Class D Vs Class AB Amplifier Measured Differences
Two families divide the power-amp landscape here, and one axis separates them: whether the output transistors stay in a continuous, partially open state or snap fully on and fully off many times per second. Once you know which side of that line a design sits, the rest of its thermal, size, and distortion behavior follows from that single choice.
- Efficiency is the headline split. Class D units deliver 90 to 95 percent of their supply power to the speaker, while Class AB units land in the 50 to 70 percent range. That gap is why Class D enclosures run smaller and cooler.
- The distortion signature differs by branch. Class AB carries a small crossover-distortion spike near the zero-crossing point. Class D trades that for a low-level switching-noise floor that a well-designed output filter pushes below what most ears can resolve.
- At normal listening levels the two families sound close. Quarter-power on a typical speaker keeps measured total harmonic distortion under two watts, where the gap between a well-built Class D and a Class AB unit usually falls below the threshold most listeners can hear.
- Size and component cost trade in opposite directions. Class D shrinks the power supply and heatsink but adds output-filter inductors and gate-driver circuits that Class AB designs skip entirely.
The Lay Of The Land
Walk into any hi-fi room and you will meet the same fork: the amplifier on one side runs its output transistors in a continuous, partially open state, and the one on the other side turns those transistors fully on and fully off thousands of times per second. Both families exist to do the same job, which is take a tiny signal from your source and hand a much larger one to the speaker cone. Listeners keep circling back to this fork because the labels on the box (“Class A,” “Class AB,” “Class D”) describe construction, not the way the music actually reaches your ears. Think of it like two ways of controlling water through a pipe. One approach turns a single valve partly open and modulates the flow smoothly. The other approach flips the valve fully open and fully shut many times per second, then uses a filter screen at the end to smooth the pulses into a steady stream. Both deliver water. The valve approach wastes energy as heat in the partially open position. The switch approach wastes very little, but the pulse train needs a cleanup stage before it becomes a clean, musical signal.
The Whole Family At A Glance
| Family | One-line working principle | Signature sound trait | Where you typically meet it |
|---|---|---|---|
| Class A | One set of output devices stays continuously on, carving the full waveform | Warm, low-thD at idle; high heat even at low volume | Vintage tube and solid-state reference rigs |
| Class AB | Output pairs ride a small bias current and add extra conduction only at higher peaks | Clean mids and highs; a small THD spike near the zero-crossing on sensitive tweeters | Most component stereo and home-theater receivers |
| Class D | Output devices switch fully on or off at a carrier far above 20 kHz; an LC filter recovers the audio | Very low heat; bass control advantage on low-impedance loads; a faint switching-noise floor | Bluetooth speakers, portable amps, modern integrated systems, many AV receivers |
Each Family Up Close
The Class A Ancestor
Class A was the first transistor-based topology to replace the vacuum tubes that had dominated audio from the 1910s through the 1960s. In a Class A stage, the output devices sit in a permanently partially open position, much like a water valve held at a fixed crack-open setting. The signal simply moves the valve a little wider or a little narrower around that midpoint. Because the devices rarely fully shut off, a large share of the supply voltage drops across them as heat even when the music is quiet or absent. A 100-watt Class A amp might draw 300 watts from the wall just to sit at the pre-amp stage. The listening impression is one of smooth, unstrained bass and a midrange that some listeners describe as relaxed. The trade-off is thermal bulk. Big heatsinks and generous power supplies keep the devices from overheating, so Class A units tend to be heavy and warm to the touch. Today you meet Class A mainly in dedicated reference amplifiers and in the pre-amp sections of some integrated designs.
The Class AB Branch
Class AB is the middle child. It keeps the general idea of continuous, partially open operation but adds a small bias current so the output pair idles just past the crossover point where the two devices hand off to each other. This reduces the “dead zone” that pure Class A still has and cuts heat. Yet at moderate to high output, the devices move back into a deeper conduction region and the heat builds again. The signature distortion tell is a small total harmonic distortion spike that appears right around the zero-crossing of the waveform. On a sensitive tweeter running at one or two watts, that spike is the most audible flaw most people can spot. In everyday listening at quarter-power on a typical bookshelf speaker, the spike usually stays below the threshold where an average ear separates it from the music. You meet Class AB in the large majority of component stereo receivers, multi-channel AV amps, and most mid-fi integrated units. The thermal management is visible: moderate-sized heatsinks and a power supply that is larger than a Class D unit but smaller than a full Class A design.
The Class D Branch
Class D works on a completely different rhythm. Instead of riding the output devices in a partially open state, it drives them as hard switches: fully on, then fully off, many thousands of times per second at a carrier well above 20 kHz. The on-time per cycle encodes the instantaneous signal level. A pair of inductors and capacitors (an LC output filter) then smooths that rapid pulse train back into a clean audio waveform. Because a switch that is fully on or fully off dissipates very little power, the efficiency of the stage jumps to 90 to 95 percent of supply-to-speaker delivery. The heatsink shrinks. The power supply bulk shrinks. The whole enclosure gets smaller and lighter. The signature listening impression is a very tight, controlled bass, especially on low-impedance woofers. The faster switching of a Class D stage gives it a higher damping factor, meaning the amp can pull the woofer cone back more decisively when the music stops. The one thing that replaces the Class AB crossover spike is a low-level switching-noise floor. A well-chosen output filter and a careful gate-driver layout keep that floor below the threshold of hearing in most listening rooms. You meet Class D in Bluetooth portable speakers, modern integrated amplifiers, many in-vehicle audio systems, and an increasing share of home-theater receivers.
