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Studio Technologies 220 User Manual

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User Guide
Issue 5, February 2013
This User Guide is applicable for serial numbers:
M220-01151 and later
Copyright © 2013 by Studio Technologies, Inc., all rights reserved
www.studio-tech.com
50306-0213, Issue 5
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Table of Contents

Introduction ................................................................... 5
System Features ...........................................................
Installation and Setup ................................................... 11
Configuration ................................................................
Operation ...................................................................... 24
Advanced Operation ..................................................... 29
Technical Notes .............................................................
Specifications ................................................................
Appendix A .................................................................... 42
Block Diagram
6
15
30
41
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Introduction

What This User Guide Covers
This User Guide is designed to assist you when installing, configuring, and using Model 220 Announcer’s Consoles with serial numbers 01151 and later. Additional background technical information is also provided. A product block diagram is included at the end of this guide.
System Overview
The Model 220 Announcer’s Console is designed to serve as the audio control “hub” for announcers, commentators, and production talent. The tabletop unit is suited for numerous applications including on-air sports broadcasting. The Model 220 is compatible with essentially all broadcast and audio system environments. Standard connectors are used to interface micro­phone, headphone, on-air, talkback, and IFB signals.
Whether it’s microphone switching, talk­back outputs, or headphone cue feed, superior audio quality is maintained throughout. A microprocessor provides the Model 220’s logic power, allowing ex­acting control of the unit’s operation. With extensive flexibility built in, creating the desired operating configuration is a simple matter. While the operating features of the unit can be extensively configured, the user is presented with an easy-to-use set of controls and indicators. Power on the inside, simplicity during use—that’s the hallmark of the Model 220.
A truly next-generation product, extensive research into the needs and desires of field production personnel was integral to the Model 220’s creation. While primarily targeted for on-air television applications, specialized features are included to allow the Model 220 to be used in a variety of other audio applications. These include on-air radio broadcasting, stadium an­nouncement, and voice-over/narration booths.
Figure 1. Model 220 front panel
Figure 2. Model 220 back panel

System Features

Microphone Input
A high-performance microphone preampli­fier 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 micro­phone power source is 48 volts nominal and meets the worldwide P48 phantom power standard. An LED indicator serves as an aid for optimizing the setting of the preamplifier’s gain. The output of the mi­crophone preamplifier is used by the main output as well as being routed to the com­pressor circuit that supports the talkback functions.
Main Output
The Model 220 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. It features a high-performance output trans­former expressly designed for professional audio applications.
Talkback Outputs
The two talkback outputs are intended to provide production trucks, control rooms, or support personnel with talent-originated cue signals. These outputs are transformer­coupled with +4 dBu nominal signal levels. They contain resistors in series with their output connector, allowing the talkback outputs from multiple units to be directly “summed.”
For non-on-air applications, a special Model 220 feature can be enabled, placing the unit in a “production” mode. This allows the main output to be used as a third talk­back output. In this configuration the unit can be even more powerful when used in corporate or theatrical events, for example serving as a master console for a produc­tion director.
Dynamic Range Control
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 outputs. In addition, the audio source for the main output can be selected to be either the output of the microphone preamplifier or the output of the compressor. While possi­bly not appropriate for major on-air situa­tions, having dynamic range control of the main output can offer increased perfor­mance for many applications. These could include stadium announcement positions, sports events using nonprofessional on-air talent, and situations where cable cross­talk is of concern.
User Controls and Status Indicators
Three pushbutton switches, four LED indicators, and two rotary controls provide the user 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, 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 two talkback outputs. These are the audio outputs used to communi­cate with producers, directors, “spotters,” or other behind-the-scenes production personnel. A status LED is associated with each talkback button. Two rotary controls allow the user to adjust the level of the headphone output.
Flexibility
A large part of the Model 220’s unique power is the ability to configure the op­eration of the main output and talkback functions. To meet the needs of the many specific broadcast and production ap­plications, 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 momen­tary 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 applica­tions 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 voice-over applications. The fourth mode provides a hybrid function, supporting both push-to-talk and tap-to-enable/tap-to­disable operation. This operation is similar to that found in many broadcast intercom system user stations.
The two buttons associated with the talk­back functions can be configured to oper­ate 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 220 is used in a production-support application.
In addition to the two modes provided for talkback 1, the button associated with talk­back 2 supports three additional operating modes. One mode allows talkback 2 to be disabled. This is useful when only talkback 1 is to be utilized. The other two modes provide special momentary and hybrid functions. With these modes the operation of the main and talkback outputs are not impacted. This allows the talkback 2 but­ton to control the state of the Model 220’s auxiliary relay, making specialized imple­mentations possible.
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.
IFB Input
A broadcast-standard “wet” (DC with au­dio) IFB circuit can be directly connected to the Model 220’s IFB input. Originated by sources such as the RTS™ 4000-series IFB system or IFB interface devices from Studio Technologies, the connected IFB circuit can provide DC power to operate the Model 220 as well as two channels of cue audio.
Cue Sources
The Model 220 allows up to four audio sources to be selected for routing to the headphone output. The sources are IFB channel 1, IFB channel 2, line input 1, and line input 2. Each source can be individu­ally assigned to the left channel, right channel, or both left and right. This allows a wide variety of stereo and mono head­phone mixes to be created.
systems, and audio consoles. The con­nected signals can be from two indepen­dent sources, or could be a stereo audio feed such as would be associated with a broadcast music event. Two trim poten­tiometers, located on the bottom of the unit, allow signals with wide nominal audio levels to be cleanly interfaced.
Headphone Output
Two rotary controls are provided for user adjustment of the headphone output levels. For application flexibility, the actual function of the two “pots” 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 adjustment 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 the controls are at their minimum position.
The two audio signals associated with the IFB input can be assigned to the head­phone output. Originating in production trailers, control rooms, or remote loca­tions, these unbalanced sources normally provide DC power and program-with-inter­rupt audio on one channel and program­only audio on the other.
For application flexibility, two line-level audio sources can also be connected to the Model 220. Possible signal sources include off-air receivers, wireless IFB
The headphone output was designed to meet the needs of contemporary head­phones 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 mini­mal current consumption. The output cir­cuits are configured to safely drive stereo or mono loads. This ensures that all types of headphones, headsets, and earpieces can be directly connected.
