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What’s this Class A, Class AB, Class B stuff?

This post focuses on the basics of the differences between various amplifier (and preamplifier) types, namely class A, B, and AB.

When we first start out learning about audio equipment, there’s an awful lot of terms to sort through.  One in particular that escaped me for years was what exactly the various ‘classes’ of amplifiers amount to.  You hear ‘Class A’ thrown around quite a bit in the preamp industry.  What does it mean?

Basically, amplifiers take the audio signal’s amplitude, the distance between the crests and the troughs of the waveform, and boost it to make it louder.  Usually you’ll end up with some amount of distortion in the process of doing so.  The various classes of amplification refer to the manner in which the signal is boosted and they relate to the efficiency of the amplifier.

Class A amplifiers run their current constantly, amplifying both the peaks and troughs of the audio signal.  Therefore, they create an accurate signal – just louder.

Class B amplifiers run their current half the time, amplifying just the peak or trough.  This produces a distorted signal but is more efficient at producing volume.

Class AB amplifiers are basically two B amplifiers, each amplifying half the signal.  Then, the signals are combined to create the overall signal. The combination isn’t perfect, which produces what’s known as ‘crossover’ distortion.  But, it is more efficient than class A and produces substantially less distortion than class B.

Preamps don’t require as much amplification as loudspeaker amps, so you’ll find that class A is more common in preamps than it is in loudspeaker amps.  Generally speaking, class A amps are more sonically accurate.  That’s why everyone throws the term around!  I really doubt you’ll ever run across anything that’s class B.  Some legendary gear is class AB, such as the 1176 compressor… so there’s no blanket statement that says that class A is automatically better.

2 Responses to :
What’s this Class A, Class AB, Class B stuff?

  1. Bryan Tewell says:

    Hows about class D? I’ve heard that used a lot on high end gear as well. Things like Acoustic Image instrument amplifiers and Channel Islands Audio amps.

  2. Dan Connor says:

    Bryan – to be honest, I don’t really understand class D stuff very well. I know they use MOSFETS instead of transformers. I have a class D amp driving my stereo system.

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