Studio Technologies 46A User manual

Copyright © 2014 by Studio Technologies, Inc., all rights reserved
www.studio-tech.com
50115-0914, Issue 1
User Guide
Issue 1, September 2014
This User Guide is applicable for serial numbers:
M46A-01151 and later

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Model 46A User Guide Issue 1, September 2014
Studio Technologies, Inc. Page 3
Table of Contents
Introduction ................................................................... 5
Installation ..................................................................... 10
Configuration ................................................................ 16
Advanced Configuration ............................................... 17
Operation ...................................................................... 20
Advanced Operation ..................................................... 24
Technical Notes ............................................................. 25
Specifications ................................................................ 27
Appendix A—Interfacing with RTS® Matrix
Intercom Systems ......................................................... 28
Appendix B—Interfacing with Riedel® Artist™
Matrix Intercom Systems .............................................. 29
Appendix C—Interfacing with Clear-Com®
Matrix Intercom Systems .............................................. 30
Block Diagram

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Model 46A User Guide Issue 1, September 2014
Studio Technologies, Inc. Page 5
Introduction
The Model 46A is designed to interface
2-wire full-duplex party-line (PL) intercom
circuits with 4-wire audio circuits associ-
ated with matrix intercom systems. Other
specialized audio system interfacing appli-
cations can also be supported. The Model
46A provides two independent full-fea-
tured 2-channel interfaces. Each interface
contains two hybrid circuits which include
automatic nulling capability. The analog
circuitry, under software control, provides
excellent audio quality and high return-loss.
The interfaces are compatible with powered
and unpowered 2-wire party-line circuits.
They are capable of supplying DC power,
allowing direct operation of devices such as
user beltpacks. Configuration settings al-
low the 2-wire and 4-wire interface circuitry
to be compatible with a range of nominal
signal levels. Audio level meters provide
user confirmation of system performance
during setup and operation. Applications for
the Model 46A include television sports and
special event broadcasting, theme park and
theater installations, corporate events, and
industrial testing environments. Standard
audio connectors are used for all input and
output signals. The Model 46A mounts in
Model 46A Back Panel
Model 46A Front Panel
one space of a standard 19-inch rack
enclosure and requires 100-240 volts,
50/60 Hz for operation.
2-Wire Interfaces
The Model 46A’s 2-wire interfaces are
optimized for direct connection with
2-channel party-line intercom circuits.
In addition, single-channel party-line
intercom circuits can also be connected.
Many broadcast applications use the
2-channel TW-Series from RTS®, includ-
ing their popular BP-325 beltpack. Other
industry-standard single- and 2-channel
PL intercom systems, including those from
Clear-Com®, are also directly compatible.
To provide optimal signal matching and
interface performance the nominal level of
the Model 46A’s 2-wire interfaces can be
selected. Using DIP switches, accessible
on the front panel, the nominal levels can
be selected to be either –10 or –14 dBu,
settings that should allow compatibility
with virtually all party-line systems. For ex-
ample, for applications that use equipment
from RTS the –10 dBu setting is appropri-
ate. For other equipment, such as that
from Clear-Com, the –14 setting is correct.
The Model 46A’s 2-wire interfaces can
correctly function with powered (“wet”)
or unpowered (“dry”) intercom circuits.

Issue 1, September 2014 Model 46A User Guide
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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

Model 46A User Guide Issue 1, September 2014
Studio Technologies, Inc. Page 7
microprocessor control to achieve sig-
nificant trans-hybrid loss. This return-loss
“null” is achieved by making a series of
adjustments to account for the resistive,
inductive, and capacitive conditions that
are present on the connected 2-wire party-
line circuit. The party-line’s conditions are
the sum of the impact made by the type
and quantity of cable, the connected user-
devices, and the intercom power source.
Whenever a user presses one of the
Model 46A’s auto null buttons digital
circuitry adjusts the analog hybrids to
rapidly achieve their maximum return-loss.
The nulling process takes less than 10
seconds for each channel of an interface.
However, it’s important to note that while
the nulling process is automatic, it only
takes place upon user request. This can
lead to more stable and consistent audio
performance when a hybrid is exposed to
the varying 2-wire conditions often found
in broadcast applications. The param-
eters obtained during the nulling process
are stored in non-volatile memory; mains
power interruptions won’t require the auto
nulling function to again be performed.
A sine-wave audio tone is generated for
use during the auto nulling process. The
frequency is software-controlled to maxi-
mize the ability of the hybrid circuits to
reach a “deep” null. In addition, at the
beginning of each auto null sequence a
short period of 24 kHz tone is sent to the
associated 2-wire party-line interface. This
serves as a microphone disable (“mic kill”)
signal for user devices such as the RTS
BP-325. By automatically disabling “open”
microphones the auto nulling process can
achieve better performance.
4-Wire Interfaces
Associated with the 4-wire portion of the
Model 46A’s interfaces 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, and other
specialized audio equipment. The input
and output circuitry is transformer-coupled
to minimize the chance of hum, noise, or
ground “loop” issues. A key characteristic
of the Model 46A’s design is the ability to
select the 4-wire input and output nominal
levels. This helps to ensure compatibility
with virtually all audio equipment. Front-
panel-accessible DIP switches allow the
nominal levels to be configured from among
four choices: 0, +4, +6, or +8 dBu. The
0 dBu setting was specifically provided for
compatibility with Clear-Com matrix inter-
com systems. The +4 dBu setting allows
“standard” audio signals to be directly
connected. Digital matrix intercom systems
from Riedel® can be effectively connected
using the +6 dBu setting. And the +8 dBu
setting allows proper level matching with the
popular ADAM™-Series of matrix intercom
systems from RTS. This setting also applies
to the related RVON-I/O VoIP product.
The Model 46A contains eight 5-segment
LED level meters. Four of the meters are
provided for each interface, with two dis-
playing the level of the signal being received
from the 4-wire source and two displaying
the level being sent to the 4-wire output.
During installation and setup the meters
are invaluable in helping to confirm that the
nominal level DIP switch settings have been
properly made. During normal operation
the meters offer rapid confirmation of audio
signal flow in and out of the unit. Additional
LED status indicators are also provided,

