Interfacing and Processing 245Impedances (often abbreviated ‘ z ’ ) are rated in ohms ( Ω ). As in this case, ohmic values
are nearly always over 1000; the counting is usually in thousands (k). 10 k o r 1 0 k Ω ( “ 10 k
ohm ” ) is easier to say than “ ten thousand ohms. ” When near a million or over, ‘ M ’ for
‘ Mega ’ is used, for example, 1 M Ω is 1000 k Ω.
8.1.2.2 Common Values
With ordinary, high NFB power amplifi ers, high input impedances (high Zin , say above
1 0 k Ω ), to 1 M Ω or more, are readily attained. For most sources, this is analogous to
very light loading. However, in most cases, power amp input impedances are commonly
at the low end of this range, at between 10 and 22 k Ω. This restricts noise and buzzes
when (particularlyun balanced) inputs are left open, unused, or fl oating, especially when
cables are unplugged at the source end. This is less of a problem with short cables and in
domestic environments.
The nominal values of amplifi er input impedances vary widely. As a rule, professional
equipment is defi ned in Table 8.3.
If balanced, Zin is the differential mode Z.
The input impedance of equipment may be described as the source’s load impedance.
This is true enough at frequencies below l kHz. However, load impedance (since the
signal source may be across a room, 100 yards down a hall, or even half-way across a
fi eld) is the totality of loading, namely including all the cable capacitance, which takes
effect increasingly above 3 kHz.
Table 8.2 : Power Amp Gains for 0-dBu Sensitivity @ Clip ⇒ Means ‘ Into ’
Gain (dB) rms voltage
swing (V)Average power ⇒
nom 8Ω (W)Average power ⇒
nom 4Ω (W)
24 1 6 12.5^19 38
30 3 2 25 78 156
33.5 4 8 37.5 176 352
36 6 5 50 312 624
40 9 7 75 703 1406
42 129 100 1250 2500
44 l61 125 1953 3906