Studio Technologies Annoucer's Console 233 User manual

Copyright © 2014 by Studio Technologies, Inc., all rights reserved
www.studio-tech.com
User Guide
Issue 5, November 2014
This User Guide is applicable for serial numbers:
M233-01151 and later
50336-1114, Issue 5

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Model 233 User Guide Issue 5, November 2014
Studio Technologies, Inc. Page 3
Table of Contents
Introduction ................................................................... 5
System Features ........................................................... 6
Installation and Setup ................................................... 12
Configuration ................................................................ 17
Operation ...................................................................... 29
Advanced Operation ..................................................... 34
Technical Notes ............................................................. 35
Specifications ................................................................ 44
Appendix A .................................................................... 45
Block Diagram

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Model 233 User Guide Issue 5, November 2014
Studio Technologies, Inc. Page 5
Introduction
What This User Guide Covers
This User Guide is designed to assist you
when installing, configuring, and using
the Model 233 Announcer’s Consoles with
serial numbers of 01151 and later. Addi-
tional background technical information is
also provided. A product block diagram is
included at the end of this guide.
System Overview
The Model 233 Announcer’s Console is
designed to serve as the audio control
“hub” for announcers, commentators, and
production personnel. The tabletop unit is
suited for numerous applications including
on-air television and radio broadcasting.
The Model 233 will also perform an out-
standing role in stadium announce appli-
cations. The Model 233 is compatible with
essentially all broadcast and production
audio system environments. Standard con-
nectors are used to interface microphone,
headphone, on-air, talkback, line input, and
intercom signals.
Whether it’s the mic preamplifier, audio
switching, talkback signals, intercom inter-
facing, or headphone cue feed, superior
audio quality is maintained throughout. A
microprocessor provides the Model 233’s
logic power, allowing precise control of
the unit’s operation. With extensive flexibil-
ity built in, creating the desired operating
configuration is a simple matter. While the
operating features of the unit can be care-
fully tailored, the user is presented with an
easy-to-use set of controls and indicators.
A wide range of resources, great perfor-
mance, and simplicity during use—these
are the hallmarks of the Model 233.
The Model 233 is a truly “next-generation
and then some” product. Exhaustive
research into the needs and desires of
field production personnel was integral to
its creation. Providing a veritable “tool kit”
of features, the unit supports a wide variety
of applications that include on-air television
and radio broadcasting, stadium announce-
ment, and simultaneous interpretation. In
addition, with the unit’s broad range
Figure 1. Model 233 front panel

Issue 5, November 2014 Model 233 User Guide
Page 6 Studio Technologies, Inc.
of capabilities many other specialized
“behind-the-scenes” applications can
also be implemented.
System Features
Microphone Input
A high-performance microphone preampli-
fier circuit provides low-noise/low-distortion
amplification over a 20 to 60 dB gain range.
The gain is adjustable in 10 dB steps. The
input is compatible with balanced dynamic
and condenser microphones. The micro-
phone power source is 48 volts nominal
and meets the worldwide P48 phantom
power standard. An LED indicator serves as
an aid for optimizing the preamplifier’s gain
setting. The output of the microphone pre-
amplifier is used by the main output as well
as being routed to the compressor circuit
that supports the talkback functions.
Main Output
The Model 233 provides a main output that
is designed to serve as the on-air, stadium
announcement, or other primary audio
feed. Nominally –2 dBu, it is designed as a
fully professional interface with high output
capability, low distortion, and low noise.
The output circuitry features a high-perfor-
mance output transformer expressly de-
signed for professional audio applications.
Talkback Functions
The talkback functions are intended to pro-
vide personnel associated with production
trucks, control rooms, live-performance,
and sports venues with talent-originated
cue signals. The Model 233 contains
two pushbutton switches that control the
talkback functions. Associated with each
pushbutton switch is a dedicated line-level
talkback output. Each pushbutton switch
can also be configured to send audio
to one specific channel associated with
the Model 233’s dual-channel intercom
interface.
The line-level talkback outputs are trans-
former-coupled with a +4 dBu nominal
signal level. They contain resistors in series
with the output connectors, allowing line-
level talkback outputs from multiple units
to be directly “summed.”
For non-on-air applications, a special
Model 233 feature set can be enabled by
placing the unit in its “production” mode.
This software-based mode allows the main
output to be used as an additional talkback
Figure 2. Model 233 back panel

