StudioTech 233 manual Symptoms of Insufficient Power

the intercom line, everything worked well. But a new problem arose when it came to finding an external linear power supply for Model 233 users worldwide. For loca- tions that are served by 100 or 120 volts, 60 Hz a 24 volt DC linear “wall-wart” pow- er supply was readily available. This is the power supply that is provided with Model 233 units shipped to North America or Japan. But as of this writing, no “perfect” 24 volt DC linear power source has been located that would serve 220/240 volts,

50 Hz applications. So a compromise had to be made. For these applications a uni- versal input switch-mode power supply is included with each Model 233. It’s far from an ideal solution, leading to noise on the intercom line’s audio channels when long cable runs are present. But at least users will have an “emergency” power source if nothing else is available.

In conclusion, users where the AC mains source is 220/240 volts should consider locating an alternate 24 volt DC power source. Several options are available that could make the task simple. It’s expected that a more sophisticated “medical-grade” switch-mode power supply will have much better control of induced noise. It’s highly likely that using one of these supplies will provide good results. As the Model 233’s current requirement is 90 milliamperes at 24 volts, a typical medical-grade power supply should be able to power multiple units. Another solution would be to obtain two of the more commonly available

12 volt DC linear power supplies, con- necting their outputs in “series” to create a 24 volt DC source. While not a glamor- ous solution, it is technically correct and should prove cost effective. If this arrange- ment is implemented remember that the

center pin of the 2.1 x 5.5 mm coaxial jack is used for the positive (+) connection.

Symptoms of Insufficient Power

A core part of the Model 233’s internal circuitry is a switch-mode power supply that produces +48 volts, +12 volts,

+5 volts, and –12 volts. This power supply circuit works very well as long as it is “fed” with sufficient input voltage and cur- rent. “Sufficient” is defined as a minimum of 24 volts on the intercom input and

20 volts on the external 24 volt DC input. The necessary current, 125 milliamperes for the intercom line and 90 milliamperes for the external source, must be supplied over their respective voltage ranges.

It’s worth discussing what will happen if any of these power sources fall below their specified minimum. Typically, if the Model 233 is being powered by an external 24 volt nominal power source, normal opera- tion will continue until the input falls to the 18-20 volt range. As the input voltage drops below this range the Model 233’s internal power supply will have reduced stability, operating in this manner until its low-voltage shutdown circuit halts opera- tion. Note that as the input voltage moves down from 24 volts the input current will rise proportionately to make up for the loss of power.

Using the intercom line to provide Model 233 power shouldn’t prove to be a prob- lem. Power supplies associated with broadcast and production intercom sys- tems are designed to support multiple belt-pack and related devices. In the “big scheme of things,” connecting a Model 233 shouldn’t add a significant load.

Issue 1, September 2007	Model 233 User Guide
Page 38	Studio Technologies, Inc.