Audio 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 modes of operation.
Small amplifiers are commonly used in electronics because they are able to amplify a relatively small input signal, such as from a sensor such as a music player, into a much larger output signal to drive a relay, lamp or loudspeaker, etc.
There are many forms of electronic circuits classified as amplifiers, from operational and small signal transducers to large pulse and power converters. The classification of a device depends on the size of the signal, large or small, its physical configuration, and how the input stream is processed, that is, the relationship between the input level and the current flowing in the load.
Device Anatomy
Audio frequency amplifiers can be seen as a simple boxor a block containing a device, such as a bipolar, FET, or operational sensor, that has two input and two output terminals (ground 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 resistance, or (R OUT).
- Gain, or (A).
No matter how complex the amplifier circuit is, a 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. Codes are set to separate them.
The increased difference between input and output signals is called conversion. Gain is a measure of how much an amplifier "transforms" 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 has been developed 50 times. An audio frequency amplifier does just that.
The conversion calculation is simply the ratio of the output divided by the input. This system does not have units as its ratio, but in electronics the symbol A is commonly used for gain. The conversion is then simply calculated as "output divided by input".
Power converters
Magnifier smallA signal amplifier is commonly referred to as a "voltage" amplifier because it tends to convert a small input to a much larger output voltage. Sometimes a device circuit is required to drive a motor or loudspeaker power, and for these types of applications, where high switching currents are involved, power converters are needed.
As the name suggests, the main task of a power amplifier (also known as a large signal amplifier) is to supply power to a load. It is the product of voltage and current applied to a load with an output power greater than the input signal level. In other words, the converter increases the power of the speaker, so this type of block circuit is used in the external stages of audio converters to drive the speakers.
Operation principle
The audio amplifier works on the principle of converting the DC power drawn from the power supply into an AC voltage signal supplied to the load. Although the conversion is high, the efficiency from the DC power supply to the AC voltage output signal is generally low.
An ideal block gives the device an efficiency of 100% or at least the power IN will be equal to the power OUT.
Class division
If users have ever looked at the specification of audio power amplifiers, they may have noticed equipment classes, usually denoted by the letter ortwo. The most common block types used in consumer home audio today are A, A/B, D, G, and H values.
These classes are not simple classification systems, but descriptions of 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 performance (and how the final characteristics are measured) remains the same.
It is to convert the waveform sent by the pre-unit without introducing interference or at least as little distortion as possible.
Class A
Compared to other classes of audio power amplifiers that will be described below, Class A models are relatively simple devices. The defining principle of operation is that all transducer output blocks must go through a complete 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 by using output devices in pairs. While both devices run a full 360-degree cycle, one device will carry most of the load during the positive part of the cycle, while the other will carry more of the negative cycle.
The main advantage of this circuit is reduced distortion compared to single-ended designs, since even order fluctuations are eliminated. In addition, Class A push-pull designs are less sensitive to noise.
Because of the positive qualities associated with Class A performance, it is considered the gold standard for sound quality in many acoustic applications. However, these designs have one important drawback - efficiency.
Requirement for Class A transistor audio amplifiers to have all output devices on at all times. This action leads to a significant loss of energy, which is eventually converted into heat. This is further exacerbated by the fact that Class A designs require relatively high levels of quiescent current, which is the amount of current flowing through the output devices when the amplifier is producing zero output. Real world efficiency rates can be in the order of 15-35%, with single digits possible using highly dynamic source material.
Class B
While all of the output mechanisms in a class A audio amplifier take 100% of the time to operate, the class B units use push-pull circuitry such that only half of the output devices are conducting at any time.
One half covers the +180 degree part of the waveform while the other half covers the -180 degree section. As a consequence, Class B amplifiers are significantly more efficient than their Class A counterparts, with a theoretical maximum of 78.5%. Given the relatively high efficiency, Class B has been used in some professional PA transducers as well as some home tube amplifiers. Despite themobvious strength, the chances of acquiring a class B block for a house are practically zero. An examination of the audio amplifier showed the cause of this, known as crossover distortion.
The problem with latency in handover between devices processing the positive and negative parts of the waveform is considered significant. It goes without saying that this distortion is audible in sufficient amounts, and while some Class B designs were better than others in this regard, Class B received little recognition from clean-sounding enthusiasts.
Class A/B
The tube audio amplifier can be found in many concert venues. It has high performance and does not overheat. In addition, the models are much cheaper than many digital blocks. But there are also deviations. Such a module may not work with all audio formats. Therefore, it is better to use equipment as part of a general signal processing complex.
Class A/B combines the best of each device type to create a unit without the disadvantages of either. With this combination of advantages, class A/B amplifiers largely dominate the consumer market.
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 ismuch less likely to have a "tear" in the loop, and therefore the crossover distortion drops to the point where it doesn't matter.
Valve audio power amplifiers can absorb this interference much faster. Thanks to this property, the sound comes out of the device much cleaner. Models of these characteristics are often used to transform the sound of acoustic and electric guitars.
Suffice it to say that Class A/B delivers on its promise, easily outperforming pure Class A constructs at ~50-70% real world performance. Actual levels, of course, depend on how much the amplifier is offset, as well as program material and other factors. It's also worth noting that some Class A/B designs go one step further in their quest to eliminate crossover distortion by operating in pure Class A mode up to a few watts of power. This gives some efficiency at low levels, but ensures that the amplifier does not turn into a furnace when a large amount of power is applied.