Model 233 User Guide Issue 5, November 2014
Studio Technologies, Inc. Page 7
output. This feature makes the unit even
more powerful when used in live-event
applications, such as serving as a master
console for an orchestra conductor or
production director.
With all the available talkback flexibility,
the exact needs of many specific applica-
tions can easily be met. And, of course,
whatever configuration is implemented,
the audio quality will be excellent.
Dynamic Range Control
To enhance the Model 233’s talkback func-
tions, a studio-quality compressor circuit
is provided to control the dynamic range
of the signal coming from the microphone
preamplifier. Far from a simple “clipper,”
the circuit utilizes a sophisticated laser-
trimmed voltage-controlled-amplifier (VCA)
integrated circuit for quiet, low-distortion
level control.
The signal from the compressor is always
used by the talkback functions. For flex-
ibility, the audio source for the main output
can be selected to be either the direct
output of the microphone preamplifier or
the output of the compressor. For on-air
applications the signal coming directly
from the microphone preamp would be
correct. But having dynamic range control
of the signal going to the main output can
offer increased performance for selected
applications. These could include stadium
announcement locations, award show
conductor positions, and use with non-
professional announcers during sports
events.
User Controls and Status
Indicators
Three pushbutton switches, four LED
indicators, and three rotary controls
provide users with a clear, easy-to-use
interface. One pushbutton switch controls
the status of the main output. This is the
audio output intended for on-air, stadium
announcement, or other primary uses.
Two LEDs display the on/off status of the
main output. Two additional pushbutton
switches control the status of the talkback
functions. These are the audio cue signals
used to communicate with producers,
directors, “spotters,” or other behind-the-
scenes production personnel. A status
LED is associated with each talkback
button. Three rotary controls allow the
user to adjust the level of the headphone
output. Two of the controls are associated
with externally connected cue signals. The
third control is associated with the Model
233’s sidetone function, allowing the user
to monitor the signal associated with the
unit’s microphone input.
Flexibility
A large part of the Model 233’s unique
power is the ability to configure the op-
eration of the main output and talkback
functions. To meet the needs of the many
specific broadcast and production ap-
plications, a variety of button operating
modes is available. The main output
button can be selected to operate from
among four modes. In the “push-to-mute”
mode the button performs a momen-
tary mute of the main output. In this way
a “cough” button function is created,
something typically required for television
sports broadcasting. In the “push-to-talk”
mode the button provides a momentary
active function for the main output. This
mode would be appropriate for applica-
tions such as stadium announcement. An
alternate action “latching” configuration
allows the button to enable or disable the
main output as desired. This is useful in
radio broadcasting, announce-booth, or

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voice-over applications. The fourth mode
provides a hybrid function, supporting
both push-to-talk and tap-to-enable/tap-to-
disable operation. This operation is similar
to that found in many broadcast intercom
system user stations.
The two buttons associated with the talk-
back functions can be configured to oper-
ate from either of two modes. One of the
modes supports a “push-to-talk” function.
This is typically used for on-air broadcast
applications. The other mode provides a
hybrid function, the operation of which is
discussed in the previous paragraph. The
hybrid mode is especially useful when the
Model 233 is used in a production-support
application.
The main button mode configures how the
main output, when it is in the “latched” on
state, responds to talkback activity. One
choice momentarily turns off the main
output when talkback is active, returning
the main output to the on state when the
talkback function has ended. The other
choice “unlatches” the main output in
response to a talkback function.
Cue Sources
The Model 233 supports the connection
of up to four external audio sources, each
of which can be selected for routing to the
stereo headphone output. The sources
are line input 1, line input 2, intercom
channel 1, and intercom channel 2. Each
source can be individually assigned to the
left channel, the right channel, or both the
left and right channels. This allows a wide
variety of stereo and mono headphone
mixes to be created.
For application flexibility, two line-level
audio sources can be connected to
the Model 233. Possible signal sources
include off-air receivers, wireless IFB
systems, and audio consoles. The con-
nected signals can be from two indepen-
dent sources, or could be from a stereo
audio feed such as would be associated
with a broadcast music event. Two level
trim potentiometers, located on the bottom
of the unit, allow signals with wide nominal
audio levels to be cleanly interfaced.
Audio signals associated with a single-
or dual-channel intercom system can
be routed to the headphone output. The
Model 233’s intercom interface is com-
patible with standard party-line intercom
systems from manufacturers such as
RTS™ and Clear-Com®. A broadcast-type
IFB (interrupted foldback) circuit can also
be connected to the intercom input. This
“listen only” signal is often found in televi-
sion broadcast applications.
Sidetone
The Model 233 includes an integrated
sidetone function that allows the output
of the compressor circuit associated with
the microphone preamplifier to be routed
to the stereo headphone output. This pro-
vides a confidence signal to the user, al-
lowing them to hear exactly what is being
sent to the main and, if desired, talkback
outputs. This sidetone signal can be a
critical element when creating an effective
communications environment.
In on-air television and radio broadcast
settings the Model 233 user’s own micro-
phone audio signal is typically returned
to the headphone output by way of a cue
audio source. But there are cases where
this signal is not available, such as when
“mix minus” cue feeds are utilized. This
type of cue feed includes everything but
the user audio, a requirement when sig-
nificant time delays, typically associated
with satellite transmission systems, are

