Studio Technologies 45A Specifications

The Model 45A is designed to interface a 2-wire full-duplex
party-line intercom circuit with a 4-wire audio circuit associated
with a matrix intercom system. Other specialized audio system
interfacing applications can also be supported. Applications
for the Model 45A include television sports and special event
broadcasting, theme park and theater installations, corporate
events, and industrial testing environments.

Key Features:

•

Portable “throw-down” or rack-mount use

• Analog hybrids with auto null capability

• Input and output level metering

• Excellent audio quality

• Flexible powering

• Standard audio connectors

• Transformer-coupled 4-wire inputs and outputs

• Integrated 2-wire intercom power source

Overview

The Model 45A provides a full-featured 2-channel interface which
includes two hybrid circuits with automatic nulling capability.
The analog hybrid circuitry provides excellent audio quality and
high return-loss. For flexibility, the Model 45A can be powered
by the connected 2-wire party-line circuit or by means of an
external source of 24 volts DC. When powered by an external
source the Model 45A is capable of supplying DC power to the
2-wire intercom circuit, allowing direct operation of devices such
as user belt packs. Audio level meters provide confirmation of
system performance during setup and operation. Standard audio
connectors are used for all input and output signals. The Model
45A’s enclosure has a “1/2-rack” 1U form factor and weighs
less than two pounds, making it well suited for use in portable
applications. Alternately, using one of the optional rack-mount
front panels, one or two Model 45A units can be mounted in a
single space of a standard 19-inch rack enclosure or one unit
can be mounted in a special 10.5-inch rack.

2-Wire Interface

The Model 45A’s 2-wire interface is optimized for direct con­nection with dual-channel party-line (PL) intercom circuits such
as the TW-series of intercoms from RTS®. In addition, other
industry-standard single- and dual-channel PL intercom circuits,
including those from Clear-Com®, are compatible.

The Model 45A’s 2-wire interface can correctly function with
powered (“wet”) or unpowered (“dry”) intercom circuits. An
auto-terminate function ensures that should a “wet” circuit not
be connected, the Model 45A’s interface circuitry will remain
stable. This unique feature makes certain that objectionable
audio signals, including oscillations and “squeals,” won’t be
sent to the connected 4-wire device.

A significant capability of the Model 45A’s 2-wire interface is
its ability to supply DC power and 200 ohm AC terminations to
“create” an intercom circuit. The 30 volt output can power a lim

­ited number of devices such as user stations and belt packs. With
up to 300 milliamperes of current available, a typical broadcast
application that uses two or three BP325 belt packs can easily
be supported. In many applications this can eliminate the need
for an external intercom power supply, reducing total system
cost, weight, and required mounting space.

Analog Hybrids with Auto Nulling

The 2-wire-to-4-wire hybrids provides low noise and distortion,
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 45A’s analog circuitry maintains extended 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, a 2-wire party-line circuit.

The Model 45A’s sophisticated auto nulling function uses analog
circuitry under microprocessor control to achieve significant
trans-hybrid loss. This return-loss “null” is achieved by making a
series of software-directed adjustments to account for the resis
tive, inductive, and capacitive conditions that are present on the
connected 2-wire party-line circuit. Whenever a user presses the
Model 45A’s auto null button digital circuitry adjusts the analog
hybrids to achieve their maximum return-loss for both interface
channels in less than 15 seconds. While the nulling process is
automatic, it only takes place upon user request. The resulting
null parameters are stored in non-volatile memory.

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50219-0411, Issue 1

4-Wire Interfaces

Associated with the 4-wire portion of the Model
45A’s interface are analog line-level inputs and
outputs. These are intended to interconnect with a
variety of 4-wire devices, including matrix intercom
systems, audio-over-fiber transmission systems,

Model 45A Back Panel

and other specialized audio equipment. The input
and output circuitry is transformer-coupled to minimize the
chance that hum, noise, or “ground-loop” issues will occur.
The +4 dBu nominal level allows standard audio signals to be
directly connected.

The Model 45A contains four 5-segment LED level meters. Two
meters display the level of the signals being received from
the 4-wire sources and two display the level being sent to the
4-wire outputs. At the time of installation and setup the meters
are invaluable in helping to confirm correct operation. During
normal operation the meters offer rapid confirmation of audio
signal flow in and out of the unit. Additional LED indicators are
also provided, offering a status indication of the party-line DC
power and auto null functions. Two other LEDs offer a direct
indication of what source is powering the Model 45A.

Pro Audio Quality

The Model 45A’s audio circuitry was designed in the spirit of
professional audio equipment rather than that found in typical
party-line intercom gear. High-performance components are
used throughout, providing low-distortion, low-noise, and
high headroom. Using passive and active filters the frequency
response is limited to nominally 100 Hz to 8 kHz. This range was
selected to provide excellent performance for human speech
while maximizing the ability of the hybrid circuits to create
substantial “nulls.” Moreover, the Model 45A’s 2-wire DC power

source offers a unique level of performance; its ability to deliver
power while maintaining audio quality is simply unmatched.

