Studio Technologies IFB Plus 2005 - Manual (Page 33)

Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 33
These references are created by resistor
divider circuits. The +18 V also passes
through a simple resistor/capacitor low
pass filter. This filtered +18 V, referred to as
+V on the Model 22’s schematic diagram,
is used by the audio circuitry, as well as to
create another reference voltage. This refer-
ence, aptly called VREF, is approximately
half of +V and acts as an analog center
voltage point—an artificial “ground.” The
+18 Vdc is also connected to a low-power,
3-terminal regulator. The +5 Vdc filtered and
regulated output voltage is used to provide
power for a Studio Technologies Model 11A
gooseneck electret condenser microphone.
Audio Circuitry
The Model 22 allows connection of two dif-
ferent types of audio sources; an electret
microphone, or a line-level signal. A switch
selects which input circuitry is active. In the
mic position, the circuitry is configured to
accept an unbalanced audio source with a
nominal level of –25 dBu. Connected via a
current limiting resistor, +5 Vdc is provided
to operate the FET preamplifier in the elec-
tret microphone’s cartridge. Microphone
audio is capacitor coupled to two sections
of low-power operational amplifier which
serve as preamplifier/output driver stages.
The two sections of op-amp are identically
configured as non-inverting amplifiers with
approximately 15 dB of gain, bringing the
signal level to nominal –10 dBu. A note
to you audio purists: the selection of the
LF353 was mandated strictly to address the
power consumption issue, not for its less
than optimal noise figure. The outputs of the
op-amps are connected to two sections of
analog switch.
In the line position the balanced line input
circuitry is active. A differential line receiver
integrated circuit is used to interface
balanced or unbalanced line-level signals.
Unlike the direct coupled configuration used
in the Model 2’s program inputs, capacitors
are used to isolate the audio signal from the
Model 22’s circuitry. The output of the ’2143
connects to a voltage divider circuit consist-
ing of two fixed resistors and a trim poten-
tiometer. This “pad” reduces the line-level
signal to match that of the electret micro-
phone, which is approximately –25 dBu.
The trim pot provides the input level trim
function, allowing a wide range of signal
levels to function correctly. The unbalanced
and attenuated line input signal is boosted
by the preamplifier/output driver op-amps.
As mentioned two paragraphs previously,
the outputs of the op-amps connect to
sections of analog switch. The outputs of
the analog switches connect to the audio
buses via capacitor/resistor combinations.
The capacitors act as level shifters, moving
the signal from a VREF center to a system
common center point. The series resistor in
the output ensures that in the event of a fault
condition the op-amp will never see less
than a 2000 ohm load. It also protects the
analog switch from transients or shorts to
+18 Vdc in the interconnecting cables. The
analog switches are held in the off condi-
tion unless the access station is initiating an
interrupt. At that time audio, via the analog
switches, is placed on the appropriate audio
bus(es).
Logic Circuitry
All logic functions are performed using sec-
tions of integrated circuit comparator, along
with two “spare” analog switches. Credit
Mitch, the consulting engineer, for continu-
ing to prove that comparators are about
the handiest “building block” available! The
comparators perform three major functions.
The first function prevents an access station