There's a comforting piece of audiophile folklore that says most amplifiers run in pure Class A at low listening levels.
It's rarely true.
A pure Class A amplifier keeps its output transistors conducting current at all times, never shutting them off across the signal swing. The benefit is no crossover distortion — the little discontinuity that happens when output devices hand off responsibility to one another. The cost is heat and inefficiency. A 50-watt Class A amp may dissipate hundreds of watts even when playing silence, but it is among the sweetest-sounding amplifiers ever made.
Class AB designs solve the heat problem by biasing the output stage with just enough idle current to keep distortion low at small signals, then letting the devices hand off as the signal swings larger. Where the dividing line falls depends on how much bias the designer chose. A heavily biased Class AB amp may genuinely run in Class A for the first watt or two — which covers a lot of real-world listening at moderate volume — before it transitions. A lightly biased one may slip out of Class A at fractions of a watt, with the crossover region landing right where our ears are most sensitive.
In our new PMG Signature series of amplifiers we put together a very interesting hybrid. Instead of your standard "just enough" class A bias to lower crossover distortion, we utzed up the figures so that our largest monoblock, "The Beast" (or more properly the M800) has a whopping 50 Watts of pure class A power for starters, and from there lots more headroom to boot. What this means is that instead of getting a 50 watt class A amplifier and hoping/praying you'll have enough headroom (you won't) 90% of what you hear is as sweet as honey and you'll never run out of gas.
The first few watts are where the music lives. Treat it like the most important watts your amplifier will ever deliver, because for most of us, it is.
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