It is far from a simple proposition to employ feedback in this way because of
the very special control required of phase shifts in the amplier and circuits,
not only throughout the useful frequency band but also for a wide range of
frequencies above and below this band. Unless these relations are maintained,
singing will occur, usually at frequencies outside the useful range. Once hav-
ing achieved a design, however, in which proper phase relations are secured,
experience has demonstrated that the performance obtained is perfectly reli-
able.