Audio Quality and Protection
The Model 220’s circuitry is carefully tai­lored to provide excellent audio perfor­mance. Professional-quality components are featured throughout. For reliability all audio routing is performed using solid-state devices. In all critical audio paths, “click­less” electronic 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 220 can derive its operat­ing power from either the IFB input or an external nominal 24 volt DC source. For redundancy, both power sources can be connected simultaneously. An internal switch-mode power supply ensures that all Model 220 features, including phantom power, are available when the unit is pow­ered by either source.
The Model 220 is compatible with IFB circuits provided by most standard broad­cast systems. However, maximum perfor­mance can often be obtained by using the IFB interface devices available from Studio Technologies. The provide high-quality audio along with an excellent source of DC power. They’re directly compatible with most matrix intercom systems, as well as standard line-level audio signals. Refer to the Studio Technologies website for details.
Auxiliary Relay
The Model 220’s circuitry includes a gen­eral-purpose relay, allowing specialized configurations to be created. Under soft­ware control, the relay can be configured to follow the state of the mic, talkback 1, or talkback 2 buttons. Taking advantage of the
locations provided for additional XLR con­nectors, a technician may easily implement a variety of functions such as mic active indication, audio muting during talkback, or audio insertion control. Several modes were specifically included to allow direct control of the relay using the talkback 2 button, without impacting any of the audio signals. The auxiliary relay is also used by the optional direct microphone output card.
Configuration
Model 220 configurations are made using a number of DIP switches and two trim potentiometers. One 8-position switch as­sembly is used to set the gain of the micro­phone preamplifier and the on/off status of phantom power. Another 8-position switch assembly configures which of the cue audio sources are routed to the headphone out­put. Two additional 8-position switch assemblies communicate the desired operating modes to the microprocessor. Two rotary trim pots are used to adjust the input sensitivity of the line inputs. All switches and trim pots are accessible via the bottom of the Model 220’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 220 uses standard connectors throughout. The microphone, IFB, and line inputs use 3-pin female XLR connectors. The main and talkback outputs use 3-pin male XLRs. 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 and production 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 220’s back panel. Multiple 3-position “headers” located on the Model 220’s circuit board provide technician­access to literally every input and output connection. Using a factory-available inter­face cable kit, these allow a Model 220 to be optimized to meet the exact needs of specific applications. For example, some applications may prefer to use a multi­pin XLR connector to interface with a headset. This could be easily accom­plished by adding the appropriate 5-, 6-, or 7-pin XLR connector and making a few simple connections. Other applications may benefit from having “mult” or “loop­through” connections, something easily incorporated into a Model 220.
Multi-Pin Headset Connectors
As previously mentioned, some broadcast applications use headsets that interface using a multi-pin connector. In most cases these connectors are 6- or 7-pin male XLRs wired to an industry-standard pinout scheme. Studio Technologies offers head­set connector assemblies that allow fast and painless installation into a spare con­nector location in the Model 220’s back panel. Details about these optional assem­blies are available on the Studio Technolo­gies website.
Options
The Model 220’s standard resources are more than sufficient to directly support a large number of applications. But in the “real world” of audio and intercommunica­tions 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 220’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 con­denser microphone that is connected to the Model 220’s mic input. Passive com­ponents, along with the auxiliary relay contact, create a “click-free” microphone­level audio signal.
• The remote switch input card uses a 4-pin XLR connector to provide access to the Model 220’s remote switch inputs.
• The tally/remote switch input card pro vides a current-limited DC voltage that serves as a main output status signal. It also provides a remote switch input connection.
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 imagine an application that can’t be served. But you’re welcome to try to “stump the chumps” in the Studio Technologies technical support department! But first please check the complete list of the available option cards listed on the Studio Technologies website.
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200-Series Announcer Console Products
The Model 220 is just one in a series of announcer console products available from Studio Technologies. For applications that require an alternate set of features the other products in the 200-series should be reviewed. Complete information is available on the Studio Technologies website.

Installation and Setup

In this section interconnections will be made using the input and output con­nectors located on the Model 220’s back panel. Microphone input, IFB input, line input, main output, and 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 external 24 volt DC power source.
System Components
Included in the shipping carton are the fol­lowing: Model 220 Announcer’s Console, user guide, button label sheet, and 24 volt DC power supply.
Microphone Input
The Model 220 is compatible with bal­anced dynamic and condenser micro­phones. Depending on the application, the microphone may be part of a headset, or be an independent handheld or stand­mounted model. The Model 220’s 48 volt nominal power source will support essen­tially all phantom-powered microphones. The quality of the Model 220’s microphone preamplifier and associated circuitry is such that special applications may benefit from using “high-end” microphones. If selected appropriately, models from man­ufacturers such as AKG, Beyer, Neumann, Sennheiser, and Shure will perform very well in Model 220 applications.
The microphone input is also compatible with line-level audio sources. This allows the output of an external microphone pre­amplifier, microphone processor, or audio 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 approxi­mately –2 dBu.
Microphone interconnection is made by way of a 3-pin female XLR connec­tor which is located on the Model 220’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 connec­tor (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 220 is not compatible with unbalanced “electret”-type microphones that require a source of low-voltage DC for operation. These microphones, some­times 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 220. 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 micro­phone and a ¼-inch 3-conductor plug for the stereo headphones. This configura­tion is very useful, allowing the headset to work directly “out of the box” with the Model 220.
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 220’s headphone output is compatible with stereo or mono head­phones, headsets, or earpieces. Connect­ing devices with a nominal impedance of 100 ohms or greater is preferred. This shouldn’t prove to 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 220’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 220’s headphone output. In this arrange­ment 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 (common). 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 coming out of the right channel goes into a “dead” short. There is a simple solution
to this issue. No audio source should be assigned to the right-channel headphone output. Refer to the Configuration section of this user guide for details.
Main Output
The main output is intended to be the “on-air” signal that 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 on the gain setting of the microphone pream­plifier, sensitivity of the microphone, and how loudly the talent speaks into the micro­phone. The transformer used in the main output is intended for professional broad­cast applications. It has a low source im­pedance and can drive lengthy cable runs with no difficulty. It is capable of driving 600 ohm loads 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 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 220’s back panel. The intercon­necting 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 con­nected to pin 1, but it will have no function. To limit the chance of grounding interaction between the Model 220 and connected equipment, pin 1 on the main output’s connector is isolated from any point in the Model 220. The fact that pin 1 “floats” will minimize the chance of hums, noises, or buzzes being present on the equipment connected to the main output.
