An audio power amplifier

Abstract

An audio amplifier, using a linear transformer B to power the power amplifier stage, a thermistor R in series with the transformer B T For positive temperature coefficient, can be old color TV demagnetization resistance, switch K1 parallel thermistor R T Both ends, the device is working properly, K1 closed short circuit, transformer B normal operation, audio amplifier power and normal work, when the need to shut down, first let switch K1 open, thermistor resistance sharply to maintain the current, this is the real shutdown, so that the transformer B core remanence low, effectively reduces the audio amplifier next time the surge current, even when the power supply is at the peak of the sine wave around.

Description

Technical Field

[0001] This invention relates to audio amplification technology, and in particular to power-on soft-start technology for audio amplifiers. Background Technology

[0002] An audio amplifier is the standard name for an audio power amplifier, also commonly referred to in the industry as an audio power amplifier or simply a power amplifier. It is a crucial component of multimedia products and is widely used in consumer electronics.

[0003] The development of audio amplifiers has gone through three eras: vacuum tube, bipolar transistor, and field-effect transistor. Before the 1960s, vacuum tube audio amplifiers were the mainstream. While they offered a mellow tone, they were bulky, consumed a lot of power, had a short lifespan, and required an output transformer. Winding a wide-frequency-response output transformer was difficult, making it hard to achieve a wide frequency response. Furthermore, the filaments in vacuum tubes consumed additional power, resulting in low overall efficiency. Vacuum tubes have largely disappeared from the mainstream market, though they still have a place in the high-fidelity (Hi-Fi) field. After the 1960s, bipolar transistor (BPT) audio amplifiers offered wide bandwidth, large dynamic range, high reliability, long lifespan, and good high-frequency response. However, their quiescent power consumption and on-resistance were high, making it difficult to improve efficiency; the theoretical upper limit was 78.53% (π / 4). After the 1980s, with advancements in field-effect transistor (FET) manufacturing technology, FET audio amplifiers emerged as a powerful force. FET amplifiers offered a similarly mellow tone to vacuum tubes, along with a wide dynamic range and theoretical efficiency comparable to BPT amplifiers. Because FETs have majority carrier conduction, they exhibited good thermal stability and low noise, quickly becoming a new force in the high-fidelity field.

[0004] The audio amplifiers mentioned above are all analog audio amplifiers, also known as analog power amplifiers, or linear power amplifiers in many documents. Here's a note about their efficiency: while the theoretical upper limit of 78.53% seems high, it's actually quite low in practical use, making it difficult to meet the demands of high-power applications such as stages and stadiums. This is because the ratio of average power to maximum power in common music signals is often above 1:10. For example, in home audio equipment, speakers achieve good sound pressure levels with an average power of 5W to 10W, while the peak power in music ranges from 50W to 100W. When an analog power amplifier outputs 100W, its theoretical efficiency is 78.5%, but when it outputs a lower power of 10W, its theoretical efficiency is only 24.83%. If we consider the 4V saturation voltage drop of the output power transistors in analog power amplifiers, then the actual efficiency of an analog power amplifier at 100W output is below 71.4%, and at a lower power of 10W, its actual efficiency is below 22.57%. Due to losses in the power supply transformer and rectifier / filter circuits, the efficiency will decrease by about 2%.

[0005] As mentioned above, analog power amplifiers have an efficiency of around 22.5% under normal use. For a large stadium with an average total output power of 600W, the rated power of the analog power amplifier would be over 6000W. In actual use, analog power amplifiers waste more than 2000W of power. This is why high-efficiency audio amplifiers emerged. In the late 1980s, the domestic journal *Radio and Television* published an article by Lu Quangen introducing Class H amplifiers from Yamaha Corporation of Japan. The invention of Class H audio amplifiers can be traced back to the 1980s, first proposed by researchers at Bell Labs in the United States. Yamaha Corporation quickly launched commercial Class H amplifiers, which became best-selling products at the time due to their light weight, high efficiency, and high power. In a typical analog audio amplifier, when operating at one-tenth of its output power, the power supply voltage largely falls dynamically across the output transistors, leading to a decrease in efficiency, down to approximately 22.5% as mentioned earlier. Class H amplifiers, however, utilize multiple power supply voltages. For example, a 100W / 8Ω analog amplifier, considering the 4V saturation voltage drop across the output transistors, should operate at ±44V. Class H amplifiers add a ±24V power supply, allowing the use of a lower ±24V power supply when the instantaneous output sine wave voltage is below ±20V. This significantly reduces the voltage drop across the output transistors, improving efficiency. With a 2:1 voltage ratio between the high and low voltage outputs, the theoretical efficiency of a Class H amplifier increases to 85.91% at full power output and 49.67% at one-tenth of its output power. This is twice as high as the 24.83% mentioned above. If we consider that the output transistor of the Class H amplifier also has a saturation voltage drop of 4V, then the Class H amplifier uses two sets of voltages of ±44V and ±24V. When it outputs 100W, its efficiency is 78.1%, and when it outputs one-tenth of the power of 10W, its efficiency is 41.39%.

