power amplifierIt is called power amplifier for short, generally referring to the most basic equipment in the sound system, commonly known as“an amplifier”Its task is to transfer the signal frommixer）To amplify the weak electrical signal to drive the speaker to make sound.It can also refer to other equipment for power amplification.
The function of the power amplifier is to transfer the sound from the sound source or the previous levelamplifierThe weak signal of is amplified to push the sound box to play.A set of sound system power amplifier plays an important role.
Physical picture of professional power amplifier
Power amplifier is the largest family of all kinds of audio equipment. Its main function is toSound sourceAfter the weak signal input by the equipment is amplified, enough current will be generated to push the loudspeaker for sound reproduction.Considering power, impedancedistortion, dynamic, different use ranges and control and adjustment functions, and different amplifierssignal processing The circuit design and production process are also different.[1]
In addition to the function of amplifying signals of ordinary power amplifiers to drive loudspeakers to amplify sound, frequency shift power amplifiers can also effectively suppress on-site howling and ensure the quality of voice transmission. Even in poor environments, they can also greatly suppress back howling and protect audio equipment from being burned out by howling.
Frequency shift power amplifier is widely used in high-endMultimedia E-classroom, training room, small conference room or other simple sound reinforcement occasions, the microphone can be directly connected to the frequency shift power amplifier, and can also effectively suppress the whistling of the microphone to the speaker, and support background music access for sound reinforcement.[1]
By conductive mode
Power amplifier
According to the power amplifier tubeelectric conductionDifferent methods can be divided intoClass A power amplifier(also called Class A)Class B power amplifier(also called Class B), Class A and Class B power amplifiers (also called Class AB) and Class D power amplifiers (also called Class D).
Class A power amplifier refers to a kind of amplifier in which no current cutoff (i.e. stop output) occurs in any power output element of the amplifier during the whole signal cycle (positive and negative half cycles of the sine wave).Class A amplifier will produce high heat and low efficiency when working, but the inherent advantage is that there is no crossover distortion.Single ended amplificationAll machines work in Class A mode,Push-pull amplificationThe utensil can be Class A, Class B or Class A and Class B.
Class B power amplifierIt means that the positive and negative half cycles of the sine signal are respectivelyPush pull outputThe two "arms" of the stage amplify the output amplifier in turn, and the conduction time of each "arm" is half a cycle of the signal.The advantage of class B amplifier is high efficiency, but the disadvantage is that it will produce crossover distortion.
Class A and B workIt is set between Class A and Class B, and the conduction time of each "arm" of push-pull amplification is greater than half a cycle of the signal but less than one cycle.Class A and Class B amplifiers have effectively solved the problem of crossover distortion of Class B amplifiers, and their efficiency is higher than that of Class A amplifiers, so they have been widely used.
Class D power amplifier, also known as digital amplifier, uses extremely high frequency conversion switch circuit to amplifysound signalIt has the advantages of high efficiency and small size.Many Class D amplifiers with power up to 1000W are as large as VHS video tapes.This type of amplifier is not suitable for use as a broadband amplifier, but it can be used in activeSubwooferThere are many applications in.[1]
Single ended amplifierThe output stage of is composed of one amplification element (or several elements but connected in parallel as a group) to amplify the positive and negative signals for two and a half cycles.Single ended amplification machine can only adopt Class A working state.
Push-pull amplifierThere are two "arms" (two sets of amplification elements) in the output stage of the. When the current of one "arm" increases, the current of the other "arm" decreases, and the state of the two switches in turn.For the load, it seems that an "arm" is pushing and an "arm" is pulling to complete the current output task together.Although Class A amplifier can adopt push-pull amplification, it is more common to use push-pull amplification to form Class B or Class A/B amplifier.[1]
By amplifier tube type
According to the different types of power amplifier tubes, they can be divided intoAmpsandStonecutter。
Power amplifier is called power amplifier for short, which is used to enhance signal power to drive sound boxElectronics。WithoutSignal source selection、Volume controlSuch auxiliary functions as power amplifier are called rear stage.
The preamplifier is the preamplifier and control part before the power amplifier, which is used to enhance the voltage amplitude of the signal and provide input signal selection,toneAdjustment, volume control and other functions.Preamplifieralso known asPreceding stage。
Pre amplification and power amplificationPartial installationAmplifiers in the same cabinet are called combined amplifiersPower amplifierThey are generally consolidated.[1]
By use
According to different purposes, it can be divided intoAV power amplifier，Hi-FiPower amplifier.
AV power amplifier is specially designed forHome theaterAmplifiers designed for use generally have more than 4 channels andSurround soundDecoding function with a display screen.The main purpose of this kind of amplifier is to truly create the sound effect of the film environment so that the audience can experience the theater effect.
along withLarge screen TVWith the popularity of various image carriers, people's demand for "sitting at home watching movies" is growing, so a multi-function power amplifier with various audio and video functions comes into being."AV" is the initials of English AudioVideo, namely audio and video."AV power amplifier" has experiencedDolby surround Dolby directional logic, AC-3, DTS process, the difference between AV power amplifier and ordinary power amplifier is that AV power amplifier has AV optionDolby Pro Logic Decoder, AC-3, DTS decoder, and five channelpower amplifier。And the finishing touchDigital sound field(DSP) circuit provides different sound field effects for various programs.However, because AV power amplifier has gone through too many and complicated processing circuits in the signal flow link of the circuit, the purity of sound has been "stained" too much, so the effect of using AV power amplifier compatible with HI-FI playback is not ideal.This is also the reason why many HI-FI enthusiasts despise AV amplifiers.
