Interfacing and Processing 269amplifi er’s capability are useful. It’s also relevant where a quick sweep to ∞ (infi nite
attenuation) may be needed as a mute—to turn off the signal in one speaker, say—
without switching off or unplugging anything.
8.6.1.4 The Right Range
In many applications, the range offered by a raw pot is far too wide. In other industries
employing pots, a vernier or a multiturn mechanism is added between the knob and shaft
to aid fi ne settings. However, these are eschewed by modem professional audio operators,
partly because of an ingrained fear of the loss of instant sweep control and because of
relatively high cost versus relative fragility. There is also the false sense of alignment
suggested by the verniers ’ 3 or 4 fi gure scale; scales on different amplifi ers would be
strictly incomparable, owing to most pots ’ poor tolerances, particularly good-sounding
log pots. In the past 20 years, variations of 5 to 25% (or 0.5 dB to 3 dB) have remained
thenorm for the resistance mismatch between different pots at the same mechanical
setting.
8.6.1.5 Linear Variants
Using a linear (A) pot and a fi xed resistor, Figure 8.9(b) shows how adjustment range is
restricted to the “ top ” 12 dB, that is, 0 dB to –12 dB. For system adjustment, this may be
more usefully expressed as /–6 dB. This range of adjustment is preferable for active
crossover-based and arrayed systems, where the gain of individual amplifi ers benefi ts
from close adjustments and only needs this limited range. In practice, switched (say)
–20 dB and ∞ settings are then required. Note that the impedance vs. rotation relation
is naturally slightly changed—the highest source impedance is here less at about 20%
(rather than 25%) of the pot.
Returning to the full-scale mode, a linear pot may alternatively be used [ Figure 8.9(c) ],
with a fi xed resistor used for “ law faking. ” This converts the linear law to a log-like
curve, if the pot and resistor values are kept within tight limits; this approach can give
approximations of an audio taper that are at least more consistent than most log pots,
which are made by butting n different-valued linear track segments together. Note that
the pot’s effective value is here a tenth of its rated value after the law faking resistor
is included. As a result, the pot shown in Figure 8.9(c) looks like a 10 kf2 pot to the
load. However, the maximum source resistance is, as with the audio taper, at the 50%
attenuation point and is just about 10% from maximum.