Studio Technologies 80 1997 - Manual (Page 14)

Model 80 User Guide Issue 4, March 1997
Studio Technologies, Inc. Page 17
design, the ’2143 provides 6dB (voltage)
of attenuation.
The output of each SSM-2143 is capacitive
coupled into one section of a dual (stereo)
log taper potentiometer (pot). This pot is
the input level control that is accessible
from the front panel. The pot adjusts the
input sensitivity for both the left and right
inputs. Note that the coupling capacitors
used to couple the ’2143s to the pot are
a nonpolar type, allowing for small DC
voltages of unknown polarity to be re-
ceived on the inputs. The wiper of each
section of pot is connected to the non-
inverting input of a section of low-noise
operational amplifier. The voltage gain of
the left channel op-amp is fixed at approxi-
mately 15dB. The combination of a fixed
resistor and trim potentiometer allows the
voltage gain of the right channel op-amp
to be adjustable over a nominal range of
13 to 17dB. This implements the right trim
function accessible from the front panel,
allowing differences in the left and right
signal levels to be accounted for. The
output of the left channel op-amp section
is defined as the left channel internal audio
bus. Its nominal level is –2dBu. The same
holds true for the right channel op-amp,
where its output is the right channel inter-
nal audio bus.
Metering
Outside of the overall system architecture,
the metering section of the Model 80 is
probably the most unique. Two identical
meter circuits monitor the audio level on
the left and right internal audio buses.
Audio signal enters each meter circuit via
an operational amplifier configured as a
half-wave synchronous rectifier. The result-
ing DC output is smoothed via a resistor/
capacitor low pass filter. The output of the
low pass filter is connected to inputs on
three sections of integrated circuit com-
parator. Four precision resistors are con-
nected in series with the +15Vdc power
supply rail to create three DC voltages.
These act as reference voltages and are
used as the switching points for the com-
parators. The outputs of two of the com-
parator sections are connected as an OR
gate and control the Lo LED. The third
comparator section controls the Hi LED.
Notice that the comparators are con-
nected such that the LEDs light simulta-
neously only over a very small voltage
range. This is the calibration “window” that
the user observes. The current through the
LEDs is directly controlled by transistors,
whose bases are driven by pull-up resis-
tors on the outputs of the comparators.
ExactCal
One of the highlights (or should we say
hype-lights!) of the Model 80 is the
ExactCal section. To a user the ExactCal
controls and LEDs allow a wide range of
input signals to be precisely interfaced
with the Model 80. ExactCal is a term that
Studio Technologies selected to describe
the combination of the audio input and
metering circuitry. The audio input circuitry
allows a user to adjust the input sensitivity
to match the level of the audio source. The
metering circuitry provides the user with
visual feedback, allowing the controls to
be set to the correct position.
Audio Outputs
Eight identical circuits provide the eight
stereo line-level outputs. Each of the eight
circuits is essentially two identical circuits
that share a common nominal output level
control lead. For simplicity, only the left
channel of a stereo output circuit will be
described.