[0006] Later, Class D audio amplifiers (also known as digital amplifiers) further improved efficiency, achieving over 88% efficiency from low power output to full power output. Although the electromagnetic compatibility of high-quality switching power supplies has been well done, audio amplifiers still largely use traditional linear transformers for power supply. Even Class D audio amplifiers (also known as digital amplifiers) themselves operate in a switching state, but their power supply still stubbornly uses linear transformers and rectifier and filter circuits.

[0007] The conversion efficiency of linear power supplies composed of these traditional linear transformers is relatively low, generally around 96%. This is acceptable for audio amplifiers due to their high reliability, and their application remains widespread. Common types of linear transformers include the traditional laminated transformer, commonly known in the industry as the "square-core transformer" (derived from a southern Chinese dialect), whose core cross-section and shape are typically square. Another type is the toroidal transformer, whose core cross-section is still square, but the overall core shape is circular. A circle has the shortest perimeter for a given area, resulting in the shortest magnetic field length, reduced losses, low internal resistance, and high efficiency, offering better performance than square-core transformers; this type is commonly known as the "toroidal transformer." One type of toroidal transformer has a circular or near-circular core cross-section. Because the circumference of each turn of the enameled wire in its coil is also minimized, its performance is extremely excellent. This is the R-type transformer, which is actually a special form of toroidal transformer. The overall shape of the R-type transformer's core is no longer circular, but rather a U-shape resembling a running track, for ease of winding.

[0008] A significant problem with audio amplifiers powered by linear transformers is the large inrush current at power-on, which can be heard as a mechanical vibration from the transformer. This necessitates a larger capacity mains control switch, increasing the cost of the circuit system. Furthermore, the large inrush current may trigger overcurrent protection devices in the power supply circuit, such as circuit breakers, leading to system malfunctions. Toroidal transformers, in particular, exhibit an even larger inrush current at the same power and no-load current, severely hindering their widespread adoption. Many professional fields, including audio and video, utilize power sequencers to ensure that equipment is powered on in a specified time sequence, preventing the main circuit breaker from tripping due to simultaneous power-on of numerous devices.

[0009] In the utility model patent application No. 200920178160.X, entitled "Surge Absorber for Toroidal Transformer" (hereinafter referred to as Patent Document 1), a solution is presented where mains power supplies the toroidal transformer through an NTC thermistor. Its working principle is relatively typical. A negative temperature coefficient thermistor, also known as an NTC thermistor, is a resistor whose resistance decreases as temperature increases. It is widely used in various electronic circuits, commonly for limiting start-up current. When powered on, the NTC thermistor is in a zero-power resistance state, with a relatively large resistance value, limiting the start-up current and keeping it below the desired value. The maximum peak value of the start-up current is calculated as the peak value of the mains power divided by the zero-power resistance value of the NTC. As current flows through the NTC thermistor, its temperature rises, its resistance decreases, and the heating power P = I... 2R also decreases accordingly, thus reducing insertion loss under normal operating conditions. Even so, since the temperature coefficient of an NTC thermistor ranges from -2% to -6.5% per degree, its insertion loss during normal operation is still significant. This utility model patent uses a relay to short-circuit the NTC thermistor at a certain time after power-on to reduce its insertion loss. In fact, this circuit has been widely used in the audio industry since the 1980s in audio amplifiers, from early analog power amplifiers to later Class D digital power amplifiers. Using a relay to short-circuit the NTC thermistor at a certain time after power-on to reduce insertion loss is a well-known and publicly disclosed technique. The surge current during power-on can shorten the lifespan of the NTC thermistor. Therefore, later audio amplifiers used high-power cement resistors (a type of wire-wound resistor) instead of NTC thermistors to increase circuit reliability.

[0010] The circuit in Patent Document 1, due to an incorrect connection of diodes D1 to D4, cannot achieve the rectified output through capacitor C1 (which provides a current-limiting constant current output), and therefore the circuit fails to function. Here, we will explain using a subsequent working circuit feature from Patent Document 1: a relay is used to short-circuit the NTC thermistor at a certain time after power-on. This feature is consistent with known technology from the 1980s. Its shortcoming is that it still cannot solve the problem of a large inrush current in linear transformers when powered on, especially if the mains voltage is not zero-crossing. Under 220V / 50Hz mains voltage, a high-quality toroidal transformer with a no-load current of only 30mA and a power rating of 500W records a peak inrush current of 1.1A to 2.7A upon power-on. This is related to the position of the mains voltage at the moment of power-on; if the peak voltage is below 30V, the inrush current is small; if the peak voltage is above 165V, the inrush current is large.

