Telex SPK-300L Desktop Speaker User Station with A5F Headset Connector

The Telex SPK-300L Portable Desktop Speaker User Station with A5F Headset Connector is two channel select portable speaker user station housed in a rugged all metal housing perfect for table-top operation. Some features include channel select switch, call light, speaker on/off switch and dual-purpose portable desktop volume control. Each station is a communication unit along a multi-conference bus and is used in the TW Intercom System. The SPK300L includes the following functional components: a microphone preamplifier with limiter, an electronic microphone switch, a bilateral current source line driver, a listen volume control, a headphone amplifier, a speaker amplifier, a speaker switch and a channel selector switch. Operational Controls Channel select switch / latching-action microphone on-off toggle switch / momentary-action microphone on-off push-button switch / speaker-headphone volume control / call switch-indicator / speaker on-off switch / sidetone adjustment. Volume Control The volume control also feeds the speaker amplifier via the speaker switch and the speaker dim network. Microphone Preamplifier/Limiter The microphone preamplifier/limiter converts the small microphone signal to a strong line level signal conditions the signal strength from loud and soft talkers to be almost the same sends the signal to the line via the microphone switch and a bilateral current source. Bilateral Current Source The bilateral current source adds signal, via the channel select switch, to the line without affecting any signals already on the line. The bilateral current source also extracts the listen signal from the line and sends it to the headphone amplifier via the volume control. Some of the user's own voice signal (sidetone) is also fed to the headphone amplifier. Channel Selector Switch The channel selector switch selects the channel on which the user will talk and listen. The headphone amplifier output drives the user's headphones. User Station Voltage Regulator The user station voltage regulator takes power from channel 1, regardless of the channel selector switch setting (exception: local power option units). The regulator not only supplies regulated power to the user station, but also prevents unwanted interaction between the user station and the intercom line which is supplying the power. Because the regulator takes power from channel 1, channel 2 can be expanded into many channels by using a switch and, for each channel, a separate wire and a termination network consisting of a 200 ohm resistor and a 10 microfarad capacitor in series.