Studio Technologies IFB Plus 2001 - Manual (Page 37)

Model 2 User Guide Issue 7, November 2001
Studio Technologies, Inc. Page 37
Each circuit capacitive couples audio into
an operational amplifier configured as an
inverting unity gain buffer. The output of
each op-amp is connected to one section
of SSM-2400-series analog switch, part
of the interrupt audio routing circuit. This
analog switch produces a click-free join-
ing of access station audio to the IFB
channels.
Adjustable voltage regulator integrated
circuits are used to provide power for the
push button switch’s status (a.k.a. tally)
lamps located on the access stations.
These regulators were selected because
of their ability to be controlled by a logic
signal, along with their inherent over-
current protection.
Three logic signals are ready to be con-
trolled by the Model 22 Access Stations:
channel 1 interrupt control, channel 2
interrupt control, and monitor mute con-
trol. These CMOS-type logic signals are
active low. Pull up resistors maintain a
logic high, as well as providing sufficient
current for the access stations to function
correctly. Series resistors and zener
diodes provide overvoltage protection
when interfacing the nasty “outside world”
signals with the Model 2’s logic gates.
Static “zaps” of 8000 volts shouldn’t
damage the Model 2—but please don’t
try to prove us wrong!
Filtered and regulated +18Vdc provides
power for the access stations. The
+18Vdc is created by a 3-terminal, inte-
grated circuit regulator which provides
over-current protection. This feature is
especially important as access station
cable shorts may be quite common
during installation and testing.
Talent Amplifier Output
Power, IFB channel 1 audio, and IFB
channel 2 audio are provided by the
talent amplifier output. An integrated
circuit adjustable voltage regulator pro-
vides nominal +18Vdc, which is sufficient
current to power any combination of up
to four Model 32 or 33 Talent Amplifiers.
An operational amplifier, set for unity gain,
drives the adjust pin to superimpose
channel 1 audio on the DC voltage. The
nominal audio level is –10dBu, with over
20dB of headroom available. The voltage
regulator has inherent over-current protec-
tion so that a shorted cable will not dam-
age the output circuitry. Another section
of operational amplifier is used to drive
channel 2 audio to the outside world. A
resistor and capacitor in series with the
output protects the op amp from a short
circuit, as well as an accidental shorting
of the power/channel 1 audio connection
to the channel 2 connection.
Line Outputs
Each IFB channel has a balanced line-
level output associated with it. The two
circuits are identical. Composite IFB audio
at the internal operating level of –10dBu is
boosted 8dB by one section of operational
amplifier. The output of the op amp is
connected to a SSM-2142 differential line
driver integrated circuit. This integrated
circuit provides an electronically balanced,
low impedance output signal. It is capable
of driving high signal levels into 600 ohms
or greater. Because of the nature of a
differential driver, an effective 6dB boost
in signal level is achieved, giving a nomi-
nal output level of +4dBu. Shorting one
side of the balanced output to circuit
common (pin 1 of the line output connec-
tor) will not damage the ‘2142. Unbal-
anced operation in this manner is perfectly