Studio Technologies 220 2004 - Manual (Page 36)

Model 220 User Guide Issue 2, August 2004
Studio Technologies, Inc. Page 37
to get things going. A standard 0.1-inch-
center jumper, commonly used on elec-
tronic equipment, is all that is required. A
200 ohm, 2 watt resistor is electronically
in series between the external 24 volt DC
input and pin 3 of the header. When used
with the lamps available from Studio Tech-
nologies, the resistor limits the lamp cur-
rent to approximately 65 mA. This lights
the lamps to a moderate intensity. If a
different type of lamp is installed, its power
source should be connected to pins 1
(common) and 2 (lamps) of the header.
Remote Control Connections
Provision has been made on the Model
220’s printed circuit board assembly to
allow external switches or contact closures
to control the main and talkback output
functions. Two 3-position headers, labeled
P17 and P18, provide access to the cir-
cuitry associated with the functions.
P17: Pin 1 is connected to the common
point of the Model 220’s circuitry. Pin 2
is connected to the circuitry associated
with the talkback 1 button. Pin 3 is con-
nected to the circuitry associated with
the talkback 2 button.
P18: Pin 1 is connected to the common
point of the Model 220’s circuitry. Pin 2 is
connected to the circuitry associated with
the main out button. Pin 3 is connected to
the circuitry associated with the auxiliary
pushbutton input. (This function is not cur-
rently supported in software.)
The input circuitry is “active low,” with a
10 k ohm resistor connected to +5 volts
acting as a pull up. A combination of resis-
tors and capacitors provide ESD protec-
tion. A qualified technician can use these
inputs for special applications. Contact the
factory for additional details.
Compressor Circuit
In this section some general information
about the Model 220’s compressor circuit
will be provided. As previously discussed,
the output of the microphone preamplifier
circuit is connected to a studio-quality com-
pressor circuit. The output of the compres-
sor is used by the talkback outputs and, if
configured, the main output. (In most cases
the main output will utilize the signal com-
ing directly out of the microphone preampli-
fier.) The gain element in the compressor
circuit is a laser-trimmed voltage-controlled-
amplifier integrated circuit. It provides accu-
rate, low-noise, low-distortion performance.
The threshold of the compressor circuit is 2
dB above the Model 220’s nominal internal
operating level of –2 dBu. A 5:1 compres-
sion ratio is implemented and, like the
threshold level, is not field adjustable. The
threshold and ratio settings were selected
so that excellent talkback audio would be
provided. By controlling the dynamic range,
intelligibility can be improved and overload-
ing of connected devices can be avoided.
An LED indicator lights whenever the com-
pressor’s threshold has been reached and
the circuit is actively controlling the dynam-
ic range. This LED is provided as an aid
when setting the gain of the microphone
preamplifier.
Auxiliary Relay
The Model 220 provides an auxiliary relay
for use in specialized applications. Some
“head scratching” or “brainstorming”
should lead to a number of interesting ways
to take advantage of this unique resource.
To implement any auxiliary relay applica-
tion does require the services of a qualified
technician. This is because the Model 220’s
enclosure must be disassembled and the
desired wiring scheme implemented. The