Studio Technologies Gen II - Manual (Page 10)

Issue 3, July 1991 Generation II User Guide
Page 12 Studio Technologies, Inc.
Meter Circuit
Two, three-stage LED level meter circuits
provide the user with an indication of sev-
eral Generation II internal operating levels.
The meters are not intended to give a
precise reading, but simply to provide gen-
eral level indications. The right meter dis-
plays the level of the right channel signal
just prior to the right channel line driver
stage. The input to the right meter comes,
via the meter select switch, from the output
of the op amp that serves as a right channel
difference/gain section. The left meter is
switch-selectable to display the level of the
input signal or the level of the left channel
signal just prior to the left channel line driver
stage. The input to left meter comes from
either the output of the line input section, or
the op amp that serves as the left channel
summing/gain section.
Op amps configured as half wave rectifiers
convert the incoming meter audio into DC.
This DC voltage is fed to three sections of
op amp which are configured as voltage
comparators. Voltage dividers provide
reference voltages for the comparators. The
comparator outputs act as current sinks to
light the LEDs. The LED current is resistor
limited to 10mA for the green and red LEDs,
and 15mA for the yellow LEDs. The differ-
ent currents compensate for the different
LED efficiencies.
The green LEDs have a low lighting thresh-
old, making them act as “signal present”
indicators. The yellow LEDs are set to light
with moderate signal levels. The red LEDs
are set to light when internal signal levels
approach clipping.
Line Input
The purpose of the line input stage is to
take a balanced audio input signal, reduce
its level by a fixed amount, and convert it to
unbalanced. For best operating perfor-
mance, the internal operating level of the
Generation II is
–
6dBu. This level combines
a good signal to noise figure with ample
peak signal headroom for circuits operating
with ±15Vdc power sources. Since different
facilities run different average operating
levels, the line input stage is switch-select-
able for 0, +4, or +8dBu operation. This
translates to attenuating the input signal by
6, 10, or 14dB.
Signal enters the Generation II via a female
XLR-type connector. A resistor position has
been left vacant on the circuit board if an
input load resistor is desired. The input
signal is direct coupled to a differential line
receiver integrated circuit. This IC is truly
optimized for audio applications, providing
many fine features, including excellent
common-mode rejection, low distortion, and
low noise. The now unbalanced signal
enters a passive attenuator section consist-
ing of a 3-position switch, fixed resistors,
and a trim pot. This attenuator reduces the
signal level to the nominal
–
6dBu internal
operating level from the nominal input levels
of 0, +4, and +8dBu. The trim pot allows the
input level to be adjusted over a ±0.75dB
range. One section of op amp buffers the
attenuated signal to correctly drive the
band-reject filter, the summing/gain and
difference/gain sections, and the system
status switch.