Most of the info already posted is essentially correct, save a few points. S/N ratio is a bit of misnomer, although it obviously expresses a relationship between the input audio signal and the noise inherent in the system (the electronics primarily, but "dirty" electricity can also contribute). To be a bit more precise, the noise in the system is best expressed as a "minus" figure below the signal. Hence, a S/N ratio of -96 db (pretty typical for CD's) means that the noise level is 96 db below the audio signal. The higher the S/N ratio number, (e.g., -100db), the lower the noise level in relation to the signal level. Some of the newer power amps, such as Bryston's 4B-ST, now have S/N ratios around -116 to -118 db, so they are actually quieter than CD's. The S/N ratio really becomes a factor when you are listening to very quiet music, since this is the time when it is most likely for the noise to be audible. On loud musical passages, such as heavy metal rock, the S/N ratio is essentially unimportant. Likewise, very weak audio signals, such as those from a low output phono cartridge (which can be less than 1.0 mv), may be affected by the noise. Hence, S/N ratio can be a significant factor in reproducing sound from LP's. Hope this helps.