[0011] Previous literature has largely used resistive devices connected in series in the linear transformer circuit to limit the maximum inrush current at startup, but it overlooks a fundamental fact: the primary winding of a linear transformer connected to mains power has a very large inductance. For example, the toroidal transformer with a no-load current of only 30mA and a power rating of 500W, measured with an LCR bridge, has a primary inductance between 1.8 and 2.2H, while its calculated inductance is around 24H. According to current theoretical knowledge, the current in an inductor cannot change abruptly. When a relatively high DC voltage (simulating the instantaneous value of mains power-on) is applied, the magnetizing current should theoretically start from zero. It is important to note that the large number of turns and the tight winding of the enameled wire in the primary winding of the linear transformer have created a large distributed capacitance. This primary distributed capacitance can also cause inrush current, but the inrush current caused by this distributed capacitance is relatively small and accounts for a small proportion, insufficient to form the "inrush current at the moment of power-on" mentioned earlier.

[0012] In summary, existing soft-start circuits for audio amplifiers powered by linear transformers have the following shortcomings:

[0013] (1) Using resistive devices to forcibly limit the surge current during power-on results in insertion loss and unsatisfactory lifespan. Furthermore, when using NTC thermistors in series, the resistance of the NTC thermistors increases at low temperatures, causing the power-on failure. A solution for low-temperature operation is provided in the proceedings of the 21st Annual Academic Conference of the China Power Supply Society, entitled "Application of Flyback Switching Power Supplies at Low Temperatures," authored by Zheng Lingxiao et al. However, the method described in the paper still suffers from insertion loss.

[0014] (2) When powered on, a strong vibration sound can still be heard from the audio amplifier due to the linear transformer;

[0015] (3) Existing technology does not reduce the surge current of the linear transformer in the audio amplifier when it is powered on.

[0016] For convenience, unless otherwise specified, the term "linear transformer" in this article refers to the linear transformer that powers the audio amplifier, or simply "transformer". Summary of the Invention

[0017] Therefore, the technical problem to be solved by the present invention is to reduce the inrush current of the linear transformer in the audio amplifier when it is powered on, thereby reducing the inrush current flowing through the NTC thermistor or other current-limiting resistor when the power is turned on.

[0018] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0019] An audio amplifier includes at least a linear transformer, a thermistor with a positive temperature coefficient, and a first switch. The linear transformer and the thermistor are connected in series and electrically to an alternating current. The first switch is connected in parallel across the thermistor via an electrical connection. When the linear transformer is required to work normally, the first switch is in a closed state; when the linear transformer is not required to work normally, the first switch is in an open state.

[0020] Preferably, the audio amplifier further includes a second switch, which is connected in series with the linear transformer and the thermistor in a series circuit. When the linear transformer is not required to work normally, the first switch is in the open state, and after a delay, the second switch is opened again; when the linear transformer is required to work normally, the second switch is in the closed state.

[0021] Preferably, the audio amplifier further includes a third switch and a first resistor. The first resistor and a thermistor are thermally encapsulated together. The third switch is connected in series with the first resistor and then in parallel with the linear transformer. When the linear transformer is not required to work normally, the third switch is in the closed state. When the linear transformer is required to work normally, the third switch is in the open state.

[0022] Preferably, the above-mentioned audio amplifier scheme with the addition of a third switch and a first resistor also includes a second switch. The second switch is connected in series with the linear transformer and the thermistor in a series circuit. When the linear transformer is not required to work normally, the first switch is in the open state, and after a delay, the second switch is opened again. When the linear transformer is required to work normally, the second switch is in the closed state.

[0023] The beneficial effects of the audio amplifier of the present invention are that it truly reduces the inrush current of the linear transformer itself when powered on, eliminates the vibration sound emitted by the linear transformer when powered on, and eliminates the need to use resistive devices to forcibly limit the inrush current when powered on. Attached Figure Description

[0024] Figure 1 This is a circuit schematic diagram of the first embodiment of the present invention;

[0025] Figure 2 This is a circuit schematic diagram of the second embodiment of the present invention;

[0026] Figure 3 This is a circuit schematic diagram of the third embodiment of the present invention;

[0027] Figure 4 This is a circuit schematic diagram of the fourth embodiment of the present invention;

[0028] Figure 5 This is a measured static hysteresis loop cluster diagram of the core of transformer B in the first embodiment;

[0029] Figure 6 This is a schematic diagram showing the decrease of AC current in transformer B over time. Detailed Implementation

[0030] Example 1

[0031] See Figure 1 This is a schematic diagram of an audio amplifier according to the first embodiment of the present invention. The audio amplifier in this embodiment includes at least a linear transformer B and a thermistor with a positive temperature coefficient R. T This is the degaussing resistor from an older color television set, model MZ72 M10R. The first switch, K1, is a common AC power switch, which can be replaced by relay contacts. The linear transformer B is an 800W toroidal transformer with a nominal no-load current of 50mA, which was measured to be 43.7mA at AC220V / 50Hz. This is related to the thermistor R... T After being connected in series, it is electrically connected to AC power, and the first switch K1 is electrically connected in parallel to the thermistor R. TAt both ends, when the linear transformer B needs to work normally, the first switch K1 is in the closed state; when the linear transformer B does not need to work normally, the first switch K1 is in the open state.

