4 Speaker Configuration

A speaker’s output power is correlated to the power supply and the load connected to the audio amplifier. The amplifier itself also limits the output power for an audio system, as the core of an audio amplifier is a MOSFET amplifying the analog signal, restricted by voltage, current, and thermal limits. Ohm’s law dictates that current is a function of voltage and resistance, making power inversely proportional to resistance (the load). Since an amplifier has limitations on its voltage and current, the resistance of the speaker is important to consider when selecting an amplifier. Drivers typically range between 2-8 Ω. A lower impedance driver draws more current and produces higher output power for as long as the amplifier is able to support it. Lower impedances are typically used in subwoofers (2-4 Ω) while higher impedances (4-8 Ω) are used for woofers and tweeters.

Generally, a Class-D amplifier can be manipulated to provide different levels of power by topology. For example, a Class-D amplifier with four half-bridges can drive 4, 2 or even just 1 channel. These topologies are referred to as single-ended (SE), bridge-tied load (BTL), and parallel bridge-tied load (PBTL). Bridging two outputs effectively doubles the voltage across the load, while configuring them in parallel allows double the current. In theory, doubling the voltage and current in mode PBTL should quadruple the power, but since amplifiers have voltage and current limitations, running in PBTL typically only allows for more power into smaller loads. These output configurations can be seen in Figure 4-1. For more information on output configurations, visit section one of the LC Filter Design application note.

Figure 4-1 Various Output Configurations