Quality criteria and the problem of consumer competencies
There are several approaches to consumer definition of quality, but none provide a guarantee of a good buy. If the fidelity of reproduction and power (loudness) can be assessed subjectively, then problems with the reliability and stability of the parameters may arise. I even encountered cases when very decent sounding expensive amplifiers of little-known high end manufacturers began to work as oscillators, began to emit a rumble in a fit of self-excitation.
If you do not go into details, then in order to understand the quality of the product, you should have minimal knowledge in the circuitry of amplifiers and the physics of the processes on which they are built, have a specific amplifier circuit in your hands and know about the features of the elements used in the design of the device. Those. Ideally, you need to be an engineer or at least an experienced radio amateur for this assessment. Most buyers do not have such competencies. This makes it possible for numerous marketing manipulations, ranging from the appearance of the device to a manipulative approach to measuring the basic parameters.
Formal criteria for the quality of an amplifier for a consumer are data from manuals or datasheets. It should be remembered that they reflect the real picture only if the measurements were carried out within the framework of accepted standards and there must be indicated the power of the device, the range of reproducible frequencies and frequency response unevenness, the coefficient of nonlinear distortion, the signal-to-weighted noise ratio, analog and digital are listed interfaces. Less commonly in the documentation, you can find data on the damping factor, crosstalk between channels and the difference in channel gain.
Power
Any data in datasheets can be distorted for the purpose of marketing manipulation. More often this happens with power, as we wrote about here . So, instead of RMS and DIN, which have clear design criteria, terms like program power can be used, which, in fact, do not mean anything, since the power calculation method is known only to the creators of the amplifier. It makes sense to look at the value of the consumed power, if it is approximately equal, slightly more, and even more so, if it is less than the declared program power, then the power data is clearly distorted, and the used measurement technique does not allow you to see any real picture.
For the consumer, this means that one should look in the RMS indication and that it is impossible to focus on the Program power value, since this value actually means the so-called. marketing power of the device. Valid values ​​are:
DIN - the power value at a real load (for an amplifier), limited by the appearance of nonlinear distortion. Measured by applying a 1 kHz signal to the device input for 10 minutes. Power is measured when 1% THD is reached. This power calculation standard is identical to the Japanese EIAJ standard adopted by the Electronic Industries Association of Japan.
DIN Music Power describes the meaning of continuous loading of a music signal without risk of damage. IEC Power is the same DIN Music Power, but with a strictly defined measurement duration of 100 hours.
RMS (Rated Maxmum Sinusoidal) is the maximum (limiting) sinusoidal power at which an amplifier or speaker can operate for one hour with a real music signal without physical damage. Usually 20 - 25% higher than DIN. RMS is almost identical to the AES power defined by the AES2-1984 standard.
In the Soviet and Russian documentation, you can also find the "Rated power" parameter - it is determined with the middle position of the amplifier volume control, at which the other parameters of the device correspond to those stated in the technical description. This is a manipulative indicator, like program power, since it can be measured at the most favorable value of harmonic distortion and can be adjusted to applicable standards. Interestingly, for all the manipulativeness, the “Soviet rating” is usually lower than other values, for example, the rated power of 35 W corresponds to approximately 110 W RMS (AES power), 90 W - IEC Power (DIN Music Power). Program power values ​​are usually twice (or more) RMS, i.e. 35W nominal can correspond to 220W Program power.
Frequency response and frequency range
It's even more interesting with the frequency range. It is known that a person is able to hear frequencies from 20 Hz to 20 kHz, while the ultrasonic components of the recording can be preserved in the HiRes music signal. At the same time, it is obvious that a wide frequency range of the amplifier is created for a reason. Raising the upper threshold of the frequency range is a way to improve the transient response of the amplifier, since the high frequency region corresponds to the transient response in the short time region. More on this here .
