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Figure 14 shows a simplified schematic of the pan pots.
Operation is simple, when the wiper of the
pan pot is set to mid-range Ra and the top half of the pot Rc form a voltage divider as do Ib and the
lower half of Rc.
At mid-range of the pot the respective resistance in each branch are the same so
that equal signal levels appear at the left and right output lines.
The attenuation that occurs because
of the voltage divider action is compensated in the gain of the mixing amplifiers.
As the wiper is
rotated more toward the upper end of Rc the signal present on the left output line decreases until
finally when the grounded wiper is all the way toward the high end the left output line is grounded,
eliminating any signal from this line.
At the same time more and more resistance is appearing in
the lower branch of the Rb-Rc voltage divider so the signal level on the right output line is increas-
ing.
The network as shown in figure 14 (a) produces the output response shown in figure 14 (b),
rotation of the pan pot produces attenuation in the left output that is equal to the increase in gain in
the right channel.
While this may seem ideal it has some serious disadvantages because when the
(a)
PAN-
(b)
Figure 14
pan pot is in the middle, the two mixer outputs will not seem as loud as either single output does
when the pan pot is either fully left or fully right.
Ideally we would like to have a more or less
constant apparent total volume regardless of the setting of the pan pots.
To get around this
problem "tapering" resistors are added from each side of the pan pot to ground as shown in
figure 15 (a).
With this modification we produce the response curves shown in figure 15 (b),
the output from either channel is very nearly the same with the pan pot in the center as it is when
the signal is routed fully to that chaunel.
Here the panning effect is as it should be with most
of the change taking place in attenuation of the lower level channel while very little boost occurs
in the selected channel.
Figure 15
(a)
]4
Figure 16 shows the complete mixer.
The resistor capacitor combinations R33 -Cl and R34 -
C2 are power supply filtering and decoupling while capacitors C3 and C4 provide d. c. isolation
of the outputs.
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