Studio Technologies IFB Plus 2007 - Manual (Page 26)

Issue 9, January 2006 Model 2 User Guide
Page 26 Studio Technologies, Inc.
You might enjoy a quick explanation of why
6 dB 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 posi-
tive to a level of +2 V. Observing the bal-
anced output at that very same instant, the
“high” output is swinging +2 V, while the
“low” output is swinging –2 V. The unbal-
anced input has an excursion of 2 V, but
the balanced output had an excursion of 4
V (+2 V to –2 V). Since voltage gain in dB
is equal to 20 log Vout/Vin, calculating our
example you get 20 log 4/2 = 6 dB! Another
side note, this 6 dB of boost is exactly why
the differential line receiver, used the Model
2’s program input circuits, contains a fixed
6 dB attenuation. The receiver IC gets rid of
the 6 dB 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 op-
erational 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 10 k
ohms a potential stability problem arises. To
eliminate the chance of a problem, connect
a 100pF capacitor in parallel with the feed-
back 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 instal-
lation 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 audio level meter
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 sig-
nals 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 +4 dBu nominal, with
+8 to +12 dBu probable on peaks. The
actual output level is not important, and a
lower level is much better than a “hotter”
average level. After observing 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., –10 dBu, 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 +4 dBu, set their trim
pots fully counterclockwise, reducing their
input sensitivity by approximately 8 dB.
On the inputs that have the –10 sources
connected, set their input trim pots to fully
clockwise, adding 8 dB of input sensitiv-
ity. You have now equalized the 14 dB (+4
to –10) level difference using the trim pots!
Using the program input switches, you now