A Brief Primer For Music Amps

Demands concerning audio power and audio fidelity of today’s loudspeakers and home theater systems are constantly growing. At the heart of these systems is the music amp. Today’s mini stereo amps have to perform well enough to satisfy these ever growing requirements. It is difficult to select an amplifier given the huge number of styles and concepts. I will describe some of the most common amp designs such as “tube amplifiers”, “linear amps”, “class-AB” and “class-D” and also “class-T amps” to help you understand some of the terms normally used by amplifier makers. This essay should also help you figure out which topology is perfect for your particular application.

Simply put, the function of an audio amp is to convert a low-power audio signal into a high-power music signal. The high-power signal is great enough to drive a loudspeaker sufficiently loud. The type of element used to amplify the signal depends on what amplifier topology is used. Some amplifiers even utilize several types of elements. Typically the following parts are used: tubes, bipolar transistors and FETs. A couple of decades ago, the most popular type of audio amplifier were tube amplifiers. Tube amplifiers use a tube as the amplifying element. The current flow through the tube is controlled by a low-level control signal. In that way the low-level audio is transformed into a high-level signal. One drawback with tubes is that they are not very linear when amplifying signals. Aside from the original music, there will be overtones or higher harmonics present in the amplified signal. Therefore tube amps have moderately high distortion. However, this characteristic of tube amplifiers still makes these popular. Many people describe tube amplifiers as having a warm sound as opposed to the cold sound of solid state amplifiers.

Furthermore, tube amps have quite low power efficiency and thereby radiate much power as heat. Tube amps, however, a fairly costly to make and thus tube amps have by and large been replaced with amplifiers making use of transistor elements which are less expensive to build.

Solid state amplifiers replace the tube with semiconductor elements, generally bipolar transistors or FETs. The earliest type of solid-state amps is often 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. If you need an ultra-low distortion amp then you might wish to investigate class-A amps since they provide amongst the lowest distortion of any audio amps. Class-A amplifiers, on the other hand, waste most of the energy as heat. As a result they frequently have big heat sinks and are quite heavy. In order to improve on the low efficiency of class-A amplifiers, class-AB amps make use of a number of transistors which each amplify a distinct area, each of which being more efficient than class-A amps. Due to the higher efficiency, class-AB amps do not require the same number of heat sinks as class-A amps. For that reason they can be manufactured lighter and cheaper. When the signal transitions between the 2 separate regions, though, some level of distortion is being produced, thus class-AB amps will not achieve the same audio fidelity as class-A amps. Class-D amps are able to attain power efficiencies above 90% by using a switching transistor that is continually being switched on and off and therefore 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). Usual switching frequencies are between 300 kHz and 1 MHz. This high-frequency switching signal needs to be removed from the amplified signal by a lowpass filter. Generally a straightforward first-order lowpass is being used. Both the pulse-width modulator and the transistor have non-linearities that result in class-D amps having larger audio distortion than other types of amps.

To resolve the dilemma of large music distortion, new switching amp styles include feedback. The amplified signal is compared with the original low-level signal and errors are corrected. “Class-T” amplifiers (also referred to as “t-amp”) utilize this kind of feedback method and for that reason can be manufactured extremely small whilst attaining low audio distortion.