ModBlog  – Amplifier Class

Class A vs. Class AB: What Bias, Heat, Distortion & Design Really Mean for the Music You Hear

When we refer to amplifier “Class”, we are really talking about how the amplifier’s output stage is biased. Class A and Class AB are different amplifier classes with unique properties, strengths and sonic signature. Amplifier “Class” andbias affect everything: tone, distortion, heat and our emotional connection to music. Each operating Class has its strengths and drawbacks. As is often true in life, we don’t always have to choose one or the other. There is a sweet spot that allows us to achieve the best of both worlds.

Think of bias as the “idle speed” of the amplifier’s output devices—whether they’re BJTs (bipolar transistors) such as the KWA 300 Reference power amp, the KWH 225i Hybrid Integrated amp, or MOSFETs, in the KWA 100 Monoblock Amplifiers. Higher bias means that the transistors are running at a higher speed or operating current, in much the same way as a car’s engine idle.  In the case of Class AB operation, there is also the rate at which the power transistors are turning on and off.

The more continuously the devices conduct, the smoother the signal and the less distortion you hear, especially around the zero-crossing point where the music waveform switches from positive to negative.

This interaction is where the difference between Class A and Class AB truly lies.

In a pure Class A amplifier, the output devices conduct 100% of the time, even when no music is playing. This creates Zero crossover distortion, natural even-order harmonics, liquid midrange and improved musicality.

The downside to Class A, is that the amp runs very hot and inefficiently. At higher power levels, this becomes problematic as it requires larger heatsinks and chassis, high operating temperatures and high AC power consumption. Class A amplifierw will often overheat the room.

Class AB addresses the issues of heat and physical size limitations by letting each device of different polarity, handle its half of the waveform, with some overlap. This reduces idle current significantly, resulting in cooler operation, higher efficiency and much higher power handling capability.

The rub here, is a thing called crossover distortion and hinges on how much overlap there is between ‘on’ stages of power transistors of different polarity. This overlap point is where crossover distortion comes into play. The larger the overlap, the lower the crossover distortion.

So, there is no free lunch!

Pure Class A is hot and inefficient. Class AB is efficient and runs cool, but does trade off some distortion and musicality.

At ModWright, we prefer to achieve the best of both worlds, by operating in High Bias Class A/B.

A high-bias Class AB amplifier pushes the idle current high enough that both output devices conduct simultaneously for a significant portion of the crossover region. This allows for the amplifier to stay in Class A for the first watts, commonly 10% or less of the total amplifier’s power output. The key here is that these are the most important watts.

High bias Class AB operation allows for greatly reduced crossover distortion, and heat is controlled and manageable. You also retain the ability to deliver substantial power on musical peaks. This approach blends the best of both worlds, offering Class A warmth and tone, low heat levels and very low measured distortion, particularly at normal listening levels.

The key to all of this, is that most listening happens within the first few watts. Keeping these watts in Class A, allows for a natural, organic sound with lifelike tone draws you into the music.

Understanding the difference between Class A and Class AB isn’t just about measurements, it’s about the enjoyment of music. Pure Class A offers unmatched purity and Class AB delivers practical power and control. High-bias Class AB combines these strengths, creating amplifiers that are musical and powerful.