[0032] In the audio amplifier of the first embodiment, the rectification and filtering processes following the linear transformer B, as well as the Class AB amplification circuit, are existing technologies. The rectified and filtered output voltage is ±88V DC. Before the rectification circuit, the secondary winding output voltage of the linear transformer B is 130V / 6.2A with a center tap. The Class AB amplification circuit has two channels, representing the left and right channels, commonly distinguished as Channel A and Channel B in the industry. Each channel's output power transistors consist of three pairs of 2SC5200 and 2SA1943 transistors, with each transistor having a quiescent current between 25mA and 50mA. In this way, each channel achieves 800W / 4Ω power. This design utilizes the fact that the ratio of the average power of the music signal to its maximum power is greater than 1:10. That is, the actual average output power of each channel is 80W, and the two channels are 160W, with an efficiency of 20%. At this time, the power consumption is 800W. When the music power is high, the circuit needs to output more power, or even the maximum power. At this time, the energy stored in the two 10000uF / 100V electrolytic capacitors originally set in the filter circuit ensures that the music power is output without distortion.

[0033] The working principle of this invention will be explained in detail here. As mentioned in the background section: When the primary inductance of a linear transformer is connected to the mains power, according to current known theory, the current in the inductor cannot change abruptly. When a relatively high DC voltage (simulating the instantaneous value of the mains power at random power-on) is applied, the excitation current should theoretically start from zero. Furthermore, the surge current generated by the distributed capacitance of the primary side is relatively small and accounts for a small proportion, insufficient to form the "surge current at the moment of power-on" mentioned earlier. Under 220V / 50Hz mains power, the first embodiment uses a high-quality toroidal transformer with an open-circuit current of 43.7mA and a nominal power of 800W. When powered on, the instantaneous value of the AC power at the moment of power-on is monitored with an oscilloscope, and only the surge current data corresponding to the instantaneous value above 130V is adopted, recording a peak surge current of 2.5A to 4.1A.

[0034] The inventors of this invention discovered that the surge current at the moment of energization is caused by the residual magnetism of the transformer core. The verification method involves using two transformers of the same specification, with no-load currents of the same or very close values. Their primary windings are connected in parallel with the same pure AC power supply in phase, and their secondary windings are connected to the same purely resistive load, reaching 80% of the rated power. Then, the AC power is simultaneously and randomly turned off. The primary windings are then connected to DC power supplies of the same voltage, with the leads being red and black wires respectively. For the first transformer, the red wire is connected to the positive DC power supply, and the black wire to the negative; for the second transformer, the connection is reversed, with the red wire connected to the negative and the black wire to the positive. If the residual magnetism of both cores is zero, then the surge current of the two transformers should be the same when the DC power supply is energized. However, actual measurements show a significant difference in the surge current between these two connections. Many companies and researchers have discovered this using similar methods, but have not provided a solution.

[0035] This indicates that when the AC power supply to the transformer is randomly interrupted, the iron core retains residual magnetism. In practical use, it is extremely difficult for electrical equipment to shut down at the exact moment the AC current crosses zero. While Chinese scientists have indeed designed overvoltage zero-crossing starting circuits, they haven't considered zero-crossing shutdown circuits. Furthermore, because linear transformers are generally inductive during operation, their voltage and current phases are different; the AC current phase lags behind the voltage. This complicates the design of zero-crossing shutdown circuits. Therefore, when electrical equipment is shut down, whether at the zero-voltage or zero-current crossing, there is a very high probability that it will not shut down at the exact moment of zero-crossing. Because zero-crossing protection cannot be achieved... Figure 5 It shows Figure 1 The measured static hysteresis loop cluster of the core of the linear transformer B shows that curve 101 represents the largest hysteresis loop. Limit tests demonstrate this. Figure 1 The core permeability of the linear transformer B is very high, and its hysteresis loop is close to a rectangle. Curve 102 is the hysteresis loop of this 800W toroidal transformer at an output power of 1200W. It can be seen that when the power is exceeded, the transformer core exhibits shallow saturation. Curve 103 is the hysteresis loop at an output power of 800W, and curve 104 is the hysteresis loop at an output power of 300W. From curves 101, 102, 103, and 104, it can be seen that when the excitation current in the coil is zero, the residual magnetism of the core is very large, with the value of 101 being greater than that of 102. From 101 to 104, the value decreases from large to small. To achieve zero residual magnetism, a reverse excitation current must be applied. Since curves 101 and 102 are close to rectangles, the reverse excitation current is extremely difficult to control. Curves 103 and 104 drop very quickly, making it difficult to achieve precise control instantaneously, i.e., to cut off the current when the residual magnetism is zero. The residual magnetism in the iron core when the power is off is the root cause of the instantaneous surge current when the power is turned on again.

