An audio frequency amplifier is a general term used to describe a circuit that produces and amplifies a version of its input signal. However, not all converter technologies are the same, as they are classified according to their configurations and operating modes.
Small amplifiers are usually used in electronics, since they are capable of increasing a relatively small input signal, for example, from a sensor such as a player, to a much larger output signal to control a relay, lamp or speaker, etc.
There are many forms of electronic circuits classified as amplifiers, from operating rooms and small signal sensors to large pulse and power converters. The classification of the device depends on the size of the signal, large or small, its physical configuration and the method of processing the input stream, that is, the relationship between the input level and the current flowing in the load.
Device anatomy
Audio amplifiers can be considered as a simple box or unit containing a device, such as a bipolar, field effect transistor or an operational sensor, which has two input and two output terminals (grounding is common). Moreover, the output signal is much larger due to its conversion on the device.
An ideal signal amplifier will have three main properties:
- Input impedance, or (R IN).
- Output impedance, or (R OUT).
- Amplification, or (A).
Regardless of how complex the amplifier circuitry is, the general block model can be used to demonstrate the relationship of these three properties.
General concepts
High-quality audio amplifiers may vary in performance. Each type has a digital or analog conversion. To separate them, code symbols are set.
The increased difference between the input and output signals is called conversion. Gain is a measure of how much an amplifier “converts” an input signal. For example, if there is an input level of 1 volt and an output level of 50 volts, then the conversion will be 50. In other words, the input signal was developed 50 times. The audio amplifier just performs this task.
A conversion calculation is simply an output ratio divided by an input. This system does not have units of measure as its ratio, but in electronics the symbol A is usually used for amplification. Then, the conversion is simply calculated as “output divided by input signal”.
Power converters
A small signal magnifier is usually called a “voltage” amplifier because it typically converts a small input voltage to a much larger output voltage. Sometimes a device circuit is required to control the motor or the power of the loudspeaker, and for applications of this type where high switching currents are involved, power converters are needed.
As the name implies, the main task of a power amplifier (also known as a large signal amplifier) is to supply power to the load. This is the product of voltage and current applied to the load with an output power exceeding the input signal level. In other words, the converter increases the power of the speaker, so block circuits of this type are used on the external stages of the audio converters to control the speakers.
Operating principle
The audio frequency amplifier works by the principle of converting the DC power consumed from the power source into an AC voltage signal supplied to the load. Although the conversion is high, its efficiency from a DC power source to an AC voltage output signal is usually low.
An ideal unit gives the device a 100% efficiency, or at least the IN power will be equal to the OUT power.
Class division
If users at least once looked into the specification of audio frequency power amplifiers, they might notice equipment classes, usually indicated by a letter or two. The most common block types used today in consumer home audio are A, A / B, D, G, and H.
These classes are not simple classification systems, but descriptions of the amplifier topology, that is, how they function at the core level. While each type of amplifier has its own set of strengths and weaknesses, their work (and how the final characteristics are evaluated) remains unchanged.
It consists in converting the waveform sent by the preliminary unit without introducing interference or at least as little distortion as possible.
Class A
Compared to other classes of audio frequency power amplifiers, which will be described below, Class A models are relatively simple devices. The decisive principle of operation is that all output blocks of the converters must go through a full 360-degree signal cycle.
Class A can also be divided into single-ended and push-pull amplifiers. Push / pull differs from the main explanation above using output devices in pairs. While both devices carry out a full 360-degree cycle, one device will carry most of the load during the positive part of the cycle, and the other more than the negative cycle.
The main advantage of this scheme is reduced distortion compared to single-ended designs, since even order fluctuations are eliminated. Class A push-pull designs are also less sensitive to noise.
Due to the positive qualities associated with the work of class A, it is considered the gold standard for sound quality in many areas of acoustic production. However, these designs have one important drawback - efficiency.
