Studio Technologies IFB Plus 2007 - Manual (Page 25)

Model 2 User Guide Issue 9, January 2006
Studio Technologies, Inc. Page 25
circuit board. With care, the resistors can be
added without removing the circuit board
from the chassis. Bend the leads so that
they are ½-inches apart, allowing them to
easily drop into the resistor locations. Trim
the leads so that once inserted and soldered
into the board, the ends of the leads will not
touch the chassis below the board. Again,
ensure that the ends of the straps do not ex-
tend below the circuit board so as to touch,
or even come near the bottom of the chas-
sis. Failure to heed this warning can result
in operational problems.
5) Replace the top cover and secure it
using the screws.
6) Reconnect the mains power and check
the Model 2 for correct operation.
Disabling the Auto Answer Feature
In the telephone line mode, telephone
interface 2’s circuitry will automatically go
off-hook upon detecting ringing voltage
on the telephone line. This feature can be
very useful, allowing another site to access
the Model 2 without operator intervention.
There may be cases where this feature is
not desired, and the auto answer function
can be disabled. Referring to page 5 of the
Model 2’s schematic diagram, observe the
ring detection circuit in the lower left section
of the page. If needed contact the factory
to obtain the schematic files. A 5K6 resis-
tor (5600 ohm, 1 watt) is in series with the
optocoupler. Remove this resistor to disable
auto answer. Remember to be careful inside
the cabinet. Disconnect mains power prior
to taking off the cover!
Voice Operated (VOX) Interrupt Release
Time
The VOX interrupt circuit was designed to
have a fast detect time and a relatively long
release time. The fast detect time ensures
that the beginning of a word will not be lost.
The slower release time keeps the circuit from
“modulating” during normal English language
speech patterns. This release time was sci-
entifically determined—we gathered a bunch
of people in our lab, “listened” to a number
of different release times, then chose the
favorite. Actually, we think the selected time
works quite well. However, you may have a
different opinion. (Do beans belong in chili?
Is Elvis really dead?) The release time is set
using one capacitor. Referring to the right side
of page 7 of the Model 2’s schematic, locate
the r33uF (0.33uF) capacitor connected to the
output of the synchronous half-wave rectifier.
Reducing the capacitance to r22uF (0.22uF)
or r1uF (0.1uF) will shorten the release time.
Increasing the capacitance to r47 (0.47uF) or
1uF, will increase the release time. Use safety
precautions if you will be performing this, or
any other, modification. If needed, contact the
factory to obtain the schematic files.
Line Output Nominal Level
The nominal level of the line outputs is
+4 dBu. This should be well-suited for most
professional applications. However, the line
output stage is such that if you require a dif-
ferent nominal output level, you can achieve it
by simply changing one resistor per channel.
Refer to page 9 of the Model 2’s schematic to
observe the two identical line output stages.
If needed contact the factory to obtain the
schematic files. Audio enters the line driver
stage at the internal nominal operating level of
–10 dBu. Each channel’s composite IFB signal
enters a section of operational amplifier which
is configured to add 8 dB of gain, bringing the
nominal level to –2 dBu. This signal connects
to a differential line driver integrated circuit. By
the very nature of a balanced line driver a 6 dB
level boost is achieved. The nominal output
level is now +4 dBu.