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Until recently, the struggle for the efficiency of amplifiers and sound quality went in two directions. Analog amplifiers provided an improvement in sound quality at the same time as a decrease in efficiency, while digital amplifiers provided high efficiency with low signal quality. Simultaneously, these problems can be solved with the joint use of digital and analog signal processing methods, and many years of development have been crowned with success. According to Tripath Technology, its Class T amplifiers combine high efficiency with audiophile sound quality. The magazine has already written about the main classes of audio frequency amplifiers. Economy Class Amplifiers B significant low-level signal distortion ("first watt") is characteristic, audiophile class amplifiers A incredibly voracious. Compromise class solutions AB does not completely solve any of the problems.
At best, only half of the power consumed by the amplifier goes to the load. The rest heats the output stage transistors. To improve the efficiency of analog amplifiers, many technical solutions have been proposed that can be summarized in three groups: 1. Parallel operation on the total load of a low-power class cascade A and powerful class B (Class Super A). 2. Work on the total load of cascades with different supply voltages (class G). 3. Control of the supply voltage of the output stage (class H). However, the complexity of the design did not justify the savings, and amplifiers of these types were not widely used even in home appliances. In the automotive sector, the situation is even worse:
Class Amplifiers D - achievement of design ideas of the "digital" era. Their main feature is the use of pulse width modulation (PWM, also known as PWM - pulse width modulation) instead of amplification. Unlike analog amplifiers, where the output signal is a "magnified" copy of the input signal, the output signal of amplifiers of the class D is a rectangular pulse. Their amplitude is constant, and the duration ("width") varies depending on the amplitude of the analog signal entering the amplifier input. The pulse frequency (sampling frequency) is constant and, depending on the requirements for the amplifier, ranges from several tens to hundreds of kilohertz. After formation, the pulses are amplified by terminal transistors operating in the key mode. The conversion of the pulse signal to analog occurs in the low-pass filter at the output of the amplifier or directly in the load. The main advantage of amplifiers of this class is high efficiency (up to 95% in the best samples). This is explained by the fact that the amplitude of the pulses is almost equal to the supply voltage and the power losses on the output transistors are minimal. Distortion increases as the signal frequency increases and the sampling frequency decreases. The output power also depends indirectly on the sampling rate - with increasing frequency, the inductance of the coils decreases and the losses in the output filter decrease. Like analog amplifiers, switching amplifiers are divided into subclasses. AD и BD, and their advantages and disadvantages are also similar. In class amplifiers AD in the absence of an input signal, the output stage continues to work, delivering pulses of the same duration to the load. This makes it possible to improve the quality of transmission of weak signals, but significantly reduces the efficiency and gives rise to a number of technical problems. In particular, you have to deal with the so-called through current, which occurs when the output transistors are simultaneously switched. To eliminate the through current in the output stage, a dead time is introduced between the closing of one transistor and the opening of another.
Practical applications are found in amplifiers of the BD class, which are simpler in design, the output stage of which, in the absence of a signal, generates pulses of very short duration or is at rest. However, in amplifiers of this type, the main drawback of the method is most pronounced - the dependence of the level of nonlinear distortion on the sampling frequency and signal frequency. In addition, distortion increases when low-level signals are transmitted. Creating a high-quality broadband class amplifier D requires significant design complexity. Therefore, in car audio systems, such amplifiers are still used only in subwoofers - in this case, the level of non-linear distortion up to several percent is quite acceptable. In class amplifiers T the sound quality has been improved by an order of magnitude while maintaining high efficiency. This is especially true when creating head unit power amplifiers. Tripath Technology manufactures 10W and 20W integrated amplifiers for portable audio equipment and head units, as well as microcircuits for creating higher power amplifiers - up to 300W.
The graphs show that class amplifiers T in terms of performance they are not inferior to the best samples of analog amplifiers. The level of distortion is minimal, and there are practically no higher harmonics in the spectrum of the output signal. As a result, the reproduction of the musical signal becomes more natural. The main difference between the new amplifiers and both analog and traditional digital ones is the low level of intermodulation distortion, which is lower than the harmonic coefficient. For class amplifiers AB, for example, the coefficient of intermodulation distortion significantly (sometimes several tens of times) exceeds the harmonic coefficient; for class amplifiers A these quantities are of the same order. Integrated amplifiers in this indicator are somewhat inferior to their "large" class counterparts. T, but traditional microcircuits do not compete at all. Therefore, it is not surprising that a large number of radio tape recorders and amplifiers of a new type were presented at the last exhibition in Las Vegas. What is the secret of the method? Using proprietary technology Digital Power Processing(TM). A lot of text is devoted to this technology in the company's materials, but there is, for obvious reasons, very little useful information. The secret contains not only details, but also the principle of signal processing. If we discard the rhetoric, then it all comes down to two interrelated processes - "prediction" (Predictive processing) and "adaptive transformation" (Adaptive Signal Conditioning Processing) Let's try to figure out "how they get stronger here." From time immemorial, priests and fortune-tellers have been engaged in predictions, and with varying degrees of success. In our case, you can find out the level of the sound signal in two ways:
Judging by the fact that the dynamic range of even integrated amplifiers exceeds 100 dB, the signal amplitude is calculated. Why do you need to know her? In class amplifiers T there is no fixed sampling rate - it changes continuously in the bandwidth up to 1,5 MHz in accordance with the "adaptive conversion" algorithm. The initial data is just the amplitude of the signal and the rate of its change. Upsampling improves sound quality and simplifies the design of the output filter. One can only guess about the essence of the processing algorithm. In addition to the above, adaptive conversion can also include internal negative feedback - digital or analog. Based on this, it can be assumed that the Digital Power Processing(TM) put one of the varieties of delta - modulation. It differs from the traditional pulse-width one in that it is not the absolute value of the signal that is transmitted, but its change relative to the previous state (hence the "delta" in the name). Negative feedback is included in it genetically, and "prediction" also has a place to be ... Chips are produced directly by Tripath Technology. A significant number of various components are produced, including ready-made amplifier modules. All signal processing functions are concentrated in one chip with a minimum of external components. Amplifiers of low and medium power are manufactured in integrated design. In high power amplifiers, the output stage is performed on discrete components. The output LC filter is in all cases mounted separately. And as an illustration to what has been said - a few numbers:
Publication: www.bluesmobil.com/shikhman
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