Understand Amplifiers

The purpose of an electronic amplifier is to make an electronic signal bigger without affecting it in any other way. Before considering exactly what is implied by .
Table of contents

Understanding Amplifier Operating “Classes”

This format was originally mandated by the US Federal Trade Commission and as such, serves to provide a rather stable reference when comparing different amplifiers from different brands. Note that this is not the case with powered subwoofers; there are no such regulations with built-in power amplifiers, thus explaining why powered subwoofer ratings are often exaggerated in the way they are reported by their respective manufacturers.

While many refer to power as RMS power, when it comes to power, it is more correct to refer to average power.


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RMS root-mean-square makes sense only with variables that have both negative and positive values as in the case of an alternating voltage. There is no such thing as negative power; hence, the 'root' and the 'square' of the RMS process would only serve to extract the sign out of a number. In their attempt to come up with even bigger numbers, some manufactures also specify peak power ; this corresponds to the audio amplifier peak power output as derived from the amplifier peak output voltage when driving a loud, transient high point. The actual relation between peak power and average power depends very much on the type of signal.


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    5. For example, in the case of a typical pink noise audio test signal i. While peak power per se is not as important as the average power, it can serve as a valid indication of an audio amplifier's ability to respond to musical peaks say during an orchestra playback and short duration special loud effects during movie watching.

    To what extent an amplifier is capable of responding to such loud transients depends on the amplifier dynamic headroom. Dynamic headroom - expressed in dB, represents the reserve transient power that an amplifier is capable of delivering for the short period of time that the transient lasts. This ability of a home theater receiver or audio amplifier to deliver bursts of power is important for an accurate sound reproduction. An amplifier rated at watts average power but that can deliver watts peaks is said to have 6dB headroom.

    An amplifier that does not have the ability to reproduce sufficient high peaks will compress those peaks to fit within its headroom; the result is a significantly reduced sonic impact. Directly related with amplifier output is sound quality. You will better enjoy a 50W amplifier of superior sound quality against a W amplifier of poor sound.

    Understanding Audio Amplifiers Specs: What makes a good home theater amplifier?

    In addition, as we will further explain in this discussion, doubling amplifier power will not double the resultant loudness, or sound volume in the room. You need ten times as much amplifier power for your ears to perceive sound twice as loud. Some would also include the amplifier dynamic headroom referred to above as part of an amplifier sound quality indicator in that an amplifier with insufficient headroom would fail to deliver the highs in the way these were intended to be.

    Distortion is a measure of the amount by which an audio amplifier output is distorted, or altered with respect to the input signal. The issue with total harmonic distortion arises as higher levels of distortions affect the higher order harmonics even more. Harmonics refer to the higher order tones - or frequency multiples of the original note; these are what differentiate say a piano note from the same note played on a different instrument.


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    While THD ratings would give you an indication of the distortion levels present, yet it would not give you any indication of the nature of the distortion. Some flagship amplifier models give the distortion levels at different order-harmonics - with the most common being the 3 rd and 5 th harmonics.

    This gives a better picture than just a total level but for the purist, even this is not enough. The only way is to trust your ears ; there is no substitute to taking the time to listen when making an amplifier purchase - inasmuch as you have to listen to make a speaker purchase. Amplifier class defines the type of amplification used in the amplifier output stage and as such impacts the distortion levels produced by a power audio amplifier.

    In very simple terms, a power amplifier output stage consists of two main sub power stages, one to amplify the negative voltage signal while the other to amplify the positive part. It is therefore the one that generates the most heat as maximum current is present all the time irrespective of the audio signal level, thus leading to a highly inefficient design in order to deliver the best sound.

    Class B pulls and pushes - hence the term push-pull stage - current between the two sub-power stages so that as one is on, the other is off. Class AB is a hybrid between the two in that it leaves both amplification stages 'on' only for the smaller part of the signal - thus working in class A for low level signals while switching to class B operation when handling larger amplitude signals.