Talkback Outputs
The talkback outputs are intended for connection to control rooms, production trailers, or other locations where talent­originated voice cues are required The talkback outputs are transformer bal­anced with a nominal level of +4 dBu. To enhance talkback audio quality, the 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 talkback outputs are capacitor coupled. In series with the talkback output leads are 300 ohm resistors, making the effective output impedance approximately 600 ohms. These resistors create a pas­sive summing network, allowing talkback outputs on multiple Model 220 units to be connected together.
The talkback outputs are connected by way of 3-pin male XLR connectors which are located on the Model 220’s back panel. Mating connectors (female) 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. But, like the main output, in order to minimize the chance that ground-interaction prob­lems will arise, pin 1 on each talkback output connector is isolated from the Model 220’s chassis and circuitry. By making pin 1 “float,” an often-feared “ground loop” problem shouldn’t arise.
The talkback outputs are intended to drive lengthy cable runs that are frequently part of a remote broadcast application. While the output circuitry is not intended to be “on-air” quality, overall audio performance should be very good. Devices connected to the talkback outputs can range from
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amplified loudspeakers to analog inputs on intercom systems, and input channels associated with audio consoles. Connect­ing the talkback outputs to devices that al­low easy control of the signal level can be helpful. For example, connecting to an in­put channel on an audio console will pro­vide the flexibility to add gain or attenuate as required. Then a talkback-associated output connection on the audio console can connect to the final destination(s).
As previously mentioned, the talkback outputs on multiple Model 220 units can be directly connected together. Using a simple “Y” or “W” cable, this passive sum­ming (adding together) of talkback sig­nals allows one audio cable to serve as a master talkback path. One side effect from using this passive summing technique is that signal attenuation will occur. The audio quality won’t suffer, but an audio “pad” is created. If two talkback outputs are connected together, a signal attenua­tion of 6 dB can be expected. Connecting three talkback outputs together will result in 9.5 dB of attenuation. 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.
IFB Input
The Model 220’s IFB input is designed to directly connect with “wet” (DC-biased) IFB circuits. These circuits provide DC power and one or two channels of audio over a standard 3-conductor microphone­style cable. Typically, the IFB circuit’s interface connector is a 3-pin male XLR 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. Some IFB circuits may
only have one audio channel. In this case, audio will generally be on pin 3 with pin 2 providing only DC power.
The power supplied by an IFB circuit, normally in the range of 28 to 32 volts DC, is usually sufficient to operate the Model 220’s circuitry. The acceptable input range is 24 to 32 volts, with a required current of 125 milliamperes. Note that the speci­fied input voltage is given when measured directly at the Model 220’s IFB input con­nector, not at the source of the IFB circuit. The one or two audio signals provided by the IFB circuit can serve as the audio sources for the headphone outputs.
While the Model 220’s IFB input was designed for connection to a “wet” IFB circuit, it’s also possible to connect line­level audio sources. Please refer to the Technical Notes section of this user guide for details.
Line Inputs
The Model 220 allows two line-level audio sources to be connected. These sources can be individually routed to the left-chan­nel, right-channel, or both the left- and right-channel 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 DC voltage that’s present on a connected source from im­pacting performance. The line inputs are compatible with signals that have a nomi­nal level of –12 dBV to +6 dBu. Two trim potentiometers, located on the bottom of the Model 220’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 connectors 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 connection from pin 1 can elimi­nate the issue. 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.
External Power Input
An external source of 24 volt DC power can be connected to the Model 220 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 requirement for the external source is nominally 24 volts, correct operation will take place over a 20 to 30 volt range. The Model 220 requires 80 milliamperes at 24 volts DC for correct operation. Included with each Model 220 is a 24 volt DC external power supply. The power supply’s DC output cable has been terminated with a Switchcraft® S760K coaxial power plug. This “locking” type of plug correctly mates with the Model 220’s 24 Vdc input jack. The locking feature is im­portant, allowing the external power source to be securely attached to the Model 220.
As previously discussed in this user guide, an IFB circuit connected to the IFB in­put can serve as the Model 220’s power source. Alternately, an external 24 volt DC source can be connected. For redundancy, both the IFB circuit and the external source can be connected at the same time. If one or the other becomes inoperative, the remaining source will provide all Model 220 power.
If both an IFB circuit and an external 24 volt DC source are connected, power will be drawn only from the external source. This minimizes the chance that lengthy cable runs or other IFB circuit issues will impact Model 220 operation. Whichever device is providing power, audio signals from the IFB circuit can still serve as the audio sources for the headphone output.
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.
Pushbutton Labeling
The three pushbutton switches used in the Model 220 were selected for several rea­sons. Foremost was the fact that they are highly reliable, using gold-plated contacts for long life in less-than-ideal environ­ments. A second reason was that apply­ing 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 applica­tions 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 stan­dard personal computer graphics program and laser printed onto 3M CG3300 trans­parency 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.
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 220 to support the needs of specific applications a number of op­erating parameters must be configured. These include microphone preamplifier gain, phantom power on/off, headphone source and output mode selection, and operating modes. Four 8-position DIP switch assemblies are used to establish the desired configuration. These switch assemblies are referred to as SW1 through SW4, with individual switches designated as SW1-1, SW1-2, etc. The switch assem­blies are accessed through openings in the bottom of the Model 220’s enclosure. The enclosure does not have to be disas­sembled to gain access to the switches.
To prevent unauthorized personnel from changing the configuration settings, a security plate is attached to the bottom
of the Model 220’s enclosure. For conve­nience, text and graphics on the security plate provides a summary of the configu­rable 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 4. Microphone preamplifier gain switch settings
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 experimenta­tion. The goal is to bring the mic’s signal up to line level, nominally –2 dBu for the Model 220’s main output. Operating at this
Figure 3. Bottom view of Model 220 showing configuration switches, trim pots, and compressor active LED
signal level will help to ensure the delivery of “clean” audio to the connected device. It’s also acceptable to connect a line-level
Microphone Preamplifier Gain
audio source to the microphone input. In this case selecting 0 dB (no gain) would
and Phantom Power
be appropriate.
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.
The output of the Model 220’s microphone preamplifier is used by the main output and, by way of the compressor circuit, the talkback outputs. So creating a nice “hot”
Microphone Preamplifier Gain
Switches SW1-1 through SW1-5 are used to select the gain of the microphone
signal will help maintain audio quality, spe­cifically a high signal-to-noise ratio, when driving the often-lengthy cable runs.
