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 jumper-selectable, 600 ohm
or 10k ohm, high impedance balanced input
that is adjustable from a range of -30 dBm to
10 dBm.
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
-15 dBm to 10 dBm. The output level is adjust-
able through the front panel. Each transmit am-
plifier is also a mixer which allows the mixing
of any one or all four receive channels mixed
into one transmit line amplifier.
7
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: 1-800-752-7560 Ext
291 for further informaion;
