Studio Technologies IFB Plus 2001 - Manual (Page 27)

Model 2 User Guide Issue 7, November 2001
Studio Technologies, Inc. Page 27
2, is internally set for a gain reduction
of 6dB. The ‘2141 is a unity gain device.
The two devices, manufactured by Analog
Devices, are pin for pin compatible so
removing one and inserting the other is
all that is required. Be certain to observe
all safety precautions when accessing
the inside of the Model 2. Should you
have trouble obtaining SSM-2141 device,
please contact Studio Technologies for
assistance.
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. Refer-
ring to page 5 of the Model 2’s schematic
diagram, observe the ring detection circuit
in the lower left section of the page. A 5K6
resistor (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 nor-
mal English language speech patterns.
This release time was scientifically deter-
mined—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) ca-
pacitor connected to the output of the
synchronous half-wave rectifier. Reducing
the capacitance to r22 (0.22uf) or r1
(0.1uf) will shorten the release time. In-
creasing the capacitance to r47 (0.47uf)
or 1uf, will increase the release time. Use
safety precautions if you will be perform-
ing this, or any other, modification.
Line Output Nominal Level
The nominal level of the line outputs is
+4dBu. This should be well-suited for
most professional applications. However,
the line output stage is such that if you
require a different 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. Audio
enters the line driver stage at the internal
nominal operating level of –10dBu. Each
channel’s composite IFB signal enters a
section of operational amplifier which is
configured to add 8dB of gain, bringing
the nominal level to –2dBu. This signal
connects to an SSM-2142 differential line
driver integrated circuit. By the very nature
of a balanced line driver a 6dB level boost
is achieved. The nominal output level is
now +4dBu.
You might enjoy a quick explanation of
why 6dB is added by a differential output
stage. We’ll use the example of an input
signal to the differential line driver chip
which, upon taking a “snapshot” view,
is swinging positive to a level of +2V.