Saturday, October 28, 2017

Differences between Class A, B, AB, D power amps

Hello and welcome to this week's article,
today we're going to talk about power amps!
This article is to be intended as a follow up to our Tube vs Solid State power amps article.

Let's start off by saying I'm no engineer, I have made some research (I will put at the end of the article my sources), and the aim of the article is just to put in simple terms the differences, acting as a reference article for the simple musician that is not too expert in electronics.

We all have read in the description of a guitar amplifier (or any power amp in general) the definition "Class A", "Class B", "Class A/B", "Class D" and so on, wondering what it means: it is a code to describe the way the power amp works (yes, there are also other classes, but these are the most relevants in sound amplification).
First let's define what is a preamp and what is a power amp: a preamp is the device that boosts microphone and instrument level signals to line level, a power amp is the one that boosts the line level signal strong enough to drive a speaker.

Class A power amps: these were the first tube power amps ever created, the least efficient in terms of power comsumption to output ratio and the simpliest to build; basically the electricity passes thorugh one or more tubes in serie, which operates continuously. It generates a lot of heat and consumes the tubes much faster than the other circuit types, but it has also some particular property, for example it has no crossover nor switchoff distortion, which are typical of Class B power amps. A famous The tonal characteristics of this kind of power amp are a "bluesy" and particularly "musical" tone.

Class B power amps: in these amps there are two or more power tubes, and this configuration conducts the power half of the time in one tube and half of the time in the other (at a very high speed), prolonging the life of the power tubes but generating a very particular distortion, which is typical of many guitar amplifiers cranked to the max. This distortion is called "crossover distortion" because it is generated by the current that, moving from one tube to another alternatively (or from one transistor to another in solid state amps), for a split second in time it is not drawn by neither of the two, thus generating that noise.
As a general rule, Class B power amps grants more headroom, higher volume, better clarity and sparkle than the Class A ones.

Class AB power amps: these kind of amplifiers are very similar to the Class B ones, but the tubes doesn't stop drawing the current before the other one is on, so there is never a moment in time in which for an instant both of them are off. This reduces the amount of crossover noise, and allows to save at the same time some tube life and power comsumption. This system is often used in high-end home sound systems.

Class D power amps: these ones operates with transistors instead of tubes, and this means that they operate on a better power efficiency (less heat is generated, less electricity is wasted) and they work through a system called "pulse width modulation", which means that the sound is divided in many single pulses (measured in hertz), and the "width" of these pulses generates louder or quieter sounds. The "space" between a pulse and another is noise, which needs to be eliminated from the system with a filter, and all this project requires a significantly more complex amount of engineering compared to the tube circuits aforementioned. Class D power amps  usually are less expensive than their tube counterparts, but they lack some of the tonal characteristics many guitar players are after (although technology is slowly catching up, and it is very likely that in the near future this differences will be completely nullified), on the other hand they are much more reliable, solid and power efficient.

I hope this was helpful!


Cambridge audio Class B and Class AB Amplification

Paul Mc Gowan


Electronic - Class A Amplifiers

Electronic - Amplifier Classes

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