Issue 1, September 2014 Model 46A User Guide
Page 8 Studio Technologies, Inc.
offering a clear view of the 2-wire DC power
and auto null functions.
Pro Audio Quality
The Model 46A’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 approximately 100 Hz to 8 kHz.
This range was selected to provide excel-
lent performance for human speech, while
maximizing the ability of the hybrids to
create substantial “nulls.” When the Model
46A’s internal DC power sources are se-
lected to provide 2-wire party-line intercom
power, enhanced audio performance can
also be expected. The quality of the DC
supply circuits is excellent, with very little
noise, hum, or “hiss” being added to the
2-wire connections. In addition, the imped-
ance characteristics of each interface’s DC
powered (“wet”) channels was tailored to
be essentially identical to that of the un-
powered (“dry”) channel. This unique situa-
tion allows the automatic nulling circuitry
to provide excellent, consistent results
for both the powered and unpowered
channels.
Attention to detail is a hallmark of the
Model 46A’s design. For example, during
the brief auto nulling process an interface
channel’s 4-wire input and output signal is
muted, preventing unwanted audio from
reaching the connected equipment. Asso-
ciated with the 2-wire interfaces is circuitry
that, under software control, applies 200
ohm terminating impedances to the 2-wire
party-line circuits. This, along with other
circuitry that monitors DC voltages pres-
ent on the 2-wire circuits, ensures that
audio instability associated with untermi-
nated circuits will rarely occur. As previ-
ously mentioned the Model 46A’s 2-wire
DC power sources offer a unique level of
performance. Their ability to deliver power
while maintaining audio quality is simply
unmatched.
Special Applications
While the Model 46A is designed to di-
rectly integrate into typical applications,
it’s ready to support the “one-in-a-million”
situations, too. To accomplish this DIP
switches, accessible on the back panel,
allow some of the automatic features to be
disabled. For example, one switch allows
the auto-terminate feature to be disabled.
While this feature can help maintain audio
quality, advanced users might need to dis-
able it. This would allow full control over
the Model 46A’s four hybrid circuits, en-
abling them to be used completely inde-
pendently. A second DIP switch changes
the way in which the front-panel auto null
push button switches operate. Normally
pressing one of the buttons begins the
process of auto nulling both channels
associated with an interface. The alter-
nate switch mode allows each auto null
sequence to be activated independently.
One tap (a quick press and release) of the
auto null button will begin the auto null
process for channel 1. Two taps of the
button will begin the auto null process for
channel 2. For installer-selected applica-
tions, three opto-coupled inputs allow se-
lected Model 46A features to be remotely
controlled. These features include remote
auto nulling for each interface and remote
“mic kill” activation.