So, the GOST 24388-88 in force until now. Household amplifiers of audio frequency signals. The general technical conditions, partially borrowed from the German standard DIN 45500 of 1977 and modified, assumes a frequency range of 10 to 40,000 Hz for amplifiers of the zero complexity group (i.e. high fidelity), and from 20 to 25,000 Hz for amplifiers of the first group of complexity. ...
In this case, the unevenness in the standard is indicated just in the range of audible frequencies and should be no more than ± 0.3 dB for zero and ± 0.5 for the first group. The current international standard for amplifiers is IEC 60268-3: 2018, the norms of the standard regarding frequency response are almost identical to the Russian (Soviet) GOST and German DIN 45500.
For the consumer, this means that it makes sense to choose an amplifier with a reproducible frequency range of at least 20 Hz to 20 kHz with an uneven frequency response of no more than ± 0.5 dB. Also, if the fidelity of reproduction is very critical, it makes sense to choose an amplifier with a range from 10 Hz to 40 kHz (and above) and unevenness in the audible spectrum (from 20 Hz to 20 kHz) no more than ± 0.3 dB. I emphasize, not because the customer has become a bat and hears above 20 kHz, but because the expansion of the frequency range improves the transient response.
THD
A significant characteristic of an amplifier, which objectively speaks of quality, is the total harmonic distortion, according to the same Soviet standard for preliminary and integral amplifiers (as separate devices) it should be up to 0.005% and for power amplifiers up to 0.007% for the zero group. And also 0.05% and 0.07%, respectively, for the first group. As in the case of frequency response, similar requirements exist in all modern (and not so) world standards for high fidelity audio equipment.
For the consumer, this means that it makes sense to look for an amplifier with a THD value with a maximum THD value of 0.07%, and with high claims and audiophile requirements for a reproduction fidelity of 0.007% and below. I must say that finding such an amplifier is quite simple, since most modern ones boast a relatively low THD.
IMD
It should be noted that in addition to harmonic distortion, amplifying equipment is a source of intermodulation distortions, which rarely fall into datasheets, and meanwhile, they seriously harm the fidelity of reproduction, are perceived as blurred sound. The DIN 45500 standard, which is considered the source of standards for HI-FI-class equipment, determined that for high-fidelity amplifiers, the “intermodulation distortion (IMD) coefficient in the reproducible frequency range of 250-8000 Hz (also outside this band with a decrease in the sound pressure level by 6 dB) ”, should not exceed 3%.
Out of 400 datasheets and manuals of amplifiers that I have seen recently, IMD values ​​were indicated in five, all of them cost more than 100K rubles. And the point is not even that the manufacturer is trying to hide the truth at all costs, but that the measurement of an additional parameter, which 0.1% of consumers of mass equipment knows at all, is regarded as not a very rational decision.
For the consumer, this means that, most likely, he will not find this parameter even in documents of rather expensive devices. You can determine intermodulation by ear for this, it is enough to use the recordings of the children's and female choir dictation. It is necessary to try to concentrate attention on individual voices, if this cannot be done, and individual voices are not heard clearly - probably, we are talking about a sufficiently large coefficient of intermodulation distortion. It is also important to understand that their source may not be an amplifier, but a speaker system, so for this subjective test it makes sense to use the best possible speaker system or compare with a certain reference amplifier on one speaker system.
Signal to Weighted Noise Ratio
Signal-to-Noise Ratio is a parameter of amplifiers that shows the level of noise in the absence of a signal. In accordance with the mentioned standards, the signal-to-weighted noise ratio should be at least 80 - 90 dB for HI-FI pre and integral amplifiers and 100 - 110 for high fidelity power amplifiers. The minimum value for pre and integral amplifiers is 63 dB and for power amplifiers 86 dB. I must say that most modern amplifiers have complete order with this parameter, and if the values ​​differ significantly from those given above, we can say that we are clearly talking about a low quality device.
It makes sense for the consumer to pay attention to the signal-to-weighted noise ratio, since attempts to make a circuit design cheaper or a not very professional approach to PCB layout in modern equipment sometimes give disastrous results. It is important that the value is at least 60-80 dB; for demanding music lovers, you should focus on 90 dB and above.