Model 233 User Guide Issue 5, November 2014
Studio Technologies, Inc. Page 9
present. In effect the Model 233’s sidetone
function adds the “minus” part to a “mix
minus” cue.
Several configurable parameters allow
the sidetone function to be tailored to the
needs of a specific Model 233 installation.
The sidetone function can be configured
to route audio to the left, right, or left and
right headphone outputs. It can also be
completely disabled. Sidetone can also
be configured to be active only when
the main output is active, or to be active
anytime the main output or either of the
talkback outputs is active.
Headphone Output
Three rotary controls are provided for
user adjustment of the headphone output.
Two of the controls adjust the levels of the
externally connected sources. The third
control is used to set the level of the side-
tone function.
For flexibility the actual functioning of the
two controls associated with the external
sources is configurable. For traditional
on-air sports applications they can be
selected to the dual level control mode
which provides independent control of
the left and right channel volume. For use
with stereo cue signals, or to support user
preference, the level/balance mode can be
selected. In this mode one control adjusts
the overall level of both the left and right
channels, while the other allows adjust-
ment of the left/right level balance. To help
minimize the chance of broadcast cues
being missed, both level control modes
can be configured so that a minimum
headphone output level is maintained.
Alternately, the headphone output can be
set to fully mute when these controls are
at their minimum position.
A headphone control reverse mode is
provided specifically for on-air television
applications where a headset with boom
microphone is used. The reverse mode
ensures that no matter which headset
orientation is used by the talent, the two
“pots” controlling the external sources will
always work intuitively. This will result in a
comfortable work environment, allowing
the “left” control to impact the level to the
talent’s left ear while the “right” control
impacts the right.
The sidetone level control is dedicated to
adjusting the level of the mic audio signal
that’s routed to the headphone outputs.
Configuration switches allow which of the
headphone outputs—left, right, or both—
will receive the sidetone signal. When the
sidetone level control is set to its minimum
position the sidetone level is always set to
be fully muted.
Provision has been made to support ap-
plications where a monaural cue feed is
desired. A configuration switch allows the
summing (combining) of the selected left
and right headphone sources. In addition
to creating a dual-channel mono output it
also allows the level controls to be config-
ured as a simple 3-channel mixer.
The headphone output is optimized to
meet the needs of contemporary head-
phones and headsets. Specifically, the
output circuits act as voltage, rather than
power, drivers. In this configuration they
can provide high output levels with very
low distortion and noise, along with mini-
mal current consumption. The output cir-
cuits are configured to safely drive stereo
or mono loads. This ensures that all types
of headphones, headsets, and earpieces
can be directly connected.

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Intercom Interface
Of special note is the Model 233’s sophis-
ticated intercom interface. It’s designed
to work correctly with industry-standard
single- and dual-channel party-line intercom
systems, including those from RTS and
Clear-Com. An intercom line connected to
the Model 233 can serve three functions:
providing cue audio signals to the head-
phone output, allowing talkback audio to
be sent to intercom users, and acting as
a Model 233 power source. Audio signals
present on the single- or dual-channel
intercom line can be flexibly routed to the
stereo headphone output. Talkback audio,
controlled by the two talkback pushbutton
switches, can be sent to either or both of
the intercom channels. Trim potentiometers,
located on the bottom of the unit, allow
adjustment of the talkback-to-intercom
sidetone (null) level.
The ability to listen to one or both pins of
a connected intercom circuit will only be
available when the intercom interface circuit
detects DC voltage on pin 2. This feature
minimizes the chance that objectionable
audio content (i.e., “howls” or “squeals”)
will be routed to the headphone output
when an intercom circuit is not connected.
Audio Quality and Protection
The Model 233’s circuitry is carefully tailored
to provide excellent audio performance.
Professional-quality components are fea-
tured throughout. For reliability all audio
routing is performed using solid-state
devices under microcontroller direction.
In all critical audio paths, “clickless” elec-
tronic switches provide noise-free control.
All audio inputs and outputs make extensive
use of protection components. This limits
the chance of damage from ESD and other
undesirable, yet real-world, hazards.
Power Sources
The Model 233 can derive its operating
power from an intercom line or an external
24 volt DC source. For redundancy, both
power sources can be connected simul-
taneously. An internal switch-mode power
supply ensures that all Model 233 features
are available, including phantom power,
when the unit is powered by either source.
Auxiliary Relay
Model 233 resources include a general-
purpose relay, allowing specialized configu-
rations to be created. Under software con-
trol, the relay can be configured to follow
the state of the main output, talkback 1, or
talkback 2 buttons. Taking advantage of the
back-panel locations provided for additional
XLR connectors, a technician may easily
create functions such as an “on-air” indica-
tor or implement loudspeaker muting dur-
ing talkback function. Special configuration
modes are even included to allow direct
control of the relay using the talkback 1 or
talkback 2 buttons without impacting any
of the Model 233’s audio signals.
Tally Output
Another unique Model 233 feature is the
tally output. It provides an indication, in
the form of a current-limited DC signal, of
the status of the main output. Whenever
the main output is active the tally output
is active. This 3.5 volt nominal, 12 milliam-
peres maximum, signal is capable of direct-
ly lighting an LED indicator or triggering an
external control system. The tally output
is provided specifically for specialized ap-
plications such as television award show
broadcasts where remote monitoring and
control of the main output is required.

