This chapter considers a number of performance-critical applications of audio signal processing to demonstrate how modern circuit techniques are used to complement the advances being made in digital audio systems. The performance of a digital audio system demands a high degree of accuracy within the analog circuitry to maximize transparency and to prevent compromise in terms of distortion, noise, and bandwidth. Gateway filters are fundamental to the successful operation of a digital audio system, where the performance of the analog circuitry should approach, if not exceed, the resolution of the digital channel. Although much signal processing can be performed in the digital domain, certain key processes ultimately require analog techniques. Prominent examples are the antialiasing and signal recovery filters and the transresistance amplifier required to perform current-to-voltage conversion in association with current output digital-to-analog converters. An important class of analog amplifiers is the voltage-controlled amplifier.