High signal-to-noise ratio is an important goal for most audio systems. However, ac power connections unavoidably create ground voltage differences, magnetic fields, and electric fields. Balanced interfaces, in theory, are totally immune to such interference. For 50 years, virtually all audio equipment used transformers at its balanced inputs and outputs. Their high noise rejection was taken for granted and the reason for it all but forgotten. The transformer's extremely high common-mode impedance - about a thousand times that of its solid-state "equivalents" - is the reason. Traditional input stages will be discussed and compared. A novel IC that compares favorably to the best transformers will be described. Widespread misunderstanding of the meaning of "balance" as well as the underlying theory has resulted in all-too-common design mistakes in modern equipment and seriously flawed testing methods. Therefore, noise rejection in today's real-world systems is often inadequate or marginal. Other topics will include tradeoffs in output stage design, effects of non-ideal cables, and the "pin 1 problem."
The task of transferring an analog audio signal from one system component to another while avoiding audible contamination is anything but trivial. The dynamic range of a system is the ratio, generally measured in dB, of its maximum undistorted output signal to its residual output noise or noise floor. Fielder has shown that up to 120 dB of dynamic range may be required in high-performance sound systems in typical homes.  The trend in professional audio systems is toward increasing dynamic range, fueled largely by increasing resolution in available digital converters. Analog signals accumulate noise as they flow through system equipment and cables. Once noise is added to a signal, it's essentially impossible to remove it without altering or degrading the original signal. Therefore, noise and interference must be prevented along the entire signal path. Of course, a predictable amount of random or "white" noise, sometimes called "the eternal hiss," is inherent in all electronic devices and must be expected. Excess random noise is generally a gain structure problem, a topic beyond the scope of this paper.