[0036] This invention achieves near-zero residual magnetism by having the first switch K1 in the open state when the linear transformer B is not required to operate normally. This indicates that the audio amplifier, according to the first embodiment of this invention, is preparing to shut down. At the instant the audio amplifier shuts down or after the shutdown command is issued, i.e., "when the linear transformer B is not required to operate normally," switch K1 is in the open state. At this time, the transformer B passes through the thermistor R... T Powered by AC power at 220V and 50Hz, in this embodiment, the thermistor R... T The demagnetizing resistor, model MZ72 M10R, has a zero-power series resistance of 10Ω. Due to its small resistance, the circuit continues to function normally at this moment. Meanwhile, the thermistor R... T The transformer generates heat, and its impedance rises rapidly within seconds, while the alternating current flowing through transformer B decreases over time. Figure 6 As shown. After 180 seconds, the thermistor R... T The resistance of the thermistor rises to about 27.5KΩ, and the measured AC current flowing through transformer B is 7.09mA. At this time, the thermistor R... T The voltage across transformer B is 194.9V, and the voltage across transformer B is 31.4V. Note that the thermistor R... T If the terminal voltage of transformer B is 194.9V, and the terminal voltage of transformer B is 31.4V, the voltage becomes 226.3V, exceeding the mains voltage of 220V. This is because the terminal voltage of transformer B leads the phase of its current. Over time, the thermistor R... T The resistance will increase further, and the current I0 flowing through transformer B will decrease further. After 360 seconds, this current drops to 2.16mA. At this point, the thermistor R... T The voltage across transformer B is 212.4V, and the voltage across transformer B is 9.56V. In this state, in the first embodiment, linear transformer B is connected to the thermistor R. T The power supply circuit's static power consumption is less than 475mW, less than 0.5W, meeting energy-saving requirements and can be considered to be in a shutdown state. Actual measurements of the first embodiment showed that after 5 minutes, the power consumption was below 250mW, and after 15 minutes, the power consumption was below 220mW. When the thermistor R... T When employing insulation measures, the leads are lengthened to prevent heat loss from a large area of ​​copper foil on the circuit board; in the thermistor R T With its square casing and white heat-shrink tubing, the steady-state power consumption drops to 165mW and does not decrease further.

[0037] When linear transformer B needs to work normally, the first switch K1 is in the closed state. At this time, since linear transformer B has been in a state of extremely low current excitation before the first switch K1 is closed, its residual magnetism is extremely small. At the moment the first switch K1 is closed, multiple measurements were taken and the peak surge current corresponding to the peak value of 305V or more at 220V AC was recorded. The maximum peak value was only 0.37A, which is about 9% lower than the traditional peak surge current of 4.1A. This directly reduces the surge current of the linear transformer itself when it is powered on, which has a good effect. Moreover, linear transformer B is very quiet when powered on and does not make any vibration noise.

[0038] To better achieve the purpose of the invention, the thermistor R T The residual current should be less than half of the no-load current of linear transformer B. In actual tests, it is better to keep it below one-quarter.

[0039] Another beneficial effect is that if the first switch K1 is used simultaneously as the power switch of the audio amplifier, the anti-sparking capacitor originally connected in parallel with the power switch can be eliminated. During normal operation, the first switch K1 is in the closed state, eliminating insertion loss, and the invention can be powered on normally at low temperatures.

[0040] In the first embodiment of the present invention, both Class AB amplifier circuits are replaced with Class D audio amplifier circuits composed of a digital power amplifier driver chip IRS2092S and two field-effect transistors IRFB4227, which improves the efficiency to about 88%. When the actual average output power per channel of the unit is 80W, the two channels are 160W, and the efficiency is 82%. At this time, the total power consumption is 210W. The temperature rise of the linear transformer B is very low. Since the replacement of the main power amplifier circuit after rectification and filtering does not affect the technical solution and working principle of the present invention, the purpose of the invention is still achieved after the replacement.

[0041] The first embodiment of the present invention still has shortcomings. When the audio amplifier is to be turned off, that is, when the first switch K1 is in the open state, the linear transformer B in the audio amplifier is connected to the thermistor R. T Power is supplied, and after a few minutes, the thermistor R is used. T The impedance rises rapidly due to heat, causing the AC current flowing through the linear transformer B to drop to the milliampere level. The terminal voltage of the linear transformer B drops to less than one-seventh of the original mains voltage, i.e., less than 31.4V. However, the audio amplifier still has static power consumption. In many fields and work environments, many people hope to truly cut off the power supply to achieve electrical safety and zero power consumption. The present invention will provide a solution in the second embodiment, as follows.

