Principle and design guide of power amplifier circuit

    The power tube of professional power amplifier mostly adopts collector output mode, and the emitter output of this household power amplifier is very different The advantage of collector output is that the mica sheet can not be used on the heat dissipation gas, so that the heat dissipation of power tube is better

    In addition, most household power amplifiers use OCL circuits of differential amplification, but professional power amplifiers generally can‘t see the opposite tube of differential amplification. Generally, professional power amplifiers use an operational amplifier (4558, etc.) as the voltage amplification stage, and the overall gain is about 34dB

    In order to ensure stability, there are many protection circuits, such as overvoltage and overcurrent protection, temperature protection, midpoint potential protection, etc In addition, the fan speed will also accelerate with the increase of temperature

    In addition, many professional power amplifiers will have a function of compressing and limiting the amplitude. That is, when the output power is detected to be close to the limit, the signal at the input end will be automatically shunted and adjusted so that the output will not be seriously clipped. The simple circuit is realized through a photoelectric coupler, and also through professional IC (LM13700) In this respect, we have done a good job in the Peavey amplifier

     In order to meet the high power requirements of the stage, the professional power amplifier can be adjusted to BTL bridge drive, so that the power can be increased by about three times. For example, after 800W+800W is changed to BTL, 2400W output is possible But now it is a single channel

    It is precisely because of the huge power that special care should be taken when adjusting the circuit. The static current must be adjusted from the minimum. If the static current is adjusted too much, the power tube will be burned in an instant, or even catch fire

    Over the years, I have produced and sold more than 1000 power amplifiers, and the circuits are also diverse. Basically, all the typical circuits have been made, so I will talk about the differences in the sound quality of various circuits. The following comparison is based on the fact that the power supply, housing, components, output stage, and other matched equipment are the same. The difference is only the circuit.

    1. Double-power asymmetrical two-stage differential circuit (such as PIONEER M22K)

    2. Dual power supply is symmetrical, the first stage is typical differential, and the second stage is common beam amplification (such as PHILIP‘s LHH1000)

    3. Dual power supply is symmetrical, the first stage common radiation common base differential, and the second stage common radiation common base (such as Jinsheng E-305V)

    4. Dual power supply is symmetrical, the first and second stage common radiation common base differential, and the third stage common radiation common base (such as Jinsheng A-100)

    Circuit 1, which is the common circuit of many imported low-grade machines below 8000 yuan. Many people think that this is a single-ended Class A voltage amplification mode, which can eliminate crossover distortion. In the actual audition, this kind of circuit gives people a soft and slow feeling. The low frequency is loose, the feelings of the human voice are rich and quite prominent. It has a certain thickness, but the volume sense is insufficient. The high frequency has the performance of fading (the measured closed-loop gain is 10-60000 Hz). There is a feeling of looking at the flowers in the fog. It is difficult to determine the outline of the instrument. In general, the timbre aspect is closer to the performance of courage. This is similar to the timbre performance of imported pure power amplifiers below 8000 yuan.

    Circuit 2. This kind of circuit is relatively less used in imported equipment. It may be that it is high or low, usually in the range of several thousand to five thousand yuan. In this machine, I use DC servo, so the low frequency performance is better than circuit 1, the control force is more appropriate, and the definition is also improved. The throat and nasal sounds in the human voice are clear and audible, and the volume sense is also good. The medium and high frequencies are transparent, but the outline of the instrument is not enough, and the overall performance is better than circuit 1.

    The gap between circuit 3 and the first two circuits has widened, and many imported high and medium grade machines with tens of thousands of yuan also use this circuit mode. In particular, the definition of the middle and high frequency band may be attributed to the use of the common radiation common base circuit. The timbre performance, the quality and quantity of the voice and the instrument are quite sufficient, and there is no misty feeling, especially the voice and the violin, worry and resentment, lightness and steadiness, and the feelings of the performer can be clearly explained. The high frequency circuit is much smoother than the first two circuits, and there is no sense of decline.

    Circuit 4 used to have an irregular period of time. Domestic enthusiasts highly respected the theory of "simplicity first". At that time, the author also belonged to the period of parroting. Therefore, circuit 1 and 2 were commonly used at that time. Later, with the increase of experience, the circuit was slowly improved a little, and circuit 3 was developed. After two years of improvement, circuit 4 was successfully made. Why are there still people advocating "simplicity first" at that time and even today? There may be two reasons: 1. Poor component performance. Ten years ago, many enthusiasts used ordinary tubes such as C1815. What‘s more, the author found that many of these universal triode diodes were only packaged by foreign chips bought by some domestic manufacturers after working as designers in some electrical manufacturers in recent years. The more fake they are, the worse their frequency characteristics are. Of course, using these tubes can‘t be guaranteed. 2。 The technical level is low. Many people do not have debugging equipment, and often can only rely on a multimeter. They are not familiar with the setting and adjustment of the working point of the circuit, and do not know if there is a slight self-excitation. Therefore, the stability of a simple circuit is easy to ensure, and the sound quality is better than that of a complex circuit.

    When the technical level has reached a certain level, we can make complex circuits, but the results are not the same. In theory alone, the linearity of complex circuits is easier to do better, or it is not convincing enough. But you might as well open some audio magazines and compare the low-end and high-end photos of imported equipment. It is easy to find that the circuit of high-end machines is much more complex, or the circuit schematic diagram introduced in some literature, The high-end engine is also relatively complex.

      To get back to the point, when I first made circuit 4 more than a year ago, it was amazing. In a month, I was asked to copy four parts. By contrast, its sound quality has an extraordinary feeling. The performance of the musical instrument is needless to say, the contour positioning makes people have a panoramic view, the performance of the human voice appears to be full of quality, the light makes people feel active, the melancholy makes people cry, all the weak tones are not let go, there is no compromise with the musical instrument, there will be no sense of favoritism, there will be a trend to enhance the control of the speaker, and the rest will not be swallowed or lengthened. Compared with the current popular current amplification circuit, the power of resolution is greater than that of the current amplification circuit, and there will be no thinner sound like the current amplification circuit.

       This circuit is also complex enough. In terms of voltage amplification stage, the number of components is a multiple of the above three circuits, and there are 21 optical triodes. If it is used to replace the operational amplifier in the previous stage or CD, there are more than 24 triodes, and each triode is used for signal amplification and has no power supply constant current, but its performance can not be replaced by the operational amplifier or other discrete circuits.

    In short, the circuit is reasonably complex, and the link of power conversion is reasonably simple, so that the effect can be pushed to the peak. The most important thing to do well is the technical level of the producer, which has been honed for a long time.