Studio Technologies IFB Plus 2001 - Manual (Page 42)

Issue 7, November 2001 Model 2 User Guide
Page 42 Studio Technologies, Inc.
female. The female connector is intended
to interconnect with the Model 2’s talent
amplifier output. The male connector is
intended to facilitate connection of addi-
tional Model 33 or Model 32s using stan-
dard microphone-type cables.
The Model 33’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. A zener
diode, in series with the LED, is used to
generate a 5.6V reference for the circuitry.
Mixer/Output Amplifier
IFB channel 1 and 2 audio is capacitor-
coupled to two audio-taper potentiom-
eters. Resistors in series with the pots
ensures that the audio level can never be
fully turned “off.” The output signals from
the pots connect to an NE55320-type
operational amplifier that is configured as
a combining amplifier. The faithful ‘5532
is used both to mix the audio sources, as
well as to drive the audio output. Signal
enters the op-amp via a switch that selects
which of he two input sources are active.
Note that the switch actually selects one
source by muting the other! The op-amp’s
output is capacitor coupled , via a series
resistor, to the output connector. The
capacitor serves as a DC blocking func-
tion, changing the output so as to be
referenced to signal common. The series
resistor helps the op-amp maintain stabil-
ity, as well as providing protection in the
event of a shorted output connecting
cable.
Troubleshooting
Troubleshooting the Model 2 and
related components is to be per-
formed only by a qualified techni-
cian. Operating the Model 2 with the cover
removed exposes you to hazardous
voltages.
Detailed block diagrams, located at the
end of this guide, and schematic dia-
grams, located in the separate schematic
packet, are provided for assistance during
maintenance and repair. Understanding
the block diagrams will lead to much
better understanding of the individual
circuits and how they work together (or,
in case of trouble, how they don’t work
together!)
Model 2 Central Controller
The Model 2 consists of many relatively
simple circuits. Detailed troubleshooting
procedures are difficult to describe be-
cause there are few specific circuits that