Why Class D Amplifiers Are Called The Audio Disruptor
Class D amplification wins when size and heat are the constraint. It trades a small high-frequency noise floor for a dramatic drop in wasted power, which is why it moved from a niche “not hi-fi” option to the default inside subwoofers, portable speakers, and compact amplifier chains. The deciding factor is thermal.
The heat problem is the whole story. Linear output stages burn most of their input power as waste heat in the transistor. Class D switching cuts that loss to a small percentage, which shrinks the cabinet or lets a portable speaker run at meaningful wattage without overheating.
The trade-off lives at the crossover. Switching creates a high-frequency noise floor that the speaker’s crossover must filter before it reaches the tweeter. The electrical match between amp and speaker becomes more consequential than with a linear source, and a poorly voiced crossover will let that ripple color the highs.
The “disruptor” label is arithmetic, not marketing. Once you see that a few percent of loss replaces the 50-plus percent a linear stage wastes, the jump from “not hi-fi” to the default in subwoofer and portable chains follows directly. No hype needed.
A Picture Of Each Side
Think of a gas stove burner. You turn the knob, and the flame size changes smoothly and continuously. The more heat you want in the pot, the more gas you burn. But a lot of that gas rarely reaches the food. It leaks out the sides of the burner, warms the pan, and heats the air around the whole stove. The burner plate itself gets hot to the touch. To run it at full power for a long time, you need a big, heavy pan to absorb all that wasted heat, or the whole setup gets dangerously hot. The size of the stove is set by how much waste heat you need to carry away, not by how much food you are actually cooking.
Now think of a bicycle pump. You do not push the handle smoothly. You push hard, stop, push hard, stop. Each push is quick and decisive. The average air flow into the tire matches what a smooth, continuous push would give, but your hand is only on the handle for a small fraction of the total time. Your hand stays cool. You do not need a massive, heavy pump frame to keep it running for long stretches. The pump can be small, light, and portable, and it still fills the tire.
The fork between the two is simple. Linear amplification works like the stove: the output voltage changes continuously, and most of the energy you feed in becomes heat in the transistor. Switching amplification works like the pump: the output turns on and off in rapid bursts, and the average force on the speaker cone is the same, but the transistor is only on a small fraction of the time. The heat problem largely goes away, and the enclosure can shrink.
How They Compare
| What you care about | Linear (Class A / AB) | Class D (switching) |
|---|---|---|
| Where the input power goes | Most of it becomes heat in the output transistor; only a fraction drives the speaker | A small percentage becomes heat; the rest goes to the speaker |
| Enclosure and case size | A large heat sink is needed to carry off waste heat, so the cabinet must be bigger and heavier | No large heat sink; the same wattage fits in a subwoofer box, a bookshelf, or a portable shell |
| What the speaker cone receives | A smoothly varying voltage with no extra high-frequency content | The same average signal plus a high-frequency switching ripple that the crossover must filter |
| Crossover design sensitivity | Mild; the filter works with a relatively clean input signal | Higher; the switching noise floor makes the amp-to-speaker electrical match more consequential, and a shallow filter can let ripple reach the tweeter |
| Battery and portable use | Limited to a watt or two before the case overheats; battery life suffers from the wasted heat | The default topology in portable speakers; meaningful wattage without thermal runaway or rapid battery drain |
| Listening signature at the driver | Warm, continuous drive; transients can soften as the stage heats during long sessions | Tight, fast transients; a faint high-frequency edge can appear if the crossover is not well matched |
The Same Track, Two Ways
Pick any song with a steady bass line, a lead vocal, and some cymbal work. Play it through both.
Bass. Through the linear amp, the bass note starts and the cone moves with a slight cushion. The initial attack is smooth. As the song builds and the bass plays harder, the output transistor gets warmer. The drive stays continuous, but the bottom notes lose a touch of snap because the stage is working harder and the heat is stealing some of the available headroom. Through the Class D amp, that same bass note hits with a sharper initial attack. The cone gets the same average force, but the switching bursts are so short that there is no thermal drag slowing the response. The low end stays tight even when the bass plays at full volume for a long stretch.
Vocals. The linear amp gives the lead vocal a slightly rounded, forward quality. The continuous drive means the voice sits in a smooth, unbroken envelope. Through the Class D amp, the vocal stays in the same position, but the decay between syllables is a touch tighter. Consonants hit more precisely because the switching transients are fast. If the crossover is well matched, the difference is subtle. If the crossover is not, the high-frequency switching ripple adds a faint metallic edge to sibilance, and the vocal sounds slightly brittle at the top of the range.
