Studio Technologies 80 1997 - Manual (Page 15)

Issue 4, March 1997 Model 80 User Guide
Page 18 Studio Technologies, Inc.
The left channel internal audio bus is
connected, by way of a resistor, to the
inverting input of an operational amplifier.
The operational amplifier is configured as
an inverting, adjustable gain circuit. One
section of high performance analog switch
controls the gain of the op-amp by shunt-
ing a resistor in the feedback loop. When
the analog switch is in the off (open) state
the op-amp is operating at essentially
unity gain. When the analog switch is in
the on (closed) state the gain is less than
unity, providing an attenuation of 14dB
(voltage). The state of the analog switch
is controlled by one section of DIP switch.
The DIP switch, which produces a 0 or
+15Vdc logic signal, is accessible to the
user by way of the front panel.
The output of the op-amp is capacitor
coupled to the input of an SSM-2142
differential (balanced) line driver inte-
grated circuits. The ’2142 provides an
electronically balanced, low-impedance
output and is capable of driving high
signal levels into 600 ohms or greater.
Because of the nature of a differential
driver, an effective 6dB boost in signal
level is achieved. The outputs of the ’2142
are directly connected to the output jack.
The nominal output level is controlled by
our friend the analog switch. When the
switch is in the off (open) state a signal at
nominal –2dBu is presented to the line
driver. With the intrinsic 6dB of gain the
nominal level at the output jack is +4dBu.
When the analog switch is in the on
(closed) state a signal at nominal –16dBu
is presented to the line driver. This results
in a –10dBu signal at the output jack.