A Short Guide For Understanding Stereo Amps

By Gerald O'Brian


Requirements regarding audio power and audio fidelity of today's loudspeakers and home theater systems are continuously increasing. At the heart of these systems is the audio amp. Modern stereo amps have to perform well enough to meet those ever growing demands. It is challenging to pick an amp given the big number of products and concepts. I will describe some of the most popular amp designs like "tube amps", "linear amplifiers", "class-AB" and "class-D" along with "class-T amps" to help you understand several of the terms normally used by amplifier makers. This essay should also help you figure out what topology is perfect for your precise application.

Simply put, the principle of an audio amp is to convert a low-power audio signal into a high-power music signal. The high-power signal is big enough to drive a loudspeaker sufficiently loud. As a way to do that, an amplifier utilizes one or several elements that are controlled by the low-power signal in order to create a large-power signal. These elements range from tubes, bipolar transistors to FET transistors.

Tube amps used to be popular a number of decades ago. A tube is able to control the current flow in accordance to a control voltage which is connected to the tube. Unfortunately, tube amps have a somewhat high level of distortion. Technically speaking, tube amps will introduce higher harmonics into the signal. Nowadays, tube amplifiers still have many followers. The main reason is that the distortion that tubes produce are often perceived as "warm" or "pleasant". Solid state amplifiers with low distortion, on the other hand, are perceived as "cold".

An additional downside of tube amps, though, is the small power efficiency. The bulk of power which tube amplifiers consume is being dissipated as heat and only a fraction is being converted into audio power. Tube amplifiers, though, a rather expensive to produce and as a result tube amps have by and large been replaced with amps making use of transistor elements which are less costly to produce.

Solid-state amps utilize a semiconductor element, like a bipolar transistor or FET as opposed to the tube and the first type is generally known as "class-A" amps. In a class-A amplifier, the signal is being amplified by a transistor which is controlled by the low-level audio signal. In terms of harmonic distortion, class-A amps rank highest amongst all kinds of music amps. These amplifiers also usually exhibit very low noise. As such class-A amplifiers are perfect for very demanding applications in which low distortion and low noise are essential. The major downside is that similar to tube amps class A amplifiers have extremely low efficiency. Consequently these amps need big heat sinks in order to radiate the wasted energy and are frequently rather bulky.

In order to further improve the audio efficiency, "class-D" amplifiers use a switching stage that is continually switched between two states: on or off. None of these two states dissipates power within the transistor. Consequently, class-D amplifiers frequently are able to achieve power efficiencies beyond 90%. The switching transistor is being controlled by a pulse-width modulator. The switched large-level signal needs to be lowpass filtered to remove the switching signal and get back the audio signal. The switching transistor and also the pulse-width modulator frequently exhibit rather large non-linearities. As a consequence, the amplified signal will have some distortion. Class-D amplifiers by nature exhibit larger audio distortion than other types of audio amplifiers.

Class-D amplifiers improve on the efficiency of class-AB amps even further by using a switching transistor which is continuously being switched on or off. Thus this switching stage barely dissipates any energy and consequently the power efficiency of class-D amps typically exceeds 90%. The on-off switching times of the transistor are being controlled by a pulse-with modulator (PWM). Standard switching frequencies are in the range of 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 utilized. The switching transistor and also the pulse-width modulator usually exhibit fairly large non-linearities. As a consequence, the amplified signal is going to contain some distortion. Class-D amplifiers by nature have larger audio distortion than other kinds of audio amps. More modern audio amps incorporate some type of mechanism in order to minimize distortion. One approach is to feed back the amplified audio signal to the input of the amp to compare with the original signal. The difference signal is then utilized in order to correct the switching stage and compensate for the nonlinearity. A well-known architecture that utilizes this type 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 made extremely small and yet attain high audio fidelity.




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