Hi Fi power amplifierIt is an amplifier designed to reproduce the true nature of music with high fidelity. It is generally designed for two channels and has nodisplay。
Power amplifier
"HI-FI power amplifier" is usEnthusiastThe amplifier ofoutput powerGenerally, it is below 2X150 watts.The design aims at "beautiful timbre and high fidelity".All kinds of high and new technologies are concentrated on this power amplifier.The price also ranges from more than 1000 yuan to several hundred thousand yuan."HI-FI power amplifier" is also divided into "split type" (to separate the front amplifier) and "combined type" (to integrate the front and rear amplifiers).Generally speaking, the "split type" in the same level of modelsSignal-to-noise ratio, and other indicators such as channel division are higher than the "merging machine" (not absolute).And easy to passThe signal lineCompare sounds.The merging machine has the advantages of convenient use and low relative cost. The output power of the cheap merging machine is generally designed below 2X100W, and many manufacturers produce high-end merging machines above 2X100W.[1]
According to the components used
According to the different components used, the power amplifier has "courage"（Tube power amplifier），“Stonecutter”(transistor power amplifier)“IC power amplifier”（Integrated circuitPower amplifier).Due to the use of new technology and new concepts in gall bladderElectronic tubeThis ancient vacuum device also shines brilliantly, and its beautiful voice makes many people worship it.SeniorEnthusiastAlmost all have one."IC power amplifier" is rarely seen in HI-FI power amplifier because its voice color is not as good as the two power amplifiers above.[1]
By user
Power amplifiers can be generally divided into three categories: "professional power amplifiers"“Civil power amplifier”"Special power amplifier".
Power amplifier
“Professional power amplifier”It is generally used for conference, performance, hall, hall, stage and hall sound reinforcement.Designed tooutput powerLarge, perfect protection circuit, good heat dissipation.Most "professional amplifiers"timbreWhen used for HI-FI playback, the sound is dry and hard.
The detailed classification of "civil power amplifier" includes "HI-FI power amplifier" and "AV power amplifier"“Karaoke amplifier”And the so-called "integrated power amplifier" that integrates various common functions.
The difference between "Karaoke power amplifier" and general power amplifier is that "Karaoke power amplifier" has a reverberator, from BBD analog reverberation to DIGITAL（Digital Reverb）, modulator,Microphone amplifier。Some manufacturers, in order to meet the market demand, combine various functions including AV power amplifier and Karaoke power amplifier into a whole, which is called "integrated power amplifier". This is a hodgepodge of power amplifiers, which has everything and can't do anything well. It is a low-grade power amplifier, not pursuing sound quality, but pursuing rich functions.
"Special power amplifier", as its name implies, is a power amplifier used in special occasions, such as an alarm,Vehicle low-voltage power amplifierWait, I won't introduce it here.[1]
performance index
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The main performance indicators of power amplifier are output power,frequency response, distortion, signal-to-noise ratio, output impedance,Damping coefficientEtc.
Music power: refers to the maximum instantaneous output power of the power amplifier to the music signal when the output distortion does not exceed the specified value.[2]
peak power
peak power : It means that the power amplifiervolumeWhen set to maximum, the maximum music power output by the power amplifier.[2]
Rated output power
Power amplifier
Rated output power: WhenharmonicAverage output power at 10% distortion.Also called maximum useful power.Generally speaking, the peak power is greater than the music power, and the music power is greater than the rated power. Generally speaking, the peak power is 5-8 times of the rated power.[2]
frequency response
frequency response: for power amplifierfrequency range , and the unevenness within the frequency range.Frequency responseThe straightness of the curve is generally expressed in decibels (db).The frequency response of home HI-FI power amplifier is generally 20Hz -- 20KHZ plus or minus 1db. The wider the range, the better.The frequency response of some top power amplifiers has reached 0 -- 100KHZ.[2]
Distortion degree
Distortion degreeThe ideal power amplifier should be the faithful restoration of the input signal after amplification without any change.However, due to various reasons, compared with the input signal, the amplified signal by the power amplifier often has different degrees of distortion, which is called distortion.In percentage, the smaller the value, the better.The total distortion of the HI-FI power amplifier is between 0.03% - -0.05%.The power amplifier has harmonic distortion,Intermodulation distortion, cross distortion,ClippingDistortion, transient distortion,Transient InterModulation Distortion Etc.[2]
Signal-to-noise ratio
Signal-to-noise ratio: refers to various signal levels and power amplifier outputsNoiseThe level ratio, expressed in db, is as large as possible.Generally, the signal to noise ratio of household HI-FI power amplifier is above 60db.[2]
Output impedance
Output impedance: The equivalent internal resistance of speakers is called output impedance.
Whether the performance index of a power amplifier is complete or not may prove that it has good timbre, which is something that beginners must realize.This is also the pursuit of many enthusiasts.[2]
AV power amplifier
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Technical characteristics of AV power amplifier
AV power amplifier
There are many professional terms and technology applications in AV power amplifier. The following are some mainstream technologies.
