Audio Engineering

268 Chapter 8

8.6.1.1 Characteristics

As “ voltage matching ” is the norm for modern audio, pots are nearly always wired in the
voltage divider mode, where the wiper is the output. At this point, the source impedance
seen varies, up to a maximum of a quarter (25%) of the pot’s rated value (i.e., the end-to-
end resistance) at half setting. At the pot’s maximum and minimum settings, the source
impedance reaches a few ohms above zero, which is usually much less than the preceding
signal source’s impedance.

8.6.1.2 Common Values

In audio power amplifi ers, the pot’s value is commonly 5 or 10 k Ω in professional and
audiophile grade equipment and 20, 50, or 100 k Ω or even higher in “ consumer ” grade
equipment. The lower pot values offer lower maximum impedances at half-setting, for
example, just 2500 Ω (2.5 k Ω ) for a (10 kf) pot. This lessens the scope for noise pickup in
the inevitably unbalanced and relatively sensitive part of the amplifi er circuitry where the
pot is placed.

8.6.1.3 Audio Taper

These considerations are true for ordinary pots with an audio taper, that is, those marked
‘ log ’ or ‘ B ’. As shown wired in Figure 8.9(a) , these normally sweep over the maximum
possible range of level setting, from a purely nominal ∞ (hard CCW or “ shut off, ”
really more like –60 to –70 dB) up to 0 dB (maximum level). The “ audio taper ” alias
logarithmic resistance change per ° rotation makes the change in sound level reasonably
constant with rotation. The full span and audio taper are relevant when a pot is needed to
act sometimes as volume control, where output levels very much lower than the power

Input

10 kB

(log)

0dB

dB

(a)

Requires

R loading

above 20 K



Input

10 kA

(lin)

6K2

0dB

(b) (c)

Requires

defined

R loading

e.g. 10 K

12 dB

Input

100 kA

(lin)

11 K

0dB

dB

 Infinity

Requires

defined

R loading

Figure 8.9 : Gain Pot Variations