Subtleties count in wide-dynamic-range analog interfaces
Transporting high-dynamic-range analog signals from one piece of equipment to another is not a trivial task. Even subtle design variations can make huge differences in the equipment's ability to reject interference from the ac power line and other sources when the equipment connects to a real-world system.
Ground noise is often the most serious problem in a system. Reducing or eliminating this noise is usually the result of a series of experiments that stops when someone says, "I can live with that." If several coupling or conversion mechanisms are working simultaneously in the circuit, these experiments become a delicate balancing act of interactions. However, if you understand the conversion mechanisms, you can prevent most of these interactions.
The term "noise" here means any undesired in-band signal interference, rather than the rigorous engineering definition. A "system" is two or more physically separate, ac-powered devices with cabled analog-signal connections between them. Although this discussion concentrates on audio systems because they typically require a large dynamic range, the principles involved apply to any system. Contrary to popular belief, digital interfaces are also susceptible to these problems; they simply exhibit different symptoms from analog interfaces.