1. Multi channel independent amplifier - high-end AV power amplifiers all use the same independent amplifier inside, and the sound quality of each channel will not interfere with each other to ensure the consistency of sound quality.
2. Audyssey MultEQXT - enables each group of speakers to obtain the best frequency response in the listening space.Accurately lock instruments and sound effects to form a seamless surround sound field.
3. Denon Link HD digital time base control - free from the influence of time base distortion, enjoy more detailed and more enveloping blue ray sound effect.
fourHDMI1.4a - Compared with the previous HDMI version, it supports the latest 4K, 3D, ARC and other technologies.
5. New 4K UHD resolution - the next generation of UHD resolution after 1080P, as of 2012, only a few AVR products support it, such as DENON AVR-3313.
6. In AVR, 4K display is divided into various technologies, including: pass through, up frequency, GUI display, etc.In a real sense, the AVR with 4K UHD display should have a pass through function. The frequency up function is actually to upgrade the ordinary video signal to the 4K specification, while the GUI can overlay the setting menu on the display device.
7. AirPlay function, wirelessStreaming Media Technology, it is more convenient to play streaming music through smart devices or computers.[2]
HIFI power amplifier
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Technical characteristics of HIFI power amplifier
HIFI power amplifier
HIFI power amplifier is slightly different from AV power amplifier. HIFI power amplifier often has the following characteristics:
1.2.0 Channel stereo. Most HIFI power amplifiers have only two channels of output, which is different from the surround sound effect of AV power amplifiers. HIFI power amplifiers pay more attention to high fidelity sound quality, lower sound distortion, and truly restore music.
2. Advanced HIFI power amplifier adopts front and rear stage separation type to ensure higher quality sound reproduction.
3. The super dynamic amplifier module (HDAM) in the HIFI power amplifier helps to produce more dynamic, accurate and refined sound.
4. A high quality HIFI power amplifier must adopt a stable mechanismMetalPanel, thereby reducingaudio frequencyThe mutual interference between circuits is very helpful to the improvement of sound quality.
5. Current feedback high-speed discrete typeCircuit, which is conducive to achieving high channel separation in superelevation domain.
6. With FC. The high-end power amplifier of B.S. system function can be connected to two front stages, and the left and right sound channels can be completely separated from the control amplifier for signal transmission and amplification.[2]
Troubleshooting
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The common faults of HI-FI audio and AV amplifier are that the whole machine does not work, no sound output, light sound, high noisedistortion, screaming, etc.
The following describes the troubleshooting ideas and skills for various faults.
The whole machine does not work
Power amplifier
The failure of the whole machine does not work is that the amplifier has no display after power on, all function keys are invalid, and there is no sound, as when power is not on.
Check the power circuit first during maintenance.availableA multimeterMeasure the DC resistance at both ends of the power plug (the power switch should be turned on), which should be hundredsOhmsThe resistance value of.If the measured resistance is much smaller and the power transformer is seriously heated, it indicates that the primary circuit of the power transformer has a local short circuit;If the measured resistance is infinite, checkFuseWhether it is fused, whether the transformer primary winding is open circuit, and whether there is disconnection between the power line and the plug.Some machines have added temperature protection devices, which are connected in the primary circuit of the power transformerCurrent fuse(It is usually installed inside the power transformer and can be seen by removing the insulating paper outside the transformer). If it is damaged, it will also open the primary circuit of the power transformer.
If the resistance at both ends of the power plug is normal, power on to measure whether the output voltage of the power circuit is normal.For system control microprocessor orlogical control For the amplifier of the circuit, check whether the power supply voltage (usually+5V) of the control circuit is normal.
If there is no+5V voltage, measureThree terminal regulated integrated circuitCheck whether the input terminal voltage of 7805 is normal. If the input terminal voltage is abnormal, check the rectifier and filter circuits.If the 7805 input terminal voltage is normal, but the output terminal has no 5V voltage or the voltage is low, disconnect the load to see whether the+5V voltage can return to normal.If the+5V voltage is normal, the fault is in the load circuit;If the+5V voltage is still abnormal, the fault is in 7805 itself.
If the+5V power supply voltage of the system control circuit is normal, check the clock andReset signalWhether it is normal, and whether the keying and display drive circuits are damaged.[2]
No sound output
Silent fault shows that there is corresponding status display but no signal output when operating each function key.
Power amplifier
Maintenance Yesprotection circuit The power on protection shall be checkedrelayWhether it can be closed.If the relay does not act, measure whether the output voltage at the midpoint of the power amplifier circuit deviates and whether the over-current detection voltage is normal.If the midpoint output voltage deviation or over-current detection voltage is abnormal, it indicates thatPower amplifier circuitIf there is a fault, check whether the positive and negative power supplies are normal.If the positive and negative voltages are asymmetric, the load circuits of the positive and negative power supplies can be disconnected to determine whether the power supply circuit itself is abnormal or the power amplifier circuit is faulty.If the positive and negative power supplies are normal, check whether each amplifier tube in the power amplifier circuit is damaged.
If the midpoint output voltage and over-current detection voltage of the power amplifier circuit are normal, but the protection relay does not pull in, the fault is in the protection circuit. Check whether the relay drive integrated circuit or drive tube is damaged, and whether each detection circuit is normal.ifRelay contactIt can be pulled in, but there is no sound output. Check whether the loudspeaker is normalrelayCheck whether the contact is in good contact and whether the squelch circuit acts.
