Modern audio amplifiers fall into several classes. If a person does not have experience in setting up audio equipment and working with acoustics, class designations will not tell you anything. Meanwhile, these letter markings hide specific characteristics and indicators that affect the quality of sound equipment. Below we will talk in more detail about what classes of sound amplifiers are, as well as methods for selecting equipment for specific tasks.
What is an amplifier class
Amplifier class is the level of the output signal formed from the input signal during the operation of the acoustic installation. In one cycle, this signal changes in a certain range.
Frequency converters are divided into categories depending on the linearity characteristics of the technology used for amplification. There are models with the required accuracy of reproducing the initial LF and ULF signals, as well as nonlinear circuits. In the second case, the sound signal is reproduced with less accuracy, but the efficiency is higher.
Minuses
Continuing the topic of reducing energy consumption, it is impossible not to note the fact that the transition from class A to class AB gave a much more significant increase in amplifier efficiency than the transition from AB to G or H. At the same time, class D surpasses all previous classes in energy efficiency much more significantly , and against its background the difference between class AB and classes G/H begins to seem completely insignificant. In light of this, the issue of technically more complex circuitry of classes G and H comes to the fore. In fact, this design is one and a half to two times more complex than a conventional class AB with all the ensuing risks of reduced reliability and stability of operation.
Amplifier classification
Classification of sound amplifiers by class involves division into two groups. The first group includes devices A, B, AB and C. They have similar conductivity indicators. The transistor is installed in a position between on and off.
TOP best amplifiers by sound quality
The second group includes devices marked D, E, F, G, S, H and T. These devices are also called switching devices. To work here, the principle of pulse modulation is used, as well as modern digital techniques for the continuous passage of the signal between the off and on positions. The result is the desired output signal in the saturation range.
Below is a table comparing the characteristics of amplifiers of the first group, which includes models A, B, AB and C. They are more often taken for consideration, while the amplifiers of the second group represent different variations with average performance for use in specific conditions.
| Characteristics | A | IN | AB | WITH |
| Theoretical efficiency | 50% | 78% | Depends on the selected mode | 100% |
| Real efficiency | 15-30% | 50-60% | 40-50% | 80-100% |
| Nonlinear distortion | small | high | average | high |
| Power consumption | constant | depends on output parameters | depends on output parameters | depends on output parameters |
| Thermal stability | low | high | average | high |
Practice
Tube circuit design is a delicate matter, so most manufacturers practice improving one particular combination of tube operating mode and amplification class. The desire of developers to obtain an ideal (according to their ideas) sound and the subsequent rejection of any alternative methods of switching on lamps is quite understandable, but when searching for a test subject, our task was exactly the opposite: to be able to compare the same set of lamps in at least two versions inclusions.
This significantly reduced the choice of candidates, however, a suitable option was found. It was the Cayin CS-100A - a device literally created for all kinds of experiments. Its design allows the use of two types of output tubes: KT88 tetrodes and EL34 pentodes. It is possible to choose between triode and ultra-linear modes with an output power of 50 or 80 W per channel, respectively. Moreover, the amplifier circuitry in both cases is push-pull, and it operates in class AB.
Among other things, the Cayin CS-100A is a good example of a modern implementation of a traditional tube amplifier. It has a classic layout with a removable cover lamp grille, and carries on board output transformers of substantial size, providing not only sufficient power, but also a wide range of reproduced frequencies. The components meet modern quality requirements: the amplifier uses carbon resistors, audiophile capacitors, a toroidal power transformer and silver cable wiring. The installation was carried out using a hinged method - just as it was done more than half a century ago. This is not so much a tribute to history as a way of shortening signal paths. In general, the Cayin CS-100A is a device that fully fits the definition of a High End tube device.
What is the difference between amplifier classes
To fully understand the classification of amplifiers, it is recommended to study each category in detail, considering the operating features of the devices, as well as the specifics of application in certain cases.
Class A
Category A amplifiers are considered common and affordable for use. They are simple in design, characterized by linearity, as well as average distortion rates. These features help achieve the desired sound quality when organizing acoustics.
More often, one type of transistor is used in the design. It is connected so that current flows to the speakers even when the main input signal is not flowing.
A device can be classified as group A if the zero current during no-load is identical to the load current during operation.
Category A amplifiers operate in the ultra-linear frequency domain, which means the bias needs to be set correctly. This is the only way to ensure that the required sound flow is achieved.
class A amplifier
Since the output equipment is turned off, it conducts current and causes power loss. Heat is generated and efficiency decreases to 40%. So the devices seem impractical when organizing high-power installations.
In order for the equipment to operate correctly, the power supply is given the necessary dimensions, and the input signal is filtered before being sent to the amplifier. Otherwise, the likelihood of an extraneous hum during operation increases.
Class B
To increase efficiency and reduce heating of the structure, it was decided to develop group B amplifiers. The devices are equipped with two additional transistors, each of which amplifies only half of the signal. The specificity of the design determines the implementation of 50% of the cycle in a positive or negative period.
There will be no bias current here, because the quiescent current of the device is zero. This led to the fact that the power of the equipment is low. The efficiency is higher than that of devices of category A.
The equipment is twice as efficient as devices of group A. But there is also a minus, which is represented by distortions during operation.
Distortion is caused by the presence of a certain corridor in the transistors. Part of the signal in this corridor will be modified, which makes adjustments to the output signal.
Class AB
By combining the properties of devices of categories A and B, engineers managed to obtain a functional device that combines the positive qualities of these devices. The design is more similar to group B amplifiers. The main difference is that the transistors simultaneously conduct the signal in close proximity to the intersection points of the oscillograms. The bias voltage here will be 5-10% of the corresponding quiescent current indicators.
