Issue 1, September 2014
Model 46A User Guide
Page 22
Studio Technologies, Inc.
DC on pin 2 will cause the status LED la-
beled pin 2 to light. A level greater than 18
volts DC on pin 3 will cause the pin 3 status
LED to light.
The author is aware that user intercom de-
vices almost always draw power from pin 2
rather than pin 3. However, in many broad-
cast applications, power is provided on
all intercom paths so that flexible channel
assignments can be made. In the external
2-wire power mode the Model 46A is sim-
ply an observer; whether intercom power is
present on pin 2, pin 3, or both pins 2 and
3 is not significant.
So that the Model 46A’s 2-wire-to-4-wire
interface circuits remain stable and don’t
generate audio artifacts, an auto terminate
feature is implemented. This maintains a
200 ohm termination on both pins 2 and 3
so long as neither of the 2-wire power sta-
tus LEDs is lit. To clarify, if either, or both,
the LEDs are lit then both terminations are
removed. It was felt that this method would
provide a fairly accurate indication that an
active intercom circuit is connected. In this
case the 200 ohm termination is expected
to be provided elsewhere, typically as
part of the external power supply, and the
Model 46A’s termination is “lifted.”
Internal Power Source
When an interface’s front-panel DIP switch
is set for internal 2-wire power the Model
46A’s 2-wire party-line interface supplies
30 volts DC on pin 2 of the output connec-
tor. A maximum current draw of 315 mA is
available. This current is sufficient to power
various intercom user devices such as
small user stations and beltpacks. A com-
mon broadcast application would be to use
RTS BP-325 beltpacks. Select the con-
nected devices so that their total current
doesn’t exceed 315 mA. That’s not always
the easiest figure to calculate but a web
search will generally find specifications for
all commonly used devices. For example, a
search finds that the original version of the
BP-325 consumes a maximum of 85 mA of
current. According to this figure up to three
of these units can be connected to each
Model 46A interface. Newer versions of
the BP-325 use surface mount component
technology and have a lower maximum
current draw of 65 mA. Four of these “mod-
ern” BP-325 units can easily be supported.
The Model 46A’s intercom power supply
circuits operate under software control.
This allows detection of fault conditions
and protection of both the Model 46A’s
circuitry and connected intercom user de-
vices. Upon initial Model 46A power up no
monitoring of the intercom power output
takes place for a period of three seconds.
This allows the Model 46A’s circuitry and
the connected intercom user devices to
stabilize. The LED associated with pin 2 will
light to indicate that the output is active.
After this initial 3-second period monitoring
becomes active. A fault condition is de-
tected if the power on pin 2 falls below 24
volts for a continuous one second inter-
val. The hardware and software responds
to this condition by turning off the power
source to pin 2 and flashing the pin 2 LED
as a warning. After a 6-second “cool-down”
interval the output returns to the same con-
dition as upon initial power up: power is
again applied to pin 2, the pin 2 status LED
will light, and monitoring won’t begin for
another three seconds. A full short-circuit
condition applied to the Model 46A’s 2-wire
connectors will result in a continuous cycle
of 4-seconds on (three seconds for start
up and one second for detection) and
6-seconds off.
