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A good amplifier is not chosen according to personal data
Are there any objective criteria for sound quality, and is it possible to somehow classify the vast range of imported equipment? You need to start this conversation, of course, from the most important thing - from the amplifier, which is the "heart" of any audio system. In fact, a qualified specialist in circuit engineering can understand in advance why a particular device sounds exactly like that and nothing more. Circuit engineering is, first of all, an area of exact knowledge that does not accept mystery, unknowability and esotericism, so densely mixed in arguments about devices comparable in price to a car. And a specialist in many cases is able, having looked at the circuit, to assess the nature of the sound, even without turning on the device, although the sound, of course, is influenced by installation and design. But what if you only have access to instructions? An uncritical attitude to the parameters declared by firms, given in them, can mislead you. The most important parameters that determine the quality of the amplifier are the coefficients of harmonic and intermodulation distortion. They are a consequence of the nonlinearity of the current-voltage characteristics of transistors and lamps. These distortions are closely related to each other and, as a rule, their measured values are of the same order. To characterize the parameters of amplifiers, a generalized term is often used - the coefficient of nonlinear or harmonic distortion (THD, THD). It is clear that the smaller the CII, the higher the sound fidelity. This indicator is also taken into account in the standards for equipment. So even in Soviet GOSTs, a lower SOI corresponded to a higher class of equipment. At the international level, there is only one generally recognized Hi-Fi quality standard - the German standard DIN 45500, which specifies specific values. Amplifiers of this class must have a harmonic distortion factor of no more than 1% in the frequency band up to 12,5 kHz at rated power and an intermodulation distortion factor of at least 3% (measured when two frequencies are fed to the amplifier input: 250 Hz and 8 kHz with an amplitude ratio of 1 :4). Such parameter ratios in the Hi-Fi standard are not accidental. They took into account the results of studies of the physiological characteristics of hearing. The majority of consumers, with such characteristics, consider the price-quality ratio of the device as the best. In other words, only an insignificant part of music lovers is ready to pay a higher price for higher quality. For high-quality amplification, it is also desirable to have a wide frequency range extending up to 40-60 kHz (with a level deviation of no more than -3 dB). This means that the amplifier must transmit signals almost equally not only in the entire audible range, but also far beyond it. The fact is that with an increase in frequency, the efficiency of feedback decreases and, consequently, an increase in the coefficient of nonlinear distortion. At the upper end of the power band, the feedback loses its effectiveness. Therefore, in order for the feedback to have sufficient depth throughout the entire operating range and to effectively suppress non-linear and intermodulation distortions, it is necessary to have a linear frequency response up to frequencies several times higher than the upper limit of the audio range. Further expansion of the operating frequency band, for example, to hundreds of kilohertz and above, significantly complicates and increases the cost of the amplifier. Another important parameter is the slew rate of the amplifier output voltage (Slew Rate), which links its frequency and power characteristics. Many cheap amplifiers transmit only small bursts of a musical signal well, and on powerful passages of hard rock they “harden” their voice, unable to get the power appropriate to the moment from the power source. Therefore, when buying, it is not only interesting, but also desirable to evaluate the dimensions of the power transformer and the total capacitance of the filter's electrolytic capacitors. In practice, it does not hurt to have some margin for this parameter. The next parameter that must be taken into account is the damping factor (Damping Factor). It is measured as the ratio of the recommended load impedance (loudspeaker coil) to the amplifier's own output impedance. If the damping is not enough, then the diffuser will make its own "body movements" that have nothing to do with the music, but depend on the elasticity of the suspension. However, in the vast majority of models, this problem is successfully solved. It can be considered sufficient if the value of the coefficient exceeds one hundred. The level of noise, background, separation of stereo channels - these are already secondary parameters that, to a certain extent, do not affect the nature of the sound and, as a rule, have an acceptable value even in the cheapest models. All of the above is not a secret for technically savvy readers, and the question of how to use this information when choosing an amplifier is much more interesting. Looking through the brochures and instructions attached to the devices, all amplifiers can be conditionally divided into two groups: one with SOI in a fraction of a percent (0,5% -0,1%) and the other - within 0,01% -0,001%. If you carefully analyze the composition of the second group, it turns out that along with expensive and classy equipment, even “pre-high-end” devices entered. In terms of sound quality, they are much worse than even the lowest Hi-Fi, and you can’t even put them next to High End. There will be such amplifiers in the first group. How to explain it? It is possible that the parameters given in the brochures are in no way related to the sound quality and it is impossible to navigate by them. Or something is wrong with the data itself. In fact, data can be shown in an attractive way without saying everything. If, for example, both power and frequency are simultaneously increased, then a massive increase in distortion occurs. Their maximum is achieved, as a rule, at the highest frequency and maximum power, that is, the obtained THD value depends entirely on the mode of its measurement. The traditional method of measuring THD at 20 kHz and at maximum output power is a good indication of the actual quality of the amplifier. If the amplifier has a THD of 0,02% at a frequency of 1 kHz and an output signal amplitude of only 1 volt (at a 4 ohm load this corresponds to only 0,25 W), then at maximum power and a frequency of 20 kHz, the specified parameter may be in the range from one to five percent, which is unacceptable for high-quality sound reproduction. Conversely, a device with a harmonic coefficient of 0,02% at maximum power and a frequency of 20 kHz, with a power of only 0.25 W and a frequency of 1 kHz, is likely to have a vanishingly small THD. The same picture is with intermodulation distortion. By all rights, they should be measured by the two-frequency method at the maximum amplitude of the output signal. If, during the measurement process, an amplitude of 1 V is selected, then the result will be a very small and very attractive value for buyers, but in no way related to the real sound quality. Hi-Fi class amplifiers must be tested in accordance with the DIN45500 standard. If there is no Hi-Fi sign, then the company decides which measurement mode to use. Naturally, it is more profitable to choose just such a mode in which the parameters look the most attractive. On the conscience of such firms, there is also the absence of instructions in the passport how exactly the THD and other characteristics were measured. And so it turns out: the amplifiers have the same THD, but they sound radically different - one breathes music, and the other chokes on it. As always, music lovers who are inexperienced in such tricks turn out to be extreme. Critical listening is not always possible in stores, and here, it would seem, passport data should impartially give out the whole truth in order to solve the problem of choice. However, this is not quite true. If next to the THD values it is indicated that the measurements were carried out at a frequency of 1 kHz, then regardless of other conditions, these data cannot in any way be used to judge the true character of the amplifier's sound. However, most often the testing modes are not indicated at all. [Typical THD curve for a 100W output amplifier] Typical THD curve for a 100W amplifier In order for the ear not to notice the distortion, it is enough that they are quieter than the natural noise in the room. We can say that with THD less than 0,03% (at rated power and over the entire frequency range), the amplifier claims high sound quality. Only claims, because the sound quality can be affected by a whole set of additional parameters and conditions, the analysis of which requires a separate discussion. So, having sifted out all those suspicious in terms of objective parameters, go ahead to listening, which can only determine the true capabilities of the selected amplifier! Author: Dmitry Avkhimkov
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