The described approach helps eliminate the problems of high signal distortion characteristic of category B devices.
AB equipment appears to be a compromise between minimal distortion and efficiency. The conversion efficiency is 50%.
Class C
Class C units are efficient but non-linear. The input current is zero and remains at this level for half the time of the signal processing cycle. The transistor is switched to standby at this moment.
This approach guarantees an efficiency of about 80%, however, changes are made to the signal during use of the device. Because of this, amplifiers are rarely used in acoustics. They are much more common in various generators, radio devices, and other signal converters.
Class D
Class D amplifiers are usually classified as nonlinear pulse devices, which are also called PWM amplifiers.
Integrated circuits are characterized by high dissipation even with ideal implementation. The approach provides some advantages to devices due to low heat generation, lightness and compact size. The devices are cheaper, and the time required to work independently is longer.
For proper functioning of the devices, a high-voltage board of 10,000 watts is required.
Other classes
It is customary to separately consider other classes of amplifiers used to solve specific problems:
- Class F. Progressive models, the efficiency of which reaches 90%.
- Class G. Improved highly linear variation of the AB device. They automatically switch along power lines, assessing the necessary parameters of the input signal. This approach reduces energy consumption and also reduces energy losses due to heating.
- Class I: Positioned in a push-pull configuration just before switching on helps the mechanism switch quickly even when the required voltage is not present at the input connector.
- Class S. Nonlinear type amplifiers, based on the operating principle, are more reminiscent of class D devices. Using the equipment, the input analog signal is converted into a digital one with the required gain. The output power increases and the efficiency reaches 100%.
- Class T. Digital amplifiers that include special chips for processing and increasing the frequencies of incoming signals. Specially designed multi-channel 3D audio components guarantee the effectiveness of the equipment in creating a full-fledged home theater or music center.
Story
Tubes, like other electronic components, have a rich history that has seen significant evolution. It all started in the 2000s of the last century, and the end of the tube era can be considered the sixties, when the last fundamental development saw the light of day - miniature radio tubes, Nuvistors, and transistors had already begun to actively conquer the market. But from the whole history we are only interested in the key stages when the main types of radio tubes were created and the basic circuits for their inclusion were developed.
The world's first triode by inventor Lee de Forest, 1908
The first type of radio tubes developed to create amplifiers were triodes. The number 3 is heard in the name for a reason - this is exactly how many active outputs the triode has. The operating principle of a triode is extremely simple. Between the anode and cathode of the lamp, the power source and the primary winding of the output transformer (to the secondary winding of which the acoustics are connected) are connected in series. The useful signal is supplied to the lamp grid. When voltage is applied to the amplifier circuit, a flow of electrons flows between the cathode and anode, and the grid located between them modulates this flow according to changes in the level of the incoming signal.
As triodes were used in various industries, it was necessary to improve their characteristics. One of these characteristics was the pass-through capacitance, the value of which limited the maximum operating frequency of the lamp. In the process of solving this problem, tetrodes appeared - radio tubes with not three, but four electrodes inside. The fourth was a screening grid installed between the control grid and the anode. This solved the problem of increasing the operating frequency to the fullest extent, which completely satisfied the creators of the technology, who developed tetrodes so that radio stations and radio receivers could operate in the short-wave range, which has higher carrier frequencies than the medium- and long-wave ones.
Triode structure
From the point of view of sound reproduction quality, the tetrode did not fundamentally surpass the triode, so another group of scientists, puzzled by the issues of reproduction of sound frequencies, improved the tetrode, using essentially the same approach - simply adding another additional grid to the lamp design, located between the shielding grid and anode. This was necessary in order to suppress the dynatron effect - the reverse emission of electrons from the anode to the shielding grid. Connecting an additional grid to the cathode prevented this process, making the lamp's output characteristic more linear and increasing power output. Thus a new type of lamp appeared: the pentode.
Which class of audio amplifiers is better?
Depending on the scope of use and the specific environmental conditions, amplifiers of all groups are suitable options. Separately, it is worth considering equipment for home and car.
For home
When choosing an amplifier for home acoustics, it is better to first consider devices of categories AB and D with “sound” markings. The first type is an analog device that guarantees high-quality sound with average distortion.
Category D devices are digital models that can have any characteristics depending on the components installed on the circuit.
For auto
On cars, classes of amplifiers A, B, AB and D are used. Models of type A are rare in practice due to their high cost and low efficiency.
Class B stereo amplifiers are characterized by high efficiency, but lose in terms of sound distortion. They are also rarely used in car acoustics.
Devices of category AB are considered common among car enthusiasts. They are characterized by average sound quality, the required power indicators, clear sound and increased efficiency. Suitable for subwoofers with power from 500 to 600 W.
Category D equipment is used for digital signal processing. The devices are compact and also characterized by increased power ratings. An efficiency of 90-98% minimizes the likelihood of the device overheating, which means there is no need for a special cooling radiator. Such models are not common among motorists due to their high cost.
Peculiarities
From the point of view of the quality and character of the sound, each type of lamp and each switching mode has its own characteristics, so obvious to the ear that even the ultra-linear mode, in fact, did not become the golden mean. Pure triodes and triode inclusion of pentodes provide the clearest and most spacious sound until it comes to energetic music with fast and significant volume changes. In other words, triodes are much better suited for calm jazz than for listening to rock.
Pentode and ultralinear modes, on the contrary, are more suitable for energetic music, but in some cases they do not sound clean, accurate and detailed enough. These complaints especially often relate to the pentode mode, and in general the sound character of both the pentode and ultralinear modes is often compared to transistor amplifiers.
