Class-D has become the default for powered speakers and portable audio because it wastes far less energy as heat. Class-AB still appears where simplicity, analogue headroom or EMI constraints dominate. The choice drives the power supply, the thermal design and the EMC test plan.
Quick Comparison
| Aspect | Class-D | Class-AB |
|---|---|---|
| Typical efficiency | High - commonly above 85 percent | Moderate - well under Class-D |
| Heat sinking | Minimal in most designs | Required as power rises |
| Idle losses | Low | Higher quiescent current |
| Output filtering | LC filter, layout critical | Little or no output filter |
| EMI/EMC effort | Higher - switching edges must be contained | Lower |
| Suitability for battery products | Strong | Weak |
| Complexity | Switching stage plus filter design | Simpler analogue stage |
Why Efficiency Is a System Decision
Efficiency decides the power supply rating, the enclosure temperature and, in portable products, the battery size. A higher-efficiency stage can remove a heat sink entirely, which saves mechanical cost and weight - often more than the amplifier itself costs. Conversely, the switching output of a Class-D stage radiates; the LC filter, the layout of the switching node and the grounding scheme determine whether the product passes radiated emissions first time.
Engineering Trade-offs
- Filter design - Class-D output filters interact with speaker impedance, so the filter has to be tuned to the actual driver, not just the datasheet load.
- EMC risk - switching edges and long speaker wires are the two usual causes of failed radiated emissions. Keep the switching loop small and consider a ferrite or shielded cable.
- Audio performance - modern Class-D parts reach distortion figures competitive with Class-AB in practical designs, but the result depends on layout and supply quality.
- Analogue simplicity - Class-AB remains attractive where an existing analogue signal chain should not be re-qualified, or where the product already has a large chassis acting as a heat sink.
- Protection - both need over-current, over-temperature and DC-offset protection; Class-D adds shoot-through considerations.
Which One Fits Your Product
Choose Class-D for battery-powered speakers, compact powered monitors and any product where heat or enclosure size is constrained - the efficiency win usually pays for the extra filter and EMC work. Choose Class-AB for low-volume analogue designs, very cost-sensitive mains-powered products with generous chassis space, or where an existing analogue platform is being carried forward. On our amplifier and active speaker platforms we build on Class-D amplification above 85 percent efficiency, with the power stage, filter and protection tuned as one block against the real driver load.
Frequently Asked Questions
Q: Does Class-D sound worse than Class-AB?
A: Not inherently. Modern Class-D stages reach distortion and noise performance comparable to Class-AB in real products. The audible result depends more on layout, supply quality and filter tuning than on the topology label.
Q: Why does my Class-D design fail EMC?
A: The two usual causes are a large switching current loop and long, unfiltered speaker wiring. Shrink the switching loop, tune the output filter to the real driver, and keep the output away from sensitive analogue nodes.
Q: Can I skip the output filter?
A: Some parts are specified filterless for short speaker leads. With long cables or for EMC compliance, a filter - or at least a common-mode choke and careful routing - is normally required.
Q: Is Class-D always cheaper?
A: The silicon can be, once heat sinking and power supply rating are counted in. The extra cost moves to filter components and EMC engineering time rather than disappearing.
Q: Which is better for a portable speaker?
A: Class-D, almost always. Battery life and enclosure temperature dominate the design, and both favour the efficient topology.