If the above parts are normal, use the signal interference method to check whether the fault is in the post amplifier circuit or the pre amplifier circuit.useA multimeterR × 1 gear, ground the red probe, and quickly touch the input end of the rear amplifier circuit with the black probe. If there is a strong sound in the speaker, the fault is in the front amplifier circuit;If the speaker does not respond, the fault is in the rear amplifier circuit.
For the integrated circuit power amplifier circuit without peripheral protection circuit (usually with thermal protection inside the integrated circuit), first measure whether the power supply voltage is normal.If the power supply voltage is normal, check with the signal interference method: add a DC intermittent signal to the signal input terminal of the power amplifier integrated circuit. If the speaker has a strong sound, it means that the power amplifier integrated circuit is normal, and the fault is in the front amplifier circuit;If there is no sound and relevant peripheral components are normal, the fault lies in the power amplifier integrated circuit itself.
Tube power amplifierIf there is no sound output, check the power supply first to see if the filament is on and the shell temperature is normal.If the filament is not lit and the tube shell is cold, check whether the filament and plate voltage of the power amplifier tube are normal.If the voltage is abnormal, further check the power circuit. If necessary, disconnect the power load circuit to determine whether it isPower circuitThe fault or the load has a short circuit.If each voltage is normal, the volume can bepotentiometerAdd DC intermittent to the center head ofinterference signalIf there is a strong response, it indicates that the rear amplifier circuit is normal, and the fault is in the front amplifier circuit;On the contrary, the fault is in the post amplifier circuit.The interference signal can be added to the grid of the push tube and the grid of the input amplifier tube respectively. The stage at which the interference signal is added has no response, indicating that the circuit behind the stage is not working properly.Suspicious components (such as electronic tubes) can be repaired by replacement method.
haveDolby surround For AV amplifier with decoding function, if the main sound channel sound is normal under Dolby surround sound state and all sound channels are silent and straight through state, it is usually Dolby surround sound decoding circuit orSystem controlThe circuit is not working properly.If all sound channels are silent in surround sound and direct mode, check the system control circuit, signal selection circuit and total volume control circuit.[2]
Toneless
Power amplifier
The so-called tone light fault refers to that the gain of the amplifier or the output power of the amplifier is reduced due to the large change or attenuation of an amplifier stage during the amplification and transmission of the audio signal.
During maintenance, first check whether the signal source and speaker are normal, and replace them.Then check various change-over switches and control potentiometers to see if the volume can be increased.
If all the above parts are normal, it should be judged whether the fault is in the front stage or the rear stage circuit.If the sound of a certain channel is light, the signal output from its front circuit can be exchanged and input to the rear circuit of another channel. If the sound size of the speaker does not change, the fault is in the rear circuit;On the contrary, the fault is in the front circuit.
There are two main reasons for the low tone caused by the post amplifier circuit: insufficient output power and insufficient gain.The input signal can be properly increased (for example, the signal output from the radio recorder to the loudspeaker can be directly added to the input terminal of the rear power amplifier circuit, the volume of the radio recorder can be changed, and the output of the power amplifier can be observed) to determine the cause.If the output sound is loud enough after increasing the input signal, it means that the output power of the power amplifier is sufficient, but the gain is reduced, and it should be checked emphaticallyRelay contactWhether the contact resistance increases, the input coupling capacitance decreases, the isolation resistance increases, the negative feedback capacitance decreases or opens, and the negative feedback resistance increases or opens.If the output sound is distorted and the volume does not increase significantly after increasing the input signal, it indicates that the output power of the later stage amplifier is insufficient. First, check whether the positive and negative power supply voltages of the amplifier are low (if only one sound channel is soft, it is unnecessary to check the power supply), whether the performance of the power transistor or integrated circuit has deteriorated, and whether the emitter resistance has increased.
The soft sound caused by the change-over switch and potentiometer in the front circuit can be easily found by visual inspection and can be cleaned or replaced.If you suspect aSignal couplingIf the capacitance fails, it can be tested in parallel with the capacitance of the same value;If the performance of amplifier tube or operational amplifier IC is poor, it can also be checked by substitution method.In addition, if the negative feedback component is faulty, the circuit gain will also decrease.[2]
High noise
The noise of amplifier includes alternating sound, burst sound, induced noise and random noise.
Power amplifier
During maintenance, it is necessary to determine whether the noise is from the previous stage or from the subsequent stage circuit.The signal connection plugs of the front and rear stages can be removed. If the noise is significantly reduced, the fault is in the front stage circuit;On the contrary, the fault is in the subsequent circuit.
AC sound refers to the noise generated by AC power to the surrounding air, which is mainly caused by poor filtering of the power supply. It is necessary to check whether the power supply rectification, filtering and voltage stabilizing components are damaged.Decoupling capacitors at the power supply end of the front and rear amplifier circuitsFaulty solderingOr failure will also produce an oscillating noise similar to AC sound.
Inductive noise is a kind of AC sound with complex components, which is mainly caused by poor grounding of transfer switch and potentiometer in the front circuit or poor shielding of signal wiring.