Model 233 User Guide Issue 5, November 2014
Studio Technologies, Inc. Page 11
Configuration
Model 233 configurations are made
using a number of DIP switches and four
trim potentiometers. One 8-position switch
assembly is used to set the gain of the
microphone preamplifier, the on/off status
of phantom power, and control of the head-
phone output mode. A 12-position switch
assembly configures which of the four cue
audio sources, as well as the sidetone au-
dio, are routed to the headphone outputs.
In addition, the last two sections of the 12-
position switch assembly are used to select
whether talkback audio will be routed to the
intercom interface. Two 8-position switch
assemblies communicate the desired oper-
ating modes to the microprocessor.
Two rotary “trim pots” are used to adjust
the sensitivity of the line inputs. Two addi-
tional rotary trim pots are provided to adjust
the sidetone (null) level for the intercom
interface’s talkback functions.
All switches and trim pots are accessible via
the bottom of the Model 233’s enclosure;
the unit does not have to be disassembled.
Changes made to any of the configuration
parameters become active immediately. To
prevent access to the configuration controls
a security plate, included with each unit, is
attached to the bottom of the enclosure.
Connectors
The Model 233 uses standard connectors
throughout. The microphone input, line
inputs, and intercom interface functions
use 3-pin female XLR connectors. The
main and line-level talkback output func-
tions use 3-pin male XLR connectors.
The headphone output utilizes a ¼-inch
3-conductor jack. The external source of
24 volt DC power is connected by way
of a 2.1 x 5.5 mm “locking” coaxial power
jack.
Additional Connector Locations
In the world of broadcast, production, and
public address audio it’s fair to say that
applications vary widely. To this end, up
to three additional XLR connectors can
be easily mounted into the Model 233’s
back panel. Multiple 3-position “headers”
located on the Model 233’s circuit board
provide technician-access to literally every
input and output connection. Moreover,
some additional features, including the
remote control inputs and tally output, are
accessible using 3-position headers. Using
a factory-available interface cable kit, these
headers allow a Model 233 to be optimized
to meet the exact needs of specific applica-
tions. For example, some applications may
prefer to use a multi-pin XLR connector to
interface with a headset. This could be eas-
ily accomplished by adding the appropriate
6- or 7-pin XLR connector and making a
few simple connections. Other applications
may benefit from having “mult” or “loop-
through” connections, something easily
incorporated into a Model 233.
Multi-Pin Headset Connectors
As previously mentioned, some applica-
tions are best supported using headsets
that interface using a multi-pin connector.
In most of these cases the desired connec-
tors are 6- or 7-pin male XLRs wired to an
industry-standard pinout scheme. Studio
Technologies offers headset connector as-
semblies that allow fast and painless instal-
lation into a spare connector location in
the Model 233’s back panel. Details about
these optional assemblies are available on
the Studio Technologies website.

Issue 5, November 2014 Model 233 User Guide
Page 12 Studio Technologies, Inc.
Options
The Model 233’s standard resources are
more than sufficient to directly support a
large number of applications. But in the
“real world” of audio and intercommunica-
tions special needs always seem to arise.
To that end, Studio Technologies offers
a number of option cards. In addition to
passive or active components, each card
contains an integral connector, allowing
simple installation into a spare connector
location on the Model 233’s back panel.
For interest, the resources provided by
some of these option cards are worth
describing.
• The direct microphone output card
provides access to the dynamic or con-
denser microphone that is connected
to the Model 233’s mic input. Passive
components, along with the auxiliary
relay contact, create a “click-free” micro-
phone-level audio signal.
• The remote switch input card uses a
4-pin XLR connector to provide access
to the Model 233’s remote switch inputs.
Other general-purpose option cards
provide 3-pin XLR, 4-pin XLR, and 8-pin
EtherCon® connectors. With the range
of option cards available it’s hard to imag-
ine an application that can’t be served.
But you’re welcome to try to “stump the
chumps” in the Studio Technologies tech-
nical support department! But first please
check the complete list of the available
option cards listed on the Studio Technol-
ogies website.
Installation and
Setup
In this section interconnections will be
made using the input and output con-
nectors located on the Model 233’s back
panel. Microphone, line-level audio sourc-
es, intercom, main output, and line-level
talkback output signals are interfaced by
way of 3-pin XLR connectors. A ¼-inch
3-conductor phone jack is provided for
the headphone output. A 2.1 x 5.5 mm
coaxial jack allows connection of an exter-
nal 24 volt DC power source.
System Components
The following is included in the shipping
carton: Model 233 Announcer’s Console,
user guide, button label sheet, and 24 volt
DC power supply.
Microphone Input
The Model 233 is compatible with balanced
dynamic and condenser microphones.
Depending on the application, the micro-
phone may be part of a headset, or be an
independent handheld or stand-mounted
model. The Model 233’s 48 volt nominal
power source will support essentially all
phantom-powered microphones. The qual-
ity of the Model 233’s microphone pream-
plifier and associated circuitry is such that
special applications may benefit from using
“high-end” microphones. If selected appro-
priately, models from manufacturers such
as AKG, Beyer, Neumann, Sennheiser, and
Shure will perform very well in Model 233
applications.
The microphone input is also compatible
with line-level audio sources. This allows
the output of an external microphone pre-
amplifier, microphone processor, or audio