High frequencies. Cymbals and overheads through the linear amp bloom naturally. The tweeter receives a clean, smooth signal with no extra content above the audio band. Through the Class D amp, the tweeter still receives the same cymbal detail, but only if the crossover filter suppresses the switching ripple. A well-designed filter rolls that ripple off before it reaches the tweeter, and the cymbals sound just as open and airy. A poorly matched filter lets the ripple through, and the cymbal takes on a thin, hissy layer that is hard to ignore at higher volume.
Where People Go Wrong
Slapping a Class D amp behind a passive speaker with a crossover built for a linear source. The crossover was designed for a clean, smooth input signal. The switching ripple, sitting at a much higher frequency, is not filtered well. The audible symptom: a faint, persistent hiss or whistle at the tweeter, most noticeable in the quiet passages between verses. The fix: use a crossover with a steeper filter slope or one specifically voiced for a switching source.
Pairing a Class D subwoofer amp with a sub that has a low, dipping impedance curve. The switching stage is more sensitive to load swings than a linear stage. When the sub’s voice coil impedance drops to its lowest point in the range, the amp’s output can momentarily sag. The audible symptom: the bass notes lose punch on the hardest hits, and the low end feels soft or lazy right at the moment the song needs the most impact. The fix: match the amp’s rated load to the sub’s worst-case impedance, not its nominal value.
Judging a linear amp by how hot the case gets. You crank the volume on a Class A or AB integrated amp, feel the top of the case getting warm, and assume the amp is pushing harder than it really is. The audible symptom: the music does not get louder or more dynamic just because the case is hot. Most of that heat is wasted input wattage that rarely reaches the speaker. The amp is not working harder; it is simply less efficient. The fix: judge output by the speaker’s cone movement, not by the amplifier’s surface temperature.
Choosing a portable speaker purely on the wattage number printed on the box. The wattage figure often refers to peak switching output, not the continuous power the battery and the small enclosure can sustain. The audible symptom: the bass sounds thin and the mids are thin, even though the box claims a high number. The sustained level is far below the peak. The fix: listen to the sustained bass level over a full song, not the peak number.
Questions Before You Choose
Does “Switching” Mean The Sound Is Digital Or Cold?
No. The word “switching” describes how the output transistors turn on and off in rapid bursts. It does not mean the signal path is quantized the way a digital-to-analog converter works. The music stays analog from source to speaker. The cold, thin reputation was earned by early designs with poor filter components and loose electrical matching. Well-voiced modern designs sound continuous and full.
Why Did Portable Speakers Jump From A Watt Or Two To Meaningful Bass So Quickly?
Because the thermal wall came down. A battery-powered box running a linear stage would overheat at just a few watts of continuous output. Wasted heat drained the battery and cooked the enclosure. Switching collapses that loss to a small percentage, so the same small case can push the cone at far higher levels without the battery dying in ten minutes or the plastic warping. The size constraint, not the audio quality constraint, was the bottleneck.
Is The Switching Noise Actually Audible To Most Listeners?
Usually not, if the crossover is well matched. The switching ripple sits well above the audio band, and a properly designed filter rolls it off before it reaches the tweeter. You would notice it only in a poorly designed system, where the filter slope is shallow or the electrical match between amp and speaker is off. Then a faint hiss in the air between notes is the giveaway.
Can A Class D Amp Drive A Full Passive Speaker, Or Only A Sub?
It can drive both, but the crossover design matters more with a switching source. A two-way passive speaker works behind a Class D amplifier if the crossover filter is steep enough to handle the switching ripple. A subwoofer-only chain is the simplest case, because the crossover only manages the low band and the ripple sits well above the sub’s range. The speaker itself is not the limitation; the crossover matching is.
Which Topology Should I Pick For A Small Bookshelf Pair In A Bedroom?
If the room is small and the listening level is moderate, a linear Class AB amp is simpler to pair with, and the thermal penalty is manageable at low wattage. If you want the same pair to reach a higher volume without the case getting hot, the Class D path keeps the whole stack cooler and more compact. The room size and target volume decide it more than the label on the box.
(Originally released on hifireport.com. Do not copy or republish without permission.)