The input for power amplifiers comes from the mixing console, usually through graphic equalizers, crossovers, and limiters. This signal is amplified to drive the speakers. All power amplifiers perform the same function but have different power capacities and features, such as visual output displays and ampere status warnings for shorts or direct current (DC) voltage on the output. The input voltage of an amplifier varies between 1 and 1.75 V root mean square (RMS). The maximum input voltage must be applied to the input to achieve maximum amplifier power. Amplifier output requires a heavy-duty cable to deliver the signal to the speakers. The cable must be capable of taking high currents to ensure a minimum of voltage drop and full power to the speakers. The harder an amplifier works, the hotter it becomes, and cooling is thus critical. Dust buildup in an amplifier restricts airflow and causes the amplifier to overheat. Amplifiers with thermal protection switch off, but those without protection develop faults. Figure 5-1 shows an amplifier with its lid removed.