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TRANSMIT SIGNAL PATH
The transmit signal path begins at the talkback microphone. The intercom interface
takes the line level signal at U1705A and sends it to the intercom sub-system via JP1704
and JP1803.
Analog switch U1801A passes the transmit audio when the INTERCOM button has been
pressed. U1802A buffers the analog switch output and U1802B operates as a bi-
directional current source that drives the intercom line via isolation transformer T1801.
RECEIVE SIGNAL PATH
Audio signals from the common party line appear on pin 3 of the XLR connector, pass
through isolation transformer T1801, and on to one input of diffamp U1803B. As far as
signals coming into the interface are concerned, the non-inverting input of U1803B is all
that matters. This audio passes on to analog switches U1801B (ignore), and U1801C (listen
when called).
The output of the analog switches drives U1803A, connected as an active variable gain
amplifier. The stage gain is variable from off to unity to 16dB. U1803A then feeds a mono
input into the headphone amplifier.
The input to the non-inverting side of the diffamp, U1803B, is also common to the output
of the bi-directional current source used to drive the line. Since the current source has a
high output impedance, it presents a minimal load to signals coming from the line.
Another input to the diffamp’s inverting input comes from the current source op-amp,
but prior to the current sensing resistor, R1807. The two signals are subtracted from each
other in the diffamp, and this partially cancels the audio signal (sidetone) that would
otherwise appear at U1803B during transmit times. Without sidetone cancellation, the
talker’s own voice would be very loud in their own headset.
CALL SIGNAL
When the outside world calls the SR40·8’s intercom station, the DC call signal travels
through T1801’s primary, turning on darlington pair Q1802 and Q1803, which turns on
opto-isolator U18. The intercom line powers this circuitry. The lowpass filter formed by
R1821 and C1807 and the filtering action of C1806 prevents the call system from being
activated by audio signals on the line. Opto-isolator U18 isolates the intercom system
ground from the SR40·8’s ground system for call signals.
On the revision 6 output PCB, the output of U18 drives the call lamp (physically located in
the INTERCOM switch) via Q1805 and simultaneously turning off the LED located in the
talkback switch, SW1 via Q1806. U18 also drives the listen-when-called switch U1801C.
From revision A on, Q1806 was replaced by a comparator, U1804D. U1804C drives the
lamp kill feature via JP1804.
When the SR40·8 originates a call signal, current source Q1801 supplies about 3 milliamps
to the intercom line whenever CALL switch, SW1801 is pressed.