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Studio Technologies, Inc. Page 9
Simple Installation
The Model 46A uses standard 3-pin XLR
connectors to allow convenient intercon-
nection in most broadcast and general
audio environments. For flexibility, access
to the 2-wire party-line intercom interfaces
can be made using the connectors pro-
vided on both the front and back panels.
In permanent installations the back-panel
connectors will typically be utilized. In other
settings, such as field television produc-
tion, front-panel-access allows intercom
beltpacks to be rapidly connected, without
requiring access to the inside of a rack
enclosure or necessitating the use of a sec-
ondary input/output (I/O) panel.
The Model 46A is housed in a rugged alu-
minum and steel enclosure that is designed
to be “road tough.” It mounts in one space
of a standard 19-inch rack enclosure. The
mains input source can range from 100
to 240 volts, 50/60 Hz allowing direct op-
eration virtually anywhere in the world.
A switch-mode power supply, contained
within the Model 46A’s enclosure, provides
conversion of the mains input power into
the DC voltage required by the intercom
power supply circuits. Also contained
within the unit is a DC-to-DC converter that
creates the voltages required by the analog
and digital circuitry. These efficient power
supplies help to ensure cool, reliable op-
eration in a variety of settings.
Design Philosophy
While the “bits and pieces” that make up
the Model 46A have been described in
conventional terms, the real strength of the
unit rests in how it integrates and performs
in the “real world.” Before beginning the
Model 46A’s design process, conversa-
tions with industry experts quickly made
it apparent that installing and configuring
existing 2-wire-to-4-wire interface units was
invariably a time-consuming, aggravating
process that required the talent of an expert
to achieve reasonable results. And even
under those constraints the resulting
audio performance was often mediocre.
Any new design had to look at the ap-
plication in a different way. This led to the
over-riding Model 46A design goal: create
a “new breed of cat,” fundamentally chang-
ing how broadcast 2-wire-to-4-wire interface
equipment fit into actual applications.
An important first step was to eliminate the
requirement that a senior technician, along
with a screwdriver, be present during every
installation. (It was universally acknowl-
edged that their time can be better spent
elsewhere!) The need to adjust trim potenti-
ometers, fabricate special cabling and con-
nector straps, use nulling earpieces, etc.,
had to be eliminated. For example, in virtu-
ally all instances, input and output levels fall
within just a few dB of their nominal values
and, as such, could be supported with a
limited number of configurable choices.
In addition, it was acknowledged that in
this application analog audio circuitry was
capable of providing excellent audio perfor-
mance, but that the required manual nulling
process was operationally taxing. By add-
ing digital control to the analog circuitry,
automatic nulling could be performed—the
best of both worlds!
The next step was to identify resources that
would improve the installation process and
make operation more reliable. This led to
the inclusion of multiple LED level meters,
allowing continuous monitoring of the input
and output signals. It also turned out that
in many applications only a small number
of user devices, such as beltpacks, are

Issue 1, September 2014 Model 46A User Guide
Page 10 Studio Technologies, Inc.
typically connected to 2-wire party-line cir-
cuits. By adding DC power sources to the
Model 46A’s 2-wire interfaces, the need
for external intercom power supplies could
often be eliminated.
The final step was to create a physical
package that would provide significant re-
sources in a form that allowed simple and
reliable integration with other equipment.
This was accomplished by including two
2-channel interfaces in a one-rack-space
enclosure. Using standard 3-pin XLR au-
dio connectors would enable rapid instal-
lation and troubleshooting. And by making
the Model 46A “universally” powered,
installation in any locale could be directly
supported.
Installation
In this section you will be installing and
interconnecting the Model 46A. The one-
rack-space unit will be mounted in an
equipment rack. Audio input, audio out-
put, and party-line intercom connections
will be made by way of 3-pin XLR connec-
tors. AC mains power will be connected
by means of a detachable cord set that is
compatible with the Model 46A’s 3-pin IEC
320 C14 inlet connector.
System Components
The shipping carton contains the Model
46A Interface and associated user guide.
Units destined for North America and
Japan also include an AC mains cord.
Your dealer or distributor should provide
an AC mains cord for other destinations.
Locating the Model 46A
The Model 46A’s mounting location will
dictate the length of the cable runs need-
ed to link the unit with the connected de-
vices. This is really only a factor in relation
to the 2-wire party-line intercom circuits.
These circuits carry unbalanced audio
which can be susceptible to interference
and crosstalk issues. Intercom circuits
also carry DC power which may make
resistive losses come into the picture. In
general, shorter intercom cable lengths
will assure more reliable and consistent
system performance. Another location
criterion is access to the Model 46A’s front
panel. An optimal location will allow con-
venient use of the auto null push buttons
and easy observation of the status and
level meter LEDs.
Mounting the Model 46A
Once a mounting location has been se-
lected installation can begin. The Model
46A requires one space (1.75 vertical
inches) in a standard 19-inch (48.3 cm)
equipment rack. Secure the unit into
the equipment rack using two mounting
screws per side.
4-Wire Audio Inputs and
Outputs
Two audio line input and two audio line
output signals are associated with the
4-wire section of each of the Model 46A’s
two interface circuits. Connections are
made using standard 3-pin male and
female XLR connectors. Refer to Appen-
dices A, B, and C for details on intercon-
necting with RTS, Riedel, and Clear-Com
matrix intercom systems.

