Adventures In Audio

Power amps: Class A, Class, B and Class D

by David Mellor

Power amps have a tough job to do, particularly in live sound. A 1000 watt amplifier, for instance, might be called upon to deliver up to 60 volts at a current of up to 16 amps. And don't forget that 1000 watts, equal to 1 kilowatt, provided by an electric radiator would probably be enough to keep your studio warm in winter. That's a lot of power to control.

There are a number of 'classes' in which power amplifiers can operate. Very small power amplifiers can operate in Class A. In Class A, there is a 'standing current'. This means that a current flows through the output transistors (or tubes) all the time that is as large as the maximum current that will ever be delivered to the loudspeakers. Imagine a river flowing through its normal channel, then diverting that flow to a watermill to do some useful work. Plainly, quantity of water, or current, is never going to be a problem.

But in a Class A amplifier, this standing current causes a different problem in itself - heat. Heat is of concern both in the output devices and the amplifier's own power supply. For this reason, it is rare to see a Class A power amplifier with an output greater than 20 watts or so. (You will sometimes see microphone pre-amplifiers described as Class A. These classes apply even for small signals, but at small signal levels there really isn't any point in using anything other than Class A. So in this context, Class A should just be expected - it isn't a bonus in any way).

The solution to the lack of efficiency of Class A is to use Class B. In Class B there is no standing current in the output devices. It is supplied to the loudspeakers as and when needed, and only as much as is needed. The problem in Class B is that there is a switching effect between the positive and negative half cycles of the waveform. To cure this, a small standing current is provided, making the amplifier Class AB, which the vast majority of power amplifiers are.

Audio Masterclass Video Courses

Learn FAST With Audio Masterclass Video Courses

With more than 900 courses on all topics audio, including almost every DAW on the market, these courses are your fast track to audio mastery.
Get a library pass for all 900+ courses for just $25.

Although Class B is more efficient than Class A, meaning that less energy is wasted, it is still only 70% efficient at best. This means that 30% of the energy is transformed into heat inside the amplifier, which means that the amplifier must have large heatsinks or a fan to dissipate the waste heat.

The Audio Masterclass Music Production and Sound Engineering Course


Ready to take your recording to the next level? Full online course leading to your Audio Masterclass certificate on successful completion.

But then there is Class D. Class D works in an altogether different way. Here, the amplifier switches rapidly between zero volts and maximum volts. There is no in-between level. But the time it spends on maximum is in proportion to the level of the input signal. So the amount of current made available by a Class D amplifier is proportional to the input. The high switching frequency is filtered out leaving just the amplified audio signal that is sent to the loudspeakers.

The efficiency of a Class D amplifier is determined by the 'rise time' - the time it takes to go from zero volts to maximum volts. The faster this can be, the more efficient the amplifier is and the less heat produced actually in the amp. Typically, Class D amplifiers can approach 90% efficiency.

This means that Class D amplifiers can be incredibly small and lightweight, because they simply do not need the massive cooling of normal Class AB amps. It has to be said that in purist circles, there is still a doubt about the ultimate sound quality of Class D amplifiers, but for live sound they seem set to take over the industry. Amp racks have never been so small, and since Class D amplifiers are lightweight, there is no barrier to their use in active loudspeaker systems.

Sunday December 25, 2005

Like, follow, and comment on this article at Facebook, Twitter, Reddit, Instagram or the social network of your choice.

David Mellor

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Adventures In Audio.

More from Adventures In Audio...

An interesting microphone setup for violinist Nigel Kennedy

Are you compressing too much? Here's how to tell...

If setting the gain correctly is so important, why don't mic preamplifiers have meters?

The Internet goes analogue!

How to choose an audio interface

Audio left-right test. Does it matter?

Electric guitar - compress before the amp, or after?

What is comb filtering? What does it sound like?

NEW: Audio crossfades come to Final Cut Pro X 10.4.9!

What is the difference between EQ and filters? *With Audio*

What difference will a preamp make to your recording?

Watch our video on linear phase filters and frequency response with the FabFilter Pro Q 2

Read our post on linear phase filters and frequency response with the Fabfilter Pro Q 2

Harmonic distortion with the Soundtoys Decapitator

What's the best height for studio monitors? Answer - Not too low!

What is the Red Book standard? Do I need to use it? Why?

Will floating point change the way we record?

Mixing: What is the 'Pedalboard Exception'?

The difference between mic level and line level

The problem with parallel compression that you didn't know you had. What it sounds like and how to fix it.

Compressing a snare drum to even out the level

What does parallel compression on vocals sound like?

How to automate tracks that have parallel compression

Why mono is better than stereo for recording vocals and dialogue