StudioTech 233 manual Compressor Circuit, P19

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

3.5volts 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

Model 233 User Guide	Issue 1, September 2007
Studio Technologies, Inc.	Page 43