Class D vs Class AB Amplifiers

Amplifier classes describe how the output devices are biased — how much of the signal waveform each device conducts for. It is an engineering description, not a quality ranking, and the letter on the box tells you far less about the sound than the specification sheet does.

Class AB in one paragraph

Two sets of output devices, one handling the positive half of the waveform and one the negative, both biased so they conduct slightly past the crossing point. That overlap is the “AB”, and it exists to avoid the switching distortion a pure Class B design produces at the zero crossing.

The devices operate as variable resistors in the current path, so the difference between the supply voltage and the output voltage is dissipated as heat. Typical efficiency runs somewhere in the region of 50–60% at full output and considerably less at normal listening levels — which is why a 45-watt Class AB amplifier has substantial heatsinks and weighs what it weighs.

The Yamaha A-S501, Marantz PM6007, Denon PMA-600NE and Cambridge AXA35 are all Class AB.

Class D in one paragraph

The output devices are switches, flipped fully on or fully off at a high frequency — usually several hundred kilohertz — with the ratio of on-time to off-time tracking the audio waveform. A low-pass filter at the output removes the switching frequency and leaves the audio.

Because a switch that is fully on drops almost no voltage and a switch that is fully off passes almost no current, very little power turns into heat. Efficiency is typically well above 90%, which is why the Fosi Audio V3 can claim large wattage figures in a chassis you could hold in one hand, and why the WiiM Amp fits 60 W per channel into a 190 mm square box weighing 1.84 kg.

“D” does not stand for digital. It is simply the next letter after C. The signal path in most Class D amplifiers is entirely analog.

What the published specs actually show

This is where the comparison gets useful, because the two topologies get specified very differently — and Class D products in this price range are consistently less completely specified.

The power-supply fine print

Here is the single most important thing to understand about budget Class D wattage claims.

Fosi publishes 300 W × 2 into 4 Ω for the V3 — a “600 W total” headline. They also publish a power input range of DC 24–48 V, and list both 32 V/5 A and 48 V/5 A supplies.

Output power scales with the square of the supply voltage. A module fed 32 V cannot produce what the same module produces at 48 V, and the headline figure necessarily assumes the higher one. Which supply is in the box you buy determines what the amplifier actually does — and that is before noticing that a 5 A supply at 48 V can deliver 240 W total, which is less than the “600 W” headline on any continuous basis.

Fosi also publishes no 8-ohm rating at all, while stating a terminating impedance of 2–8 Ω. So an 8-ohm speaker will see meaningfully less than the headline number, and Fosi does not say how much less. None of this makes it a bad amplifier — its published SINAD of 88 dB or better is genuinely good — but the number on the box is not a number you can compare to a Yamaha’s.

Does Class D sound worse?

We have not heard any of these amplifiers and we do not make sonic claims. What we can tell you is what the published measurements support and what they do not.

On distortion, modern Class D is not behind. Fosi publishes THD at or below 0.003% and a SINAD of 88 dB or better; Yamaha publishes 0.019% THD for the A-S501 at 50 W. Those are both far below audibility, and the Class D figure is the lower one.

The historical criticisms — output filter interaction with the speaker’s impedance curve causing frequency response variation, and switching artifacts near the top of the audio band — were real in early designs and have been substantially addressed in modern modules. What we cannot verify is how well any particular implementation addresses them, because neither Fosi nor WiiM publishes a frequency response measured into a real load rather than a resistor.

That last point is the honest summary of this whole comparison. It is not that Class D measures worse. It is that at this price, Class D products publish less.

When each makes sense

Class D when size, heat or power consumption matter; when you need a lot of watts cheaply for insensitive speakers like the Micca RB42 at 83 dB; or when the amplifier is part of a streaming product where an efficient module is the only thing that fits.

Class AB when you want a fully specified amplifier: a published damping factor, a published 4-ohm rating with a stated condition, a phono stage with a published input sensitivity, and a THD figure quoted across the whole audio band rather than at one frequency.

And note that the class tells you nothing about the features that usually decide a purchase. Whether an amplifier has a phono input is a completely separate question from how its output stage is biased — the Fosi V3 has no phono input, and neither does the WiiM Amp.

Whichever topology you choose, the decision that actually determines whether the system goes loud enough is the power-versus-sensitivity arithmetic, and it does not care what class the amplifier is.