Studio Technologies IFB Plus 2001 - Manual (Page 38)

Issue 7, November 2001 Model 2 User Guide
Page 38 Studio Technologies, Inc.
acceptable, as long as the short is made
directly at the output connector.
Monitor Amplifier
An integrated circuit power amplifier
provides 4 watts RMS of relatively low
distortion audio to drive a monitor loud-
speaker. A switch selects the IFB channel
to be monitored. The output of the switch
connects one section of SSM-2400-series
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 capacitor and resis-
tor. The wiper of the pot connects 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
combination 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 con-
sumption 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 10 LEDs, lighting them in 3dB 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 arrange-
ment. This arrangement minimizes current
consumption; the same current is con-
sumed whether one LED, or all five are
lit. The LED current is set to approximately
13mA 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
push buttons lie on, you guessed it, the
push button switch board. The boards
are interconnected via a 12-position pin
and socket arrangement. The electronics
board contains power supply, interface,
and audio circuitry. The Model 22 inter-
connects with the Model 2 Central Con-
troller and, if present, other Model 22s via
9-pin D-subminiature 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 +18Vdc is provided
by the Model 2 Central Controller. A diode
in series with the incoming power protects
the circuitry from improper wiring. The
+18V is used directly by the two four-
section comparator integrated circuits,