234 Chapter 7
While such types of tone control are undoubtedly useful and can make signifi cant
improvements in the performance of otherwise unsatisfactory hi-fi systems, the inability
to attain a fl at frequency response when this is desired, even at the midposition of the
octave-band controls, has given such arrangements a very poor status in the eyes of the
hi-fi fraternity. This unfavorable opinion has been reinforced by the less than optimal
performance offered by inexpensive, add-on units whose engineering standards have
refl ected their low purchase price.
7.18.3 Channel Balance Controls
These are provided in any stereo system to equalize the gain in the left- and right-hand
channels and to obtain a desired balance in the sound image. (In a quadraphonic system,
four such channel gain controls will ideally be provided.) In general, there are only two
options available for this purpose: those balance controls that allow one or the other of
the two channels to be reduced to zero output level and those systems, usually based on
differential adjustment of the amount of negative feedback across controlled stages, in
which the relative adjustment of the gain, in one channel with reference to the other, may
only be about 10 dB.
This is adequate for all balance correction purposes, but does not allow the complete
extinction of either channel.
The fi rst type of balance control is merely a gain control, of the type shown in Figure
7.50. A negative feedback type of control is shown in Figure 7.69.
7.18.4 Channel Separation Controls
While the closest reproduction, within the environment of the listener, of the sound
stage of the original performance will be given by a certain specifi c degree of separation
between signals within the ‘ L ’ and ‘ R ’ channels, it is found that shortcomings in the
design of the reproducing and amplifying equipment tend universally to lessen the degree
of channel separation rather than the reverse.
Some degree of enhancement of channel separation is therefore often of great value, and
electronic circuits for this purpose are available, such as that, due to the author, shown in
Figure 7.70.