[0042] Example 2

[0043] See Figure 2This is a schematic diagram of an audio amplifier according to a second embodiment of the present invention. The audio amplifier in this embodiment includes at least a linear transformer B and a thermistor with a positive temperature coefficient R. T The first switch is K1. R T This is a degaussing resistor, model MZ75 5R, found in older color televisions. The first switch, K1, is a common AC power switch and can be replaced by relay contacts. Linear transformer B is a 1200W toroidal transformer with a nominal no-load current of 80mA; its measured current at AC220V / 50Hz is 81.2mA. This is related to the thermistor R. T After being connected in series, it is then connected to a 220V 50Hz AC power supply via a second switch K2. The first switch K1 is connected in parallel to the thermistor R. T At both ends,

[0044] When linear transformer B is required to operate normally, the first switch K1 is in the closed state; when linear transformer B is not required to operate normally, the first switch K1 is in the open state. It also includes a second switch K2, which is connected to linear transformer B and the thermistor R. T In a series circuit, when the linear transformer B is not required to work normally, the first switch K1 is in the open state, and after a certain delay, the second switch K2 is opened. When the linear transformer is required to work normally, the second switch K2 is in the closed state, as mentioned above, at which time the first switch K1 should be in the closed state.

[0045] In the audio amplifier of the second embodiment, the rectification and filtering processes following the linear transformer B, as well as the Class H amplifier circuit, are existing technologies. The rectified and filtered output voltages are ±88V DC high voltage and ±33V relative DC low voltage. Before the rectifier circuit, the secondary winding output voltage of the linear transformer B is 130V / 50V / 6.2A with a center tap, i.e., five terminals from top to bottom: 65V in-phase, 25V in-phase, 0V with center tap, 25V out-of-phase, and 65V out-of-phase. The high and low voltages are switched by an electronic switch consisting of four LM311 chips, four FQP55N10 MOSFETs, and four common-cathode dual diodes FFPF20UP20DN. The Class H amplifier circuit has two channels, left and right, which are also commonly distinguished as A and B channels in the industry. Each output power transistor consists of three pairs of 2SC5200 and 2SA1943 transistors, with each power transistor having a quiescent current between 25mA and 50mA. This achieves 800W / 4Ω power per channel. This design utilizes the fact that the ratio of the average power of the music signal to its maximum power is greater than 1:10, meaning the actual average output per channel is 80W, and two channels are 160W, with a measured efficiency of 60%. At this point, the power consumption is 267W. When music power is high, the circuit needs to output even more power, even the maximum power. At this time, the ±88V DC high voltage supplies power to the positive and negative half-cycles respectively. Combined with the energy storage of the two 8200uF / 100V electrolytic capacitors originally installed in the filter circuit, this ensures undistorted music power output. Due to the increased power of the transformer, the temperature rise is also lower.

[0046] Under 220V / 50Hz AC mains power, this embodiment uses a high-quality toroidal transformer with an open-circuit current of 81.2mA and a nominal power of 1200W. When powered on, the instantaneous value of the AC current is monitored with an oscilloscope. Only the surge current data corresponding to the instantaneous value above 130V is adopted, and the peak surge current is recorded from 4.6A to 11.1A.

[0047] Similar to Example 1, the surge current at the moment of energization is caused by the residual magnetism of the transformer core. For example... Figure 5 As can be seen from the working principle described in the first embodiment, the residual magnetism of the iron core when the power is off is still the fundamental reason for the instantaneous surge current when the power is turned on again.

