500W subwoofer design using Class D amplifiers

With active subwoofers demanding typical power levels up to 500W, building the amplifiers to meet the requirement is not without difficulty. Consider using a 50% efficient Class AB amplifier, and it’s not hard to imagine a power source that generates masses of heat, is very large, heavy and prohibitive in cost.

While Class D amplifiers certainly help, by increasing efficiency levels up to the 90% mark, the fact they are generally deployed in an open loop mode means that other performance constraints impinge.

In open loop mode, any noise that’s generated by the power supply will influence what you hear – there is no supply rejection. This means that the Class D supply, though smaller than that of the Class AB design, has to be well regulated and low in output impedance – and is therefore expensive. Furthermore, to maintain very low distortion and noise (THD+N), attention must be paid to bridge design. Care must be taken in dead time control to balance distortion with dissipation in the bridge.

Consider then a closed loop Class D amplifier. It can be shown that the addition of a feedback loop has a very positive influence on the performance of our 500W subwoofer and greatly simplifies the design process.

For the power supply, a basic switcher can now be used - or an unregulated supply - so long as it’s capable of delivering the requisite volts and amps. In this case 500W driving 4 ohms requires a 35V supply with a peak capacity of 17.5A.

The feedback also assists with balancing distortion and dissipation in the bridge. In Class D amplifiers a certain amount of cross conduction is allowed in the bridge, the more cross conduction allowed, the lower the distortion but the higher the dissipation. Feedback reduces the distortion level to such a degree that cross conduction can be significantly reduced, resulting in cool running and an exceptional THD+N performance (Figure 1).

Figure 1: THD+N up to 500W

The design’s 90% efficiency implies dissipation in the amplifier of around 50W, mostly in the bridge components. Assuming an ambient temperature of 40°C inside the subwoofer and a maximum operating temperature of 85°C, then we need a heatsink capacity of around 1°C/W. In practice the power content of music is never continuous so a smaller heatsink is sufficient.

Figure 2 shows the architecture of the feedback design. It’s worthwhile noting that while this implementation delivers 500W into 4 ohms, the output stage is scalable and so, with appropriate supply rail modification, the design can be adapted to a wide range of output powers and load impedances. A single ended input signal is buffered into the ZXCD1000 modulator. The buffering is provided by an AD8512 which enables the level shift and gain control required as well as providing the summing point for the feedback signal.

Figure 2: Amplifier schematic diagram