Issue 1, September 2014
Model 46A User Guide
Page 6
Studio Technologies, Inc.
Powered circuits have a DC voltage pres-
ent, typically provided by power supplies
such as the RTS PS31 or Clear-Com PS-
232. This DC power, normally 30-32 volts,
provides energy for connected devices
such as user stations or beltpacks. In this
type of application the Model 46A is config-
ured to operate in its external 2-wire power
source mode. In this mode the Model
46A’s circuitry maintains the required
high-impedance load and, as in all cases,
draws no power from the party-line circuit.
An auto-terminate function ensures that
should a “wet” circuit not be connected,
the Model 46A’s interface circuitry will
remain stable. This unique feature makes
certain that objectionable audio signals,
including oscillations and “squeals,” will
infrequently be sent to the connected
4-wire device.
A significant capability of the Model 46A’s
2-wire interfaces is their ability to supply
DC power and 200 ohm AC termination
to the connected intercom circuits. Re-
ferred to as the Model 46A’s internal 2-wire
power source mode, the 30 volt output can
power devices such as user stations and
beltpacks. In many applications this will
eliminate the need for external intercom
power supplies. Besides reducing total
system cost, this feature can also lower
system weight, reduce required mounting
space, and decrease the mains energy
requirement.
With each 2-wire interface’s ability to sup-
ply up to 315 milliamperes (mA) of current,
a typical broadcast application that uses
two sets of three BP-325 beltpacks can
easily be supported. Each interface’s 30
volt DC intercom power source ensures
that systems requiring long intercom cable
runs will function correctly. The circuitry’s
output regulation is such that little change
in the output voltage will occur over its en-
tire rated output current. Also, the unique
design of the power supply circuitry virtu-
ally eliminates the noise and “hiss” associ-
ated with typical intercom power supply
performance. Under software control the
output of the DC power supplies are moni-
tored for over-current and short-circuit
conditions. This allows protection shut-
down of the output DC, as well as provid-
ing an alert by way of a visual indicator.
Analog Hybrids with Auto
Nulling
A key reason that the Model 46A’s inter-
faces achieve excellent audio performance
is the design of the 2-wire-to-4-wire hybrid
circuits. They provide low noise and dis-
tortion, good frequency response, and
high return-loss (“nulling”), even when
presented with a wide range of 2-wire
conditions. Unlike telephone-line (“POTS”)
oriented DSP-based hybrid circuits, the
Model 46A’s analog circuitry provides ex-
tended frequency response. With a pass
band of 100 Hz on the low end and 8 kHz
on the high end, natural-sounding voice
signals can be sent to, and received from,
the 2-wire party-line circuits.
A hybrid’s ability to isolate the transmit
signal from the receive signal in the 2-wire-
to-4-wire interface is critical. The quality of
this isolation, technically known as return-
loss or trans-hybrid loss, is measured in
dB. A high value is important, especially
in applications where multiple 2-wire-to-
4-wire interfaces are used together. Re-
mote sports broadcast applications are
especially sensitive to this requirement.
The Model 46A’s sophisticated auto null-
ing function uses analog circuitry under
