Class D vs Class AB Amplifier Topologies — Class D vs Class AB

What Class D and Class AB are

A Class AB amplifier sits between Class A (each output device conducts the entire input cycle) and Class B (each device conducts exactly half the cycle). Each of the two active output devices conducts more than half of the input cycle but less than the full cycle. The small overlap is the bias current that prevents crossover distortion at the zero-crossing where one device hands off to the other.

Because that bias current dissipates power continuously, Class AB's theoretical efficiency ceiling sits below the Class B 78.5% number for full-amplitude sine waves in transistor amplifiers. That figure is the ceiling at clipping with a full sine; at realistic music listening levels actual efficiency is much lower.

In a Class D amplifier the output transistors do not amplify linearly — they function as electronic switches that are either fully saturated (on) or fully cut off (off). Because a saturated device drops near-zero voltage and an off device passes near-zero current, the V × I product across the device is small in either state, which is the source of Class D's efficiency advantage. Practical Class D output stages run at efficiencies well over 90%, and the advantage holds across the full power range, not only at peak — Class AB's losses are highest at the low and mid output levels where music actually sits.

How each topology produces audio

A pure Class B push-pull stage produces crossover distortion at the zero-crossing because each output device needs roughly 0.7 V of base-emitter voltage before it begins conducting, leaving a flat dead-zone. Class AB fixes this by biasing both devices into a small idle current so their operating regions overlap, which allows a gradual transition between the push and pull devices and greatly reduces the crossover distortion. The bias must be tuned carefully — too much overlap causes overheating, too little causes a rise in crossover distortion.

Class D works on a completely different principle. The analog input is converted to a stream of pulses that represents the signal by pulse-width modulation, pulse-density modulation, delta-sigma modulation, or a related modulation technique. The switching output is then reconstructed by a passive LC filter — a simple low-pass network of an inductor and a capacitor that provides a path for the low frequencies of the audio signal while leaving the high-frequency pulses behind.

The signature design move of modern reference Class D — Hypex NCore and Purifi Eigentakt — is closing the feedback loop after the output filter, so the filter's load-dependent behavior, inductor resistance, and switching artifacts all sit inside the correction loop. Hypex describes NCore as a non-hysteresis fifth-order self-oscillating control loop that applies feedback directly at the loudspeaker output. Purifi's Eigentakt cites ultra-high loop gain greater than 75 dB across the entire audio band — corresponding to a 110 MHz gain-bandwidth product — with the result that THD remains extremely low at any frequency and any power level right until clipping.

Audible and measurable differences in 2026

On bench measurements modern reference Class D modules now equal or beat the best linear designs. The Hypex NCx500 module's published spec lists THD+N of 0.0006% at 1 W across 20 Hz–20 kHz, with third-party finished-amplifier measurements reporting figures down to ~0.00020% and roughly 137 dB signal-to-noise; Purifi's 1ET400A is rated at THD+N below 0.00017% with 131 dB(A) dynamic range. Those numbers sit at or above the Benchmark AHB2 — a Class AB design with feed-forward correction — which Audio Science Review measured at 113 dB SINAD and called the best they had tested at the time.

The objectivist measurement community's consensus is that audible differences track measured performance, not amplifier class. Forum reference summaries hold that within their rated performance envelope, any decently engineered amp will sound like every other decently engineered amp, and that quality designs in Class A, AB, and D all sound the same in double-blind tests. Once frequency response is flat, noise is inaudible, and distortion is inaudible, there is no other way for the amplifier to mess up the sound. The audiophile press more often asserts that Class AB has a warmer or more analog character; both views are widely held and a brief like this should not present either as undisputed.

Early Class D suffered audible problems — load-dependent frequency response from the LC filter interacting with speaker impedance, intermodulation distortion at high frequencies, and switching artifacts inside the audio band — which produced a harsh reputation. Purifi states explicitly that Eigentakt eliminates common stigmas such as harsh sound and frail reliability that plagued traditional Class D. Hypex frames NCore as the first Class-D amplifier not just to nudge the best linear amplifiers but to surpass them in every aspect relevant to sound quality.

When to choose each in 2026

High-channel-count immersive amplifiers are practical only in Class D. The Trinnov Amplitude16 packs 16 channels of Class D power rated at 200 W into 8 ohms and 400 W into 4 ohms, delivering 2,800 W continuous all channels driven in a 3RU chassis. As one reviewer summarised, building a 16-channel amplifier this compact but also this powerful requires Class D — anything else would not be practical. The same thermal logic drives every modern 11-plus channel AVR.

Class AB still owns the discrete-component audiophile two-channel market — McIntosh, Pass Labs, Luxman, Krell, Accuphase, and most boutique headphone-amp makers — where chassis size, heat, and efficiency are not constraints, and where the visual and cultural appeal of large heat sinks and meters is part of the product. For home audio setups where ultimate sound quality is desired and space or budget is less of a concern, Class AB remains a popular choice among discerning listeners.

Between linear and switching topologies sit two hybrids that keep a Class AB output stage but vary the supply rails to cut idle dissipation. Class G amplifiers use rail switching to decrease power consumption and increase efficiency and are more efficient than Class AB; Class H amplifiers create an effectively variable analog supply rail by modulating the rails so that they sit only a few volts above the output signal. Class G sits between AB and D on efficiency — more efficient than Class AB but less efficient than Class D.