Audio content has evolved over the years. It’s gone from the days of buying 12-inch LP vinyl at the local record store to being able to stream almost any content from the cloud instantaneously. Although some audiophiles are bringing back the days of vinyl as a novelty, the vast majority of audio today is consumed digitally. Even though the way consumers receive audio has changed, one thing has remained constant: a desire to enjoy the content as it was originally recorded. A key component of making this a reality is the audio amplifier in the sound system. With one piece of silicon having such a large impact on the audio quality, it’s important to understand the tradeoffs and how to choose the right one for a design.

An end product’s features are usually defined before it is designed — an example being the target loudness for a voice-controlled smart speaker that should be audible across the room. In terms of electrical system design, the loudness correlates with output power and speaker/voice coil efficiency. Without knowing a lot of details about the end system, output power is the closest specification one could use to determine the loudness (usually approximated in dB) of the finished product. Hence, the output power level is usually one of the first criteria used to select an audio amplifier. The industry standard for specifying the power level of an amplifier is the output power at 1% or 10% total harmonic distortion plus noise (THD+N). Typically, the number of channels at that power level is also referenced, so a stereo device could be specified as 2 x 100 W at 10% THD+N. One important thing to note when referring to an amplifier's power level is whether it is specified as a peak or continuous value.

Class-D amplifiers are usually up to about 90% efficient and the approximate 10% loss of energy is converted to heat. To protect the integrated circuit, Class-D audio amplifiers have a temperature threshold at which they shut down. So a device may be able to hit a certain peak output power level, say 30 W, but not be able to sustain this level for an extended period of time without going into over-temperature shutdown. The highest output power level a system could support and sustain without reaching the over-temperature state is considered to be the maximum continuous output power level.

Figure 2-1 Continuous and Peak Power in an Audio Signal

Since the continuous output power level relies on heat dissipation, it depends on factors of the end system; for example: PCB thickness, enclosure size, and ventilation. A major attribute that does impact the continuous output power level from the amplifier is the thermal pad orientation. Devices with a thermal pad on the bottom are soldered directly to a PCB, while devices with the thermal pad on the top require a heatsink. Using a heatsink can increase the continuous output power level, but may in turn require extra space and heatsink material.

Figure 2-2 Pad-Down vs. Pad-Up Audio Amplifier