Model 46A User Guide Issue 1, September 2014
Studio Technologies, Inc. Page 11
4-Wire Line Inputs
As previously mentioned, each of the
Model 46A’s two interfaces allows two
analog line-level audio sources to be con-
nected. The source for these signals will
typically be ports on a matrix intercom
system. It’s also possible that the signals
will come from other devices, such as a fi-
ber optic or copper-based audio transmis-
sion system. The 4-wire input circuitry is
balanced, capacitor coupled, transformer
isolated, and has an impedance of 13 k
ohms. The line inputs are compatible with
signals that have a nominal level of 0, +4,
+6, or +8 dBu. Configuration switches,
located on the Model 46A’s front panel,
allow the circuitry to be adjusted to match
the connected nominal level. The switches
work in tandem to control both the input
and output nominal levels.
Audio sources are connected to the line
inputs by way of 3-pin female XLR con-
nectors which are located on the unit’s
back panel. Refer to Figure 1 for a detailed
view. Note that pin 1 of these connectors
is internally connected to the Model 46A’s
earth/chassis ground point.
Prepare the mating connectors (males)
so that pin 2 is signal high (+ or hot),
pin 3 is low (– or cold), and pin 1 is shield.
If connecting a source in this manner results
in hum or noise it’s possible that removing
the shield connection from pin 1 can elimi-
nate the issue; “floating” pin 1 will remove
a potential ground current path from the
Model 46A’s earth/chassis ground point
through the shield of a cable. Also, if a hum
or noise issue does arise be certain to con-
firm that, unless absolutely necessary, the
mating connector’s “shell” isn’t connected
to the cables’ shield or pin 1. Termination of
this “fourth” pin of a 3-pin XLR connector is
often the cause of seemingly inexplicable
noise issues.
With an unbalanced source connect pin 2
to signal high (+ or hot) and both pins 1
and 3 to shield. If connecting an unbalanced
source in this manner results in hum or
noise, connect pin 2 to high (+ or hot) and
pin 3 to shield; leave pin 1 unterminated.
4-Wire Line Outputs
Each of the Model 46A’s two interfaces
provides two analog line-level audio outputs.
These outputs are intended to be connected
to inputs on the devices associated with the
4-wire audio signals. The outputs are ca-
pacitor coupled, transformer balanced with
a nominal level of 0, +4, +6, or +8 dBu. As
previously mentioned, the 4-wire nominal
level configuration switches, located on the
Figure 1. Detail of back panel showing line inputs and outputs

Issue 1, September 2014 Model 46A User Guide
Page 12 Studio Technologies, Inc.
Model 46A’s front panel, control the level
of both the line outputs and the line inputs.
The 4-wire line outputs are capable of driv-
ing inputs that have impedances as low as
600 ohms, however connecting to loads
of 2 k ohms or greater is preferred. The
line outputs are connected by way of 3-pin
male XLR connectors which are located
on the Model 46A’s back panel. Refer to
Figure 1 for a detailed view.
The mating connectors (females) should
be prepared so that signal high (+ or hot)
is expected on pin 2. Signal low (– or cold)
should be expected on pin 3. The cables’
shields can be connected to pin 1. How-
ever, in order to minimize the chance that
ground-interaction problems will arise, pin
1 on each of the line output connectors is
isolated from all Model 46A circuitry and
ground points. “Floating” pin 1 virtually
eliminates the chance that a “ground-loop”
problem will occur.
2-Wire Party-Line Intercom
Connections
The Model 46A’s 2-wire party-line (PL)
intercom interfaces are designed to directly
connect with standard single- and 2-chan-
nel party-line intercom devices. Each of the
two 2-channel PL interfaces is compatible
with intercom circuits that have their own
DC power sources. Alternately, the Model
46A can provide the DC power required
by connected intercom user devices. The
internal 30 volt DC intercom power source
is limited to 315 mA of current. This moder-
ate amount of power requires that the type
and number of connected user devices be
selected appropriately.
For convenience, the 2-wire PL intercom
circuits can be connected to the Model 46A
by way of four 3-pin male XLR connectors;
two are located on the back panel and two
on the front. They are organized in groups
of two connectors per interface, one on
the back and one on the front. They are
wired in parallel (“multed”) and provide
access to the identical signals.
2-Channel Intercom Systems
If compatibility with RTS TW systems is
desired the mating connectors (females)
should be wired so that common is on
pin 1, DC with channel 1 audio is on pin
2, and channel 2 audio is on pin 3. This
wiring scheme is correct whether the con-
nected devices are to be powered by an
external source or by means of the Model
46A’s internal power sources.
Single-Channel Intercom Systems
There are two ways of connecting to the
Model 46A’s 2-wire PL intercom connec-
tors when compatibility with Clear-Com
single-channel intercom devices is de-
sired. The most direct method is to pre-
pare the female XLR mating connectors so
that common is on pin 1, power is on pin
2, and audio is on pin 3. With this connec-
tion scenario only audio channel 2 (the
channel associated with pin 3 of the Model
46A’s 2-wire PL intercom interface con-
nectors) will be utilized. Pin 2, the Model
46A’s audio channel 1, will only be used
for connecting DC power to the connected
single-channel devices.
In some single-channel PL intercom sys-
tem applications it may be desirable to
take full advantage of the two channels
associated with each Model 46A interface.
In these applications one might want to
view the Model 46A as providing four 2-
wire-to-4-wire interface circuits. The Model
46A can certainly be used in this fashion,
but adapter cables will have to

