Guardian Telecom Inc.
Installation & Operation
TBA Models
Page 10
6.3. Push to Talk Switches
The Talk Back Amplifier has a 24 volt resistive connection to Push to Talk (PTT) switches.
The use of gold or silver plated switch contacts within an environmentally sealed housing is
recommended for long term reliability.
The switches will conduct about 0.5 milliamp when pressed, and have 24 volts across the terminals
when not pressed.
Each of these connections should be evaluated separately for safety.
6.4. Microphone
For all of the models except the TBA-33 the microphone input is terminated with a low impedance
of less than 10 Ohms at 1 KHz when the Central Station PTT is not pressed.
The TBA-33 has a 1K Ohm impedance across its terminals when the Central Station PTT is not
pressed. There is no DC supply or bias applied to the microphone line pair as the microphone input
is optimized for a dynamic microphone.
The level on the microphone is -80 dB reference 1 volt per bar (0.1 millivolt per bar). This is 0.1
millivolt from 150 Ohms series impedance with a 94 dB SPL activation. EIA sensitivity is -152 dB.
The Telex 785L is one microphone type that meets this specification.
The connection to the microphone is a two wire shielded cable, with the shield terminated at the
TBA enclosure only.
Ensure the microphone cable is not routed near a source of interference such as a generator, high
voltage cable or AC supply transformer.
Each of these connections should be evaluated separately for safety.
6.5. Speaker
The Talk Back Amplifier has a 10 watt at 325 ohm audio output to the speakers. This is also known
as a 70 volt intercom system.
The speakers used should be transformer coupled to the acoustic transducer. The acoustic
transducer may be a cone type speaker or a compression driver with an expansion cone. The
compression driver type of speaker is about 6 dB more efficient than a cone speaker for sound
input to electrical output when it used as a microphone.
The speaker on the Monitored Station and Stations #1, #2 and #3 are used as audio input devices.
The level of audio from these speakers is near 1 millivolt for 94 dB SPL at 1 foot from the front of
the speakers. Different types of speakers will have different sensitivities and the user must
evaluate the performance of the speaker when at the location.
The transformer on the speaker is capable of picking up surrounding magnetic fields, especially
from a 60 Hz transformer nearby. Ensure that the speakers and transformers are separated from
AC supply transformers and other sources of magnetic fields.
The connection to the speaker is a two wire shielded cable, with the shield terminated at the TBA
enclosure only.
Ensure the speaker cable is not routed near a source of interference such as a generator, high
voltage cable or AC supply transformer.
Each of these connections should be evaluated separately for safety.