The requirement for audio frequency amplifiers with class A transistors is that all output devices operate continuously. Such an action leads to significant losses of energy, which ultimately is converted into heat. This is further exacerbated by the fact that Class A structures require relatively high levels of quiescent current, which is the amount of current flowing through the output devices when the amplifier provides a zero output. Performance indicators in the real world can be about 15-35%, while it is possible to use single digits using highly dynamic source material.
Class B
While all the output mechanisms in an audio frequency amplifier with class A transistors take 100% of the time during operation, a push-pull circuit is used in class B units so that only half of the output devices conduct current at any time.
One half covers the + 180-degree portion of the waveform, while the other covers a -180 degree cross section. As a result, Class B amplifiers are significantly more efficient than their Class A counterparts, with a theoretical maximum of 78.5%. Given its relatively high efficiency, Class B has been used in some professional sound amplification converters, as well as in some home tube amplifiers. Despite their obvious strength, the chances of acquiring a Class B block for a home are practically nil. A study of the audio amplifier showed the cause of this, known as crossover distortion.
The problem with the delay in handover between devices processing the positive and negative parts of the waveform is considered significant. It goes without saying that such distortion is heard in sufficient quantities, and although some Class B designs were better than others in this regard, Class B did not receive much recognition from fans of clear sound.
Class A / B
A tube amplifier for sound frequencies can be found at many concert venues. It has high performance and does not overheat. In addition, models are much cheaper than many digital blocks. But there are deviations. Such a module may not work with all audio formats. Therefore, it is better to use equipment as part of a common signal processing complex.
Class A / B combines the best of each type of device to create a unit without the disadvantages of either. With this combination of benefits, Class A / B amplifiers dominate the consumer market to a large extent.
The solution is actually quite simple in concept. Where Class B uses a push-pull device with each half of the output stage conducting 180 degrees, Class A / B mechanisms increase it to ~ 181-200 degrees. Thus, there is a much lesser likelihood of a “break” in the cycle, and therefore the crossover distortion drops to such an extent that it does not matter.
Audio tube power amplifiers can absorb this interference much faster. Thanks to this property, the sound from the device is much cleaner. Models of such characteristics are often used to transform the sound of acoustic and electric guitars.
Suffice it to say that class A / B fulfills its promises, easily surpassing the efficiency of clean class A constructions with indicators of the order of ~ 50-70% achieved in the real world. Actual levels, of course, depend on how biased the amplifier is, as well as on program material and other factors. It is also worth noting that some class A / B developments are taking another step forward in their quest to eliminate crossover distortion by working in pure Class A mode up to several watts of power. This gives some efficiency when working at low levels, but it ensures that the amplifier does not turn into a furnace when a large amount of energy is supplied.
Classes G and H
Another pair of designs designed to increase efficiency. From a technical point of view, neither Class G nor Class H amplifiers are officially recognized. Instead, they are variations on the A / B class theme using bus voltage switching and bus modulation, respectively. In any case, in conditions of low demand, the system uses a lower voltage on the bus than a similar class A / B amplifier, which significantly reduces power consumption. When high power conditions arise, the system dynamically increases the voltage on the bus (that is, switches to the high voltage bus) to handle transients with large amplitudes.
There are disadvantages too. The main one is high cost. In the original circuit switching networks, bipolar transistors were used to control the output streams, which increases complexity and cost. High-quality tube amplifiers of sound frequency of this type are common, although the price starts from 50 thousand rubles. The unit is considered a professional technique for working on stage or for recording in a studio. There are problems with transistors. With prolonged load, some of them may fail.
Today, the price often decreases to some extent when using high-current MOS transistors to select or change the guides. Using MOSFETs not only improves efficiency and reduces heat, but also requires fewer parts (usually one device per stream). In addition to the cost of switching on the bus, the modulation itself, it is also worth noting that some class G amplifiers use more output devices than a typical class A / B design.
One pair of devices will operate in typical A / B mode, powered by low voltage buses. Meanwhile, the other is in reserve to act as a voltage amplifier, activated only depending on the situation. Only the G and H classes associated with powerful amplifiers can withstand high loads, where the increased efficiency justifies itself. Compact designs can also use G / H class topologies as opposed to A / B, given that being able to switch to low power mode means they can get along with a slightly smaller heatsink.
