Studio Technologies 47A - Manual (Page 18)

Issue 1, September 2014 Model 47A User Guide
Page 18 Studio Technologies, Inc.
pins 2 or 3. This function is directly com-
patible with RTS TW-Series intercom ap-
plications in which the three conductors of
a cable support both DC power and two
channels of audio. It’s also compatible in
situations where all three conductors of a
Clear-Com single-channel intercom circuit
are connected to the Model 47A. In this lat-
ter case only one of the Model 47A’s audio
channels will be used.
There may be situations where it’s neces-
sary for the two channels associated with
each Model 47A interface be used with
separate 2-wire party-line circuits. An ex-
ample of this might be where two “loops”
of single-channel beltpacks, such as units
from Clear-Com, need to be connected.
Adapter cables as shown in Figure 2 would
be used. In this case the intercom circuit’s
DC power will not be connected to the
Model 47A, but only to the user devices. In
this scenario, a DC voltage won’t be pres-
ent on pins 2 or 3. Another example of this
might be where adapter boxes with DC
blocking are used to interconnect intercom
circuits with the Model 47A. In both these
cases a DC voltage won’t be present on
either pins 2 or 3 and the Model 47A will
automatically apply 200 ohm terminations.
In both these cases the termination will be
in error and will result in incorrect audio lev-
els due a “double-termination” condition.
To prevent this problem the auto termi-
nate function can be disabled. Referring
to Figure 7, when DIP switch 1 is in its off
(down) position the auto terminate func-
tion is active. When DIP switch 1 is in its on
(up) position the auto terminate function is
disabled. Repeating for clarity, the auto ter-
minate function applies to both interfaces
1 and 2. With the auto terminate function
disabled the two interfaces, when set for
external power, will operate normally with
the exception that pins 2 and 3 will never
be terminated by the Model 47A’s circuitry.
The auto terminate function should be
disabled only when absolutely necessary;
it’s possible that a significant downside
could be experienced. With auto terminate
disabled it’s important that properly ter-
minated 2-wire party-line intercom circuits
be connected to pins 2 or 3 on the Model
47A’s 2-wire PL connectors. If they are not
connected, it’s likely that audio oscillations,
noise, and distortion will be generated in
the Model 47A’s 2-wire-to-4-wire converter
circuitry. These audio artifacts will not
cause any damage, but will be sent out the
4-wire line output connectors. Users of the
4-wire equipment may be less than pleased
with what they hear!
Figure 7. DIP Switch 1—Auto Terminate
Figure 6. Detail of back panel showing the
advanced mode DIP switches