The Difference That Counts
The physical difference that does the real work is the state of the output transistors at any given instant. In Class AB, they sit at a partial voltage drop, and that drop turns directly into heat. In Class D, they sit at either full supply voltage or near zero, so the product of voltage times current (the thing that makes heat) is tiny. Think of a bicycle brake. A Class AB stage is like squeezing the brake lever partway and holding it there. You get smooth, proportional slowing, but your hand gets tired and the caliper heats up because the pad rubs continuously. A Class D stage is like fully opening and fully closing the brake many times per second and using a small damper at the wheel to smooth the pulses into a steady deceleration. Your hand is barely loaded because the pad is either fully gripping or fully released. The surface labels mislead because “Class D” sounds like it should be noisier or more digital, while “Class AB” sounds like a premium linear design. In reality, at quarter-power on a typical speaker the measured total harmonic distortion of a well-built Class D and a Class AB unit usually lands within the range where most listeners cannot separate the two. The audible gap only opens up at very low power on a very sensitive tweeter, or at the extreme peaks where a Class AB unit starts to push its crossover distortion spike further above its quiet floor.
Edge Cases And Hybrids
Three common designs blur the family lines, and the trick is to ask which side of the switching-versus-continuous axis they actually lean toward.
Class A/AB hybrid pre-amp and power section. Many integrated amplifiers run a Class A pre-amp stage (continuous, partially open) feeding a Class AB power stage. The pre-amp side is Class A, the power side is Class AB. When you ask which family the unit belongs to, look at the power section, because that is where the heat, the crossover distortion, and the efficiency number live. This unit sits firmly on the continuous side of the axis.
Class D with a Class AB output buffer. Some automotive and portable designs use a Class D switching core but place a small Class AB buffer at the very last output terminal to shave off residual switching ripple. The core is still switching, so the efficiency and size advantages are mostly intact. Ask: where does most of the signal energy get converted? If it is in the switch stage, the unit leans Class D. The buffer is just a cleanup step, analogous to the LC filter but one stage later.
Class D with “linear” marketing labels. A few manufacturers market a Class D unit with a Class AB-sounding name because the output filter smooths the pulse train so well that the final waveform looks continuous on an oscilloscope. The answer is in the schematic: if the output devices are driven as on/off switches at a carrier above 20 kHz, it is Class D no matter how “analog” the final output trace appears. The axis is the drive signal to the switching devices, not the shape of the output waveform after the filter.
What Listeners Ask
Does Class D Really Sound More Digital Or Harsh?
The switching noise floor is the only real candidate for a “digital” artifact, and a well-designed output filter and gate-driver layout push that floor well below the threshold of hearing in a normal room. At quarter-power on a typical bookshelf speaker, the measured distortion of a Class D unit usually sits in the same band as a Class AB unit. If you can hear a difference, it is more likely a design-quality gap than a topology gap. The axis that matters is how thoroughly the switching ripple is filtered, not the letter in the class name.
Why Does A Class D Amp Run So Much Cooler Than A Class AB Unit Of The Same Wattage?
Because the output devices in a Class D stage are either fully on (near zero voltage drop) or fully off (zero current), the product of voltage times current, which is where heat comes from, is very small. The 90 to 95 percent efficiency number is a direct result of that. A Class AB unit sitting at 50 to 70 percent means a large share of the supply power turns into heat that the heatsink must carry away. Fewer watts of heat means a smaller heatsink, a lighter enclosure, and less fan noise or no fan at all.
Will A Class D Amp Handle My Low-Impedance Speaker Better Than A Class AB One?
Usually, yes, on the damping-factor axis. The faster switching of a Class D stage lets the amp pull current back from a low-impedance woofer cone more decisively when the signal reverses. On a four-ohm or two-ohm driver, that tighter control can make bass feel more taut and less bloated at the tail of each note. The difference is most noticeable at quarter-power, where a Class AB unit might be running near the edge of its comfortable thermal budget and the damping factor starts to sag. A Class D unit has more thermal headroom, so the damping factor holds more steadily.
Can I Hear A Difference Between A Class D And A Class AB Amp At Normal Listening Volume?
In most rooms, with most music, and at the volume where the amp is delivering quarter-power to a typical speaker, the measured total harmonic distortion gap between a well-built unit from each family falls below the threshold where most ears separate it from the source material. The crossover-distortion spike of a Class AB stage and the switching-noise floor of a Class D stage are both very small at that power level. The audible difference usually shows up at the edges: very low power on a high-sensitivity tweeter, or very high peaks where a Class AB unit is thermally compressed. For the vast majority of casual and reference listening, the two families land close enough on the distortion axis that the choice becomes about size, heat, and cost rather than tone.
Does The Efficiency Number Actually Matter If I Am Running The Amp At Low Volume?
It matters for heat, not for sound. A 90-percent-efficiency Class D unit drawing 50 watts from the wall to deliver 45 watts to the speaker will run nearly cool. A 60-percent-efficiency Class AB unit drawing the same 45 watts of output will waste roughly 30 watts as heat that the enclosure must shed. At very low listening levels the absolute waste is small for both, so the thermal difference is modest. The efficiency gap becomes dramatic when you need headroom for dynamic peaks: a Class D unit can deliver a short 200-watt burst without its power supply or heatsink reaching a stress point, whereas a Class AB unit of the same continuous rating may be pushed to its thermal limit during that burst.
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