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Class B, AB, and A

— By Paul McGowan —
Ever wonder what the classes of amplifiers mean? There's Class A, Class AB, Class B (and a few others). Here's a bit of history and an explanation. Early transistor power amplifiers had their share of problems: crossover distortion, poorly designed output stages, slew rate limitations, transient slewing, little attention paid to power supplies. These failings of solid state amps fueled the tube devotee's negative feelings toward them; and rightfully so. Until these issues could be fixed, few if any tube people would consider moving from the problems of tube amplifiers because there really weren't any better solutions being offered. Things would change, but not for a little while. Perhaps the biggest failing of early solid state designs, aside from their tendency to occasionally self ignite, was crossover notch distortion; a problem easily overcome by increasing the level of class A bias. Crossover notch distortion was fairly common amongst early solid state amplifiers that took advantage of the fact transistors had two types: positive and negative. Tube designers had only one type available to them: positive going tubes. The fact that solid state amplifier designers could use a positive transistor for the top half of the waveform and a negative transistor for the bottom half removed the need for a special inverting circuit some tube amps and push pull solid state amps used, plus benefited from an increase in efficiency. Less circuitry meant better sound. Only, if the two output devices were not used properly, this new type of bright and harsh sounding distortion would pollute our music. Here is a picture of severe crossover notch distortion. amp26 This happens when the positive transistor shuts off before the negative transistor turns on. This type of output would be called a Class B output and was almost never used in any quality audio products. However, the next scope photo was more indicative of what actual crossover notch distortion looked like. DISTORtion1 Note the breaks in the middle of the waveform. This area is heard as a harsh and bright coloration to the music, can be rather jarring and was fairly prevalent in early power amplifiers. To fix this problem we would turn each of the two transistors on just a little bit. This is called Class A biasing and the amount you turn it always on determines how we classify the amplifier: no bias and it is a Class B amplifier. A few watts of constant bias and we refer to it as Standard Class AB bias. Both transistor always on, even if there is no signal, full Class A. Early amplifiers, as well as many today, run at a few watts of class A bias; just enough to eliminate the crossover notch and no more than that. The net result is three types of amplifiers: Standard or High Class AB, Full Class A. To tell, it's reasonably easy. Just put your hand on the amplifier's heat sink after it's been idling with no music playing. If it's just warm, probably standard Class AB bias. Good and hot, yet still able to keep your hand on the sink, High Bias Class AB. Hotter'n a firecracker, Class A. Here's a picture of a true off-the-chart fully class A amplifier from our friend Nelson Pass. Aleph amps egg So now, when you see a solid state amplifier manufacturer say they have a 'Class AB amplifier' (as most do) you can ask the obvious question. "How much class A bias?" In the case of our new BHK Signature power amplifier, slated for release this spring, the answer is 40 watts per channel (High, Class AB bias). Which means that for the first 40 watts of power delivered to the loudspeaker, the amp is a true class A device. Since most loudspeakers play quite loudly at 40 watts and the majority of your listening is under 40 watts, the amplifier is essentially class A for all quiet to medium loudness music. Peaks and large dynamics are then not Class A, but since most of the benefits of class A bias are heard in inner detail and quiet passages, there's no need. Simple, eh? Now you know.

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