Manual

Model 45A User Guide Issue 3, August 2015
Studio Technologies, Inc. Page 19
“loops” of single-channel beltpacks, such
as units from Clear-Com, need to be con-
nected. In this case the intercom circuit’s
DC power will not be connected to the
Model 45A, but only to the user devices. In
this scenario, a DC voltage won’t be pres-
ent on pin 2 or pin 3. Another example of
this might be where adapter boxes with
DC blocking are used to interconnect in-
tercom circuits with the Model 45A. In both
these cases a DC voltage won’t be pres-
ent on either pin 2 or 3 and the Model 45A
will automatically apply 200 ohm termina-
tions. These terminations would be in
error, resulting in incorrect audio levels
due to a double-termination condition.
To prevent this problem the auto terminate
function can be disabled. When configura-
tion DIP switch 4 is in its off (down) posi-
tion the auto terminate function is active.
When switch 4 is in its on (up) position the
auto terminate function associated with
each interface is disabled.
The auto terminate disable function should
be used 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 of the Model
45A’s 2-wire (PL) interface. If they are not
connected, it’s likely that audio oscilla-
tions, noise, and distortion will be gener-
ated in the Model 45A’s 2-wire-to-4-wire
converter circuitry. These audio artifacts
will not cause any damage but will be
sent out of the 4-wire line outputs. Users
of the 4-wire equipment may be less than
pleased with what they hear!
4-Wire Mute during Auto Null
As part of the auto nulling process audio
tones are generated and sent out the
2-wire intercom interfaces. These tones,
depending on the nulled state of the
hybrid circuits, will also appear on the
outputs of the 4-wire interface circuits. To
prevent these unwanted tones from being
sent to the inputs of the connected 4-wire
devices by default the 4-wire outputs mute
when the auto nulling process is active.
There may be cases, primarily at the fac-
tory, where it’s desirable to have the null-
ing tones appear on the 4-wire outputs.
Configuration DIP switch 5 allows the mut-
ing of the 4-wire outputs to be disabled.
When switch 5 is in its off (down) position
the Model 45A operates in its standard
fashion. When switch 5 is in its on (up)
position the 4-wire outputs will not mute
during the auto nulling process. As ex-
pected, when the Model 45A is deployed
in the fi
eld switch 5 should remain in its off
(down) position. However, no damage to
the Model 45A or connected equipment
will occur when muting during auto null is
disabled.
Figure 10. 4-wire mute during auto null switch
settings