    Class D amplifiers convert the input signal into a sequence of higher voltage output pulses; for this reason, these are often mistakenly referred to as digital amplifiers. The resulting filtered signal is then an amplified replica of the input. For this purpose, Class D audio amplifiers are mainly used in lower-end HTiBs and in powered subwoofers, though some high-end designs are starting to appear. The larger the ratio, the better is the separation between the sound signal - whether that being music, a movie soundtrack, etc. Background noise in an amplifier can be of various origins and include among others, hum pickup, noise on the AC supply, and even electrical noise caused by the chaotic movement of the free electrons within the various electronic components constituting the amplifier itself.

    Be careful with the way manufacturers report audio amplifiers and AV receivers power ratings. Despite the mandated format by the US FTC, many manufacturers still manage to find ways of exaggerating power ratings. One very common trick is to reduce the lower frequency range from 20Hz to 40Hz - just an apparent minimal reduction but one that would greatly reduce the demand on the amplifier imposed by ultra-low bass content. Others may specify peak power with only one channel driven at a time - thus easing the load on what would be effectively an underrated amp power supply. Look for the term 'all channels driver'; unless it is specifically written in the amplifier specs, there is no guarantee that the amplifier will be able to run all channels at their maximum rated average power.

    At times, in AV receivers and multi-channel surround sound audio amplifiers, instead of all channels driven, you find the term 2 channels driven; this is not enough when dealing with multi-channel audio amplifiers for surround sound. And yes, check the audio amplifier weight - a big enough power supply capable of delivering the full rated power output on all channels is always big and heavy. In some case, manufacturers would specify lower speaker impedance than 8 Ohms - with 6 Ohms being the most common alternative. Lower impedance speakers draw more current, thus enabling the AV receiver or amp manufacturer to specify a higher power output.

    An amplifier capable of a maximum W average power in a 6 Ohm load may only deliver 75W average power in an 8 Ohm speaker load. One active device conducts during half of an input cycle, the other during the other half. Class AB amplifiers resemble class Bs, except their active devices are biased so both conduct during an overlapping portion of each input cycle.

    Understanding Audio Amplifiers Specs

    This sacrifices a certain amount of potential gain for better linearity i. Class AB sacrifices some of that efficiency for lower distortion. A push-pull amplifier can be built using amplifier ICs, rather than discretes, as in the traditional class B amp. A bridge-amplifier configuration effectively doubles the voltage swing at the load.

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    However, a better configuration would enable both amplifiers to see the same input signal. Class G and H amplifiers, variations on the standard class AB, feature additional supply rails. Class G amps employ several power rails at discrete voltage steps and switch between them as needed.

    Volts, Amps, and Watts Explained

    Instead of providing multiple rails, class H amps track the input signal and modulate the voltage on the supply rails. Class G and H amplifiers often find their way into audio applications. However, a related but previously almost forgotten alternative called the Doherty amplifier has been revived for cell-phone applications. In the input, half the input signal drives one device in the Class B; half the other. On the output, the signals are summed. Somewhat like a class G or H amp, the class B amp sustains the output most of the time, but the class C amp cuts in on high signal peaks.

    The benefit of the Doherty is increased efficiency, relative to a pure class B. An analogy would be the continuous tapping of a big bell with a small hammer at a rate equal to the resonant frequency of the bell. More generally, though, it reflects how sharply an L-C resonant circuit is tuned where L implies the presence of an inductance and C implies one or more capacitors.

    The difference between class E and class C amps is the active device becoming a switch, rather than operating in the linear portion of its transfer characteristic. Class F amplifiers resemble class E amplifiers, but use a more complex load network. In part, this network improves the impedance match between the load and the switch.

    It improves efficiency because the switch runs at saturation or cutoff for a longer period. Class D is most often used in audio applications. At the amplifier output, a low-pass filter attenuates the switching frequency and its harmonics, leaving only the amplified version of the input waveform. A drawback of Class D concerns the challenging task of suppressing radiated and conducted interference from the switching circuitry. Electronic Design brought to you by. Class A through H, what do they mean? More information about text formats. Text format Comments Plain text.

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