Headphone Source Selection, Phantom Power On/Off

It is visible by observing the bottom of the Model 233’s enclosure when the security plate has been removed. Technically, this red LED lights whenever the compressor circuitry is controlling the dynamic range of the signal coming from the microphone preamplifier. The threshold is set to be 2 dB above the Model 233’s nominal internal operating level. So a good “rule of thumb” is to adjust the gain of the microphone preamplifier so that the compressor active LED lights (“flashes”) when the connected microphone is sending signal peaks. During normal operation the LED should not remain fully lit when typical audio sig- nals are present on the mic input.

It’s important to remember that the com- pressor active LED is used to assist in set- ting the mic preamplifier gain to the optimal value. It doesn’t necessarily indicate that the main output’s signal is being compressed. Unless specifically configured to perform otherwise, the output of the compressor is only used for the talkback output functions.

It’s expected that the 20 and 60 dB gain settings will not often be used. But there are always exceptions and that’s why they were included. It’s possible that with a very “hot” microphone, such as a phantom- powered condenser-type, 20 dB of gain could be correct. It’s also possible that a microphone with a very low level output, such as a ribbon-type, would need 60 dB of gain. But in general, the 30, 40, and 50 dB gain settings will serve most applications.

Note that if no gain switch is set to its ac- tive (on) position the Model 233’s preampli- fier will operate at unity (0 dB) gain. In this mode the preamplifier will remain stable, but is intended only for use during factory testing. A valid exception would be where

a line-level signal is connected to the microphone input. This could occur with a special Model 233 application. But with a microphone connected as the input source one should never use the 0 dB setting. The issue is that with no gain added to the microphone input signal the relative noise floor on the circuitry associated with the main output and talkback functions will be much too high. These circuits are designed for handling line-level signals, expecting to receive audio from the output of the micro- phone preamplifier. In conclusion, the 0 dB gain setting doesn’t highlight a problem, but simply reflects the unit’s gain structure.

Phantom Power On/Off

The Model 233 can provide nominal 48 volt phantom power to the microphone input. Switch SW1-8 controls whether or not phan- tom power is active. By phantom power’s very nature it could be left applied to the microphone input at all times. But generally people prefer to turn it off unless required for a specific microphone.

Figure 5. Phantom power switch settings

Headphone Source Selection

Switch assembly SW2 is used to configure the sources that are routed to the ste- reo headphone output. Five headphone sources are available: line input 1, line in- put 2, intercom channel 1, intercom chan- nel 2, and sidetone. Each of these sources can be assigned to the left, right, or both the left and right channels of the stereo headphone output.

Model 233 User Guide	Issue 1, September 2007
Studio Technologies, Inc.	Page 19