Unfortunately, there’s no “perfect” gain setting that this guide can recommend. The two issues that impact the setting are output sensitivity of the connected micro­phone and the acoustical output level of the microphone’s user. With some headset microphones, such as the Sennheiser HMD 26, selecting an initial setting of 40 dB is appropriate. 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 locat­ed adjacent to switch assembly 1. It is vis­ible by observing the bottom of the Model 220’s enclosure when the security plate has been removed. Technically, this red LED lights whenever the compressor cir­cuitry is controlling the dynamic range of the signal coming from the microphone preamplifier. The threshold is set to be 2 dB above the Model 220’s nominal in­ternal operating level. So a good “rule of thumb” is to adjust the gain of the micro­phone preamplifier so that the compres­sor active LED lights (“flashes”) when the connected microphone is sending signal peaks. During normal operation the LED should not remain fully lit when audio is present on the mic input.
were included. It’s possible that with a very “hot” microphone, such as a phan­tom-powered condenser type, 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 active (on) position the preamplifier will operate 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 outputs will be much too high.
Phantom Power On/Off
The Model 220 can provide 48 volt phan­tom power to a connected microphone. 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.
It’s important to remember that the com­pressor active LED is used to assist in set­ting the gain to the optimal value. It doesn’t necessarily indicate that the main output’s signal is being compressed. Unless specifi­cally configured to do otherwise, the out­put of the compressor is only used for the talkback outputs.
Figure 5. Phantom power switch settings
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
Headphone Source Selection
Switch assembly SW2 is used to config­ure the source or sources that are routed to the stereo headphone output. The headphone sources are IFB channel 1, IFB channel 2, line input 1, and line input
2. The IFB channels are provided by way of the IFB input connector located on the Model 220’s back panel. The line inputs are connected by way of two connectors also located on the back panel. Associ­ated with line inputs 1 and 2 are level trim potentiometers. They are provided 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.
Each of the available input sources can be assigned to the headphone output’s left channel, right channel, or both the left and right channels. The Model 220’s circuitry allows any combination of input assign­ments to be made. For example, consider the situation where a single-channel IFB system, with both program and interrupt audio on pin 3, is connected to the Model
220. In this case it may be desirable to assign this IFB signal to both the left and right channels. This would entail setting switches SW2-2 and SW2-6 to their on positions. All other switches would remain in their off positions.
A more complex application might have a 2-channel IFB circuit connected to the Model 220. In addition, a line-level audio signal from a golf event “spotter” is con­nected to line input 1. In a case such as this, it would be typical for IFB channel 1 to be assigned to the headphone’s left channel, IFB channel 2 assigned to the right channel, and line input 1 also
Figure 6. Left and right channel headphone source selection settings
assigned to the right channel. This would allow both IFB channel 2 and “spotter” audio to be heard in the headphone’s right-channel output. To achieve this would require that switches SW2-1, SW2-6, and SW2-7 be placed in their on positions. Note that using another announcer console unit from Studio Technologies at the “spotter” location could also prove effective. It would provide all the necessary microphone pre­amplifier, talkback routing, and headphone monitoring resources.
Note that in some cases a user may wish to wear a headset or a pair of headphones in a left/right orientation opposite of what is usual. In this situation the transducer designated for the left ear would actually supply audio to the user’s right ear, and vice versa. A specific application where this occurs is where on-air talent needs to have a headset’s boom microphone come
across the right side of their face, rather than the more-typical left side. In this case it’s important to select the left- and right­channel headphone source assignment accordingly. With the Model 220’s flexible source selection there’s no reason why users, such as on-air talent, shouldn’t have their cue sources assigned correctly.
There may be cases where a monaural “single-muff” headset or headphone will be connected to the Model 220’s head­phone output. In this case the desired source(s) should be routed only to the left channel. No sources should be assigned to the right channel. This will eliminate the short-circuit current that could occur when a 2-conductor (monaural) plug is mated with the Model 220’s 3-conductor (stereo) headphone output jack.
Headphone Output Mode
Switch SW1-6 allows a monaural head­phone output to be created. This is accom­plished by summing (adding) the selected left- and right-channel cue signals. The combined signals are sent to both the left­and right-channel headphone output driver circuits. The outputs of these circuits con­nect, by way of 100 ohm series protection resistors, to the headphone output jack.
The headphone output monaural mode feature was specifically included so that a special “2-channel headphone mix” mode can be created. By enabling the mono mode, the two front-panel user level controls (“pots”) can be used to create the desired “mix” of signals being sent to the headphone outputs. Many, many appli­cations, especially in production settings, can benefit from this capability. The desired cue sources must be carefully assigned to take advantage of the monaural mode. The first cue source should be assigned,
Figure 7. Headphone output mode settings
using the DIP switches, to the left channel. Its output level will be adjusted by the left control. The second cue source should be assigned to the right channel. Its output level will be adjusted by the right control.
There is one limitation related to the head­phone output mode. It’s the fact that the output will be 2-channel monaural. What­ever signal is present on the headphone output’s left channel will also be present on the right channel. (The exception is if the right channel output is disabled using the DIP switch setting.) A stereo head­phone mix can’t be created. But in most cases this limitation won’t overshadow the benefit of being able to create the mix. For signal-flow clarification please review the block diagram located at the end of this user guide.
Operating Modes
The sixteen switches associated with switch assemblies SW3 and SW4 are used to configure the Model 220’s operat­ing modes. Technically, these switches “talk” to the microcontroller integrated circuit and associated software that give the Model 220 its “smarts.” The software has been carefully designed to provide a number of different ways in which the unit can function. It’s critical to carefully review the available options and choose the ones that best meet the needs of a specific application. Note that switches can be changed even while the Model 220 is powered up and operating. The unit’s
operating characteristics will change in “real-time” in response to configuration changes.
Main Output Button Mode
Switches SW3-1 and SW3-2 configure how the main output button functions.
button is released. If the button is mo­mentarily “tapped” the main output will change state. Upon power up the main output will be in its muted state.
Talkback Output 1 Button Mode
Switch SW3-3 configures how the talkback output 1 button functions.
Figure 8. Talkback output 1 button mode settings
Figure 7. Main output button mode settings
There are four available modes:
• Push to mute: In this mode the main output is normally active. The main output will mute whenever the button is pressed and held. This is the “cough” mode typically used for on-air sports broadcasting applications.