The popping sound refers to the intermittent "popping" sound. In the front level circuit, check whether the signal input plug is in poor contact with the socket, change-over switch, potentiometer, etc., and whether the coupling capacitor has faulty soldering, leakage, etc.The post amplification circuit shall be checkedRelay contactWhether there is oxidation, and whether the input coupling capacitor has leakage or poor contact.In addition, the soft breakdown of the differential input transistor or the constant current transistor in the later stage circuit will also produce a "pop" noise similar to an electric spark.
Random noise refers to random and irregular signals, usually composed of input transistorsField effect transistorOr the background noise caused by poor performance of the operational amplifier integrated circuit can be replaced with components of the same specification during maintenance.[2]
distortion
Circuit diagram
The distortion fault is caused by the offset of the operating point of an amplifier stage or the asymmetric operation of the push-pull output stage of the power amplifier.During maintenance, the specific fault location can be determined according to the change of amplifier output power and distortion.
If the electronic tube amplifier is distorted and the output power decreases (soft sound), check whether one of the amplifiers in the push-pull power amplifier is aging, the operating point is wrong, orOutput transformerThe partial short circuit causes its work imbalance;If the output power becomes larger at the same time of distortion, it is mostly negativeFeedback circuitResistance variation, capacitance failure or cathode self generated bias inBypass capacitanceIt is caused by short circuit.
If the distortion of the transistor amplifier increases significantly with the increase of the volume, check whether the operating point of a transistor at the driving stage is offset (usually in the power amplifier without protection circuit) or the capacitance distortion in the feedback circuit;If there is distortion regardless of the volume, the fault is in the front amplifier circuit. Check whether the operating point of each amplifier is offset.
The abnormal working voltage of the integrated circuit amplifier or the internal damage of the power amplifier integrated circuit will also cause distortion (refer to the machine without protective circuit).[2]
Howl
The whistling fault is in the circuitSelf excitationAs a result, it can be divided into low-frequency whistling and high-frequency whistling.
Low frequency whistling refers to the low frequency "thump" or "beep" sound, which is usually caused by poor power filter or decoupling (noise is often accompanied by whistling). Check whether the power filter capacitor, voltage regulator and decoupling capacitor are open circuit or invalidInternal resistance of power supplyenlarge.The performance of the power amplifier IC is poor, and the low-frequency whistling fault will also occur. At this time, the operating temperature of the IC will be very high.
The high frequency of high-frequency whistling is usually caused by the failure of the high-frequency damping capacitor of the amplifier circuit or the deterioration of the performance of the front-end operational amplifier IC.It can be checked by connecting small capacitors at both ends of the vibration elimination capacitor or decoupling capacitor of the subsequent amplifier circuit.In addition, when the negative feedback component is damaged, changed or desoldered, it will also cause high-frequency positive feedback and high-frequency whistling.[2]
Noise reduction
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Some cheap power amplifiers hum when they are turned on, which not only affectstone quality, and it's annoying.Several treatment methods are introduced:
Power supply and grounding point treatment
The filter capacitance of many power amplifiers is small, sometimes only about 1000 μ F, and a 0.22 μ F CBB capacitor is connected at both ends. This can not only reduce the noise of power amplifiers in static state, but also improve the transient strength of power amplifiers in large dynamic state.Even after the above treatment, some power amplifiers still have noise, which may be due to improper grounding point. Generally, the grounding point should be near the filter capacitor and the "one point grounding method" should be used.
Output stage
If the static potential of the output stage deviates from the zero point, great noise will be generated. At this time, there may be a problem with the zero adjustment resistance or the output tube. The output point potential of the power amplifier can be carefully adjusted, which should be below 100mv. If the zero point cannot be adjusted, carefully check whether some components of the power amplifier, such as the tube, are damaged.
Pre amplification part
First, short-circuit the input end of the preamplifier to the ground to see if the noise disappears. IfnoiseIt disappears, and the noise can be identified as coming from the input signal line, which can be replaced with a three coreShielding wireNote that the shielding layer can only be grounded at one end.In addition, the coupling capacitor shall be the one with small leakage, such asTantalum capacitor, MKP capacitor, etc. The volume potentiometer shell should be grounded.[2]
maintain
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1. The user should place the power amplifier in a dry and ventilated place to avoid working in a humid, high temperature and corrosive environment of oil smoke chemicals.
2. The user should place the power amplifier in a safe, stable and not easy to drop table or cabinet for use, so as not to damage or fall on the ground, damage the machine or cause greater man-made disasters, such as fire, electric shock, etc.
3. The user should keep the power amplifier away from the environment with serious electromagnetic interference. For example, the electromagnetic interference emitted by the aging fluorescent lamp ballast will cause the CPU program of the machine to be disordered, resulting in the machine not working properly.
4. When wiring the PCB, pay attention that the power supply pin should not be too far away from the water tank. If it is too far away, add 1000-470U and place it at its feet.[2]
Important parameters
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Several important parameters of power amplifier
1. Input sensitivity refers to the minimum input signal required by the power amplifierlevel, which requires thatSound sourceThe signal is amplified enough to push the necessary conditions for the subsequent power amplifier.
