by a two-pole double-throw switch or the like. When the variable range of the cancel signal phase can be limited, a semi-fixed phase shifter or the like may be used instead of the second phase inverting circuit 35a.

(Measurement results of performance)
Here, an actual measurement result of the performance of the apparatus for canceling the hum noise of the frequency 50/60 Hz shown in the lower part (reference numerals 32 to 35b) of the apparatus of [Example 3] in FIG. 5 will be described. A 50 Hz AC power source was connected to the connection terminal 21a in FIG. The 50 Hz cancellation signal is the output of the second bandpass filter 34b and the output of the variable gain adder 26, and its total harmonic distortion rate (actual measurement value) is about 0.022% (−73 dB), and the second Harmonics, random noise, flicker, and the like were removed from the output of the transformer circuit 32 (similar to waveform A in FIG. 4), and a low distortion sine wave cancel signal could be obtained.

(Results of evaluation experiment)
Next, a description will be given of the results of an evaluation experiment in which 100 Hz and 50 Hz hum noise was canceled by an audio amplifier using an AC-ignited direct heat triode vacuum tube DA30 as an output tube using the apparatus of [Example 3] of the present invention.
The output of the variable gain adder 26 is injected into the input connector of the audio amplifier by the cancel signal injection circuit 28 to cancel the hum noise. Since the characteristics of the left and right channels are almost the same, the result of the left channel will be described.
The residual noise (including hum noise) of the DA30 audio amplifier before cancellation was 3.1 mV as a result of measuring the average voltage with an AC millivolt meter at the 8Ω speaker output terminal. The spectrum analysis result of the hum noise was that the 100 Hz component was 1.4 mV and the 50 Hz component was 0.57 mV, and the hum noise could be heard at a listening position about 4.5 m away from the speaker. The result of the hearing test was cloudy sound quality.

After canceling the hum noise with a fundamental frequency of 100 Hz by the apparatus of [Example 3], the average voltage of residual noise (including hum noise) by the AC millivoltmeter was 0.52 mV (cancellation ratio about 1/6). . The 100 Hz component of hum noise was reduced to 0.021 mV, and the cancellation ratio was about 1/67 (-37 dB). At the listening position about 4.5 m away from the speaker, 100 Hz hum noise could not be heard.
After canceling the hum noise with the fundamental frequency of 50 Hz by the apparatus of [Example 3], the 50 Hz component was reduced to 0.023 mV, and the cancellation ratio was about 1/25 (-28 dB).
After canceling the hum noise at the fundamental frequencies of 100 Hz and 50 Hz by the apparatus of [Example 3], the average voltage of residual noise (including hum noise) by the AC millivoltmeter is about 0.20 mV (cancellation ratio 1/16). became.
In this case, the voltage (effective value) of the harmonic component included in the cancel signal is 0.70 μV in the case of 100 Hz and 0.13 μV in the case of 50 Hz, and this level is described in [Background Art]. When the output of the audio amplifier is 10 W and the impedance of the speaker is 8Ω, the voltage between the speaker terminals is 8.9 V, −142 dB at 100 Hz, −157 dB at 50 Hz, and the dynamic range (80 dB or more) necessary for the audio signal, This is a small level that does not cause a problem when considering the dynamic range of human hearing (about 120 dB).
As a result of the hearing test after canceling only the 100 Hz component of hum noise, the turbid sound quality including hum noise before cancellation became high sound quality with a wide dynamic range with silence after cancellation. The result of the hearing test after canceling the 100 Hz and 50 Hz components of the hum noise was a further high quality sound with a quiet feeling.