Class D
This type of device allows you to create your own modular systems. With the help of equipment, high-quality processing of the entire effluent takes place. Designing audio power amplifiers allows you to create your own multimedia system for work or entertainment. However, there are some nuances. Often erroneously referred to as digital amplification, class D converters provide a guarantee of block efficiency, while in actual tests, factors exceeding 90% are achieved.
First, it’s worth parsing the question of why this applies to class D if the “digital gain” is wrong. It was just the next letter in the alphabet, with the class C used in audio systems. More importantly, how 90% + efficiency can be achieved. While all the previously mentioned classes of amplifiers have one or more output devices that are constantly active, even when the converter is actually in standby mode, class D units quickly switch them to the “off” and “on” states. This is quite convenient and makes it possible to use the module only at the right moments.
For example, the calculation of Class T audio frequency amplifiers, which are an implementation of the Class D developed by Tripath, unlike the base unit, use switching frequencies of the order of 50 MHz. Output devices are usually controlled by pulse width modulation. This is when square waves of different widths are generated by a modulator that represents an analog signal for reproduction. With strict control of output devices in this way, an efficiency of 100% is theoretically possible (although, obviously, it is unattainable in the real world).
Having delved into the world of Class D audio frequency amplifiers, you can also find mention of analog and digital controlled modules. These control units have an analog input signal and an analog control system, usually with some degree of feedback error correction. Class D amplifiers with digital conversion, on the other hand, use digital control, which switches the power stage without error control. This decision also finds approval, according to many customer reviews. However, the price segment is much higher.
A study of the audio frequency amplifier showed that analog-driven class D has a performance advantage over digital analogs, as it usually offers lower output impedance (impedance) and an improved distortion profile. This increases the initial values of the system at its maximum load.
The parameters of sound frequency amplifiers are much higher than the basic models. It should be understood that such calculations are required only to create music in the studio. Ordinary buyers can skip these specifications.
Usually this is an L-circuit (inductor and capacitor) located between the amplifier and speakers to reduce noise associated with class D. The filter is of great importance. Poor design can jeopardize performance, reliability, and sound quality. In addition, feedback after the output filter has its advantages. Although designs that do not use feedback at this stage can tune their response to a specific impedance when such amplifiers have a complex load (i.e. a speaker rather than a resistor), the frequency response can vary significantly depending on the load on the speaker. Feedback stabilizes this problem by providing a smooth response to complex loads.
Ultimately, the complexity of Class D electrical audio amplifiers has its advantages. Efficiency and, as a result, less weight. Since relatively little energy is spent on heat, much less energy is required. Thus, many Class D amplifiers are used in conjunction with switching power supplies (SMPS). Like the output stage, the power supply itself can be quickly turned on and off to control the voltage, which leads to a further increase in efficiency and the ability to reduce weight relative to traditional analog / linear power supplies.
In aggregate, even powerful Class D amplifiers can weigh just a few pounds. SMPS , .
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In today's market, a simple A / B class audio amplifier dominates, for good reason. It works very well, is relatively cheap, and its efficiency is quite adequate for low-power applications (> 200 watts). Of course, since the manufacturers of converters are trying to expand the scope of supply, for example, with the Emotiva XPR-1 monoblock with a power of 1000 W, they turn to class G / H and D designs to avoid the double use of their amplifiers as systems that can quickly heat equipment. Meanwhile, there are Class A fans on the other side of the market who can forgive the device’s lack of efficiency in the hope of a cleaner sound.
Total
After all, converter classes are not necessarily that important. Of course, there are actual differences, especially when it comes to cost, amplifier efficiency and, therefore, weight. Of course, class A technology with a power of 500 W is a bad idea, unless, of course, the user does not have a powerful cooling system. On the other hand, differences between classes do not determine sound quality. In the end, it comes down to the development and implementation of their own projects. It is important to understand that converters are just one device that is part of the audio system.