244 Chapter 8
However, as just discussed, a specifi c lower value, as low as 30 mV, may be best (at least
in high NFB circuits) from the viewpoint of circuit and device physics for absolute best
linearity.^2 However, the higher voltages that are mostly needed by desensitized high
swing amplifi ers (e.g., driving 2 V or SdBu and above to clip) confer the highest SNR ,
hence dynamic range,and also the highest RF EMI and CMV immunity. So the best of
both these worlds appears not to be immediately reconcilable.
As most amplifi ers are not pure voltage sources, when driven with continuous, high-
level test signals into a real (or simulated) loudspeaker load (as opposed to an ideal,
simple resistive load), the sensitivity (for a given clip level) can appear to increase at
some frequencies, as the maximum output voltage with a conventional amplifi er having
an unregulated supply is reduced by typically by –0.5 to –2 dB. The average shortfall
is likely to be less with program, at least at mid- and high frequencies. It follows that
there is a complex frequency-conscious and dynamic peak-to-mean disparity in practical
amplifi ers ’ sensitivity ratings. The purer the voltage source, the less this can happen.
8.1.1.9 Gain and Swing
Table 8.2 shows the gain requirements both in dB for some “ round-fi gured ” voltage
swings, and how the nominal power then varies into 4 and 8 ohms.
For other sensitivities, gains are determined easily by appropriate subtraction or addition,
for example, for 4 dBu, subtract 4 dB from the indicated gain(s) and for –10 dBu, add
10 dB to the indicated gain(s).
8.1.2 Input Impedance ( Zin )
8.1.2.1 Introduction
The amplifi er’s input impedance is the loading presented by the amplifi er to the signal
sourcedriving (or “ looking up ” or “ into ” ) it.
Table 8.1 : Range of Input Sensitivities
Category In volts In dBu
Home hi-fi 30 mV to 2 V –28 to 8
Home studios 100 mV to l V 18 to 2
Pro-audio 775 mV to 5 V 0 to 1 6