Audio Engineering

220 Chapter 7

infi nitely high, it will attenuate the maximum signal voltage (Emax ) obtainable from the
source, in the ratio

E

ER

max RZ

n

1 source





1

()

where Zsource is the output impedance of the driving circuit. This factor favors the use of a
high value for R 1 to avoid loss of input signal.

However, the following amplifi er stage may have specifi c input impedance requirements
and is unlikely to operate satisfactorily unless the output impedance of the gain control
circuit is fairly low. This will vary according to the setting of the control, between zero
and a value, at the maximum gain setting of the control, due to the parallel impedances of
the source and gain control.

Z

R

out RZ

1

() 1 source





.

The output impedance at intermediate positions of the control varies as the effective
source impedance and the impedance to the 0-V line are altered. However, in general,
these factors would encourage the use of a low value for R 1.

An additional and common problem arises because the perceived volume level associated
with a given sound pressure (power) level has a logarithmic characteristic. This means
that the gain control potentiometer, R 1 , must have a resistance value that has a logarithmic,
rather than linear, relationship with the angular rotation of the potentiometer shaft.

7.18.1.1 Potentiometer Law

Since the most common types of control potentiometer employ a resistive composition
material to form the potentiometer track, it is a diffi cult matter to ensure that the grading
of conductivity within this material will follow an accurate logarithmic law.

On a single channel this error in the relationship between signal loudness and spindle
rotation may be relatively unimportant. In a stereo system, having two ganged gain
control spindles, intended to control the loudness of the two channels simultaneously,
errors in following the required resistance law, existing between the two potentiometer
sections, will cause a shift in the apparent location of the stereo image as the gain control
is adjusted, which can be very annoying.