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Model 233 User Guide Issue 1, September 2007
Studio Technologies, Inc. Page 43
P19:
Pin 1 is connected to the common point of
the Model 233’s circuitry. Pin 2 is connect-
ed to the circuitry associated with the main
output pushbutton. Pin 3 is connected to
circuitry associated with the tally output.
The remote switch input circuits are “active
low,” with a 10 k ohm resistor connected to
+5 volts acting as a pull up. A combination
of resistors and capacitors provide ESD
protection. The tally output is nominally
3.5 volts DC and is current limited to 12
milliamperes. This output can directly drive
LED indicators. Using a series resistor in
the range of 150 to 330 ohms is recom-
mended. This will limit the current while
still providing adequate LED light output.
Compressor Circuit
In this section some general information
about the Model 233’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 functions 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 233’s nominal inter-
nal operating level of –2 dBu. A 5:1 com-
pression 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
compressor’s threshold has been reached
and the circuit is actively controlling the
dynamic range. This LED is provided as
an aid when setting the gain of the micro-
phone preamplifier.
Auxiliary Relay
The Model 233 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
application does require the services of a
qualified technician. This is because the
Model 233’s enclosure must be disas-
sembled and the desired wiring scheme
implemented. The relay operates under
software control, following the configura-
tion selected using two of the DIP-type
switches. Four operating modes are avail-
able: relay disabled, relay follows main
output status, relay follows talkback 1
status, and relay follows talkback 2 status.
These choices were previously discussed
in this user guide and should be reviewed.
The relay provides two “form-C” contacts,
each consisting of a common, normally
open (not shorted), and normally closed
(shorted) connection. Obviously the two
form-C contacts change state in unison;
two independent relay functions are not
provided. These relay contacts are ac-
cessible on the Model 233’s main printed
circuit board assembly by way of two
3-position header connectors labeled P15
and P16. The contacts are titled A and
B, but there is no significance between
the two. With both headers pin 1 is nor-
mally closed, pin 2 is common, and pin
3 is normally open. For additional details
on connecting to the 3-position headers