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A Class D amplifier uses output transistors as fast switches: they turn on and off to create a pulse-based signal that represents the audio. In a conventional design, a low-pass filter then recovers the audio signal and reduces high-frequency switching energy. “Class D” describes how the output stage operates—not whether the amplifier receives digital audio.
How a Class D amplifier works
Audio is not normally a train of switching pulses, so the amplifier first uses a modulator to encode the input into them. In a basic pulse-width modulation (PWM) design, the circuit compares the audio signal with a triangle or sawtooth waveform. The resulting pulses become wider or narrower as the audio level changes.
Output transistors, often MOSFETs, switch the output node between the power supply and ground. In a traditional filtered design, an LC low-pass network passes the audio-band component while attenuating much of the high-frequency switching energy.
Analog Devices gives 250 kHz to 1.5 MHz as a representative switching-frequency range in its Class D fundamentals article. That range is an example from the source, not a universal specification for every amplifier.
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PWM is common, but not the only approach
Pulse-width modulation is a useful way to understand the basic operation, but Class D designs can use other modulation methods, including pulse-density and sigma-delta techniques. Some designs also use feedback to help manage performance.
Some designs are filterless
An external LC output filter is common in conventional designs, but it is not universal. Some amplifiers use filterless modulation. “Filterless” does not mean that any speaker or wiring arrangement is safe: follow the particular amplifier’s documentation for permitted loads and installation.
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Does Class D mean the amplifier is digital?
No. The class label refers to the output stage’s switching operation, not the format of the input. An amplifier can accept an analog audio signal and use a modulator to turn it into pulses for the switching stage. The Audio Engineering Society’s Pro Audio Reference describes Class D operation as switching, which is why it is also called switching power amplification.
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Why Class D can be efficient—and why efficiency varies
Switches can dissipate less power than output devices operated as linear gain elements, especially at higher output levels. But the ideal case assumes switches with no losses and instantaneous transitions. Real amplifiers lose power through transistor on-resistance, switching transitions, and control-circuit overhead, so efficiency depends on the design and how hard it is operating.
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Analog Devices’ discussion of the AD1994 gives a specific illustration: it reports 90% output-stage efficiency at the onset of clipping and 78% at a 1 W average load in an example with a 10 W maximum load. Those figures describe that article’s comparison, not a guaranteed rating for Class D amplifiers generally. The Audio Engineering Society says practical designs can approach 90%, but that general statement likewise does not establish the efficiency of a particular model.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to compare beyond the class label
“Class D” alone does not tell you whether an amplifier will suit a speaker or system, or how it will sound. For a specific model, check:
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- Double-panel wiring, properly solve the wiring caused by the potential balance and crosstalk between channels
- Output power and load: Confirm the rated power at the speaker impedance you intend to use.
- Efficiency at relevant levels: A peak or clipping-level figure may not represent typical listening or operating conditions. At low output power, system overhead can narrow Class D’s advantage; Analog Devices notes that Class AB can be competitive in some such cases.
- Distortion and noise: Look for measurements under stated loads and operating conditions rather than inferring performance from the class.
- Output-filter and EMI behavior: Determine whether the design requires an output filter and how it handles switching-related electromagnetic interference.
- Feedback, protection, and supply needs: Check the implementation’s feedback and protection features, power-supply requirements, and thermal limits.
For a board or module, use its documentation to match the supply voltage, speaker impedance, and required output power. Do not assume a conventional LC filter is required—or that a filterless design removes load restrictions.
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