Studio Technologies IFB Plus 2005 - Manual (Page 26)

Issue 8, December 2005 Model 2 User Guide
Page 26 Studio Technologies, Inc.
fed to a full wave diode bridge and a electro-
lytic filter capacitor which produces nominal
+40 Vdc. The negative pin of the bridge is
connected as the circuit common, as well
as being strapped to the metal chassis and
the ground pin of the power entry connec-
tor. The unregulated +40 Vdc is fed to an
integrated circuit regulator which produces
+26 Vdc. This voltage is used by the talent
amplifier output circuit.
The third power supply generates unregu-
lated nominal +25 Vdc and regulated
+18 Vdc. Its transformer has dual 17 V
secondaries which are connected in parallel.
The 17 Vac is fed to a full wave diode bridge
and a electrolytic filter capacitor which pro-
duces nominal +25 Vdc. The negative pin
of the bridge is connected as the circuit
common, as well as being strapped to the
metal chassis and the ground pin of the
power entry connector. The unregulated
+25 Vdc is used by the monitor amplifier
and lamp output circuits. The lamp outputs
are used to drive indicator lights in Model 22
Access Stations.
The +25 Vdc is also fed to an integrated cir-
cuit regulator which produces +18 Vdc for
use by the access stations. A capacitor on
the output of the regulator provides stability.
The +18 V regulator acts as much as a cur-
rent limiter as it does a regulated power sup-
ply. The regulator limits the amount of short
current that can be drawn by the access
stations. In this way a short-ed access sta-
tion or related cabling will not bring “down”
or damage the Model 2.
For service purposes, test points are in-
cluded on all the power supply buses. In
addition, circuit common test points are
included—two on the left side of the board
and one on the right side.
Program Inputs
The Model 2 contains four identical pro-
gram input circuits. The program inputs
are designed to receive balanced signals
with a nominal level of +4 dBu. A trim po-
tentiometer associated with each program
input allows ±8 dB of input variation. Now
is a good time to note that the Model 2’s
internal operating level is –10 dBu. This was
selected to optimize audio performance—
maximizing the signal-to-noise ratio and al-
lowing ample headroom. The exact purpose
of the program input circuits is to receive
the program audio, separate out hum and
noise, unbalance it, and attenuate it to the
–10 dBu internal operating level. Each circuit
contains a differential input integrated cir-
cuit, followed by a variable gain reduction
stage. The program input signals are direct
coupled to the differential (balanced) line
receiver integrated circuits. These devices
have excellent common mode rejection,
low noise, and high slew rate. They contain
two internal 12 k ohm series input resistors,
individually laser trimmed for accuracy. By
design, they provide 6 dB of level reduction.
The nominal –2 dBu signal is capacitive
coupled to an operational amplifier config-
ured as a variable attenuator. The coupling
capacitors are nonpolar type, allowing for
DC voltages of unknown polarity to be
received on the program inputs. The output
of the op-amps have nominal signal levels
of –10 dBu and are direct coupled to the
program select switches. Small capacitors in
the feedback loop of the op-amps ensures
stability at the less than unity gain imple-
mentation.
Program Select, Program Muting, and
Program Level Control
Each IFB channel contains a group of six
program select switches. These switches
allow one or more of the program sources