• Push to talk: In this mode the main out put is normally muted. The main output will become active whenever the button is pressed and held.
• Alternate action: In this mode the main output will change between its active and muted state whenever the button is pressed. Upon power up the main output will be in its muted state.
• Hybrid: This mode is a combination of push to talk and alternate action. It’s similar to the way talk buttons function on user stations associated with broad­cast and production intercom systems. If the button is pressed and held, the main output will become active until the
Two modes are available:
• Push to talk: In this mode talkback out
­put 1 is normally muted. The output will become active whenever the button is pressed and held.
• Hybrid: This mode is a combination of push to talk and alternate action. If the button is pressed and held, talkback output 1 will become active until the but-
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ton is released. If the button is momen­tarily “tapped” the output will change state. Upon power up talkback output 1 will be in its muted state.
Talkback Output 2 Button Mode
Switch SW3-4 configures the way the talk
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back output 2 button functions.
Figure 9. Talkback output 2 button mode settings
Two modes are available:
• Push to talk: In this mode talkback out put 2 is normally muted. The output will become active whenever the button is pressed and held.
• Hybrid: This mode is a combination of push to talk and alternate action. If the button is pressed and held, talkback output 2 will become active until the but­ton is released. If the button is momen­tarily “tapped” the output will change state. Upon power up talkback output 2 will be in its muted state.
Main Button Mode
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Switch SW3-6 is used to configure how the main button responds to talkback ac­tivity. Specifically it applies only when the system is selected for the on-air mode and the main output button mode has been configured for alternate action or hybrid. When the main button mode is set to nor­mal and a talkback function is active, the main output will, if “latched” on, be tempo­rarily placed in its off (muted) state. When the talkback function is no longer active the main output will return to its previous latched on state.
Talkback Output 2 Function Mode
Switch SW3-5 configures the overall operation of talkback output 2.
Figure 10. Talkback output 2 function mode settings
Two modes are available:
• Enabled: In this mode the talkback output 2 button and the line-level audio output associated with the function operate normally. Whenever the button is active the audio output is also active. The exception is if the system mode is set so that talkback output 2 provides a “hot mic” output or serves as a second main output function.
• Disabled: In this mode the talkback output 2 button and associated audio output are disabled. The exception is if the auxiliary relay is configured to follow the status of the talkback output 2 button. In this case the button will be active to control the re­lay; the audio output will remain disabled.
When the talkback forces main button to latch off mode is selected, and a talk­back function becomes active, the main output will, if latched on, be placed in its off (muted) state and the latch condition forced off. When the talkback function is no longer active the main output will remain in the latched off state.
The differences in how the main button modes impact operation and user comfort are relatively subtle. The nuances of both modes should be considered before a final selection is made. If possible, experi­menting with both modes in a test environ­ment might prove to be very helpful.
Figure 11. Main button mode settings
Auxiliary Relay Control Mode
Switch SW3-7 and SW3-8 configure the operating mode of the auxiliary relay.
Figure 12. Auxiliary relay control mode settings
Four modes are available:
• Relay disabled: In this mode the relay is disabled and will never change state.
• Follows main output status: In this mode the relay will follow the status of the main output. Specifically, the relay will change state (energize) whenever the main output is active.
• Follows talkback output 1 button sta
­tus: In this mode the relay will follow the status of the button associated with talkback output 1. Specifically, the relay will change state (energize) whenever the button is active.
the way the controls function. With just these three switches a wide range of oper­ating modes can be configured. Carefully reviewing the capabilities of the available functions may prove worthwhile.
Dual-Channel or Stereo Mode
Switch SW4-1 is used to select whether the controls provide a dual-channel (“level/level”) or stereo (“level/balance”) mode of operation. In the level/level mode the two controls operate independently, each controlling the level of one of the headphone output channels. This mode is generally used for on-air broadcast applications where independent cue signals are provided to the left- and right­headphone channels. In the level/balance mode the left rotary control sets the overall output level for both headphone channels. The right rotary control is used to adjust the balance (the relative levels) of the left and right channels. This mode is generally best suited for applications where a stereo cue source is being provided.
• Follows talkback output 2 button sta
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Figure 13. Headphone control mode settings
tus: In this mode the relay will follow the status of the button associated with talkback output 2. Specifically, the relay will change (energize) state whenever the button is active.
Reverse Left/Right Mode
Switch SW4-2 is used to select whether the rotary controls are in the normal or reverse left/right mode of operation. When selected to the normal mode, and level/level mode
Headphone Output Operating Modes
The user is provided with two rotary level controls (“pots”) that are associated with the headphone output. Switches SW4-1, SW4-2, and SW4-3 are used to configure
is also selected, the left control adjusts the level of headphone output’s left channel. (This is the signal that appears on the tip lead of the ¼-inch 3-conductor jack.) The right control adjusts the level of the right channel. When selected to the normal
Figure 14. Headphone control operation settings
mode, and the level/balance mode is also selected, turning the balance control in the counterclockwise direction increases the perceived level of the left channel, and vice versa.
As you may have already guessed, when selecting the reverse left/right mode of op­eration everything is reversed! To be more specific, when selected for reverse mode, and the level/level mode is also selected, the left control adjusts the headphone output’s right channel (output jack’s ring lead) while the right control adjusts the left channel. When selected to the reverse mode, and the level/balance is also se­lected, turning the balance control in the counterclockwise direction increases the perceived level of the right channel, and vice versa.
The reverse mode is provided specifically for cases where a headset’s left and right ear pieces are placed on a user’s head in a reverse orientation. This ensures that the user is provided with a consistent and easy-to-use set of headphone level controls.
Minimum Level Mode
Switch SW4-3 is used to configure the headphone output’s minimum level. In the –40 dB mode the minimum headphone output level is 40 dB below maximum. The headphone output channels will never fully mute. This ensures that any audio signal present on the selected Model 220 inputs
Figure 15. Headphone output minimum level settings
will always be present on the headphone output. In most on-air broadcast applica­tions this is the appropriate setting.
When the full mute mode is selected and the level/level mode is also selected, mov­ing either control to its fully counterclock­wise position will cause its associated channel to fully mute.
When the full mute mode is selected and the level/balance mode is also selected, turning the level control to its fully coun­terclockwise position will cause both headphone channels to mute. Turning the balance control to either its fully clock­wise or fully counterclockwise position will cause the appropriate channel to mute.
