Electrical device that takes a low power signal and amplifies it up to a higher power level, often (but not always) sufficient to drive speakers. Usually rated in Watts. It is important to note that there are many other variables that determine an amplifiers use, reliability, output characteristics etc.

Unfortunately the Watts used to rate an amplifier aren't always the same Watts! Watts can be measured as RMS (Root Mean Square) Watts or as PMPO (Peak Music Power Output) Watts. While RMS is clearly defined PMPO is not, however as a rule of thumb divide PMPO by 3 to get a rough RMS rating!

The term PMPO is generally shunned by audio professionals, and mostly used by marketing droids - especially when trying to "talk up" a budget offering..

Amplifier Classes

Power Amplifiers output stages are usually given a class. there are a number of different classes which all have advantages and disadvantages. here are a few of the different types of classes.

Class A

In this class the output stage of the amplifier draws a constantly high current from the power supply regardless of whether an audio signal is present or not. Low-current class A output stages are used in many audio circuits. This constant current draw is known as a steady bias current and is used as transistors are very non-linear devices, more so when working at a low current. As a steady current is passed through them they are biased into their working area at which they are most linear.

Class A amplification can be very inefficient for this reason and can typically run quite warm, however this higher temperature is constant which some may argue is more of an advantage. Class A is usually associated with high end audio amplifiers, including some hifi power amplifiers, some examples are the later range of Rotel and Krell amplifiers, again these are only usually seen in the home hifi market.

Class B

Class B is very much the opposite of Class A in the sense that the current flowing through the transistors is entirely dependant on the audio signal presence and level. It is in this case the driving signal which will bias the transistors into conduction to drive the amplifier outputs. Class B is very efficient, it is sometimes found in battery operated equipment. Before we get carried away with the advantage over Class A, Class B amplifier stages at low levels are operating the transistors in their non linear range. Usually the transistors are in multiple pairs to provide the output current. A pair will handle positive and negative sides of the signal with respect to zero. As the output goes through the zero point "crossover distortion" is present. To conclude, Class B amplfiers are very often poor quality sound. This is not a problem for applications where high quality sound isn't necessary. Radio Communication, paging, intercom, and even some speaker systems.

Class A-B

Class A-B is very common in the live sound reinforcement world. A low voltage bias is used to bias the transistors into their linear range, but a class B type system is also used which is dependant on the input signal. In effect you get the best of both worlds here. The majority of power amplifiers (non switch mode etc) use this technology.

Class C

Class C is an amplifier design which is made to drive a very narrow frequency band, this is often used in RF work.

Class D

Pulse Width Modulation. In this style of amplifier an ultrasonic frequency is used to drive the output transistors. this signal is modulated by the audio signal. This requires a ((LPF)) to be used after the output stage.

Class G and H

A different method is used with these 2 classes. Voltage rails are used in which the higher the input signal the more voltage rails are used. this can be quite efficient as for low level work, only a number of voltage rails are in operation. Class G amplifiers can sometimes be Lighter weight than a Class A-B equivalent. Class H has voltage rails which monitor the input signal at all times maintaining just enough headroom with respect to the current input signal. This Improves efficiency further.

Class T

A modern Variation of Class D amplifiers (see above) where the ultrasonic frequency alters with the audio signal. A low audio signal will give a high PWM signal of around 1.2Mhz and a high level audio signal as low as 200KHz. this is said to be more efficient than Class D.

Slew Rate

Slew rate is a measure of how an amplifier responds to transients in an audio signal. A transient may require an amplifier output to swing from 0 to 120watts in a few microseconds. the slew rate is defined in Volts per microsecond and an amplfier with an output of 200watts may be required to have a slew rate of at least 30 Volts per microsecond. Of course, a higher powered amplifier will need a much higher slew rate as the swing from zero is much greater.

It is however important to note that most modern power amplifiers are capable of much faster transients (a higher slew rate). Extremely high slew rates of several hundred Volts per microsecond are often found. These are sometimes created by fast reacting transistors. Excessive slew rates like this are often unnecessary and the poor reliability of some fast transistors often makes the extra expense a step backwards.

External Links

• Rane Notes (scroll down a bit)
• Wikipedia article on Audio Power