Model 46A User Guide Issue 1, September 2014
Studio Technologies, Inc. Page 13
be prepared. These adapters will “split” the
Model 46A’s 2-wire PL intercom connectors
into two 3-pin male XLR connectors, one
for each audio channel. Pin 1 of the female
3-pin XLR intended to mate with the Model
46A will connect to pin 1 of both 3-pin male
XLR connectors. Pin 2 of the female XLR
will go to pin 3 of the male XLR designated
as channel 1. Pin 3 of the female XLR will
go to pin 3 of the male XLR designated as
channel 2. Refer to Figure 2 for details. Us-
ing two adapter cables the Model 46A can
be directly interconnected with four Clear-
Com intercom circuits. However, power for
the connected devices must be provided
by external power sources. The Model
46A’s ability to supply intercom power will
not be utilized.
Note: It’s critical that the correct configu-
ration settings be made when using the
Model 46A’s interfaces to support four
independent intercom circuits. Specifically,
the 2-wire power source configuration DIP
switches must be set for external. In addi-
tion, the auto terminate disable DIP switch
must be placed in its on (up) position.
Refer to the Configuration and Advanced
Configuration sections of this user guide
for details.
Remote Control Inputs
The Model 46A allows connection of three
externally provided DC signals. These
signals can provide remote control opera-
tion of three functions: auto nulling for
interface 1, auto nulling for interface 2, and
a special “mic kill” function. Remote con-
trol of the auto nulling functions provides a
resource identical to that of the front-panel
pushbutton switches. The exact manner in
which the buttons and the remote control
inputs operate depends on the setting of
auto null button mode configuration DIP
switch. The “mic kill” function is unique,
only being available using the remote
control input. It causes a 500 millisecond
“burst” of 24 kHz signal to be sent se-
quentially to both of the 2-wire partly-line
interface channels associated with each
of the Model 46A’s two interfaces. To
clarify, a “mic kill” signal is sent to a total
of four intercom channels whenever the
function is activated. Independent control
of sending “mic kill” signals to interface 1
or interface 2 is not supported. User inter-
com devices compatible with this 24 kHz
“mic kill” signal include RTS TW-Series
beltpacks such as the BP-325.
The opto-coupled remote control inputs
are designed for direct connection with
3.3 and 5 volt DC logic signals. An internal
475 ohm resistor, in series with each opto-
coupler’s photodiode, acts to limit the
current flow. Signals of up to 32 volts DC
can be safely connected as long as the
current is limited to 20 mA maximum. If
necessary, an external resistor can serve
to limit the current. For example, with a 12
volt DC signal using a 560 ohm, ¼-watt re-
sistor in series with the connection would
be appropriate. With a 24 volt DC control
signal a series resistor of 1.8 k (1800) ohm
Figure 2. Adapter cable wiring diagram

Issue 1, September 2014 Model 46A User Guide
Page 14 Studio Technologies, Inc.
is recommended. For correct operation a
minimum current of 2 mA is recommended.
Access to the remote control inputs is pro-
vided by means of a 10-pin male “header”
connector which is located on the Model
46A’s circuit board. Refer to Figure 3 for
a view of the connector’s location. The
“keyed” and “shrouded” header follows
a common industry-standard specifica-
tion: 2 rows of five pins each with 0.1 inch
between rows and pins. The mating con-
nector is intended to be an insulation-dis-
placement (IDC) socket connector such as
the AMP 746290-1. The connector would
be “crimp” terminated onto a piece of
10-conductor flat ribbon cable. This rib-
bon cable can safely exit the Model 46A
by laying flat between the chassis and the
cover. The ribbon cable can then “fan out”
to loose wires or, better yet, be terminated
onto another connector such as a 9-pin
D-subminiature type.
There are several ways of preparing to
connect to the remote control inputs. The
easiest means is to purchase an intercon-
necting cable assembly from Studio Tech-
nologies, Inc. The cable assembly consists
of a length of ribbon cable with a 10-pin
socket on one end and a 9-pin D-sub fe-
male on the other. Contact the factory or
check the website for details. Alternately,
a competent technician can easy fabricate
an appropriate remote control input inter-
connect wiring assembly. An example of a
partially pre-made assembly is part number
A1AXH-1036G-ND which is available from
Digi-Key (www.digikey.com).
It’s important to ensure that the source
of the remote control signals can meet
the Model 46A’s logic-state and timing
requirements. The Model 46A recognizes
an idle remote control input as one that
has no current flowing through it. An
active remote control input is one that
Figure 3. Location of 10-pin male header connector on the Model 46A printed circuit board