FIG. 6 is a block diagram showing the configuration of a hum noise canceling apparatus for an audio amplifier according to the fourth embodiment of the present invention. In the figure, as the synchronous signal generating means 12 and the cancel signal source generating means 13 of FIG. 1, a transformer / double multiplier / waveform shaping circuit 42 and a Wien bridge oscillation circuit 43 are used. FIG. 6 shows an embodiment suitable for generating a cancel signal with an even lower total harmonic distortion rate. This figure shows a case where the cancel frequency is 100/120 Hz.
In FIG. 6, an AC power supply is connected to the connection terminal 41a. The voltage transformation / double frequency / waveform shaping circuit 42 transforms the AC power supply voltage input from the connection terminal 41a to several volts, then doubles it by a full-wave rectifier circuit using a bridge diode, and forms a waveform by a primary RC low-pass filter. Perform shaping.
The Wien bridge oscillation circuit 43 is basically a low-distortion sine wave (100/120 Hz) self-excited oscillator, but operates as an injection-locked oscillator by inputting a synchronization signal from the voltage transformation / double multiplication / waveform shaping circuit 42. . The injection locking input from the transformation / double frequency / waveform shaping circuit 42 to the Wien bridge oscillation circuit 43 is performed through the input side of the positive feedback loop of the Wien bridge oscillation circuit 43 through as high resistance as possible. The output of the Wien bridge oscillation circuit 43 is the first low-pass instead of the first transformer circuit 22 and the frequency doubler circuit 23 of the apparatus of [Embodiment 2] of FIG. 2 or [Embodiment 3] of FIG. Connect to input of filter 24a. The Wien bridge oscillation circuit 43 can be easily realized by a general-purpose operational amplifier or the like.
If the transformer / double frequency / waveform shaping circuit 42 of [Embodiment 4] in FIG. 6 has a frequency multiplication function of n times and the self-excited oscillation frequency of the Wien bridge oscillation circuit 43 is adjusted to that frequency, the AC power supply frequency N times as many cancellation signal sources can be generated.

FIG. 7 is a block diagram showing the configuration of a hum noise canceling apparatus for an audio amplifier according to a fifth embodiment of the present invention. This figure uses a third transformer circuit 52 as the synchronization signal generation means 12 of FIG. 1, and a PLL frequency synthesizer circuit 53 a and a counter circuit 53 b as the cancellation signal source generation means 13. FIG. 7 shows an embodiment suitable for the case where a cancel signal source having a plurality of frequencies is simultaneously generated. This figure shows a case where there are three types of cancel frequencies.
In FIG. 7, an AC power supply (50/60 Hz) is connected to the connection terminal 51a. The third transformer circuit 52 transforms the AC power supply voltage input from the connection terminal 51a to several volts.
The output of the third transformer circuit 52 is input to a phase comparator of a PLL (Phase Locked Loop) frequency synthesizer circuit 53a. Since the PLL frequency synthesizer circuit 53a can be easily realized by using an LSI (Large Scale Integrated Circuit), description of its principle is omitted. When the division period in the feedback loop from the VCO (Voltage Controlled Oscillator) inside the PLL frequency synthesizer circuit 53a to the phase comparator is set to 1/8 frequency, the frequency of the output E of the PLL frequency synthesizer circuit 53a Is 400/480 Hz, which is eight times the frequency of the AC power supply.
The output E of the PLL frequency synthesizer circuit 53a is input to the counter circuit 53b. Assuming that the counter circuit 53b is a 4-bit counter and the output is output F, output G, output H, and output I from the lower bits, the output F is 200/240 Hz, the output G is 100/120 Hz, and The output H is a 50/60 Hz square wave.

Here, it associates with the case of [Example 3] in FIG. If the output G of the counter circuit 53b is connected to the input of the first low-pass filter 24a, and the output H of the counter circuit 53b is connected to the input of the second low-pass filter 34a, the first transformer circuit of [Example 3]. 22, instead of the frequency doubler circuit 23 and the second transformer circuit 32, the third transformer circuit 52, the PLL frequency synthesizer circuit 53a, and the counter circuit 53b in [Embodiment 5] can be realized.
When there are three or more cancel frequencies, the first low-pass filter 24a, the first band-pass filter 24b, the first phase inverting circuit 25a, and the first phase shift are performed in [Embodiment 3] of FIG. The frequency characteristics of the low-pass filter and the band-pass filter of each frequency channel may be set in accordance with the cancellation frequency by increasing the number of frequency channels configured by the device 25b.
In [Embodiment 5] in FIG. 7, when the output of the counter circuit 53b may be one frequency, the counter circuit 53b is omitted, and the PLL frequency synthesizer circuit 53a is divided by a frequency divider inside the PLL frequency synthesizer circuit 53a. Is set to a required frequency.