Classes G and H
Another pair of designs designed to improve efficiency. From a technical point of view, neither class G nor class H amplifiers are officially recognized. Instead, they are variations on the Class A/B theme using bus voltage switching and bus modulation, respectively. In any case, in low demand conditions, the system uses a lower bus voltage than a similar class A/B amplifier, which significantlyreduces power consumption. When high power conditions occur, the system dynamically increases the bus voltage (i.e. switches to the high voltage bus) to handle high amplitude transients.
There are flaws too. Chief among them is the high cost. The original network switching circuits used bipolar transistors to control the output streams, adding complexity and cost. High-quality tube audio frequency amplifiers of this type are common, although the price starts at 50 thousand rubles. The block is considered a professional technique for working on stage or recording in a studio. There are problems with transistors. Under prolonged load, some of them may fail.
Today, the price is often reduced to some extent by using high current MOSFETs to select or change guides. The use of MOSFETs not only improves efficiency and reduces heat, but also requires fewer parts (typically one device per thread). In addition to the cost of bus switching, the modulation itself, it's 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 the low voltage busbars. Meanwhile, the other one is on standby to act as a voltage booster, activated only depending on the situation. Withstand high loads only classes G and H,associated with powerful amplifiers, where the increased efficiency pays off. Compact designs may also use class G/H topologies as opposed to A/B given that the ability to switch to low power mode means they can get away with a slightly smaller heatsink.
Class D
This type of device allows you to create your own modular systems. With the help of the equipment, high-quality processing of the entire outgoing stream takes place. Designing audio frequency power amplifiers allows you to create your own multimedia system for work or entertainment. However, there are some nuances here. Often erroneously referred to as digital amplification, class D converters are a guarantee of unit efficiency and achieve gains in excess of 90% in actual testing.
First it's worth considering why this is class D if "digital amplification" is incorrect. It was just the next letter in the alphabet, with the C class used in audio systems. More importantly, how 90%+ efficiency can be achieved. While all of the previously mentioned amplifier classes 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 off and on. 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 amplifiers, which areTripath's class D implementation, unlike the basic device, uses 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 presents an analog signal for playback. With strict control of output devices in this way, 100% efficiency is theoretically possible (although obviously not achievable in the real world).
Digging into the world of class D audio amplifiers, you can also find mention of analog and digital controlled modules. These control blocks have an analog input signal and an analog control system, usually with some degree of feedback error correction. On the other hand, digital conversion class D amplifiers use digital control, which switches the power stage without error control. This solution also finds approval, according to the reviews of many buyers. However, the price segment is much higher here.
Audio amplifier research has shown that analog-driven Class D has a performance advantage over digital analog, as it typically offers lower output impedance (resistance) and improved distortion profile. This raises the initial values of the system at its maximum load.
The parameters of the audio frequency amplifiers are much higher than those of the basic models. It should be understood that such calculations are required only for creating music in the studio. For ordinary buyers, thesecharacteristics can be skipped.
Usually an L-circuit (inductor and capacitor) placed between the amplifier and speakers to reduce the noise associated with class D operation. The filter is of great importance. Poor design can compromise efficiency, 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 loudspeaker 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.
In the end, the complexity of Class D audio amplifiers has its benefits. Efficiency and, as a result, less weight. Since relatively little energy is spent on heat, much less energy is required. As such, many class D amplifiers are used in conjunction with switched mode power supplies (SMPS). Like the output stage, the power supply itself can be quickly turned on and off to regulate voltage, resulting in further efficiency gains and the ability to reduce weight over traditional analog/linear power supplies.
In aggregate, even powerful class D amplifiers can weigh only a few kilograms. The disadvantage of SMPS power supplies compared to traditional linear supplies isthat the former usually do not have much headroom.
Tests and numerous tests of class D audio amplifiers with linear power supplies compared to SMPS modules have shown that this is indeed the case. When two amplifiers were handling rated power, but one with a linear power supply could produce higher dynamic power levels. However, SMPS designs are becoming more commonplace and you can expect to see better next generation Class D units using similar shapes in stores.
Comparison of the efficiency of classes AB and D
Although the efficiency of a Class A/B transistorized audio power amplifier increases as the maximum output power is approached, Class D designs maintain high efficiency over most operating ranges. As a result, efficiency and sound quality are increasingly leaning towards the last block.
Use one transducer
When properly implemented, any of the above blocks outside of class B can form the basis of a high fidelity amplifier. Aside from potential performance pitfalls (which are primarily a design decision rather than class-specific), block type selection is largely a matter of cost versus efficiency.
In today's market, the simple Class A/B audio amplifier dominates, and for good reason. It works very well, is relatively cheap, and itsefficiency is quite adequate for low power applications (>200W). Of course, as converter manufacturers try to push the envelope with, for example, the 1000W Emotiva XPR-1 monoblock, they are turning to G/H and D class designs to avoid duplicating their amplifiers as systems capable of heating up equipment quickly. Meanwhile, on the other side of the market, there are class A fans who can forgive the lack of efficiency of the device in the hope of a cleaner sound.
Result
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, 500W class A appliances are a bad idea, unless, of course, the user has a powerful cooling system. On the other hand, differences between classes do not determine sound quality. In the end, it comes down to developing and implementing your own projects. It is important to understand that transducers are only one device that is part of the audio system.
