Amplifier Classes Explained: A, A/B, D — and Arcam’s Class G

Amplifier “classes” sound like grades — as if Class A were the best and everything else a compromise. They are nothing of the sort: they are different engineering answers to one stubborn problem, and knowing the trade-offs makes you a sharper buyer.

The problem being solved

An output transistor is most linear — most faithful — when it is always conducting. It is most efficient when it only works as needed. Faithful runs hot and wastes power; efficient risks tiny handover errors as devices switch on and off. Every class is a different position on that see-saw.

Class A — purity with a radiator

Everything conducts all the time: gloriously linear, gloriously inefficient. Expect heat, weight and modest power — and, done well, a lovely sound. Rare now outside high-end designs and valve amplifiers, which operate on related principles.

Class A/B — the sensible workhorse

Devices share the work, overlapping slightly at the handover to dodge the errors. The overwhelming majority of amplifiers ever sold — including many superb ones in our listening room — are well-executed A/B.

Class D — the efficient modern

Switching designs reaching efficiencies over 90%, which is how tiny streaming amplifiers like the WiiM Amp make real power without a heatsink the size of a loaf. Long past its rough early reputation — today’s best Class D is genuinely good.

Class G — the clever compromise Arcam champions

Class G runs a conventional output stage from multiple power rails: low rails for everyday listening (cool, efficient), switching seamlessly to high rails when a crescendo demands current. You get A/B-style linearity with far less waste heat — which is why Arcam has made fifth-generation Class G the heart of its A25+, SA35 and SA45. Our Arcam range guide goes deeper.

What to do with all this

Never buy a class; buy a sound. The letters explain why an amplifier behaves as it does — only listening tells you whether you’ll love it. Arrange a demonstration and hear A/B, D and G back to back through the same loudspeakers.