[0048] This invention achieves near-zero residual magnetism by having the first switch K1 open when the linear transformer B is not required to operate normally. This indicates that the audio amplifier, according to the second embodiment of this invention, is preparing to shut down. At the instant the audio amplifier shuts down or after the shutdown command is issued, i.e., "when the linear transformer B is not required to operate normally," switch K1 remains open, while switch K2 remains closed. At this time, the transformer B passes through the thermistor R... TPowered by AC current with an effective value of 220V and a frequency of 50Hz, in Example 2, the thermistor R T The demagnetizing resistor, model MZ75 5R, has a zero-power series resistance of 5Ω. Due to its small resistance, the circuit still functions relatively normally at this instant. The thermistor R... T The terminal voltage is 17.6V the instant switch K1 is opened, and at the same time the thermistor R... T The heating power is 61W, its impedance rises rapidly within seconds, while the AC current flowing through transformer B decreases over time, similarly... Figure 6 As shown. After 180 seconds, the thermistor R... T The resistance of the thermistor R rises to approximately 17.1 kΩ, and the measured AC current flowing through transformer B is 11.81 mA. At this point, the thermistor R... T The voltage across transformer B is 201.9V, and the voltage across transformer B is 25.3V. Note that the thermistor R... T If the terminal voltage of transformer B is 201.9V, and the terminal voltage of transformer B is 25.3V, the voltage becomes 227.2V, exceeding the mains voltage of 220V. This is because the terminal voltage of transformer B leads the phase of its current. Over time, the thermistor R... T The resistance will increase further, and the current I0 flowing through transformer B will decrease further. After 360 seconds, this current drops to 3.86mA. At this point, the thermistor R... T The voltage across transformer B is 211.2V, and the voltage across transformer B is 10.56V. In this state, in the second embodiment, linear transformer B is connected to the thermistor R. T The power supply circuit's static power consumption is less than 855mW, less than 1W, which does not meet energy-saving requirements. Although it can be considered to be in a shutdown state, there is still a significant power consumption. At this point, opening the second switch K2 achieves zero power consumption. Since the circuit's operating current is very small (3.86mA) when the second switch K2 is open, the residual magnetism of the linear transformer B is close to zero. When power is restored, the surge current of the linear transformer B at the moment of power-on has dropped to a very low level, achieving the invention's objective. In actual testing of the second embodiment, with the second switch K2 not initially open, the power consumption is below 600mW after 5 minutes and below 500mW after 15 minutes. If the second switch K2 is opened at this point, zero power consumption can be achieved, and the residual magnetism of the linear transformer B is even closer to zero.

[0049] When linear transformer B needs to operate normally, the first switch K1 is closed and the second switch K2 is closed. At this time, since linear transformer B was in a state where the residual magnetism was close to zero before the first switch K1 and the second switch K2 were closed, its residual magnetism is extremely small. At the moment the first switch K1 and the second switch K2 were closed, multiple measurements were taken and the peak surge current corresponding to a peak value of over 305V at 220V AC was recorded. The maximum peak value was only 0.83A, which is lower than the traditional peak surge current of 11.1A, down to below 7.5%. The effect is very good. Moreover, linear transformer B is very quiet when powered on, without any vibration noise, thus achieving the purpose of the invention.

[0050] Another beneficial effect is that, since the second switch K2 is the power switch of the audio amplifier in Embodiment 2, and it is turned off in the current state, the anti-sparking capacitor that was originally connected in parallel with the second switch K2 as the power switch can be eliminated, or a small-capacity anti-sparking capacitor can be used. In addition, since the peak surge current is controlled when the power is turned on, the current margin of the second switch K2 does not need to be large, thus reducing costs.

[0051] Example 3

[0052] The first embodiment of the present invention still has shortcomings. When the audio amplifier is to be turned off, that is, when the first switch K1 is in the open state, the linear transformer B passes through the thermistor R. T Power is supplied, and after a few minutes, the thermistor R is used. T The impedance rises rapidly due to heat, causing the AC current flowing through linear transformer B to drop to the milliampere level. The terminal voltage of linear transformer B drops to less than one-seventh of the original mains voltage, i.e., below 31.4V. This voltage is still relatively high. The present invention will provide a solution in the third embodiment, as follows.

[0053] like Figure 3 As shown, in Figure 1 Based on the corresponding embodiment one, it also includes a third switch K3, a first resistor R1, and a thermistor R T The components are thermally packaged together. The third switch K3 is connected in series with the first resistor R1, and then in parallel with the linear transformer B. When the linear transformer B is not required to operate normally, the third switch K3 is in the closed state; when the linear transformer B is required to operate normally, the third switch K3 is in the open state. The first resistor R1 can be a PTC thermistor. Here, an MZ23 type PCT thermistor is selected, with a nominal resistance of 500Ω and a Curie point of 80℃. Thermal packaging means that the two components are packaged together by encapsulation material or a heat sink, so that the heating of either component will cause the other component to heat up simultaneously. Other rectification, filtering, and Class AB amplification circuits are the same as in the first embodiment.

[0054] The principle is explained as follows: when the audio amplifier is to be turned off, i.e., the first switch K1 is in the open state, the linear transformer B passes through the thermistor R. T When power is supplied, the first resistor R1 also heats up rapidly to 80℃ due to the high terminal voltage. The first resistor R1 and the thermistor R T The components are thermally sealed together, utilizing a thermistor R. T The impedance is affected by the heat generated by the thermistor R1. T The impedance rises rapidly, causing the AC current flowing through linear transformer B to drop to the milliampere level. Because of the parallel connection of the first resistor R1, the voltage across linear transformer B decreases further. At this point, due to the significant drop in voltage across the first resistor R1, its own heat generation is very slight; its temperature mainly comes from the thermistor R. T Thermistor R T The Curie point is slightly lower than the Curie point of the first resistor R1, where the thermistor R... T The Curie point is 75°C. The resistance of the first resistor R1 is about 1kΩ at this temperature. It is connected in parallel with the linear transformer B through the closed third switch K3. After the first switch K1 is opened, the terminal voltage of the linear transformer B drops to 5.5V after 180 seconds. Compared with the first embodiment where the terminal voltage of the linear transformer B is 31.4V, the terminal voltage of the linear transformer B is further reduced, and its remanence is further reduced to a minimum.

