SM-57's and '58's are low impedance (Z) microphones. As opposed to most findings at the local RatShack outlet. Professional microphones and some not so professional use a low impedance to match that of the input device. A high impedance microphone is one that you will find plugged into a sound card, cheap PA, guitar amp if pressed for a sound source, and most low-end four-tracks.
Some will change the impedance and the connection of a Lo-Z mic and cable by attaching an adapter that will allow a quarter inch connection to be made to the input device and vice-versa.
Typically a Lo-Z microphone and cable will use a balance XLR connection. Hi-Z will use a quarter inch tip-sleeve connection.
If you're calling bingo at the local American Legion hall a Hi-Z mic is fine. Most recording folk and professional engineers use a quality Lo-Z microphone into a balanced Lo-Z connection for recording music.