Lamp wonders
Sometimes in expensive tube devices the signal / weighted noise ratio is lower, due to the imperfection of archaic circuitry solutions, when this parameter is sacrificed for some other useful, from the point of view of creators or eccentric consumers, effects, for example, some characteristic sound that is rated as more “musical”, “warmer”, “genre compatible”. By the way, a similar story happens with nonlinear distortions. So, the harmonic distortion even in ultra-expensive tube amplifiers can reach 3 and even 5%.
Amplifier classes
Class A Class A
amplifiers are traditionally considered to have the greatest fidelity. In theory, simple circuitry and, as a rule, single-ended operation without clipping the signal can minimize harmonic distortion (both THD and IMD), as well as reduce the order of harmonics ... The downside of the solution is tiny efficiency, which rarely exceeds 15 - 17%, and, accordingly, huge dimensions and mass become additional problems. Energy consumption is also growing naturally.
For consumers striving for maximum fidelity of reproduction, not constrained in funds and not afraid of huge mass and dimensions, this option is ideal. For everyone else, it is not rational and unacceptable.
Class B
In class B, the operating mode is push-pull, the element (lamp, npn transistor) reproduces either positive or negative (pnp transistors) input signals. In this case, the conduction angle is equal to 180 ° or slightly exceeds this value, in connection with which the IMD and THD increase. The advantage of this mode is its relatively high efficiency, which in theory can reach 75%. Today this class has been almost completely replaced by class D class A / B amplifiers.
Class A / B
From class AB, it is clear that this is an attempt to combine high efficiency and low harmonic distortion. To eliminate the stepped transition that exists in class B, a cut-off angle of 90 degrees or more is applied when switching amplifying elements. Accordingly, the operating point is at the beginning of the linear section of the current-voltage characteristic. For this reason, locking of the amplifying elements is excluded and a quiescent current flows through them, sometimes quite significant. This slightly reduces efficiency compared to Class B, but significantly reduces harmonic distortion. The disadvantage of this class is the insignificant problem of stabilization of the quiescent current, which is solved in various ways.
Class D
The most common, cheap and high-performance, as well as one of the most controversial classes of amplifiers, is class D. Such amplifiers are often referred to as digital amplifiers, since PWM modulation is used for amplification. They consist of a filtering unit, a 4-channel PWM controller, a current amplifier, an output low-frequency filter, a protection unit and a power supply. Key advantage: extremely high, in comparison with other classes of efficiency, in theory, capable of reaching 90% or more. Also class D has a number of problems, namely:
- Non-linearities caused by the modulation method (timing errors).
- Inconsistency of the timing characteristics of the output transistor control circuits.
- LC low pass filter nonlinearity.
- Electromagnetic pickup, incl. interference from the power supply.
"Congenital diseases" are only the first and second points, their influence becomes less and less with the improvement of the manufactured amplifying chips. Many of the problems, thanks to which these amplifiers have earned a not very good reputation, were defeated by manufacturers back in the late 2000s. The non-linearity of the LC low-pass filter is solved schematically and depends solely on the circuitry of a particular amplifier. Electromagnetic pickup and interference from the power source are also solved schematically and structurally. Accordingly, class D is not a sentence at all.
Dry residue
The main quality criteria for amplifiers are parameters such as power, frequency response, THD. It also makes sense to pay attention to IMD and signal-to-weighted noise ratio. The standards created in different countries after 40 years describe the values ​​of these, which high fidelity amplifiers must comply with, such standards include DIN 45500, GOST 24388-88, IEC 60581, IEC 60268-3: 2018, in accordance with the norms of which the most modern amplifiers. A high fidelity amplifier can be built in any class, including class D, which are currently the most common. I tried to choose the criteria that are most significant for the fidelity of the amplifier. Certainly not described everything, so the damping factor,I left channel separation by gain and crosstalk between stereo channels for other materials. If you have something to add - I will sincerely appreciate the additional information in the comments.
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