2. HarmonicDistortion degreeThis is an extremely important indicator of power amplifier, and the harmonic distortion isnonlinear distortion It is caused by the nonlinear characteristics of the amplifier during operation, and the distortion results in newHarmonic component, making the voice lose its original timbre, and breaking in serious cases.There are also odd and even harmonics distortion, odd harmonics will make people irritable, disgusted, easy to be perceived.Some amplifiers sound irritable and tired, which is caused by the large distortion.The biggest impact on the power amplifier is the distortion. Generally, the harmonic distortion is required to be below 0.05% for high fidelity, and the lower the better.In addition to harmonic distortion, there are intermodulation distortion, cross distortion, clipping distortion, transient distortion,Phase distortionThey are the culprits that affect the quality of power amplifiers.The first thing to check the effectiveness is its distortion. For example, the total harmonic distortion of the Italian Sinfoni power amplifier is below 0.01%.
3. The problem of output power and power is most difficult for car audio practitioners to understand. Here we need to explain it one by one:
A、Rated output power, called (RMS), refers to the audio signal output by the amplifierTotal harmonic distortionThe maximum power that can be output within the range.It is generallyAC signal0.707 times the peak value.
B、Average powerThe average power generally refers to the average power consumption at each frequency point. It is somewhat similar to the rated output power, but it generally refers to the time.
C、Peak output power: The maximum music power that can be output by the power amplifier is called peak output power, which does not consider distortion, and is usually about 1.414 times of (RMS) power.
D、Peak to peak power, which is positiveVoltage peakThe peak power to the negative voltage, which is four times the peak output power.Its appearance is for commercial purposes and has no practical significance.
4. Signal to noise ratio, the larger the value, the better, generally expressed in (S/N), expressed in decibels of the ratio of signal power Ps to noise power Pn, S/N=10lgPs/Pn=20lgVs/Vn (db), where Vs and Vn are signal voltage andNoise voltage。
With the increase of signal to noise ratio and input signal level, the signal to noise ratio increases gradually, but when the input signal level reaches a certain value, the signal to noise ratio basically remains unchanged.According to the requirements of high fidelity, the signal-to-noise ratio should be more than 90dB. Imported high-end power amplifiers can often reach 110-120dB, and its performance can be imagined.In short, the higher the signal to noise ratio, the smaller the noise mixed in the signal, the better the playback quality, so that the playback music is clean and layered.
5. Frequency response, commonly known as power in the early daysbandwidthWhen the harmonic distortion does not exceed the specified value, it refers to the 1/2 rated power band width of the power amplifier, that is, the band included between two frequency points with a high and low end drop of - 3dB, which is called power bandwidth.
6. Damping coefficient, mainly for low frequencies, is an extremely important technical parameter that directly affects the bass sound quality.As we all know, the larger the diameter of the horn, the better the bass. However, it often shows unnatural sound, especially at 100-400Hz low frequency, which is easy to cause sound staining and make people sound unnatural.
In the design of power amplifier, engineers take some technical measures for the power amplifier, such as selecting multi tube parallel connection, low internal resistance (milliohm level) high-power tube, improving the working voltage, selecting high-quality wire, etc., and trying to improve the damping coefficient so that it can produce a "resistance" effect against the inertial motion of the horn, so that the motion of the sound basin can synchronize with the audio signal,Try to make the sound basin return to the zero position (i.e. the center position) as soon as possible after the end of the driving signal. The blocking effect is the Damp Factor, D=Rs/Ri, Rs=horn impedance, Ri=internal resistance of the power amplifier output. The larger D is,PhonophoreThe better the synchronization effect with the signal, the purer and cleaner the bass, and the better the playback effect.
7. Slew rate, the conversion rate of power amplifier greatly affects the quality and performance of high tone playback.The faster the conversion rate is, the better the treble quality is, and the more accurate the fleeting high-frequency information can be captured.High grade power amplifiers can achieve more than ten to dozens of V/us, and low and medium grade power amplifiers are generally not marked. The value of this conversion rate is closely related to the design and materials, but it should not be too high, because too high will produce an ultra high frequency signal above 20KHz that cannot be heard by the human ear, which will not only have no effect on improving the sound quality, but will easily burn out the tweeter.[2]
Shopping points
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First, check whether the interfaces are complete.
OneAV power amplifierThe basic input/output interfaces should include the following: coaxial, optical fiber, RCA multi-channel input interfaces for digital or analog audio signal input;The horn output interface is used to output signals to the audio system.
The second is to see whether the surround sound format is complete.
The popular surround sound formats are DD and DTS, both of which are 5.1 channels.Now these two formats have developed to DD EX and DTS ES, both of which are 6.1 channels.
The third is to see whether the power of all sound channels is individually adjustable.
Some low-priced amplifiers divide two channels into five channels, and each channel can be adjusted separately if it is large or small.
The fourth is the weight of the power amplifier.
In general, heavier models should be selected as far as possible, because heavier equipment has a strong power supply first, and most of the weight of power amplifier comes from the power supply and chassis. Heavy equipment means that the transformer used is larger or the capacitor with larger capacity is used, which is a way to improve the quality of the amplifier.Secondly, the chassis is heavy. The materials and weight of the chassis have a certain impact on the sound. The chassis made of certain materials can help to isolate the circuit inside the chassis from the radio waves walking outside.The chassis has high weight or stable structure, which can also prevent equipment from being affected by unnecessary vibration and sound.The third is the heavier power amplifier, which usually uses rich and solid materials.
Power isAudio systemThe most important parameter in, which represents the capacity of the audio system with load.This is also the first thing we should pay attention to when buying.However, if each manufacturer uses different measurement standards to identify product performance, it is often difficult to make an objective comparison without sufficient knowledge.The same is true for power amplifiers. When viewing the power identification of power amplifiers, the following three points should be noted:
First,Battery voltage.
