Studio Technologies IFB Plus 2005 - Manual (Page 32)

Issue 8, December 2005 Model 2 User Guide
Page 32 Studio Technologies, Inc.
of the balanced output to circuit common
(pin 1 of the line output connector) will not
damage the “chip.” Unbalanced operation in
this manner is perfectly acceptable, as long
as the short is made directly at the output
connector.
Monitor Amplifier
An integrated circuit power amplifier pro-
vides 4 watts RMS of relatively low distor-
tion audio to drive a monitor loudspeaker. A
switch selects the IFB channel to be moni-
tored. The output of the switch connects
one section of SSM-2404 analog switch
which provides the click-free monitor output
muting function. The output of the analog
switch connects to a log taper potentiometer
via a low-pass filter created using a capaci-
tor and resistor. The wiper of the pot con-
nects to the power amp via a DC blocking
capacitor. Three resistors and a capacitor
provide a half supply bias voltage. The open
loop bandwidth of the power amp is limited
via a capacitor in the feedback loop. The
output of the amp is capacitive coupled to
the output jack. A resistor/capacitor combi-
nation connected across the amp’s output
provides protection from oscillation.
Miscellaneous Logic Circuitry
Simple but effective logic circuitry controls
the Model 2’s functions. All gates, with the
exception of the relay drivers, use CMOS
technology for low power consumption
and high reliability. Page 10 of the Model
2’s schematic contains the bulk of the logic
circuitry, and is fairly self-explanatory.
Meter Circuits
Two identical meter circuits monitor the
audio level of the composite IFB channels.
Audio signal enters a meter circuit via an
operational amplifier configured as a half-
wave synchronous rectifier. The resulting DC
output is smoothed via a resistor/capacitor
low pass filter. This DC signal is connected
to a specialized meter driver integrated
circuit. It will directly control up to ten LEDs,
lighting them in 3 dB steps. Five of the steps
were selected for Model 2 use. The meter
integrated circuit is set to operate in its bar
graph mode, with the five LEDs connected
in a series arrangement. This arrangement
minimizes current consumption; the same
current is consumed whether one LED, or
all five are lit. The LED current is set to ap-
proximately 13 mA by two fixed resistors.
Model 22 Access Station
General Description
The Model 22’s circuitry is contained on
two printed circuit boards. The bulk of it
lies on the electronics board; the two
pushbuttons lie on, you guessed it, the
pushbutton switch board. The boards are
interconnected via a 12-position pin and
socket arrangement. The electronics board
contains power supply, interface, and au-
dio circuitry. The Model 22 interconnects
with the Model 2 Central Controller and, if
present, other Model 22s via 9-pin D-sub-
miniature female connectors. Contained on
the Model 22’s circuits board are two such
connectors, wired in parallel. This allows
“daisy-chaining” of the wiring through one
unit to get to others, without requiring the
use of wiring “splitters.”
Power Filtering, Voltage Dividers, and
Power Supply
Filtered and regulated +18 Vdc is provided
by the Model 2 Central Controller. A diode in
series with the incoming power protects the
circuitry from improper wiring. The +18 V is
used directly by the two 4-section compara-
tor integrated circuits, as well as to create
two reference voltages; +6 V and +12 V.