6. Theory of Operation
6.1 General
The 1505 module uses a low loss, wide band transformer for each channel to interface to the phone line. Gain elements are low noise, low distortion, high gain operational amplifiers for quality performance. Each channel utilizes one half of dual compandor IC, configured as a compressor/limiter, to amplify and limit the maximum receive input signal, in order to maintain operator adjusted system levels.
Each receive amplifier output is routed through a user programmed switch matrix, to summing amplifiers. The output of the summing amplifi- ers are transmit line amplifiers, and a low loss, wide band transformer for each channel to in- terface to the respective phone line.
A handset/microphone amplifier circuit is in- cluded, with provision for jumper selection of carbon, electret, or dynamic microphone ele- ments. Also included is a compressor/limiter circuit to maintain system levels. The output of the handset/microphone amplifier circuit is routed to the switch matrix.
An on board voltage regulator maintains proper voltages for optimum performance.
6.2 Receive Amplifier
This amplifier is a
Each receive amplifier has an unbalanced input with an input level of 0.025 Vac to 2.5 Vac each. The amplifier output is adjustable and is accessed through the front panel. The output level for each receive amplifier is normally setup for a level of 0.776 Vrms at the test point. For example, channel one’s test point is TP3. Operation is the same for all four receive inputs.
6.3 Compression Limiter Circuit
This circuit amplifies and limits the output of the receive inputs ensuring uniform audio lev- els from all channels regardless of the input level received from the external line input. For a 30 dB gain, once achieving the compression level, only a 3 dB increase in output level, without distortion, results in the circuit. Normally the input level, at 0 dB, allows the limiter to amplify in a linear fashion below that level, and above the 0.776 will limit the output to no more than a 3 dB rise above that point. A jumper allows the bypass of the compressor circuit. The compressor circuit is the same for all four receive channel circuits. Each receive input can be muted by an external switched low input from the rear panel. A front panel LED mute indicator alerts the operator that the channel is muted. The output from each receive channel is bussed through various switches and is switch selectable by the user to have the au- dio appear at any one of four line outputs or a speaker output.
6.4 Transmit Line Amplifier
This bridging amplifier drives a 600 ohm bal- anced line with an output level adjustable from
NOTE
The speaker amplifier circuit is no longer useable, the IC required to drive this cir- cuit has been obsolete by the manufacture and there is no substitute being manufac- tured at this time. Should you have any quiestions regarding the speaker amplifier circuit please call:
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