Gear & technology

Class A

Also called Class A amplifier, pure Class A

A Class A amplifier keeps its output transistors or tubes conducting all the time, so the signal never has to hand over between devices. It is the simplest and, in theory, cleanest output stage, and also the least efficient: most of the power becomes heat.

In Class A every output device conducts for the whole waveform: its current swings around a steady bias and never reaches zero, so there is no handover. That standing current is why efficiency peaks at only 25 to 50 percent.

Because the output devices are always on, there is no crossover distortion, the glitch a Class AB stage can add as one device switches off and the other on. The price is efficiency, which peaks at 25 to 50 percent depending on the design and is far lower at normal listening levels, so a 25 W Class A amp draws well over 100 W from the wall and needs large heatsinks.

Class A is common in headphone amplifiers and tube amps, where the power involved is small, and in a few speaker amplifiers from makers such as Pass Labs. Fans describe a smooth, effortless sound; measurements show low distortion but not necessarily lower than a good Class AB design.

Related terms