Studio Technologies IFB Plus 2001 - Manual (Page 28)

Issue 7, November 2001 Model 2 User Guide
Page 28 Studio Technologies, Inc.
Observing the balanced output at that very
same instant, the “high” output is swing-
ing +2V, while the “low” output is swing-
ing –2V. The unbalanced input has an
excursion of 2V, but the balanced output
had an excursion of 4V (+2V to –2V).
Since voltage gain in dB is equal to 20 log
Vout/Vin, calculating our example you get
20 log 4/2 = 6dB! Another side note, this
is 6dB boost is exactly why the SSM-2143
differential line receiver, used the Model
2’s program input circuits, contains a fixed
6dB attenuation. The ‘2143 gets rid of the
6dB gain created by a differential driver,
saving headroom and correctly matching
equipment levels.
Anyway, to modify the nominal output
level requires you to change the gain of
an operational amplifier by changing the
value of its feedback resistor. Refer to
the chart on page 9 of the schematic
diagrams for several output levels and
the corresponding resistor values. If you
need an output level that requires less
than unity gain from the op-amp you
can do it, but be careful. If you choose
a feedback resistor less than 10K0 ohms
a potential stability problem arises. To
eliminate the chance of a problem, con-
nect a 100pf capacitor in parallel with
the feedback resistor to reduce the open
loop gain.
As with all changes to internal Model 2
circuitry, taking a safety first approach
is required. Disconnect mains power
before removing the Model 2’s cover.
Setting the Program Input Level Trim
Pots
There is no hard and fast rule on how to
best adjust the trim pots associated with
the program inputs. As mentioned in the
installation section, the pots are provided
simply to allow the relative levels of the
program input signals to be equalized.
This will allow minimal level changes when
an operator is switching between the four
program inputs. A simple method of using
the trim pots to “calibrate” the program
input levels is to start by connecting an AC
VTVM to the line output of IFB channel 1.
Set the front-panel program level control
associated with IFB channel 1 to the
middle position (12 o’clock or 50 percent
of rotation). Set the four program level trim
pots to 50 percent of their rotation. Set the
four program sources to deliver their
normal operating signal level to the Model
2’s program inputs. These signals could
be in the form of alignment tones, or
actual program material. Use the program
select switches to select, one at a time,
the four program inputs. The line output
should give an output level of +4dBu
nominal, with +8 to +12dBu probable
on peaks. The actual output level is not
important, and a lower level is much better
than a “hotter” average level. After observ-
ing the relative strengths of the four
sources, use one or more of the trim pots,
if required, to get an equal average level
from the four sources.
If you have a source that is especially low
in level, e.g., –10dBu, all is not lost. While
getting a –10 to +4 level booster box is a
good idea, the level trim pots can come to
the rescue. On the inputs that have signal
sources of nominally +4dBu, set their trim
pots fully counterclockwise, reducing their
input sensitivity by approximately 8dB.
On the inputs that have the –10 sources
connected, set their input trim pots to fully
clockwise, adding 8dB of input sensitivity.
You have now equalized the 14dB (+4 to
–10) level difference using the trim pots!
Using the program input switches, you