Handbook for Sound Engineers

250 Chapter 10

inaccurate readings, therefore the current through the
unit should be kept to 1 mA or less. Self heating can
also be controlled by using a 10% duty cycle rather than
constant bias or by using an extremely low bias which
can reduce the SNR. The connection leads may cause
errors if they are long due to the wire resistance.

Potentiometers and Rheostats. The resistance of
potentiometers (pots), and rheostats is varied by
mechanically varying the size of the resistor. They are
normally three terminal devices, two ends and one
wiper, Fig. 10-10. By varying the position of the wiper,
the resistance between either end and the wiper changes.
Potentiometers may be wirewound or nonwirewound.
The nonwirewound resistors usually have either a carbon
or a conductive plastic coating. Potentiometers or pots
may be 300° single turn or multiple turn, the most
common being 1080° three turn and 3600° ten turn.

Wirewound pots offer TCRs of r50 ppm/°C and
tolerances of r5%. Resistive values are typically
10 :–100 k:, with power ratings from 1 W to 200 W.

Carbon pots have TCRs of r400 ppm/°C to
r800 ppm/°C and tolerances of r20%. The resistive
range spans 50:–2 M:, and power ratings are gener-
ally less than 0.5 W.

Potentiometers may be either linear or nonlinear, as
shown in Fig. 10-11. The most common nonlinear pots
are counterclockwise semilog and clockwise semilog.
The counterclockwise semilog pot is also called an
audio taper pot because when used as a volume control,
it follows the human hearing equal loudness curve. If a
linear pot is used as a simple volume control, only about
the first 20% of the pot rotation would control the
usable volume of the sound system. By using an audio
taper pot as in Fig. 10-11 curve C2, the entire pot is
used. Note there is only a 10%–20% change in resis-
tance value between the common and wiper when the
pot is 50% rotated.

Potentiometers are also produced with various taps
that are often used in conjunction with loudness
controls.

Potentiometers also come in combinations of two or

more units controlled by a single control shaft or

controlled individually by concentric shafts. Switches

with various contact configurations can also be assem-

bled to single or ganged potentiometers and arranged for

actuation during the first few degrees of shaft rotation.

A wirewound potentiometer is made by winding

resistance wire around a thin insulated card, Fig.

10-12A. After winding, the card is formed into a circle

and fitted around a form. The card may be tapered, Fig.

10-12B, to permit various rates of change of resistance

as shown in Fig 10-11. The wiper presses along the wire

on the edge of the card.

Contact Resistance. Noisy potentiometers have been a

problem that has plagued audio circuits for years.

Although pots have become better in tolerance and

construction, noise is still the culprit that forces pots to

be replaced. Noise is usually caused by dirt or, in the

case of wirewound potentiometers, oxidation. Many

circuits have gone up in smoke because bias-adjusting

resistors, which are wirewound for good TCR, oxidize

and the contact resistance increases to a point where it is

more than the value of the pot. This problem is most

Figure 10-10. Three terminal potentiometer.

High

Low

Wiper

Low

High

Wiper

Low

Rear view

Figure 10-11. Tapers for six standard potentiometers in

resistivity versus rotation.

100

80

60

40

20

0

010 20 30 40 50 60 70 80 90100

Left terminal

Percent clockwise rotation

Right terminal

C5

C3

C6

C1 C4

C2

Linear taper, general-purpose control for television

picture adjustments. Resistance proportional to

shaft rotation.

Left-hand semilog taper for volume and tone

controls. 10% of resistance at 50% rotation.

Right-hand semilog taper, reverse of C2. 90% of

resistance at 50% of rotation.

Modified left-hand semilog taper for volume and

tone controls. 20% of resistance at 50% of rotation.

Modified right-hand semilog taper, reverse of C4.

80% of resistance at 50% of rotation.

Symmetrical straight-line taper with slow resistance

change at either end. Used principally as tone

control or balance control.

C1.

C2.

C3.

C4.

C5.

C6.

% resistance

Percent counter-clockwise rotation

100 90 80 70 60 50 40 30 20 10 0