Selecting the full mute mode may be appropriate for applications where mini­mizing the chance of audio “leakage” is important. This could occur when the connected headset or headphones are at times placed on a desk or tabletop.
Main Output Source
Switch SW4-4 is used to select which audio source is routed to the main output. The choices are the output of the micro­phone preamplifier or the output of the compressor circuit. For most on-air ap­plications the output of the microphone preamplifier is the desired source. This will provide the most natural audio quality with the potential for a large amount of dynamic range.
Figure 16. Main output source settings
Figure 17. System mode settings
In some applications it may be desirable for the output of the compressor circuit to be routed to the main output. Appropriate applications could include on-air broad­cast situations where nonprofessional talent is present. Controlling the dynamic range of the audio signals on the main output can limit the chance of cable cross­talk and equipment overload. Another typi­cal application where using the output of the compressor would be when the Model 220’s system mode is selected for produc­tion. In this case the main output would be used as an additional talkback output and dynamic range control could be beneficial.
System Mode
Switch SW4-8 is used to configure the overall operating mode of the Model 220. Understanding how the two modes impact overall system operation will ensure that correct operation and maximum usability will occur. When selected to the on-air mode, the main output will mute when­ever a talkback function is active. The LED indicators associated with the main output will light accordingly. The on-air mode should be selected for all on-air broadcast applications. It’s imperative that the main output be muted whenever on-air talent is using one the talkback functions to com­municate with production personnel.
When the system mode is set for pro­duction, the main output is never muted in response to a talkback output being active. This mode allows the main output
to be used, for example, as an additional talkback output. In this way the main and talkback outputs can be used indepen­dently, with neither impacting the other. This also allows all three buttons to be used simultaneously.
In summary, when selected for the correct application, each system mode can prove to be very useful. A thorough study of how they impact the Model 220’s operation can lead to many interesting and powerful uses.
Conclusion
Once the switches have been set to the desired configuration, it may be time to reattach the security plate. The exception is if the trim pots associated with the line inputs are going to be adjusted. Details are provided later in this user guide. The plate attaches using the four rubber bum­pers. They should be hand-tightened only; no tools are to be used.

Operation

At this point the desired input, output, and power connections should have been made. The button labels may have been revised. Finally, the configuration switches should have been set. Normal operation of the Model 220 can now begin. The unit will begin functioning as soon as a power source is connected. As previ­ously discussed, the power source can
be provided by an IFB circuit, an external 24 volt DC power source, or both. It’s im­portant to highlight the fact that the Model 220 is an active device. Audio signals will not be present on the outputs if correct power has not been supplied. Specifically, the microphone does not passively “cut through” to the main output connector!
Upon Model 220 power up, the four status LEDs will light in succession as a firmware “boot up” indication. The unit will then begin normal operation. Depending on the selected configuration, one LED as­sociated with the status of the main output may be lit. The user is now presented with three buttons, four LEDs, and two rotary controls. These are simple to operate and understand, as will be described in the following paragraphs.
Pushbutton Switches and Status LEDs
Three pushbutton switches are used to control the main and talkback outputs. The way each operates depends on the selected configuration. Four LED indica­tors are located adjacent to the buttons. They reflect the status of the main and talkback output functions.
mode is configured to on-air, the red LED, located on the left, will be lit when the main output is muted. This indicates that it’s safe to speak as one sees fit.
If the Model 220’s system mode is config ured to production, the red LED will never light. This is to reflect the fact that the main output button has now taken on a function similar to that of one of the talkback output buttons. To clarify, when the Model 220 is set to the production mode, the red LED will never light; the green LED will light whenever the main output is active.
Main Output Button Modes
• Push to mute: If this mode is selected the main output is normally active. The main output will mute whenever the button is pressed and held.
• Push to talk: If this mode is selected the main output is normally muted. The main output will become active when­ever the button is pressed and held.
• Alternate action: If this mode is selected the main output will alternate between its active and muted states whenever the button is pressed. Upon power up the main output will be in its muted state.
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Main Output Button and LED Indicators
The button on the left, factory labeled as COUGH, functions according to the se­lected configuration. Two LED indicators, located directly above the button, are as­sociated with the status of the main out­put. The green LED, located on the right, is lit whenever the main output is active. This could be considered as an “on-air” or mic active indicator. At the least it should serve as a “careful what you say” warning! If the Model 220’s system
• Hybrid: This mode is a combination of push to talk and alternate action. It’s similar to the way talk buttons function on user stations associated with broad­cast or production intercom systems. If the button is pressed and held, the main output will become active until the button is released. If the button is momentarily “tapped” the main out­put will change state. Upon Model 220 power up the main output will be in its muted state.
Main Output vis-à-vis Talkback Activity
This short section applies only in the case where the Model 220’s system mode is configured for on-air and the main output button mode is set to alternate action or hybrid.
How the main output responds to talk­back activity depends on the configu­ration of the main output mode. In the normal mode talkback activity will always cause the main output to be placed in its off state. If the main output was in the “latched” on state when talkback began, once talkback activity ends that state will resume; the main output will again be in its on (“latched”) state. If the main output mode is configured to force off, talkback activity will cause the main output to be placed in its off state. Once talkback activ­ity ends the main output will remain in its off (“unlatched”) state. In critical on-air broadcast applications it’s important that a Model 220 user be aware of how the selected mode impacts operation.
Talkback 1 Button and LED Indicator
The button in the center, factory labeled TALKBACK 1, controls talkback output 1. The manner in which the button functions depends on the way it was configured. An LED indicator, green in color, is located directly above the button. It lights when­ever the talkback output 1 is active. If the Model 220 is selected to the on-air mode, whenever the talkback output 1 function is active the main output will be placed in the muted state. If the Model 220 is selected to the production mode, the status of talkback output 1 will not impact the main output.
Talkback 1 Button Modes
• Push to talk: If this mode is selected talkback output 1 is normally muted. The output will become active whenever the button is pressed and held.
• Hybrid: This mode is a combination of push to talk and alternate action. If the button is pressed and held, talkback output 1 will become active until the but­ton is released. If the button is momen­tarily “tapped” the output will change state. Upon Model 220 power up talk­back output 1 will be in its muted state.