Model 46A User Guide Issue 1, September 2014
Studio Technologies, Inc. Page 15
has current flowing through it for a mini-
mum of 30 milliseconds. A special case
arises when the auto null button operating
mode has been set to independent. This
requires one front-panel button tap to auto
null channel 1 and two button taps to auto
null channel 2. The remote control equiva-
lent for one tap is current flowing for a mini-
mum of 30 milliseconds. The equivalent for
two taps is current that flows for 30 millisec-
onds, then a minimum no-current period of
30 milliseconds, followed by current flow for
a minimum of 30 milliseconds. The second
period of current flow must come within one
second of the start of the first.
Once the interconnect method and the
source of the control signals have been
selected, refer to Figure 4 for connection
details. Ensure that the connected signals
follow the specified polarity or damage to
the opto-coupled inputs may occur.
It’s important that a competent technician
be available to perform the actual remote
control input connection process. As ex-
pected, mating the interconnecting cable
assembly with the Model 46A’s 10-pin
“header” requires removing the unit’s cover.
As such, this must be done only after con-
firming that all sources of mains power have
been disconnected from the unit.
AC Mains Power
The Model 46A operates directly from AC
mains power of 100 to 240 V, 50/60 Hz. As
a “universal mains input” device, there are
no switches to set or jumpers to install. A
3-pin IEC 320 C14 inlet connector on the
Model 46A mates with a detachable mains
cord set. For units shipped to North Amer-
ica and Japan a cord is supplied that has
a North-American (NEMA 15L) standard
plug on one end and an IEC 320 C13 con-
nector on the other. Units bound for other
destinations require that the appropriate
cord set be obtained. The wire colors in
the mains cord should conform to the
internationally recognized color code and
should be terminated according to the fol-
lowing chart:
Connection Wire Color
Neutral (N) Light Blue
Line (L) Brown
Earth/Ground (E) Green/Yellow
Safety Warning: The Model 46A does
not contain an AC mains disconnect
switch. As such, the AC mains cord
plug serves as the disconnection de-
vice. Safety considerations require that
the plug and associated inlet be easily
accessible to allow rapid disconnec-
tion of AC mains power should it prove
necessary.
As soon as AC mains power is applied
the Model 46A will begin its power-up
sequence. As a “boot up” indication the
power LED and each of the status LEDs
will momentarily light. After the sequence
has completed the power LED will again
light and remain lit. The unit is now fully
functional.
10-Pin 9-Pin D-Sub
Function Header (P9) Interface Cable
Remote Auto Null Interface 1 + 7 4
Remote Auto Null Interface 1 – 9 5
Remote Auto Null Interface 2 + 2 6
Remote Auto Null Interface 1 – 4 7
Remote “Mic Kill” + 6 8
Remote “Mic Kill” – 8 9
Figure 4. Remote control input pin-out chart

Issue 1, September 2014 Model 46A User Guide
Page 16 Studio Technologies, Inc.
Configuration
For the Model 46A to support the needs
of specific applications a number of op-
erating parameters must be configured.
These include the 2-wire party-line power
source, the nominal 2-wire level, and the
nominal 4-wire level. Three 4-position DIP
switch assemblies are used to establish
the desired configuration. One DIP switch
assembly is associated with interface 1, a
second is associated with interface 2, and
a third associated with advanced operat-
ing features that apply to both interfaces.
Configuration – Interface 1
and 2
The DIP switch assemblies associated
with interface 1 and 2 are accessible on
the Model 46A’s front panel. They provide
identical capability for their respective
interfaces. Refer to Figure 5 for a detailed
view. In this section the four switches as-
sociated with interface 1 will be covered in
detail. This information applies to interface
2 as well. The four switches allow selec-
tion of the 2-wire party-line power source,
the nominal level for the 2-wire party-line
channels, and the nominal level of the
4-wire inputs and outputs.
2-Wire Party-Line Power Source
DIP switch 1 is used to select whether or
not the Model 46A provides DC power to
pin 2 of connectors associated with the
2-wire party-line intercom channels. Two
3-pin male XLR connectors, one located
on the Model 46A’s front panel and one
on the back panel, are used to intercon-
nect the Model 46A with the intercom user
devices. When switch 1 is in its off (down)
position the Model 46A will not provide DC
power. This switch setting is appropriate
when an external intercom power supply
is providing power to the intercom circuit.
In addition to power, it’s expected that the
external power supply will also provide the
required 200 ohm terminating impedance
to both pins 2 and 3.
When switch 1 is in its on (up) position the
Model 46A will provide a 30 volt, 315 mA
maximum source of DC power to pin 2 of
the 2-wire PL intercom connectors. The
Model 46A will also provide 200 ohm termi-
nating impedances for both pin 2 and pin 3
of the party-line connectors. The DC power
and termination capabilities allow direct
powering of 2-channel intercom beltpacks,
such as the RTS BP-325. In addition, listen-
only beltpacks, such as the Model 34 from
Studio Technologies, Inc. can be directly
connected. There’s also no problem con-
necting single-channel intercom beltpacks
or user stations.
2-Wire Nominal Audio Level
DIP switch 2 allows adjustment of the nomi-
nal audio level for the two 2-wire party-line
channels associated with interface 1. When
the switch is in its off (down) position the
nominal level is –14 dBu. This level is com-
patible with intercom beltpacks from manu-
facturers such as Clear-Com. When the
Figure 5. Detail of front panel showing four DIP
switches