  The apparatus of [Example 3] to [Example 5] according to the method of [Example 1] described above was prototyped, and an evaluation experiment and a hearing test were performed under various conditions. As a result, the total harmonic distortion rate of the low distortion sine wave cancellation signal is in the range of 0.02% (−74 dB) to 0.2% (−54 dB) when considering sound quality, feasibility, and cost. Then, it was found that hum noise of about several mV to 10 mV can be canceled to a level of about 0.5 mV or less without adding unnecessary frequency components and irregular noise that deteriorate the sound quality to the audio signal. Under this condition, even when the total distortion rate of the low-distortion sine wave cancellation signal is 0.2% (−54 dB) which is the largest, when the hum noise level is assumed to be 10 mV at a single frequency, The effective value of the harmonic distortion component applied to the audio signal is 0.02 mV. The level is −113 dB when the speaker impedance is 8Ω and the output of the audio amplifier is 10 W (8.9 V), as described in [Background Art], and the dynamic range (80 dB or more) necessary for the audio signal and Considering the human auditory dynamic range (about 120 dB), etc., this is considered to be a reasonable result.

FIG. 2 is a block diagram showing a method for canceling hum noise of an audio amplifier according to the first embodiment of the present invention. It is a circuit diagram which shows the example of a cancellation signal injection circuit. It is a block diagram which shows the structure of the cancellation apparatus of the hum noise of the audio amplifier by the 2nd Embodiment of this invention. FIG. 6 is a waveform diagram showing waveforms of respective parts of the hum noise canceling apparatus of the audio amplifier according to the first, second, and third embodiments of the present invention. It is a block diagram which shows the structure of the cancellation apparatus of the hum noise of an audio amplifier by the 3rd Embodiment of this invention. It is a block diagram which shows the structure of the cancellation apparatus of the hum noise of the audio amplifier by the 4th Embodiment of this invention. It is a block diagram which shows the structure of the cancellation apparatus of the hum noise of an audio amplifier by the 5th Embodiment of this invention.

Explanation of symbols

11a, 21a, 41a, 51a Connection terminal 12 Synchronization signal generation means 13 Cancel signal source generation means 14 Low distortion sine wave cancellation signal generation means 15 Variable phase shifter means 16 Variable gain adder means 17a, 27L, 27R Cancel signal output terminal 18 Cancel signal injection means, 28 Cancel signal injection circuit, 28a Cancel signal input terminal 28b, 28c Connector 22 First transformer circuit, 32 Second transformer circuit, 52 Third transformer circuit 23 Frequency doubler circuit 24a First Low-pass filter, 34a second low-pass filter 24b first band-pass filter, 34b second band-pass filter 25a first phase inverter circuit, 35a second phase inverter circuit 25b first phase shifter, 35b second Phase shifters 26, 26L, 26R Variable gain adder 42 Waveform shaping circuit 43 Wien bridge oscillation circuit 53a PLL frequency synthesizer circuit 53b Counter circuit A Example of output waveform B of first transformer circuit 22 and synchronization signal generating means 12 B of frequency doubler circuit 23 and cancel signal source generating means 13 Example C of output waveform Example D of output waveform of first band-pass filter 24b and low distortion sine wave cancellation signal generation means 14 Example D of output waveform of variable gain adder 26 and variable gain adder means 16 E F output, G output, H output, I output R28a Resistor

Claims (1)

In the device for canceling hum noise that sets the phase and amplitude of the cancel signal so that hum noise can be canceled at the output part of the audio amplifier,
A first transformer circuit that generates a signal synchronized with the AC power source causing the hum noise;
A frequency doubler circuit for generating a cancel signal source synchronized with a synchronization signal output from the first transformer circuit;
A first low-pass filter and a first band-pass filter for generating a low distortion sine wave cancellation signal by removing harmonics, irregular noise, AC power supply flicker and the like from the output of the frequency doubler circuit;
A first phase inversion circuit and a first phase shifter for giving a phase shift amount necessary for canceling hum noise to an output of the first band pass filter;
A variable gain adder that adjusts and adds the output of the first phase shifter to an amplitude required to cancel hum noise;
Be a total harmonic value of the distortion factor 0.02% to 0.2% of said low distortion sine wave canceling signal at the output of the variable gain adders, and the by generating a low distortion sine wave cancellation signal , to remove only the fundamental wave component or harmonic component of the ham noise included in an audio signal, and wherein the Rukoto be adjusted independently the phase and amplitude from each other in the low-distortion sine wave canceling signal, an audio amplifier Hum cancellation device.