Requirements concerning audio power and audio fidelity of latest speakers and home theater systems are constantly growing. At the heart of those systems is the audio amp. Today’s small stereo amps have to perform well enough to meet these always increasing demands. With the ever increasing quantity of models and design topologies, like “tube amplifiers”, “class-A”, “class-D” along with “t amp” designs, it is getting more and more difficult to choose the amplifier which is best for a specific application. This post will describe some of the most common terms and spell out a few of the technical jargon that amp suppliers frequently utilize.
The fundamental operating principle of an audio amplifier is fairly simple. An audio amplifier is going to take a low-level audio signal. This signal regularly comes from a source with a rather large impedance. It subsequently translates this signal into a large-level signal. This large-level signal can also drive speakers with small impedance. The kind of element used to amplify the signal depends on what amplifier architecture is utilized. Several amps even employ several kinds of elements. Usually the following parts are used: tubes, bipolar transistors and FETs. Tube amps used to be common some decades ago. A tube is able to control the current flow according to a control voltage that is attached to the tube. Regrettably, tube amplifiers have a fairly high amount of distortion. Technically speaking, tube amps will introduce higher harmonics into the signal. Nowadays, tube amplifiers still have a lot of fans. The most important reason is that the distortion that tubes bring about are frequently perceived as “warm” or “pleasant”. Solid state amps with low distortion, on the other hand, are perceived as “cold”.
Furthermore, tube amplifiers have quite low power efficiency and consequently radiate much power as heat. Moreover, tubes are pretty costly to produce. As a result tube amplifiers have mostly been replaced by solid-state amps which I am going to glance at next. Solid state amplifiers replace the tube with semiconductor elements, generally bipolar transistors or FETs. The first type of solid-state amplifiers is often known as class-A amps. In class-A amps a transistor controls the current flow according to a small-level signal. A few amps make use of a feedback mechanism to reduce the harmonic distortion. If you need an ultra-low distortion amplifier then you might wish to investigate class-A amplifiers as they offer amongst the smallest distortion of any audio amps. Class-A amplifiers, however, waste the majority of the energy as heat. For that reason they frequently have large heat sinks and are fairly bulky.
To improve on the small efficiency of class-A amplifiers, class-AB amps utilize a number of transistors which each amplify a distinct area, each of which being more efficient than class-A amps. As a result of the larger efficiency, class-AB amplifiers do not require the same number of heat sinks as class-A amplifiers. Consequently they can be made lighter and less costly. When the signal transitions between the two distinct areas, however, a certain amount of distortion is being produced, thus class-AB amplifiers will not achieve the same audio fidelity as class-A amplifiers.
Class-D amps are able to attain power efficiencies higher than 90% by employing a switching transistor that is continuously being switched on and off and thus the transistor itself does not dissipate any heat. The on-off switching times of the transistor are being controlled by a pulse-with modulator (PWM). Standard switching frequencies are between 300 kHz and 1 MHz. This high-frequency switching signal has to be removed from the amplified signal by a lowpass filter. Generally a simple first-order lowpass is being used. The switching transistor and in addition the pulse-width modulator typically exhibit quite large non-linearities. As a result, the amplified signal is going to have some distortion. Class-D amplifiers by nature exhibit higher audio distortion than other types of audio amps.
To resolve the dilemma of high music distortion, newer switching amp styles include feedback. The amplified signal is compared with the original low-level signal and errors are corrected. A well-known architecture that makes use of this kind of feedback is generally known as “class-T”. Class-T amplifiers or “t amps” achieve audio distortion which compares with the audio distortion of class-A amps while at the same time exhibiting the power efficiency of class-D amplifiers. Consequently t amplifiers can be manufactured extremely small and still attain high audio fidelity.
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