Talkback 2 Button and LED Indicator
The button on the right, factory labeled TALKBACK 2, controls the function as­sociated with the talkback 2 button. The manner in which the button functions de­pends on how it was configured. An LED indicator, green in color, is located directly above the button. If the talkback 2 function mode has been configured for enabled, it lights whenever talkback 2 is active. If the talkback output 2 function mode has been configured for disabled, it will light only if the auxiliary relay has been assigned to follow the talkback 2 button.
If the Model 220 is selected to the on-air system mode, whenever talkback 2 is ac­tive the main output will be placed in the muted state. If the Model 220 is set to the production system mode the status of talk­back 2 will not impact the main output.
Talkback 2 Button Modes
• Push to talk: If this mode is selected the function associated with talkback 2 is normally off. The function will become active whenever the button is pressed and held.
• Hybrid: This mode is a combination of push to talk and alternate action. If the button is pressed and held, the function associated with the talkback 2 button will become active until the button is released. If the button is momentarily “tapped” the function will change state. Upon Model 220 power up talkback 2 will be in its off state.
Headphone Output Level Controls
Two rotary controls (“pots”) are located on the Model 220’s front panel and are as­sociated with the headphone output. The way the controls function depends on the selected configuration. One configuration parameter sets the controls to operate in a level/level or a level/balance mode. Anoth­er parameter allows the left/right assign­ment of the controls to be reversed. A third parameter selects whether the headphone output channels will maintain a minimum output level or can be fully muted.
Level/Level Mode
When set to the level/level mode, the two controls operate independently. Each control sets the output level of one chan­nel of the stereo headphone output. If con­figured to the normal mode, the control on the left side is used to adjust the level of the headphone output’s left channel. The control on the right is used to adjust the level of the right channel. If configured to the reverse left/right mode, the control on the left side adjusts the headphone output’s right channel. The control on the right adjusts the left channel.
When in the level/level mode, and the minimum output level is set for –40 dB, turning each control to its fully counter­clockwise position will place its respective output level 40 dB below its maximum.
This setting ensures that talent will never be fully “isolated” from potentially impor­tant cue signals. If present on one of the Model 220’s inputs and assigned to the headphone channels, some audio signal will always be present on the headphone output. If set to the full mute mode, turning each control to its fully counterclockwise position will cause its respective output to fully mute.
Each level control has a mechanical step (detent) that is located at the halfway (50%) position of its rotation range. This is intended to serve as an aid to Model 220 users. In an ideal installation, setting the controls to their detent position will result in a comfortable headphone output level. The user, in response to a changing op­erating environment, can then move the level controls to get more or less level as desired. The detent position will always remain as a useful reference point. To achieve this condition the audio level on the connected IFB circuit, or line inputs, will have to be calibrated as required. This is somewhat counter to the usual mental­ity of just providing the user with whatever level comes up by default. Spending a few extra minutes “trimming” the audio levels can result in much happier, and more pro­ductive, talent. As previously mentioned, a level trim potentiometer is associated with each line input. If the line inputs are utilized as cue sources, adjusting the trim pots may be helpful in achieving the de­sired adjustment range of the level con­trols. Refer to the Technical Notes section of this user guide for details.
Level/Balance Mode
When set to the level/balance mode, the two controls operate together to adjust the desired headphone output level. The con­trol on the left adjusts the overall level of
both the left and right channels. The bal­ance control, located on the right, adjusts the relative left/right level balance. In this mode the controls operate in a manner reminiscent of a consumer audio ampli­fier or receiver. If set to the normal mode, rotating the balance control in the coun­terclockwise direction reduces the level of the right channel providing the user with more perceived level in the left channel. If set to the reverse mode, rotating the balance control in the counterclockwise direction reduces the level of the left chan­nel providing the user with more perceived level in the right channel.
When in the level/balance mode and the minimum output level is set to –40 dB, turning the level control to its fully coun­terclockwise position will place both head­phone output channels to 40 dB below maximum. This ensures that talent will never be fully “isolated” from potentially important cue signals. In addition, rotat­ing the balance control to either its fully clockwise or fully counterclockwise posi­tion will cause the applicable channel to be 40 dB below its maximum. If set to the full mute mode, turning the level control to its fully counterclockwise position will cause both the left and right channels to fully mute. In addition, rotating the balance control to either its fully clockwise or fully counterclockwise position will cause the applicable channel to fully mute.
Each control has a mechanical step (detent) that is located at the halfway (50%) position of its rotation range. The balance control will typically be set to its center, detent position, making the level of the left and right channels equal. In an ideal installation, the level control can also
be set to its detent position and provide a comfortable headphone output level. This will allow the user, in response to their preference or a changing environment, to adjust the level and balance controls as desired. The detent positions will always remain as a useful reference point. To achieve this condition the audio level on the connected IFB circuit, or line inputs, must be adjusted as required. Spend­ing a few extra minutes “trimming” the connected audio levels, rather than just providing whatever happens to come up, should prove worthwhile. The result will be talent that is more relaxed, and an overall production that works more smoothly. As previously mentioned, a level trim potenti­ometer is associated with each line input. If the line inputs are utilized as cue sourc­es, adjusting the trim pots may be helpful in achieving the desired adjustment range of the level controls. Refer to the Technical Notes section of this user guide for details.
Monaural Headphone Output Level Control Operation
As previously discussed, the headphone output can be set for monaural operation. This option is specifically provided so that a “2-channel headphone mix” mode can be created. If this has been enabled, the two front-panel level controls are used as a cue source mixer. Independent cue inputs will be assigned to each chan­nel, serving as the mix sources. How the controls respond when placed in their fully counterclockwise position will depend on the configuration. By default the output level will be 40 dB below its maximum level. An alternate configuration will make the outputs mute.

Advanced Operation

Adjusting the Line Input Trim Pots
As has been previously mentioned, as­sociated with the line inputs are trim pots that allow the input levels to be adjusted. The two trim pots are accessible by way of round openings in the bottom of the Model 220’s enclosure. By adjusting these trim pots, signals with a nominal level of –12 dBV to +6 dBu can be effectively used as cue sources. Unfortunately, there are no definitive rules regarding how best to adjust the trim pots, but some suggestions may prove to be valuable. Depending on how the line inputs are utilized, the trim pots can be used to either adjust the ab­solute level of each line input signal, or to adjust the relative level of the signals when compared to other sources. The following examples may provide some clarification.
