Studio Technologies IFB Plus 2001 - Manual (Page 35)

Model 2 User Guide Issue 7, November 2001
Studio Technologies, Inc. Page 35
relay controls the connection of the tele-
phone line to a 600 ohm to 600 ohm
coupling transformer. In series with the
center taps of the transformer is a loop
current detector. This circuit is quite inter-
esting, providing both desensitization and
over-current protection for the optocoup-
ler that actually detects current flow. The
output of the optocoupler is conditioned
by two resistors and a capacitor to provide
“debounce” of the loop current status
signal. An LED, labeled LC, indicates the
presence of loop current. The secondary
of the transformer is connected to one
section of operational amplifier which is
configured as a variable gain buffer. A
trim potentiometer allows the gain to
be adjusted from 4 to 20dB. This pro-
cesses the telephone line audio to get it
to the Model 2’s nominal internal level of
–10dBu. Zener diodes, connected directly
across the transformer’s secondary,
provide overvoltage protection for the
op-amp and related circuitry.
The interface mode switch controls two
functions: relay status and termination
impedance. In the telephone line mode,
the interface control switch located on the
front panel is active, allowing the operator
to control the relay. In the telephone line
mode, a 1000 ohm resistor is placed
across the secondary of the transformer.
This, along with the op-amp’s 2200 ohm
input resistor, provides the correct termi-
nation impedance for a standard tele-
phone line. In the standard audio mode,
the 1000 ohm resistor is not connected
to the circuit, and the 2200 ohm input
impedance of the op amp’s input resistor
provides the input impedance. In the
standard audio mode, the switch is
disabled, the relay is held energized,
and the LED labeled STD is lit.
A simple logic circuit implements the
telephone interface control function. Two
NAND gates form a flip-flop, the two states
being on-hook and off-hook. In the tele-
phone line mode, pressing the interface
control switch to the manual off-hook
position sets the flip-flop to the off-hook
state, and causing the relay to energize
via one gate of a power driver integrated
circuit. If loop current is detected the
circuit will stay in the off-hook state. If
current is not detected the circuit will time
out and return to the on-hook state. Press-
ing the interface switch to the manual
hang-up position forces the circuit to the
on-hook state. In the standard audio mode
the logic circuitry is effectively disabled,
the relay is continually energized, and the
LED labeled STD is lit.
Now we’ll discuss interface 2 specifics.
The tip and ring of the telephone line
enters the interface via a protection circuit
consisting of a fuse, power resistor, and
360Vdc breakdown resistor (varistor). In
parallel with the breakdown resistor is an
optical coupler-based ring voltage detec-
tion circuit. It provides a logic signal to
indicate when the line is “ringing.” A
sealed, bifurcated contact relay controls
the connection of the telephone line to
a 600 ohm to 600 ohm coupling trans-
former. In series with the center taps of
the transformer is a loop current detector.
An LED, labeled LC, indicates the pres-
ence of loop current. Zener diodes, con-
nected directly access the transformer’s
secondary, provide overvoltage protec-
tion. The secondary of the transformer is
connected to the input of an operational
amplifier which is configured as a variable
gain buffer. A trim potentiometer allows
the gain to be adjusted from 4 to 20dB.