Power Amplifiers


The sound of power amplifiers is one of those ethereal things that seems to defy description. This is largely hypothesis, since there are so many influences that, although present and audible; are almost impossible to quantify. Especially in combination, some of the effects will make one amp sound better; and another worse – I doubt that we can list all the possibilities, but this article might help some of you a little – at least to decipher some of the possibilities.

When people talk about the sound of an amplifier, there are many different terms used. For a typical (high quality) amplifier, the sound may be described as ‘smeared’; having ‘air’ or ‘authoritative’ bass. These terms – although describing a listener’s experience; have no direct meaning in electrical terms. The term ‘presence’; is created in guitar amps (for example) by boosting the frequencies around 3kHz – it’s not something found in power amplifiers.

Electrically, we can discuss distortion, phase shift, current capability, slew rate and a myriad of other known phenomena. I don’t have any real idea as to how we can directly link these to the common terms used by reviewers and listeners.

Some people have claimed that all amplifiers actually sound the same, and to some extent (comparing apples with apples) this is ‘proven’ in double-blind listening tests. I am a great believer in this technique, but there are some differences that cannot be readily explained. An amp that is deemed ‘identical’ to another in a test situation, may sound completely different in a normal listening environment. It is these differences that are the hardest to deal with; since we do not always measure some of the things that can have a big influence on the sound.


Class-AOutput device(s) conduct for complete audio cycle (360°)
Class-BOutput devices conduct for 180° of input cycle
Class-ABOutput devices conduct for more than 180° but less than 360° of input cycle
Class-COutput device(s) conduct for less than 180° of input cycle (RF only)
Class-E, FSub-classes of Class-C, RF only
Class-DOutput devices switch at high frequency and use PWM (pulse-width modulation) techniques (Note that Class-D does NOT mean ‘digital’)
Class-GMake use of switched power rails, with amplifiers typically having multiple power supply rails
Class-HUse modulated power rails, where the supply voltage is maintained at a voltage slightly greater than required for the power delivered
Class-IA proprietary variant of Class-D (it appears that this is not officially recognised)
Class-TAnother proprietary amp class, and also a variant of Class-D (this is also not officially recognised)
BTLBridge-Tied-Load.  Not a class of operation, but sometimes thought to be.  Can be applied to any class of amplifier

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