Audio Amplifi er Performance 331that the NFB factor varies strongly with frequency, an inconvenient fact that many audio
commentators seem to forget.
It is crucial to remember that a distortion harmonic, subjected to a frequency-dependent
NFB factor as described earlier, will be reduced by the NFB factor corresponding to
its own frequency, not that of its fundamental. If given a choice, generate low-order
rather than high-order distortion harmonics, as the NFB deals with them much more
effectively.
NFB can be applied either locally (i.e., to each stage, or each active device) or globally;
in other words, right around the whole amplifi er. Global NFB is more effi cient at
distortion reduction than the same amount distributed as local NFB, but places much
stricter limits on the amount of phase shift that may be allowed to accumulate in the
forward path.
Above the dominant pole frequency, the VAS acts as a Miller integrator and introduces a
constant 90° phase lag into the forward path. In other words, the output from the
input stage must be in quadrature if the fi nal amplifi er output is to be in phase with the
input, which to a close approximation it is. This raises the question of how the
90° phase shift is accommodated by the NFB loop; the answer is that the input and
feedback signals applied to the input stage are subtracted, and the small difference
between two relatively large signals with a small phase shift between
them has a much larger phase shift. This is the signal that drives the VAS input of
the amplifi er.
Solid-state power amplifi ers, unlike many valve designs, are almost invariably designed
to work at a fi xed closed-loop gain. If the circuit is compensated by the usual dominant
pole method, the HF open-loop gain is also fi xed, and therefore so is the important NFB
factor. This is in contrast to valve amplifi ers, where the amount of NFB applied was
regarded as a variable and often user-selectable parameter; it was presumably accepted
that varying the NFB factor caused signifi cant changes in input sensitivity. A further
complication was serious peaking of the closed-loop frequency response at both LF
and HF ends of the spectrum as NFB was increased due to the inevitable bandwidth
limitations in a transformer-coupled forward path. Solid-state amplifi er designers go cold
at the thought of the customer tampering with something as vital as the NFB factor, and
such an approach is only acceptable in cases such as valve amplifi cation where global
NFB plays a minor role.