What Is a Class D Amplifier? Working Principle, Efficiency and OEM Design Basics

Put your hand on the back of a 500 W powered speaker after an hour of playback. A Class A or Class AB design is usually too hot to touch; a Class D amplifier is often just warm. That one difference explains most of what matters.

A Class D amplifier is a switching amplifier. Its output transistors are driven fully on or fully off at a high frequency, and the audio information is carried by the width of those pulses rather than by the linear operating point of the devices. An LC low-pass filter at the output strips the switching carrier and leaves the amplified audio. Because the transistors almost never sit in the linear region, very little energy turns into heat: typical efficiency lands between 80% and 95%, against roughly 25-30% for Class A and 50-70% for Class AB.

What Is a Class D Amplifier, Exactly?

The letter D does not stand for "digital." It is simply the next letter in the sequence of amplifier classes, and the audio path inside a Class D stage is analog: a pulse-width modulation (PWM) process, not a stream of ones and zeros. What is genuinely digital in a modern powered speaker is usually the DSP feeding the amplifier, not the amplifier itself.

A typical Class D power amplifier module contains five functional blocks:

• Pulse-width modulator - compares the incoming audio with a high-frequency triangle carrier and produces a square wave whose duty cycle follows the instantaneous signal level.
• Gate driver and MOSFET output stage - switches the supply rails at several hundred kHz, arranged as a half bridge or, at higher power, as a full bridge (BTL) configuration.
• LC output filter - averages the pulse train back into an audio waveform and keeps switching residue away from the driver.
• Feedback network - samples the output and corrects errors, which is what separates a clean Class D design from a harsh one.
• Switch-mode power supply - often a PFC front end plus an LLC resonant stage in professional modules, holding the rail steady across a wide AC input range.

A - PFC and LLC resonant power supply; B - PWM modulator and gate driver; C - MOSFET output stage; D - LC output filter with feedback.

How a Class D Amplifier Works, Step by Step

1. Input conditioning. The line or balanced input is buffered and, in DSP-equipped modules, passes through EQ, crossover, limiting and delay before the power stage.
2. Modulation. A comparator mixes the audio with a triangle carrier. Loud passages produce wide pulses; silence produces a 50% duty cycle that the filter averages to zero.
3. Switching. The gate driver turns the MOSFETs hard on and hard off, with dead time inserted so both devices in a half bridge never conduct at once.
4. Reconstruction. The LC filter passes audio and blocks the carrier, with its cutoff placed well above 20 kHz but well below the switching frequency.
5. Error correction. Feedback returns a scaled copy of the output to the modulator, lowering distortion and improving damping factor at the terminals.

Two details decide whether the result sounds clean or coarse: switching frequency and loop design. Raising switching frequency shrinks the filter and improves high-frequency behavior, but increases switching loss and EMI. Pre-filter feedback is simple but load-sensitive; post-filter feedback corrects the filter's own errors and is standard in modules that must stay flat into 4 ohm or 8 ohm loads.

Class D vs Class A, AB and H: What Actually Changes

Class is not a quality rating. It describes how the output devices conduct, and that single choice drives efficiency, heat, cabinet size and power supply cost.

The gap compounds quickly. At 90% efficiency, a 1000 W module turns roughly 110 W into heat; at 55% the same output means more than 800 W of heat. That is the difference between a fanless moulded cabinet and a rack unit with forced cooling.

Class D was once dismissed for EMI, load-dependent frequency response and higher distortion near the top of the band. Those complaints came from low switching frequencies and pre-filter feedback, and they have largely been engineered out. What matters more than the class label today is the implementation: loop topology, switching frequency, power supply quality and thermal design.

Where Class D Belongs in a Powered Speaker or Subwoofer

For an OEM building active loudspeakers, the cabinet usually makes the decision. A column speaker or a moulded two-way box has no room for a linear supply and a large heatsink, so a single switching board delivering, for example, 400 W to the low-frequency driver and 100 W to the high-frequency driver is the practical answer. The same logic applies to BTL subwoofer channels, where one module can push four figures of wattage into an 8 ohm load without a fan.

Three module families cover most of that ground:

EON520: LP400W+HP100W PFC+LLC Resonance Wide Voltage Class D Power Amplifier Module● High Efficiency Switching Power Supply with PFC+LLC Resonance: 800W ● Self-adaptive Wide Voltage Input: 100V-240V ● Mid and Low-Frequency Power Amplifier: CLASS D 40...View Product →

EON522SUB: LP 500W/4ohm LLC Resonant Active Subwoofer Power Amplifier Module●LLC Resonant High Efficiency and High Reliability Switching Power Supply: 800W ●Bass Amplifier: CLASS D 500W-4 Ohms ●U-shaped Aluminum Radiator, Stable Power Output, ...View Product →

EON521+1500:LP 1500W/8ohm Pro Audio BTL Subwoofer Power Amplifier Module●LLC Resonant High Efficiency and High Reliability Switching Power Supply: 1800W ●BTL Power Amplifier: CLASS D 1500W-8 Ohm ●U-shaped Aluminum Radiator, Stable Power Ou...View Product →

Matching the module to the transducer matters as much as matching wattage. A two-way module pairs a low-pass channel with a high-pass channel and expects a crossover point that suits your HF driver; a subwoofer module expects a band-limited input and the load impedance it was designed around. Mismatched impedance and crossover points are the most common reason a Class D module measures well on the bench and sounds wrong inside the cabinet.

What to Check Before You Specify a Class D Module

Efficiency at the load you actually use

Datasheets quote peak efficiency at one operating point. Ask for the curve across 10% to 100% of rated power, and at 4 ohm, 8 ohm and bridged loads, because efficiency falls as load impedance drops.

THD+N and residual noise

0.05% THD+N or lower is a normal target for a modern module. Residual noise matters more than distortion in a quiet listening room, so check the unweighted noise floor rather than only the headline figure. For a wider comparison of topologies, the amplifier class guide covers A, AB, D and H side by side.

Switching frequency, EMI and dead time

Higher switching frequency means a smaller filter and better high-frequency performance, but tighter dead-time control and more careful PCB layout. If the module will sit near a radio tuner or a wireless microphone receiver, ask for conducted and radiated EMI data.

Power supply topology and protection

PFC plus LLC resonance is the usual answer for wide-voltage operation and stable rails. Confirm that overcurrent, overvoltage, DC offset and thermal protection are included, and check what happens at brownout rather than only at nominal mains.

When a standard part does not fit the cabinet, the load or the DSP feature list, the practical route is to adapt an existing platform: custom amplifier module development usually starts from a proven power stage and adjusts output channels, EQ modes and interface rather than designing a new amplifier from scratch.

Class D Amplifier FAQ