[0055] The heating of the first resistor R1 causes the static power consumption of the audio amplifier to decrease rapidly. At the same time, because the terminal voltage of the linear transformer B is further reduced, its residual magnetism is further reduced. When the power is turned on again, that is, the first switch K1 is in the closed state and the third switch K3 is in the open state, the peak surge current is extremely small, the linear transformer B is very quiet and does not make any vibration noise, thus achieving the purpose of the invention.

[0056] Example 4

[0057] The third embodiment of the present invention still has shortcomings. When the audio amplifier is to be turned off, that is, when the first switch K1 is in the open state, the linear transformer B passes through the thermistor R. T Power is supplied, and after a few minutes, the thermistor R is used. T The impedance rises rapidly due to heat, causing the AC current flowing through the linear transformer B to drop to the milliampere level. Due to the installation of the third switch K3 and the first resistor R1, the terminal voltage of the linear transformer B drops to less than one-thirtieth of the original mains voltage, i.e., below 7.33V. However, the audio amplifier still has static power consumption. In many fields and work environments, many people hope to truly cut off the power supply to achieve electrical safety and zero power consumption. This invention will provide a solution in this embodiment, as follows.

[0058] Similar to the second embodiment, the third embodiment adds a second switch K2 to the power supply circuit. When the linear transformer B is not required to operate normally, the first switch K1 is in the open state, and after a delay, the second switch K2 is opened again; when the linear transformer B is required to operate normally, the second switch K2 is in the closed state. Other rectification, filtering, and Class AB amplifier circuits are the same as in the first embodiment.

[0059] That is, when the terminal voltage of the linear transformer B is further reduced to the desired value in the third implementation, its residual magnetism is further reduced to a minimum. When the peak surge current of the next start-up reaches the desired value, the second switch K2 is opened, and zero power consumption of the circuit can be achieved.

[0060] Another beneficial effect is that, since the second switch K2 is the power switch of the audio amplifier in Embodiment 2, and it is turned off in the current state, the anti-sparking capacitor that was originally connected in parallel with the second switch K2 as the power switch can be eliminated, or a small-capacity anti-sparking capacitor can be used. In addition, since the peak surge current is controlled when the power is turned on, the current margin of the second switch K2 does not need to be too large, thus reducing costs.

[0061] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art and hobbyists, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention, such as the thermistor R. T The MZ92 type motor starting thermistor is selected, or a thermistor specially developed for this invention is used. For example, the first resistor R1 is not the same as the thermistor R. T The components are packaged together to achieve the same purpose; after the first resistor R1 and the third switch K3 are connected in parallel, they are no longer connected in parallel with the linear transformer B, but rather with the linear transformer B and the thermistor R. T The two-terminal circuits connected in series can also achieve the purpose of the invention by connecting them in parallel; for example, the first switch K1, the second switch K2, and the third switch K3 can be replaced by relay contacts controlled by a microcontroller or other timing circuits, which can also achieve the purpose of the invention.

Claims

1. An audio amplifier, comprising at least a linear transformer, a positive temperature coefficient thermistor, and a first switch, wherein the linear transformer and the thermistor are connected in series and electrically connected to an alternating current, and the first switch is connected in parallel across the thermistor via an electrical connection, wherein the first switch is in a closed state when the linear transformer is required to operate normally, and in an open state when the linear transformer is not required to operate normally.

2. The audio amplifier according to claim 1, characterized in that: It also includes a second switch, which is connected in series with the linear transformer and the thermistor in a series circuit. When the linear transformer is not required to work normally, the first switch is in the open state, and after a delay, the second switch is opened again; when the linear transformer is required to work normally, the second switch is in the closed state.

3. The audio amplifier according to claim 1, characterized in that: It also includes a third switch and a first resistor. The first resistor and the thermistor are thermally encapsulated together. The third switch is connected in series with the first resistor and then in parallel with the linear transformer. When the linear transformer is not required to work normally, the third switch is in a closed state. When the linear transformer is required to work normally, the third switch is in an open state.

4. The audio amplifier according to claim 2, further characterized in that: It also includes a third switch and a first resistor. The first resistor and the thermistor are thermally encapsulated together. The third switch is connected in series with the first resistor and then in parallel with the linear transformer. When the linear transformer is not required to work normally, the third switch is in a closed state. When the linear transformer is required to work normally, the third switch is in an open state.

5. The audio amplifier according to claims 3 and 4, characterized in that: The first resistor is a PCT thermistor, and the Curie point of the PCT thermistor is lower than the Curie point of the first resistor.

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