The voltage of the car battery is constantly changing. For the power amplifiers with two common marks: 14.4V/100W and 12V/100W, there are two completely different power descriptions.Since the voltage of the vehicle during driving is basically about 12V, the measured power value at 12V is closer to the real situation.Moreover, the power amplifier with continuous voltage of 12V as the benchmark can achieve greater power when it reaches above 12V.
Second, harmonic distortion rate THD.
When comparing the continuous output power of the power amplifier, it needs to be conducted under the same (or relatively close) THD value.The difference of sound quality measured under different THD values is very obvious. Sometimes the maximum power of its logo is very high, but it is likely that its distortion and noise are also very high.Therefore, when checking the maximum power, pay attention to its marked THD value.
Third, frequency range.
Power amplifierContinuous powerThe output shall be tested within the frequency range actually used.For the power of the power amplifier, it is required to identify the complete detection range. When only identifying a certain frequency, the power value is meaningless.After determining the same benchmark, we can compare the power of power amplifiers.Generally, the principle of high power output is followed when purchasing audio systems.The higher the output power of power amplifiers, the stronger their ability to drive speakers.The power of power amplifier shall be greater than that of loudspeakerIndicated powerIf the selected power is too small, it is easy to burn out when using the high-power output for a long time, which will also lead to poor sound quality, distortion and other faults.
Factors of high-quality power amplifier
Of course, it is impossible to better understand the quality of power amplifier only by the introduction of high-power words.The high-quality power amplifier must also be able to quickly reflect the peak value of the music signal, and at the same time be able to respond to the strongBassAnd can provide a stable output in a low distortion/low noise state.In order to meet these requirements, we must have the following: First, the power supply with excellent performance.This is the key to the sound quality of the left and right power amplifiers.The power supply part and amplification part shall be designed separately to reduce noise.Adopt largeStep down transformerIncrease the supply of stable current, and large capacitors can react more quickly to supply the current needed for amplification.Secondly, the built-in parameter equalizer.Car audio is very different from home audio. The installation position of speakers is very limited, so sound adjustment is very important.In addition, due tohead pillowAnd the sound masking effect of the window and theInstallation angleThe resulting sound wave confusion will affect the sound effect of the car audio system.At this time, the parameter equalizer works, which can compensate the wave crest and wave trough of the sound wave caused by the above reasons, and adjust the smooth sound field.Moreover, it is built-inFrequency divider。No matter how excellent the function of the power amplifier is, when it is actually installed in the vehicle, it cannot achieve the best effect due to various audio problems and speaker configuration problems.In order to overcome these problems, a frequency divider should be used in addition to the parameter equalizer.The built-in frequency divider has two advantages: first, the system is expandable, and the power amplifier and speakers can be combined freely.Second, make the adjustment simple and easy.This can improve the sound quality of the whole system.The use of an external frequency division system is not recommended in the selection of vehicle power amplifiers because of its complex wiring, easy to mix with noise, large space required for installation, and system price rise.[2]
Power amplifier configuration
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Speaker systemTo reproduce various music programs with high quality, the peak factor is about 10-15dB according to the properties of the music signal. From the perspective of ensuring the sound quality, the power amplifier shoulddynamic range There is no amplitude limiting inside, that is, the maximum output power of the power amplifier should be 5-8 times the rated power of the speaker. Although the sound quality of this power configuration is very good, its investment will be large, so the power ratio is generally set at 1-2 timesspeaker unit Rated power of.One or two times of this range may be out of space, so we can give you a more specific experience.
1. In some projects with low requirements and limited investment, the power of the power amplifier is at least equal to the rated power of the speaker, but great attention should be paid to keeping the sound undistorted. Too small power configuration does not seem to damage the speaker unit, but in fact, too small power is prone to overload clipping, generating a large number of harmonics, and burning the tweeter unit.
2. In general engineering, it is recommended that the power of the power amplifier should be 1.5 times, and the bass part should be more than 1.5 times, so as to get enough sense of strength.
3. Extremely demanding sound ground, such asrecording roomMonitoring, concert hall, etc., ideally twice the power of the speakerInternational Electrotechnical CommissionOne of the recommended values in the matching standard formulated by IEC is consistent).[2]
Power amplifier matching
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When designing and installing a sound system, it is inevitable to encounter the matching problem between the amplifier and the speaker.In terms of timbre, we will pay attention to whether the matching is warm and cold, soft and hard, and finally make the whole set of equipment restore the timbre to neutral, which is only from the perspective of art.From the technical point of view, the elements of power amplifier and speaker matching include: first, power matching, second, power reserve matching, thirdimpedance matching 4. Damping coefficient matching.If we realize the above four points when matching, the performance of the equipment used can be fully and maximally exerted.
Power matching
In order to meet the requirements of high fidelity listening, the rated power should be determined according to the best listening sound pressure.We all have the feeling that the volume is low, the voice is weak, thin, dynamic, no enthusiasm, low frequency is significantly lacking, the fullness is poor, and the voice seems to shrink inside.When the volume is appropriate, the sound is natural, round, soft, full, powerful and dynamic.However, when the volume is too high, the sound is not soft and has an ear shaking feeling.So replaySound pressure levelIt has a great relationship with the sound quality. The sound pressure level of the specified listening area should preferably be 80~85dB. We can calculate the rated power of the speaker and the rated power of the power amplifier from the distance from the listening area to the speaker and the characteristic sensitivity of the speaker.
