Studio Technologies 770 - Manual (Page 19)

Model 770 User Guide Issue 1, October 1997
Studio Technologies, Inc. Page 19
Model 770
mating connector (plug) so that tip is
connected to the desired channel of the
intercom system, and sleeve is connected
to intercom system common; leave the
ring unterminated.
It is common in smaller broadcast applica-
tion to use the RTS/Telex PS15 Power
Supply. The PS15 provides capacitor-
coupled access, by means of a ¼-inch
3-conductor jack, to the two intercom
buses. To connect the Model 770’s inter-
com link output to the PS15 is incredibly
simple. Prepare an interconnecting cable
using two ¼-inch 3-conductor plugs that
are wired in this fashion: Sleeve connected
to sleeve, tip on the Model 770 end to tip
on the PS15 end; leave the ring leads
unconnected. This cable will connect IFB
audio into channel 1 of the intercom sys-
tem. To connect IFB audio into channel 2
of the intercom system: Tip on the Model
770 end to ring on the PS15 end.
In applications where a PS15 isn’t used,
the intercom link output can be directly
connected to the bus of an intercom
system. For example, the XLR connectors
associated with RTS/Telex systems use
pin 1 for common, pin 2 for DC with chan-
nel 1 audio, and pin 3 for channel 2 audio.
In this scenario, an interconnecting cable
would have the tip lead of the ¼-inch plug
connected to XLR pin 2 or 3 (depending
on the channel desired), and sleeve to
XLR pin 1.
While the intercom link output was de-
signed to allow a direct connection to an
intercom bus or power supply, there are
other applications where it can be useful.
The secret is to change the intercom link
output’s current signal into a voltage. But
be warned, a very sophisticated method of
conversion is required...Not! All you need
is a single 200 ohm resistor connected
across the tip and sleeve of the mating
plug! This resistor terminates the current
output into its expected load. Using Ohm’s
Law you know that E(voltage) = I(current)
R(resistance), so a voltage is produced
across the resistor. You end up with a nice
unbalanced audio source with a nominal
level of –18dBu.
This unbalanced output can be directly
connected to the program input on an
intercom power supply, such as the RTS/
Telex PS31. It’s actually better to connect
to an intercom system in this manner,
rather than directly to the intercom bus, as
the PS31 has a program input level control
which would allow the level of the IFB
signal to be adjusted for user preference.
Don’t minimize the usefulness of convert-
ing the intercom link output to an unbal-
anced IFB audio source. You can use it to
connect to an intercom system, or to other
gear that needs an IFB “feed.” The impor-
tant parameter is that it has a nominal level
of –18dBu, rather than +4dBu as found
with the Model 770’s other audio outputs.
While some equipment is specified to
have an input compatible with “0,” “+4,”
or “line-level” audio signals, they are really
not designed to be connected to signals
this “hot.” As an example, take the MS-222
Master Intercom Station from Clear-Com.
It has a program input that is specified for
microphone or line-level, with a rated input
level of 0dB in the line position. But try to
connect a signal with a nominal level of
0dBu and see what happens. It’s so loud
you’d think you were blowing up the input
stage! But connect our nominal –18dBu
signal and all is well. (By the way, we have
no “beef” with Clear-Com in general. They
make many fine products.)