Model 233 User Guide Issue 5, November 2014
Studio Technologies, Inc. Page 13
console to be directly connected. When
the configuration DIP switches are set for
0 dB (no gain) the nominal level of the
source audio signal should be approxi-
mately –2 dBu.
Microphone interconnection is made
by way of a 3-pin female XLR connec-
tor which is located on the Model 233’s
back panel. The mating connector (male)
should be wired so that pin 2 is signal
high (+ or hot), pin 3 is signal low (– or
cold), and pin 1 is shield. It’s possible that
an unbalanced microphone will also work
correctly. In this case, the mating connec-
tor (male) should be wired so that pin 2
is signal high (+ or hot), and signal
common/shield is connected to both
pins 1 and 3.
The Model 233 is not compatible with
unbalanced “electret”-type microphones
that require a source of low-voltage DC
for operation. These microphones, some-
times found in low-cost headsets, are
not generally suitable for on-air or other
demanding applications.
As of the writing date of this user guide,
the Sennheiser HMD 26 headset is very
popular for on-air sports broadcasting
use. A fine product, it works very well with
the Model 233. Note that adding the suffix
“-XQ” to the headset’s full part number
(HMD 26-600-XQ) specifies a 3-pin male
XLR connector for the dynamic micro-
phone and a ¼-inch 3-conductor plug for
the stereo headphones. This configura-
tion is very useful, allowing the headset
to work directly “out of the box” with the
Model 233.
If the writer may digress for a moment
to recount a story… an audio dealer
once shared a secret with me concerning
headsets. He loved selling the “lower-end”
(less expensive) models of name-brand
headsets, which he did by the veritable
“boatload.” Why? Because these usually
broke soon after going into service! He
knew that on a regular basis he’d receive
orders for more of them. Had these
users, from the beginning, purchased
only premium-quality headsets, their
total cost of ownership would have been
much less. Enough said…
Headphone Output
The Model 233’s headphone output is
compatible with stereo or mono head-
phones, headsets, or earpieces. Connect-
ing devices with a nominal impedance
of 100 ohms or greater is preferred. This
shouldn’t be an issue as essentially all
contemporary devices already meet this
condition.
Devices are connected to the headphone
output by way of a ¼-inch 3-conductor
phone jack located on the Model 233’s
back panel. As is standard for stereo
headphones, the left channel is connected
to the “tip” lead of the ¼-inch headphone
jack. The right channel is connected to the
“ring” lead of the jack. Common for both
channels is connected to the “sleeve”
lead.
Devices with ¼-inch 2-conductor “mono”
plugs can also be used with the Model
233’s headphone output. In this arrange-
ment only the tip lead (left channel) will
be active. The 2-conductor plug will
physically connect (“short”) the ring lead
(right channel) to the sleeve lead (com-
mon). Technically this won’t damage the
circuitry associated with the right-channel
headphone output. (100 ohm protection
resistors are electrically in series with the
headphone output circuits.) However,
energy will be wasted if an audio signal

Issue 5, November 2014 Model 233 User Guide
Page 14 Studio Technologies, Inc.
coming out of the right channel goes into
a “dead” short. There is a simple means
of eliminating this issue; when configuring
the headphone sources simply do not as-
sign any to the right channel headphone
output. Refer to the Configuration section
of this user guide for details.
Main Output
The Model 233’s main output is intended
to be the “on-air” or primary signal that
typically connects to the input of an
audio console. The output is transformer
balanced with a nominal signal level of
–2 dBu. The actual level will depend upon
the gain setting of the microphone pream-
plifier, sensitivity of the microphone, and
how loudly the user speaks into the micro-
phone. The transformer used in the main
output is intended for professional audio
applications. Its source impedance is
very low and can drive lengthy cable runs
without difficulty. It is capable of driving
loads as low as 600 ohms but performs
best with loads of 2 k ohms or greater.
(This should not prove to be an issue as
virtually all contemporary audio equipment
has a relatively high input impedance.)
As the secondary winding of the output
transformer connects directly to the main
output connector, care should be taken
so that a DC voltage is never present on
the interconnecting cable.
The main output is interfaced by means
of a 3-pin male XLR connector located on
the Model 233’s back panel. The intercon-
necting cable’s mating connector (female)
should be wired so that signal high (+ or
hot) is on pin 2 and signal low (– or cold)
is on pin 3. The cable’s shield can be con-
nected to pin 1, but it will have no func-
tion. Also, the cable’s shield must not be
terminated on the metal “shell” connec-
tion of the mating connector. To limit the
chance of grounding interaction between
the Model 233 and connected equipment,
pin 1 on the main output’s connector is
isolated from any point in the Model 233.
The fact that pin 1 “floats” will minimize
the chance of hums, noises, or buzzes be-
ing present on the equipment connected
to the main output.
Line-Level Talkback Outputs
The two line-level talkback outputs are
intended to be connected to control
rooms, production trailers, or other loca-
tions where talent-originated voice cues
are required. The outputs are transformer
balanced with a nominal level of +4 dBu.
To enhance talkback audio quality, a com-
pressor circuit controls the dynamic range
of the signal coming from the microphone
preamplifier.
For protection against accidental connec-
tion to cables that have DC power present,
the line-level talkback outputs are capaci-
tor coupled. In series with the talkback
output leads are 300 ohm resistors, mak-
ing the effective output impedance ap-
proximately 600 ohms. These resistors
create a passive summing network, allow-
ing talkback outputs on multiple Model
233 units to be connected (“bridged”)
together.
The line-level talkback outputs are con-
nected by way of two 3-pin male XLR con-
nectors which are located on the Model
233’s back panel. The mating connectors
(female) should be prepared so that signal
high (+ or hot) is provided on pin 2.
Signal low (– or cold) should be provided
on pin 3. The cables’ shields can be con-
nected to pin 1. But, like the main out-
put, in order to minimize the chance that
ground-interaction problems will arise,
pin 1 on the line-level talkback output

