Interfacing and Processing 243
provides 100 W at normal drive levels (0 dBu say). The 13-dBu/3.5-V rms input drive
needed for full output makes it safer and more likely that the high swing will be kept in
reserve as an inviolate headroom.
In other words, in lieu of increased gain when output swing is increased, such an
amplifi er will need to be driven harder, that is, rated less sensitive. If the headroom
achieved is ever used, then the higher input drive levels can cause increased distortion in
the input stage. This effect will be noted most in esoteric amps with low feedback, but is
still there in conventional high NFB amps.
8.1.1.6 Gain and Fidelity
As noted, the positive side of having high swing amplifi ers desensitized, by not increasing
gain commensurate with the increased voltage swing is that headroom occurs by default
if the system’s level/gain settings are not then altered. Reduced gain also reduces the risk
of speaker damage by accidental loud blasts, dropped mics, styli, etc. Also, the audibility
of the system’s residual noise is lowered.
8.1.1.7 CM Stress
In conventional power amplifi ers with high NFB, “ common mode distortion, ” measurable
as %THD N ,^1 occurs because of common-mode voltage stress on the input stage,
whether differential or single ended, with the latter suffering CM stress if, as is common,
it is noninverting. The threshold voltage, ‘ Vth ’ —above which the input voltage to such
an op-amp-type input becomes highly nonlinear when open loop may be sonically
signifi cant. 2,3 These setbacks may not be revealed with conventional tests, notably
%THD N, which can contrarily show lowered distortion at high input drive test levels,
because the noise ( N) may “ out-reduce ” the rising common mode distortion.^1
8.1.1.8 Real Figures
The sensitivity of every amplifi er needs to match the zero (normal) levels of sources it is
intended to be driven by. These vary. The upshot of all the factors is a spread of amplifi er
sensitivities that users know all too well ( Table 8.1 ).
Ideally, there could be just one input sensitivity for all these uses. One that most could
accept is the de facto professional standard of 0-dBu alias 775 mV. As a general rule,
most lightweight domestic hi-fi and home studio equipment is likely to be more sensitive
than 0 dBu, with pro equipment likewise less sensitive.