Studio Technologies IFB Plus 2001 - Manual (Page 41)

Model 2 User Guide Issue 7, November 2001
Studio Technologies, Inc. Page 41
IFB channel 1 or 2 audio from a Model 2
Central Controller. The unit contains
circuitry to extract DC power from the
three conductors that carry power and two
channels of audio. Along with a power
amplifier integrated circuit, the unit con-
tains a channel select switch, power LED,
and level control. The Model 32 contains
two 3-pin XLR-type connectors, one male
and one female. The female connector is
intended to interconnect with the Model
2’s talent amplifier output. The male con-
nector is intended to facilitate connection
of additional Model 32 or Model 33s
Talent Amplifiers using standard micro-
phone-type cables.
The Model 32’s circuitry is contained on
two printed circuit boards, which are
interconnected via a 5-conductor flexible
jumper cable. All active circuitry lies on the
electronics board; the connectors lie on
the connector board—no surprises here!
Power Supply
The Model 2 Central Controller provides
+18Vdc modulated with channel 1 audio
on pin 2 of the XLR connector. Channel 2
audio is provided on pin 3, with pin 1
supplying common. From the Model 2,
the nominal audio levels are –10dBu. A
3-terminal adjustable integrated circuit
voltage regulator is configured to provide
+10V, nominal, from the incoming +18V
that is modulated with channel 1 audio.
The important characteristic about this
circuit is its constant input impedance
characteristic. The input impedance is
fixed at a moderately high value, approxi-
mately 500 ohms, and does not vary
appreciably with load. This is important so
that the channel 1 audio signal is not
significantly attenuated, nor distorted by
normal fluctuations in the power draw. An
LED indicator light provides indication that
+10V power is present.
Power Amplifier
An industry-standard (for good reason)
LM386N-1 power amplifier integrated circuit
is used to drive the audio output. Signal
enters the amplifier circuit via a switch that
selects the input source, either IFB channel
1 or 2. The signals are coupled to the
switch via coupling capacitors, providing
DC blocking, and preventing audio “clicks”
when switching between channels. The
output of the switch is connected, via a low
pass filter, to a log taper audio potentiom-
eter. This pot allows user adjustment of the
output level. A resistor in series with the
potentiometer prevents the output level
from being completely shut off, a useful
broadcast feature. A low pass filter on the
output of the amplifier helps to provide
stability. The amplifier’s output is capacitor
coupled, via a series resistor, to the output
connector. The capacitor changes the
audio output signal from being biased at
approximately +5V, to being biased at
signal common. The series resistor limits
the output current in the event of a shorted
output connecting cable.
Model 33 Talent Amplifier
General Description
The Model 33 Talent Amplifier is a self-
contained module which allows head-
phones or ear pieces to be driven with
IFB channel 1 or 2 audio from a Model 2
Central Controller. The unit contains cir-
cuitry to extract DC power from the three
conductors that carry power and two chan-
nels of audio. Along with a power amplifier
integrated circuit, the unit contains a chan-
nel select switch, power LED, and level
control. The Model 33 contains two 3-pin
XLR-type connector, one male and one