Model 233 User Guide Issue 5, November 2014
Studio Technologies, Inc. Page 15
connectors are isolated from the Model
233’s chassis and circuitry. By making
pin 1 “float,” often-feared “ground loop”
problems shouldn’t arise. Note that the
metal shell of the mating connector must
also be “floating.”
The line-level talkback outputs are in-
tended to drive the lengthy cable runs that
are often part of broadcast and produc-
tion audio applications. While the output
circuitry is not intended to be “on-air”
quality, overall audio performance should
be very good. Devices connected to the
line-level talkback outputs can range from
amplified loudspeakers, analog inputs
on intercom systems, and input channels
associated with audio consoles. Connect-
ing the outputs to devices that allow easy
control of the signal level can be helpful.
For example, connecting to an input on
an audio console provides the flexibility
to add gain or attenuate as required. A
dedicated talkback-associated output on
the audio console can then be routed to
the final destination.
As previously mentioned, the line-level
talkback outputs on multiple Model 233
units can be directly connected together.
Using a simple “Y” or “W” cable, this pas-
sive summing (adding together or “bridg-
ing”) of talkback signals allows one audio
cable to serve as a master talkback path.
A side effect from using this passive sum-
ming technique is that signal attenuation
will occur. The audio quality won’t suffer,
but an audio “pad” is created. If two line-
level talkback outputs are connected to-
gether, a signal attenuation of 6 dB can be
expected. Connecting three talkback out-
puts together will result in 9.5 dB of atten-
uation. And four talkback outputs “multed”
together will lead to 12 dB of attenuation.
In most cases this attenuation won’t pose
a problem. Typically a device that receives
the talkback signal, such as an amplified
loudspeaker, will have an adjustable input
sensitivity so this attenuation won’t prove
to be a problem.
Line Inputs
The Model 233 allows two line-level audio
sources to be connected. These sources
can be individually routed to the left chan-
nel, right channel, or both the left and right
channel of the headphone outputs. The
inputs are balanced, transformer-coupled
with a nominal impedance of 10 k ohms.
Capacitors, in series with the transformer’s
input leads, prevent a DC voltage present
on a connected source from impacting
performance. The line inputs are compat-
ible with signals that have a nominal level
of –12 dBV to +6 dBu. Two trim potentiom-
eters, located on the bottom of the Model
233’s enclosure, allow signals over this
wide nominal level range to be effectively
utilized.
Audio sources are connected to the line
inputs by way of 3-pin female XLR connec-
tors which are located on the unit’s back
panel. 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 connec-
tion from pin 1 can eliminate the issue. With
an unbalanced source connect pin 2 to sig-
nal high (+ or hot) and both pins 1 and 3 to
shield. If connecting an unbalanced source
in this manner results in hum or noise, con-
nect pin 2 to high (+ or hot) and pin 3 to
shield; leave pin 1 unterminated.

Issue 5, November 2014 Model 233 User Guide
Page 16 Studio Technologies, Inc.
Intercom Interface
The Model 233’s intercom interface is
designed to directly connect with standard
single- and dual-channel party-line inter-
com lines. The one or two audio signals
provided by the intercom line can serve as
audio sources for the headphone outputs.
Each signal can be individually assigned to
the left channel, the right channel, or both
the left and right channels. The Model 233
can also be configured to send talkback
audio to either or both of the intercom
channels. In addition, the intercom line can
provide the DC power required to operate
the Model 233’s circuitry.
An intercom line is connected to the Model
233 by way of a 3-pin female XLR connec-
tor which is located on the back panel. The
mating connector (male) should be wired
so that common is on pin 1, DC with chan-
nel 1 audio is on pin 2, and channel 2 audio
is on pin 3. With single-channel intercom
lines common is on pin 1, DC power is on
pin 2, and audio is connected to pin 3.
The Model 233’s intercom interface is di-
rectly compatible with broadcast and pro-
duction party-line intercom lines associated
with systems from manufactures such as
RTS and Clear-Com. Intercom lines associ-
ated with other systems should be equally
compatible. RTS TW-series systems are
normally interfaced using 3-pin XLR con-
nectors. These connectors are wired with
common on pin 1, DC power and channel 1
audio on pin 2, and channel 2 audio on pin
3. Most Clear-Com party-line systems are
single-channel with common on pin 1, DC
power provided on pin 2, and audio pro-
vided on pin 3.
The DC power supplied by the connected
intercom line is generally sufficient to
operate the Model 233’s circuitry. The
acceptable input range is 24 to 32 volts,
with a required current of 125 milliam-
peres. Note that the specified input volt-
age is given when measured directly at
the Model 233’s intercom input connec-
tor (with the connector terminated on the
Model 233) and not at the source of the
intercom system’s power.
External Power Input
An external source of 24 volt DC power
can be connected to the Model 233 by
way of a 2.1 x 5.5 mm coaxial power jack
which is located on the back panel of
the unit. The center pin of the jack is the
positive (+) connection. While the require-
ment for the external source is nominally
24 volts, correct operation will take place
over a 20 to 30 volt range. The Model 233
requires 90 milliamperes at 24 volts DC
for correct operation. Included with each
Model 233 is a 24 volt DC external power
supply. The power supply’s DC output
cable has been terminated with a Switch-
craft® S760K coaxial power plug. This
“locking” type of plug correctly mates with
the Model 233’s 24 Vdc input jack. The
locking feature is important, allowing
the external power source to be securely
attached to the Model 233.
As previously discussed in this user guide,
an intercom line connected to the Model
233 can serve as the unit’s power source.
Alternately, an external 24 volt DC source
can be connected. For redundancy, the
intercom line and the external source
can be connected at the same time. If
one of them becomes inoperative the
remaining source will provide power for
the Model 233.