Model 46A User Guide Issue 1, September 2014
Studio Technologies, Inc. Page 17
switch is in its on (up) position the nominal
level is –10 dBu. This setting is appropriate
when beltpacks from RTS, such as the BP-
325, or listen-only talent amplifier units from
Studio Technologies, Inc. are connected.
For best Model 46A performance it’s impor-
tant that the 2-wire nominal level selection
be made correctly.
4-Wire Nominal Audio Level
DIP switches 3 and 4 are used to configure
the nominal level of the input and output
circuitry associated with interface 1’s two
4-wire input and output connections. Four
level settings are available: 0, +4, +6, and
+8 dBu. Prior to setting these DIP switches
it’s important to determine the nominal au-
dio level of the equipment connected to the
Model 46A’s 4-wire input and outputs. This
may take a little investigation but is impor-
tant for obtaining the best possible Model
46A performance.
The 0 dBu setting is provided specifically
for use when a Model 46A is intercon-
nected with a Clear-Com matrix intercom
system. The +4 dBu setting is appropriate
when standard audio lines with “+4” nomi-
nal levels are interconnected. The +6 dBu
setting can be appropriate when intercon-
necting with equipment manufactured in
Europe, such as the digital matrix intercom
systems from Riedel. The +8 dBu setting is
provided specifically when interconnecting
the Model 46A with audio signals associ-
ated with an RTS ADAM-Series of matrix
intercom system. This includes the related
RVON-I/O unit.
When DIP switches 3 and 4 are in their off
(down) position the nominal level is set for
0 dBu. When switch 3 is on (up) and switch
4 is off (down) the nominal level is +4 dBu.
When switch 3 is off (down) and switch 4 is
on (up) the nominal level is +6 dBu. When
both switches 3 and 4 are in their on (up)
position the nominal level is configured for
+8 dBu.
Advanced
Configuration
Four DIP switches are provided on the
Model 46A’s back panel. Note that switch-
es 1, 2, and 4 are utilized; switch 3 is not
used. They allow the two interfaces to be
configured for several advanced operat-
ing modes. Refer to Figure 6 for a detailed
view of these DIP switches. The available
configurations include auto terminate, auto
null button operation, and factory test.
Figure 6. Detail of back panel showing the
Advanced Mode DIP switches
Auto Terminate
The auto terminate function is designed to
ensure that each 2-wire-to-4-wire interface
circuit remains stable under most oper-
ating conditions. Specifically, 200 ohm
impedances are automatically applied to
both pins 2 and 3 of a Model 46A 2-wire
party-line interface when that interface is
configured for external power and no ex-
ternal source of intercom power is detect-
ed on pins 2 or 3. This function is directly
compatible with RTS TW-Series intercom

Issue 1, September 2014 Model 46A User Guide
Page 18 Studio Technologies, Inc.
applications 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 46A. In this lat-
ter case only one of the Model 46A’s audio
channels will be used.
There may be situations where it’s neces-
sary for the two channels associated with
each Model 46A 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 46A, 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 intercom
circuits with the Model 46A. In both these
cases a DC voltage won’t be present on
either pin 2 or 3 and the Model 46A will au-
tomatically apply 200 ohm terminations. In
both these cases the termination will be in
error and will result in incorrect audio levels
due a “double-termination” condition.
To prevent this problem the auto terminate
function can be disabled. Referring to Fig-
ure 7, when DIP switch 1 is in its off (down)
position the auto terminate function is ac-
tive. When switch 1 is in its on (up) posi-
tion the auto terminate function associated
with each interface is disabled. Repeating
for clarity, the auto terminate function ap-
plies 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 46A’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
46A’s 2-wire PL connectors. If they are not
connected, it’s likely that audio oscilla-
tions, noise, and distortion will be gener-
ated in the Model 46A’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!
Auto Null Button
One pushbutton switch is associated with
each of the Model 46A’s two interface
circuits. An advanced configuration pa-
rameter allows selection of the button’s
operation. The choices are dual auto null
mode and independent auto null mode.
When dual auto null mode is selected a
single tap of one of the two front-panel
pushbutton switches initiates a routine
that begins with channel 1’s auto null se-
quence taking place followed immediately
by channel 2’s auto null sequence being
performed. Note that the mode selected
Figure 7. DIP Switch 1—Auto Terminate

