242 Chapter 8
(cf. peak) and also specifi ed with a steady sine wave, and for power amps in particular,
with loading—all unless stated otherwise. If a peak (or any other non-rms) voltage value
is cited, the maximum output to which it is referred must also be cited likewise, so like is
being compared with like.
8.1.1.3 Variables
The sensitivity of an amplifi er depends (as defi ned earlier) on gain and swing. If an
amp’s output power rating, hence voltage swing capability into a given load impedance,
were increased, maintaining the sensitivity requires more gain from the amplifi er. This is
a consideration for the maker and the installer who uses different sizes of a given design.
8.1.1.4 Do-It-Yourself Gain Resetting
For those uses with two or more different models and/or makes of amplifi er, it is likely
that sensitivities (however referred) will differ. Gain controls may not be present or it may
be desired not to use them. If so, to align the system (ideally within a fraction of a dB),
all the amps enter clip at about the same drive level and the gain(s) of one type of amp
will need changing. Usually, any gain controls are assumed to be at maximum. Then any
“ accidental adjustments ” can only cause reduced, not excess, gain.
In most well-designed, conventional high NFB power amps, gain may be changed up
or down easily by changing one (global feedback) resistor per channel. The part being
changed is usually in the output section. Changing gain by up to 10 dB or down by
as much as –6 dB should have relatively little effect on sonic quality, assuming that RF
stability is not upset. However, noise will be altered pro-rata.
In low- and zero-feedback designs, the availability of gain changing is far less, and the
effect on both measured and sonic performances of even a modest 10-dB ( 3) adjustment
will be far more marked.
8.1.1.5 Gain Restriction
In some power amp designs, gain changes may be unavailable because they would upset
RF stability, imperil a fi nally balanced gain/feedback structure, or violate some arbitrary
%THD N limit or other basic performance indication. Thus amplifi ers from a product
family spanning a range of output power ratings may have very similar gains ( to
–3 dB); thus sensitivities (mV, V) almost commensurate with their ascending voltage
swing. The upshot of this approach is (for example) a 2-kW 8f Ω amplifi er, which only