Model 233 User Guide Issue 5, November 2014
Studio Technologies, Inc. Page 17
The Model 233’s circuitry establishes
the priority in which the unit draws its
operating power. If an external source
of 24 volt DC power is connected, it will
always serve as the primary source. This
minimizes the impact that the unit’s power
draw could have on a connected intercom
line. If no external source of 24 volt DC
is connected then power will be drawn
from the intercom line. And, of course, no
matter which source is providing power
full operation of the intercom interface can
take place.
Pushbutton Labeling
The three pushbutton switches used in the
Model 233 were selected for several rea-
sons. Foremost was the fact that they are
highly reliable, using gold-plated contacts
for long life in less-than-ideal environ-
ments. A second reason was that apply-
ing customized labels to the button caps
would be very simple. The labels, text
printed on clear material, are placed under
the clear caps on the top of the buttons.
From the factory the left button is labeled
COUGH, the center button is labeled
TALKBACK 1, and the right button is
labeled TALKBACK 2. This was selected
to be appropriate for many on-air applica-
tions in English-speaking locations. But
it’s expected that these may need to be
changed to meet the needs of specific
applications.
As a “head start” for some applications,
a clear sheet with a number of commonly
used button designations printed on it is
included in the shipping carton. These
were created at the factory using a stan-
dard personal computer graphics program
and laser printed onto 3M CG3300 trans-
parency film. The desired button labels
can be cut out with a pair of scissors or an
X-ACTO® knife following the printed guide
lines that indicate the required size.
The clear lens on top of each button cap
can be removed with a fingernail or small
screwdriver. Be certain not to scratch the
button if a screwdriver or other small tool
is used. The clear label can be removed
and replaced. The button cap is then
snapped back into the top of the button
housing using finger-pressure only. No
tool is required to replace the button cap.
If you need to make your own labels the
process is quite simple. Use a personal
computer to create the desired text. The
finished label size should be 0.625-inches
(15.8 mm) square. The completed artwork
can then be printed on transparency film
sheets using a laser or inkjet printer. These
sheets are readily available from most
office supply stores. A pair of scissors or
an X-ACTO knife will complete the task.
Configuration
For the Model 233 to support the needs
of specific applications a number of op-
erating parameters must be configured.
These include microphone preamplifier
gain, phantom power on/off, headphone
source and output mode selection, and
operating modes. One 12-position and
three 8-position DIP switch assemblies
are used to establish the desired con-
figuration. These switch assemblies are
referred to as SW1 through SW4, with
individual switches designated as SW1-1,
SW1-2, etc. The switch assemblies are ac-
cessed through openings in the bottom of
the Model 233’s enclosure. The enclosure
does not have to be disassembled to gain
access to the switches.

Issue 5, November 2014 Model 233 User Guide
Page 18 Studio Technologies, Inc.
Microphone Preamplifier Gain
Switches SW1-1 through SW1-5 are used
to select the gain of the microphone
preamplifier. The choices are 20, 30, 40,
50, and 60 dB. Only one switch should
be enabled at a time. There’s no problem
changing the gain setting while the unit
is operating. Audio clicks or pops might
occur during gain transitions, but this
shouldn’t be a major issue as long as
associated monitor loudspeakers are
temporarily attenuated or muted.
Selecting the correct amount of gain for an
application might take a little experimenta-
tion. The goal is to bring the microphone’s
signal up to line level, nominally –2 dBu,
on the Model 233’s main output. Operat-
ing at this signal level will help to ensure
the delivery of “clean” audio to the con-
nected device. The output of the Model
233’s microphone preamplifier is used
by both the main output and, by way of
the compressor circuit, the talkback func-
tions. So creating a nice “hot” signal will
help maintain audio quality, specifically
the signal-to-noise ratio, when driving the
often-lengthy cable runs.
Figure 4. Microphone preamplifier gain switch
settings
To prevent unauthorized personnel from
changing the configuration settings, a
security plate is attached to the bottom
of the Model 233’s enclosure. For conve-
nience, text and graphics on the security
plate provides a summary of the configu-
rable parameters and related information.
Refer to Appendix A for a representative
view. The security plate is held in place by
means of four rubber bumpers (“feet”) that
have built-in screws. Using your fingers,
remove the four bumpers so that the plate
can be removed. Refer to Figure 3 for a
detailed view of the configuration switch
assemblies.
Figure 3. Bottom view of Model 233 showing
configuration switches, trim pots, and
compressor active LED
Microphone Preamplifier Gain
and Phantom Power
Five switches are used to set the gain of
the microphone preamplifier. One switch
is used to select the on/off status of the
phantom power supply.

