To play audio, you need to know the knowledge of HIFI amplifier.
Power amplifier, the abbreviation of power amplifier. Its basic function is to amplify the signal from the preamplifier about 1V, so that it can generate enough undistorted output power to drive the speaker to work normally. A power amplifier generally consists of six major parts: input stage, pre-push stage, push stage, output stage, negative feedback circuit, and protection circuit.
The basic requirements for the power amplifier are: sufficient output power and good dynamic characteristics, and require its various distortions to be as small as possible, the signal-to-noise ratio S/N as high as possible, and to have Flat frequency response characteristics and more.
Power amplifiers can be divided into tube power amplifiers and transistor power amplifiers according to their main electronic components, which are commonly known as “tube amplifiers” and “stone amplifiers”. The power amplifier circuit can be divided into several different types such as class A power amplifier, class B power amplifier, and class A and B power amplifier due to different bias conditions.
1. Characteristics of tube power amplifiers
Transistor power amplifiers have dominated the market today, but in Hi-Fi high-fidelity playback systems, tube power amplifiers Amplifiers still have a place. This is because the signal overload tolerance of the tube power amplifier is obviously better than that of the transistor power amplifier, so the required power reserve is more than double that of the transistor power amplifier. Compared with transistor power amplifiers of the same power, the low-frequency sound of the tube power amplifier is softer and the high-frequency sound is thinner, while the transistor power amplifier obviously has “transistor sound” or “metal sound”, that is, the sound is somewhat blunt and burr.
In addition, the negative feedback depth of the tube power amplifier is not large, so there is generally no transient intermodulation distortion, and the gain of the transistor power amplifier is often large, and its electro-acoustic index is achieved by increasing the negative feedback depth. This makes the transistor power amplifier prone to transient intermodulation distortion. In addition, the protection circuit of the electronic tube power amplifier is simple, the thermal stability of the electronic tube is good, and it is generally not easy to be damaged, while the damage rate of the high-power transistor is relatively high. The weaknesses of tube amplifiers are high power consumption, poor anti-vibration and anti-mechanical shock performance.
2. Characteristics of transistor power amplifiers
For three different types of transistor power amplifiers: Class A, Class B, and Class A and B, Class A power amplifiers have Fixed transistor bias, so that there is collector current flowing during the entire period of the sinusoidal input signal, even when there is no input signal, there is still a certain amount of collector quiescent current. For class B power amplifiers, the fixed bias of the transistor is almost adjusted so that the collector current is just in the cut-off state when there is no input signal. Only in the positive half cycle of each alternating cycle of the external input signal, there is a collector current. circulation.
The class A and B power amplifiers work between class A and class B amplifying states, so that the applied signal has a collector current flow during this period of time greater than half a cycle but less than a full cycle. If only one transistor is used as the output stage of the power amplifier, in order to avoid the distortion of the amplified signal, the only way to design the amplifier is to work in a Class A working state. This is the case for most tube power amplifiers and some audiophile-grade transistor power amplifiers. For class B and class A and B power amplifiers, in order to finally obtain an undistorted output signal, a push-pull amplifier circuit composed of two transistors must be used.
The main advantage of Class A power amplifier is that the circuit is simple and easy to operate, the nonlinear distortion is small, and it is suitable for low-power linear audio amplifiers. Now Class A power amplifiers are mainly used in high-end power amplifier products.
The main difference between class B power amplifiers and class A power amplifiers is that the quiescent current is small, so when there is no signal, the power consumption is small, and higher efficiency can be obtained; however, when the class B power amplifier is working, due to The two transistors are turned on and off alternately, so crossover distortion will occur at the connection of the output signal waveforms of the two tubes; and when the power amplifier tube is converted from reverse-bias to zero-bias and then to forward-bias, it increases with the signal frequency. high, the output signal will be delayed in time, resulting in so-called switching distortion.
Therefore, in the actual Hi-Fi high-fidelity sound reproduction system, the class B power amplifier is generally not used, but the class A power amplifier or the class A and B power amplifier with small linear distortion is used. Class A and B power amplifiers reduce crossover distortion by changing the bias method. It compromises the high fidelity of Class A power amplifiers with Class B power amplifiers, thereby solving the above-mentioned contradiction between high efficiency and large distortion to a certain extent.
The main circuit form of class A and B power amplifiers is a single-ended push-pull circuit with complementary or quasi-complementary output. The common ones are OTL no output transformer circuit, OCL no output capacitor circuit and BTL balanced transformerless circuit and many more. The following mainly introduces the OCL no output capacitor circuit.
OCL circuit without output capacitor is a transistor power amplifier circuit without output coupling capacitor. It is developed on the basis of OTL circuit. The main difference is that OCL has no output capacitor and adopts a set of symmetrical The main advantage is that the lower limit frequency is low, the entire frequency characteristic curve is flat, and the distortion is small.
3. Basic protection circuit of power amplifier
a. Speaker protection circuit
For OCL power amplifier The characteristics of direct coupling with the speaker generally require the addition of a speaker protection circuit. This circuit is to prevent the high DC voltage at the output end when the power amplifier fails, causing the DC current to flow into the speaker, which will cause the speaker’s voice coil to shift in light, or burn out the speaker in severe cases. The detection point of this protection circuit is the output end of the OCL, that is, the midpoint. When the DC potential between the point and the ground exceeds a certain value (such as positive or negative 1V), the protection circuit will act to cut off the circuit leading to the speaker. Play the role of protecting the speaker.
The speaker protection circuit generally does not share the DC power supply with the power amplifier, so that when the power amplifier circuit fails, the protection circuit can still work normally. The horn protection circuit generally also has a delay function. When the power amplifier is turned on, the horn will be turned on after a delay of several seconds, which protects the horn from being damaged by the large surge current at the moment of turning on, and also avoids the horn when the power is turned on. The sound of “bang bang” appears in the .
b.Overload protection circuit
In the transistor power amplifier, the high-power output tube works under the condition of high voltage, high current and heavy load. easily damaged. When the instantaneous amplitude of the input signal is particularly large or the output of the power amplifier is short-circuited in the presence of a feed-in signal, the high-power tube will be over-current, and sometimes a high reverse voltage will occur, causing the high-power tube to be damaged by over-voltage. . Set up overvoltage and overcurrent protection circuits for this reason. The protection circuit generally adopts electronic protection, does not need any mechanical contacts and relays, operates quickly, can effectively protect the high-power tube, and has the function of automatically returning to normal.
Class A
Also known as Class A, during the entire cycle of the signal (the positive and negative half cycles of the sine wave), any power from the amplifier A class of amplifiers in which none of the output components present current cutoff (ie, the output stops). Class A amplifiers generate high heat and have low efficiency, but have the inherent advantage of no crossover distortion. Single-ended amplifiers all work in class A, and push-pull amplifiers can be class A, class B, or class A and B.
Class B
Also known as Class B, the positive and negative half cycles of the sinusoidal signal are alternately amplified by the two “arms” of the push-pull output stage. A class of amplifiers that output the conduction time of each “arm” for half a period of the signal. The advantage of class B amplifier is high efficiency, the disadvantage is that it will produce crossover distortion.
Class A and B
Also called class AB, it is between class A and class B, and the on-time of each “arm” of push-pull amplification Greater than half a period of the signal and less than one period. Class A and B amplifiers effectively solve the problem of crossover distortion of class B amplifiers, and the efficiency is higher than that of class A amplifiers, so it has been widely used.
