Studio Technologies IFB Plus 2001 - Manual (Page 39)

Model 2 User Guide Issue 7, November 2001
Studio Technologies, Inc. Page 39
as well as to create two reference volt-
ages; +6V and +12V. These references
are created by resistor divider circuits.
The +18V also passes through a simple
resistor/capacitor low pass filter. This
filtered +18V, referred to as +V on the
Model 22’s schematic diagram, is used
by the audio circuitry, as well as to create
another reference voltage. This reference,
aptly called VREF, is approximately half
of +V and acts as an analog center volt-
age point—an artificial “ground.” The
+18Vdc is also connected to a low-power,
3-terminal regulator. The +5Vdc filtered
and regulated output voltage is used to
provide power for a Studio Technologies
Model 11A gooseneck electret condenser
microphone.
Audio Circuitry
The Model 22 allows connection of two
different types of audio sources; an elec-
tret microphone, or a line-level signal.
A switch selects which input circuitry is
active. In the mic position, the circuitry is
configured to accept an unbalanced audio
source with a nominal level of –25dBu.
Connected via a current limiting resistor,
+5Vdc is provided to operate the FET
preamplifier in the electret microphone’s
cartridge. Microphone audio is capacitor
coupled to two sections of low-power
operational amplifier which serve as
preamplifier/output driver stages. The
two sections of op amp are identically
configured as non-inverting amplifiers
with approximately 15dB of gain, bringing
the signal level to nominal –10dBu. A note
to you audio purists: the selection of the
LF353 was mandated strictly to address
the power consumption issue, not for its
less than optimal noise figure. The outputs
of the op-amps are connected to two
sections of analog switch.
In the line position the balanced line input
circuitry is active. Our friend the SSM-2143
differential line receiver is used to interface
balanced or unbalanced line-level signals.
Unlike the direct coupled configuration
used in the Model 2’s program inputs,
capacitors are used to isolate the audio
signal from the Model 22’s circuitry. The
output of the ‘2143 connects to a voltage
divider circuit consisting of two fixed
resistors and a trim potentiometer. This
“pad” reduces the line-level signal to
match that of the electret microphone,
which is approximately –25dBu. The trim
pot provides the input level trim function,
allowing a wide range of signal levels to
function correctly. The unbalanced and
attenuated line input signal is boosted by
the preamplifier/output driver op-amps.
As mentioned two paragraphs previously,
the outputs of the op amps connect to
sections of analog switch. The outputs of
the analog switches connect to the audio
buses via capacitor/resistor combinations.
The capacitors act as level shifters, mov-
ing the signal from a VREF center to a
system common center point. The series
resistor in the output ensures that in the
event of a fault condition the op will never
see less than a 2000 ohm load. It also
protects the analog switch from transients
or shorts to +18Vdc in the interconnecting
cables. The analog switches are held in
the off condition unless the access station
is initiating an interrupt. At that time audio,
via the analog switches, is placed on the
appropriate audio bus(es).
Logic Circuitry
All logic functions are performed using
sections of integrated circuit comparator,
along with two “spare” analog switches.
Credit Mitch, the consulting engineer, for