Performance Flexibility

While the Model 45A is designed to directly integrate into typical
applications, it’s ready to support those “one-in-a-million” situa
tions too. To accomplish this DIP switches, located on the front
panel, allow selected features to be revised as required.

Simple Installation

The Model 45A uses standard 3-pin XLR-type connectors to
allow convenient interconnection. For flexibility, access to the
2-wire party-line intercom interfaces can be made using the
connectors provided on both the front and back panels. The
Model 45A is housed in a rugged yet lightweight aluminum
enclosure that is designed to be “road tough.” The Model 45A
can be used as a standalone portable unit for “throw-down”
applications. Three rack-mount options are also available
allowing one or two units to be mounted in one space (1U) of
a standard 19-inch rack enclosure or one unit to be mounted in
a special 10.5-inch rack.

The Model 45A can operate from power provided by the
connected 2-wire party-line circuit or from the included external
24 volt DC power supply. Its mains input source can range from
100 to 240 volts, 50/60 Hz allowing operation virtually anywhere
in the world.

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Specifications

General Audio:

Frequency Response: ±2.5 dB, 100 Hz to 8 kHz
Distortion (THD+N): <0.2%, measured at
1 kHz, 4-wire input to 2-wire interface pin 2
Signal-to-Noise Ratio: >55 dB, measured at
1 kHz, 4-wire input to 2-wire interface pin 2

2-Wire Party-Line (PL) Intercom Interface:

Type: 2-channel party-line, unbalanced (pin 1
common; pin 2 DC with channel 1 audio; pin 3
channel 2 audio)
Compatibility: single- and dual-channel intercom
systems such as from RTS® and Clear-Com®
Impedance – Normal: >10 k ohms
Impedance – 2-Wire (PL) Power Source Mode:
200 ohms
Nominal Level: –10 dBu
“Mic Kill” Signal: square wave, 24 kHz, ±1%
2-Wire Power Source: 30 volts DC nominal,
300 milliamperes maximum

Hybrids: 2

Topology: 3-section analog circuitry compen­sates for resistive, inductive, and capacitive
2-wire party-line loads
Nulling Method: automatic upon user initiation,
processor implements digital control of analog
circuitry; settings stored in non-volatile memory
Nulling Line Impedance Range: 120 to 350 ohms
Nulling Cable Length Range: 0 to 3500 feet
Trans-Hybrid Loss: >40 dB, typical at 800 Hz

Studio Technologies, Inc. Model 45A 2-Wire to 4-Wire Interface, Issue 1, Page 2

4-Wire Inputs: 2

Type: transformer-coupled, capacitor isolated
Impedance: 13 k ohms
Nominal Level: +4 dBu
Maximum Level: +22 dBu

4-Wire Outputs: 2

Type: transformer-coupled, capacitor isolated
Impedance: 50 ohms nominal
Nominal Level: +4 dBu
Maximum Level: +20 dBu into 2 k ohms

Meters: 4

Function: displays level of 4-wire inputs and
outputs
Type: 5-segment LED, modified VU ballistics

Connectors:

4-Wire Line Inputs: 3-pin female XLR-type
4-Wire Line Outputs: 3-pin male XLR-type
2-Wire (PL) Interface: 3-pin male and female
XLR-type
External 24 Volt DC Input: coaxial power jack,

2.1 x 5.5 mm, locking bushing, compatible with
Switchcraft® S760K plug

Power Requirements:

External: 24 volts DC nominal, acceptable range
20 to 28, maximum required current 750 milliam
peres @ 20 volts. Universal mains input/24 volt
DC power supply shipped with each unit.
2-Wire Party-Line (when serving as source):
24 to 32 volts DC, 175 milliamperes

Dimensions – Portable “Throw-Down”
Version (Overall):

8.7 inches wide (22.1 cm)

1.72 inches high (4.4 cm)

8.3 inches deep (21.1 cm)

Mounting Options:

(M45ARM-1), dual-unit (M45ARM-2),
or “1/2-rack” (M45ARM-3) rack-mount front
panels; single-unit and dual-unit panels use
one space (1U) in a standard 19-inch rack,
“1/2-rack” panel uses one space (1U) in a
special 10.5-inch rack

Weight: 1.8 pounds (0.82 kg), rack-mount front
panel adds 0.2 pounds (0.09 kg)

Specifications subject to change without notice.

single-unit

Studio Technologies, Inc.

Skokie, Illinois USA

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© by Studio Technologies, Inc., April 2011

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