Model 233 User Guide Issue 5, November 2014
Studio Technologies, Inc. Page 19
Unfortunately, there’s no “perfect” gain set-
ting that this guide can recommend. The
two issues that impact the setting are out-
put sensitivity of the connected microphone
and the acoustical output level of the mi-
crophone’s user. With some headset micro-
phones, such as the Sennheiser HMD25,
selecting an initial setting of 40 dB is appro-
priate. Users who speak loudly might need
to have the gain reduced to 30 dB. Quiet
users might need 50 dB of gain.
An LED indicator is provided as an aid in
correctly setting the gain of the microphone
preamplifier. Red in color, this LED is locat-
ed adjacent to switch assembly 1. It is vis-
ible by observing the bottom of the Model
233’s enclosure when the security plate
has been removed. Technically, this red
LED lights whenever the compressor cir-
cuitry is controlling the dynamic range of
the signal coming from the microphone
preamplifier. The threshold is set to be
2 dB above the Model 233’s nominal in-
ternal operating level. So a good “rule of
thumb” is to adjust the gain of the micro-
phone preamplifier so that the compres-
sor active LED lights (“flashes”) when the
connected microphone is sending signal
peaks. During normal operation the LED
should not remain fully lit when typical
audio signals are present on the mic input.
It’s important to remember that the com-
pressor active LED is used to assist in set-
ting the mic preamplifier gain to the optimal
value. It doesn’t necessarily indicate that
the main output’s signal is being com-
pressed. Unless specifically configured
to perform otherwise, the output of the
compressor is only used for the talkback
output functions.
It’s expected that the 20 and 60 dB gain
settings will not often be used. But there are
always exceptions and that’s why they were
included. It’s possible that with a very “hot”
microphone, such as a phantom-powered
condenser, 20 dB of gain could be correct.
It’s also possible that a microphone with a
very low level output, such as a ribbon-type,
would need 60 dB of gain. But in general,
the 30, 40, and 50 dB gain settings will
serve most applications.
Note that if no gain switch is set to its ac-
tive (on) position the preamplifier will oper-
ate at unity (0 dB) gain. This is provided for
compatibility when line-level signals need
to be connected to the microphone input.
But with a microphone connected as the
input source one should never use the 0 dB
setting. The issue is that with no gain added
to the microphone input signal, the relative
noise floor on the main and talkback out-
puts will be much too high.
Phantom Power On/Off
The Model 233 can provide nominal 48
volt phantom power to a connected micro-
phone. Switch SW1-8 controls whether or
not phantom power is active. By phantom
power’s very nature it could be left applied
to the microphone input at all times. But
generally people prefer to turn it off unless
required for a specific microphone.
Figure 5. Phantom power switch settings

Issue 5, November 2014 Model 233 User Guide
Page 20 Studio Technologies, Inc.
Headphone Source Selection
Switch assembly SW2 is used to configure
the sources that are routed to the stereo
headphone output. Five headphone sourc-
es are available: line input 1, line input 2,
intercom channel 1, intercom channel 2,
and sidetone. Each of these sources can
be assigned to the left, right, or both the
left and right channels of the stereo head-
phone output.
The line inputs are interfaced using two
connectors also located on the back panel.
Associated with line inputs 1 and 2 are
level trim potentiometers. They are provid-
ed so that audio sources with a wide range
of nominal levels can be effectively used as
cue sources. Please refer to the Advanced
Operation section of this user guide for
details on using the trim pots.
Audio associated with intercom channels
1 and 2 is provided by way of the intercom
interface whose connector is also located
on the back panel. Two trim pots are as-
sociated with the intercom channels. They
allow adjustment of the intercom sidetone
(null) level. This impacts the amount of
talkback audio signal that is returned to
a headphone output when a talkback-to-
intercom function is active.
The sidetone audio source comes from
the output of the compressor circuit as-
sociated with the microphone preamplifier.
This allows an announcer or other Model
233 user to receive a confirmation signal
of what is coming out of the main, and if
configured, talkback outputs.
As previously discussed, each of the avail-
able input sources can be assigned to
the headphone output’s left channel, right
channel, or both the left and right chan-
nels. The Model 233’s circuitry allows any
combination of input assignments to be
Figure 6. Left and right channel headphone
source selection settings
made. For example, consider the situation
where a single-channel intercom line, with
audio present only on pin 3, is connected.
In this case it may be desirable to assign
this intercom audio source to both the left
and right channels. This would entail set-
ting switches SW2-4 and SW2-9 to their on
positions. All other switches would remain
in their off positions.
A more complex application might have
a broadcast-type 2-channel IFB circuit
connected to the intercom input and a
line-level audio signal from a golf event