Studio Technologies IFB Plus 2001 - Manual (Page 31)

Model 2 User Guide Issue 7, November 2001
Studio Technologies, Inc. Page 31
produces nominal +34Vdc. 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 unregu-
lated +34Vdc is used by the talent ampli-
fier output circuit.
The third power supply generates unregu-
lated nominal +25Vdc, and regulated
+18Vdc. Its transformer has dual 17V
secondaries which are connected in
parallel. The 17Vac is fed to a full wave
diode bridge and a electrolytic filter ca-
pacitor which produces nominal +25Vdc.
The negative pin of the bridge is con-
nected as the circuit common, as well
as being strapped to the metal chassis
and the ground pin of the power entry
connector. The unregulated +25Vdc 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 +25Vdc is also fed to an integrated
circuit regulator which produces +18Vdc
for use by the access stations. A capacitor
on the output of the regulator provides
stability. The +18V regulator acts as much
as a current limiter as it does a regulated
power supply. The regulator limits the
amount of short current that can be drawn
by the access stations. In this way a short-
ed access station or related cabling will
not bring “down” or damage the Model 2.
For service purposes, test points are
included on the power supply buses:
unregulated +34Vdc, unregulated
+25Vdc, 12Vdc, and +18Vdc. In addition,
two circuit common test points are in-
cluded—one on the left side of the board
and one on the right side.
Wonderful Integrated Circuits from
Analog Devices
The Model 2 takes advantage of an excel-
lent audio specific series of integrated
circuits from Analog Devices. Using these
parts allowed literally hundreds of compo-
nents to be saved. More importantly,
performance levels were achieved that
would have been difficult, if not impos-
sible, to obtain with more conventional
circuitry. These parts have the common
prefix of SSM, indicating their roots in a
company called Solid State Microelectron-
ics for Music, purchased, by way of
Precision Monolithics, a few years ago.
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 +4dBu. A trim
potentiometer associated with each pro-
gram input allows ±8dB of input variation.
Now is a good time to note that the Model
2’s internal operating level is –10dBu. This
was selected to optimize audio perfor-
mance—maximizing the signal-to-noise
ratio and allowing ample headroom. The
exact purpose of the program input cir-
cuits is to receive the program audio,
separate out hum and noise, unbalance
it, and attenuate it to the –10dBu internal
operating level. Each circuit contains a
differential input integrated circuit, fol-
lowed by a variable gain reduction stage.
The program input signals are direct
coupled to SSM-2143 differential (bal-
anced) line receiver integrated circuits.
The ‘2143 has excellent common mode
rejection, low noise, and high slew rate. It
contains two internal 12k ohm series input
resistors, individually laser trimmed for
accuracy. By design, the ‘2143 provides
6dB of level reduction.