Model 46A User Guide Issue 1, September 2014
Studio Technologies, Inc. Page 19
for auto null button operation will also
apply to the remote auto null inputs.
The independent auto null mode allows
the auto null function to be initiated for
each channel as desired. A single tap will
start the auto null routine for channel 1.
Two taps will start the routine for channel 2.
When DIP switch 2 is in its off (down) posi-
tion the dual auto null mode is selected.
This is provided specifically for cases where
the Model 46A’s 2-wire party line interfaces
will be used with dual channel intercom
user devices. As an example: the RTS TW-
Series provides two independent audio
channels, as well as power, over a single
3-conductor cable. In a case such as this
it’s useful for both of the hybrid circuits
associated with an interface to be auto
nulled at approximately the same time.
The dual auto null mode allows a single
tap of the button to initiate nulling of both
hybrid circuits.
Referring to Figure 8, when DIP switch 2
is in its on (up) position the independent
auto null mode is selected. This would
be appropriate for applications in which
the two audio paths associated with each
interface are used with independent party-
line intercom circuits. This situation might
arise when two Clear-Com single-channel
intercom circuits are connected to one of
the Model 46A’s 2-channel interfaces. The
hybrid circuit associated with each chan-
nel can be auto nulled as desired. Another
example would be in an RTS TW-Series
application that uses source assignment
panels. The “SAP” panels would be used
to route multiple intercom channels to vari-
ous sets of user devices as desired. In this
case, the two channels associated with
each Model 46A interface will often end up
not routed to the same user device; inde-
pendent auto nulling is certainly desired.
Factory Test
Back-panel DIP switch 4 is used to select
between normal mode and factory test
mode. Referring to Figure 9, when DIP
switch 4 is in its off (down) position the
Model 46A operates in its standard fash-
ion. When DIP switch 4 is in its on (up)
position the factory test mode is selected.
As expected, when the Model 46A is
deployed in the field DIP switch 4 should
remain in its off (down) position. No dam-
age to the Model 46A or connected equip-
ment will occur when factory test mode is
active.
Figure 8. DIP Switch 2—Auto Null Button
Figure 9. DIP Switch 4—Factory Test

Issue 1, September 2014 Model 46A User Guide
Page 20 Studio Technologies, Inc.
Operation
Technician intervention is typically not
required during normal Model 46A opera-
tion. The unit is designed for continuous
operation with no routine maintenance
necessary. Activating the auto null func-
tions may be warranted should connected
user devices or wiring associated with the
2-wire party-line intercom be changed.
Upon power-up the Model 46A will go
through a short initialization sequence
before normal operation will begin. The
power and status LEDs will each light
sequentially and, upon completion, the
power LED will light steadily. The settings
for the four hybrid circuits are stored in
non-volatile memory and will be recalled
during the power-up process.
Level Meters
The Model 46A contains eight 5-seg-
ment LED level meters. These meters
are provided as a support aid during
installation, configuration, operation, and
troubleshooting. The meters represent the
signals coming in from, and going out to,
the 4-wire connections. The meters are
organized in four groups each represent-
ing one input and one output. They are
calibrated to reflect the level in dB relative
to the configured nominal operating level.
This is similar to the way in which now-leg-
acy VU meters functioned. As an example
let’s take the situation where the nominal
operating level of interface 1 is configured
for +4 dBu. The LEDs labeled –18, –12,
–6, and 0 of the TO 4-wire ch 1 meters are
lit. (For this example, the level on pin 2 of
interface 1’s 2-wire PL circuit is just large
enough to light the 0 LED.) This indicates
that a signal with the approximate level
of +4 dBu is present on the associated
4-wire output connector. Another example
would be a Model 46A interface configured
for +8 dBu operation and –18 and –12 dB
LEDs are lit. This would indicate that a sig-
nal with an audio level of approximately
–4 dBu is present on its associated output.
Each level meter contains four green LEDs
and one yellow LED. The four green LEDs
indicate signal levels at or below the con-
figured nominal level. The top LED is yel-
low in color and indicates a signal that is
6 dB or greater than the nominal level. An
audio signal that causes the yellow LED to
light doesn’t necessarily indicate an exces-
sive level condition, but it does provide a
warning that at some stage reducing the
signal level may be prudent. Operation with
normal signal levels should find the meters
lighting near their 0 dB point. Signal peaks
may cause the yellow LEDs to flash. But a
yellow LED that lights fully during normal
operation will typically indicate a signal
level or configuration problem.
If the FROM 4-wire meters display consis-
tency lower or higher levels than their 0 dB
points it’s possible that a configuration is-
sue exists. One potential problem is incor-
rectly set 4-wire nominal audio level DIP
switches for one or both of the Model 46A’s
interfaces. If the nominal level is set too
low then sufficient audio headroom might
not be available. It can also result in ex-
cessively-high audio signals being sent to
2-wire party-line circuits. Setting the nomi-
nal level for too high a value will reduce the
signal-to-noise performance. And, as ex-
pected, it may also result in audio signals
being sent to the 2-wire circuits at too low
a level. Begin a review by confirming that
the two 4-wire level DIP switches associat-
ed with each interface are set as required.