Recording Consoles 789resistance. ResistorsRc and Rd and VR a , the pan pot itself, form this one-tenth value
network. Because the slider of the pan pot is grounded and the output signal is obtained
from either end of the pan pot, it is clear that in either extreme position of VR a , all the
signal will appear on one channel and none on the other. It is then only a matter of
deciding whether the control law of the pan pot is of the correct type. The calculation of
the control law obtained from the circuit is complicated because the signal level fed to
left and right channels is not proportional to the resistive value of each part of VR a. This
is because the total value of the network Rc , Rd, and VR a , although reasonably constant, is
not the same irrespective of the setting of VR a and so the resistive attenuator comprising
the top part of VR b and its lower part, shunted by the pan pot network, is not constant as
VRa is adjusted. Furthermore, as VR a is varied, so the output resistance of the network
changes and, since this network feeds a virtual-earth summing amplifi er, this effect also
has an infl uence on the signal fed to the output because the voltage-to-current converting
resistor feeding the virtual-earth node changes value. The control law of the fi nal circuit
is nonlinear: the sum of left and right, when the source is positioned centrally, adding
to more than the signal appearing in either channel when the source is positioned at an
extreme pan position. This control law is very usable with a source seeming to retain
equal prominence as it is “ swept across ” the stereo stage.
27.6.8 Mix Amplifi ers
The mix amplifi er is the core of the audio mixer. It is here that the various audio signals
are combined together with as little interaction as possible. The adoption of the virtual-
earth mixing amplifi er is universal. An example of a practical stereo mix amplifi er is
shown in Figure 27.20. Here, the summing op-amp is based on a conventional transistor
pair circuit. The only diffi cult decision in this area is the choice of the value for Rb. It is
this value, combined with the input resistors, that determines the total contribution each
input may make to the fi nal output.
27.6.9 Line-Level Stages
Line-level audio stages are relatively straightforward. Signals are at a high level, so noise
issues are rarely encountered. The signifi cant design parameters are linearity, headroom,
and stability. Another issue of some importance is signal balance, at least in professional
line-level stages, which are always balanced. Of these, output-stage stability is the one
most often ignored by novice designers.