Power reserve matching
loudspeaker box: In order to make it able to withstand the impact of burst strong pulse in the program signal without damage or distortion.Here is an empirical value for reference: the nominal rated power of the selected speaker should be three times the power calculated theoretically.
Power amplifier:Tube power amplifierCompared with transistor power amplifier, the required power reserve is different.This is because the overload curve of the electronic tube power amplifier is relatively flat.For the peak of overloaded music signal, the tube power amplifier does not obviously produce clipping phenomenon, but only makes the tip of the peak rounded.This is what we often call flexible shear peak.However, after the transistor power amplifier is at the overload point, the nonlinear distortion increases rapidly, causing serious clipping of the signal. It does not round the peak but neatly cuts it flat.Some people use the composite impedance composed of resistance, inductance and capacitance to simulate speakers, and test the actual output capacity of several high-quality transistor power amplifiers.The results show that when the load has phase shift, there is a 100W nominal power amplifier, and the actual output power is only 5W when the distortion is 1%!Therefore, the selection of the reserve amount of the transistor power amplifier:
For the average of the systemSound pressure levelAnd the maximum sound pressure level.It depends on the content of the program and the working environment.The minimum redundancy is 10dB. For modern pop music, disco and other music, 20~25dB redundancy is required, so that the sound system can work safely and stably.
impedance matching
It refers to the rated load impedance of the power amplifier, which should be consistent with the rated impedance of the speaker.At this time, the power amplifier is in the optimal design load line state, so the maximum undistorted power can be given. If the rated impedance of the speaker is greater than the rated output impedance of the power amplifier, the actual output power of the power amplifier will be less thanRated output power。If the rated impedance of the speaker is less than the rated output impedance of the power amplifier, the audio system can work, but the power amplifier is in danger of overload. The power amplifier must be completeOvercurrent protectionMeasures are taken to solve the problem. For the electronic tube power amplifier, the impedance matching requirements are stricter.
Matching of damping coefficient
The damping coefficient KD is defined as: KD=rated load impedance of power amplifier (equal to rated impedance of loudspeaker)/internal resistance of power amplifier output.
Because the output internal resistance of the power amplifier has actually become the power of the speakerdamperThe KD value determines the resistance of the speaker.The higher the KD value, the heavier the electrical damping. Of course, the higher the KD value of the power amplifier is, the better it is. If the KD value is too high, the electrical damping of the speaker will be too heavy, so that the establishment time of the pulse front will increase and the transient response index will be reduced.Therefore, large KD value should not be pursued unilaterally when selecting power amplifier.As a home hi fi power amplifierDamping coefficientThere is an empirical value for reference. The minimum requirement is that the KD value of transistor power amplifier is greater than or equal to 40,Electronic tubeThe KD value of the power amplifier is greater than or equal to 6.
The basic conditions to ensure good steady-state and transient characteristics of sound reproduction should pay attention to the equivalent mechanical quality factor (Qm) andamplifierThe damping coefficient (KD) should be considered as a part of the whole sound system.The equivalent resistance of the feeder of the loudspeaker shall be small enough to be negligible compared with the rated impedance of the loudspeaker.In fact, the power loss of the loudspeaker feeder should be less than 0.5dB (about 12%) to achieve this coordination.[2]
matters needing attention
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Precautions for use of power amplifier
1. It is better to use 220V to ensure safe voltageAC regulated power supplyOr DC high-voltage voltage stabilizing module.
2. The error of V+and V － shall not be greater than 1V, and the positive and negative power supply and ground shall be firmly welded. The power can be energized only after the welding is completed and confirmed to be correct.
3. In the initial stage after the power on of the power amplifier IC is normal, its stability is relatively poor compared with that of the discrete component power amplifier. Therefore, at least“Burn-in ”Or low volume playback for more than 10 minutes, which can play its excellent performance stably and efficiently.
4. In the production of power amplifier, it is necessary to strictly ground a point. The ground wire is made of multi strand thick copper wire with good effect, and even can use double bridge rectification combined with floating ground technology to maximize its signal-to-noise ratio.
5. If the local power grid is seriously polluted, the low-voltage power grid is connected with electric welding machineSilicon rectifierAnd other electrical equipmentPower filterIf it cannot be eliminated, the power supply isolation transformer shall be used, but the power shall have margin.
6. Pay close attention to the switching sequence of the audio equipment. For the power amplifier made with Hi Fi power amplifier IC, remember to turn it on last and turn it off first.
7. Before the newly purchased power amplifier IC is put on the machine, it is better to use a socket instead of welding, and fix the radiator. If the heat is serious and DC is output after power on, it can be removed and returned to the mail order unit.
8. A horn protector must be installed in order to prevent the loudspeaker from being damaged by the output DC of the amplifier IC.
9. The system equipment must be well grounded.Because the three-phase load of low-voltage distribution line is asymmetric, the neutral line will be electrified, and the potential will be zero after grounding, which is very beneficial to improving the signal noise ratio.The method is: Φ 10mm 1.5m long round steel is welded with 2.5mm2 multi strand copper wire (not riveted or wrapped) and inserted into outdoor wet ground.[2]