Let’s begin with an application that has a stereo cue source connected to the line inputs. The source selection DIP switches are configured to create a stereo head­phone output with line input 1 assigned to the left channel and line input 2 assigned to the right channel. Begin the trim pot adjustment process by moving the user level controls (located on the front panel) to their detent (50% of rotation) positions. Then, with the stereo cue source provid­ing signal at its normal level, adjust the trim pots to provide a comfortable level to the connected headphones. The user can now, in response to changing condi­tions, adjust the front-panel level controls
Figure 18. Bottom view showing line input trim pots
as desired. Returning the controls to their detent positions will always provide the “reference” level to the headphone output.
A second example has the IFB input and line input 1 both providing cue sources. Channel 1 of the IFB circuit supplies pro­gram-with-interrupt audio that is routed to the headphone output’s left channel. Channel 2 of the IFB circuit supplies program-only audio that is routed to the right channel. Line input 1 is connected to an audio source associated with a sports­event “spotter” position. This source is routed to the headphone output’s right channel. The input trim pot associated with line input 1 can now serve a critical role—adjusting the relative level of the “spotter” audio as compared to the level of IFB channel 2. The trim pot allows the desired “mix” to be created, providing the user with an effective cue signal.

Technical Notes

Grounding and Shielding
As previously discussed in this user guide, the pin 1 connections on the main and talkback outputs’ 3-pin male XLR connec­tors are “floating,” i.e., not connected to anything within the Model 220’s enclosure. Some audio experts might take offense to this, grousing that this should have been left to the user or installer to be connected or disconnected as desired. However, repeated field testing found that floating pin 1 on the outputs was the key to main­taining quiet audio. From Fenway Park in Boston, to the Orange Bowl in Miami, and then northwest to Husker Stadium in Nebraska, lifting pin 1 did the trick.
A simple solution is available if an applica­tion does require that a ground be avail­able on the main and talkback outputs’ interconnecting cables. All Model 220 XLR connectors have a ground connection that is made to the interfacing connector’s metal “shell.” And most XLR connectors have a pin or connection point available to access its metal shell. By connecting the cable shield to the mating connector’s shell terminal, the common connection typically found on audio interconnections is created.
IFB Channel Crosstalk
By the very nature of its design, a stan­dard “wet” IFB circuit is prone to exhibit crosstalk between its two audio channels. This occurs because the audio paths are unbalanced (“single-ended”) and typically transported on a single shielded twisted­pair audio cable. The primary cause of the crosstalk is the capacitance between the wires in the cable pair. The greater the capacitance, due to cable type and length,
the more crosstalk there will be. It’s not surprising to find in sports broadcasting venues that audio from one channel in an IFB circuit can be heard “bleeding” into the other channel. Does this generally cre­ate a problem? No, as each channel in an IFB circuit generally carries related audio content. For example, on-air talent hearing in their left ear a small amount of program audio from channel 2 while an interrupt from channel 1 is active, typically wouldn’t perceive this as an issue.
There are several ways of reducing IFB channel crosstalk. Probably the easiest way is to use cable pairs that are not twist­ed. Twisted pairs are great for differential (balanced) signals, but not so great for unbalanced transmission. This is gener­ally because the more twists in a pair the greater the effective cable capacitance. In a stadium or arena setting, choosing standard “telco” pairs may actually work better than “high-performance” audio or data cable!
Another option is to use two cable pairs for each IFB circuit. If the pairs are not shielded the wiring is simple. Common would be connected to one side of each pair, and then signal from each channel would connect to the other side of the pairs. If the pairs also contain shields the wiring could be done somewhat differ­ently. One option is to connect common to both cable shields, IFB channel 1 (DC with audio) to one full pair, and IFB chan­nel 2 to the second full pair. A better op­tion might be to have common connect to both shields and one side of the pair that serves IFB channel 2.
Other options are available if an applica­tion demands low crosstalk. If resources in the broadcast or production facility allow one method would be to run the
IFB circuits “dry” (no DC) and differential (balanced). This would gain the benefits of differential transmission, including mini­mizing the crosstalk. The balanced line­level signals can be directly connected to the Model 220’s line inputs. However, if it’s desired to connect cue sources to the Model 220 using an IFB circuit, the line­level audio signals will need to be “wet­ted up” into standard IFB circuits. This is easily accomplished using one of several high-performance IFB interface units from Studio Technologies. For further informa­tion please refer to the Studio Technolo­gies website.
IFB Audio Levels
The Model 220 is designed to operate best with IFB audio levels that are nomi­nally –10 dBu. This is the nominal level of most IFB systems. But actually having the correct level present on an IFB circuit is of­ten a “hit-or-miss” proposition. During field testing of prototype announcer’s consoles, Studio Technologies’ personnel found that a wide range of nominal audio levels were present on “real-world” IFB circuits. Many were fine, being reasonably close to the desired –10 dBu. But some were much too low, while others were much too “hot.” We observed one unfortunate baseball “color” commentator being sent interrupt audio signals so “hot” relative to program audio as to almost make his ears bleed! This situation should not have been allowed to happen.
In defense of field technical personnel, measuring the audio level of an IFB circuit hasn’t traditionally been an easy proposi­tion. But that situation has now changed. After experiencing this condition in the “field,” Studio Technologies’ engineers were motivated to design the Model 72
Level Meter/Interface. This compact de­vice plugs directly into IFB and intercom circuits and provides two useful functions: level meters and “dry” audio outputs. Two 5-segment LED meters allow direct obser­vation of the audio signal levels present on IFB or intercom circuits. The display range is optimized for the signal levels found on these circuits, rather than traditional “VU” scaling.
The Model 72 also provides two trans­former-coupled “dry” audio outputs, one for each IFB or intercom channel. These outputs are useful for a variety of produc­tion and testing applications. For example, the outputs can serve as the interface between a traditional “wet” IFB system and a wireless in-ear monitor system. The outputs can also be connected to a moni­tor panel, allowing visual and aural moni­toring of the IFB audio signals.
In conclusion, we’re sorry for this shame­less promotion of the Model 72 Level Meter/Interface! But necessity was defi­nitely the “mother” when it came to the unit’s invention. Working “in the field” with­out such a device, we felt “blind” when connecting to IFB circuits. That no longer has to be the case and we think that you’ll find owning one a very worthwhile invest­ment. For further information please refer to the Studio Technologies website.
Phantom Power
The Model 220 provides a 48 volt nomi­nal source of phantom power to support condenser microphones. It’s designed to meet the P48 requirements as specified in the IEC 61938 standard. The circuitry is very simple: 6.85 k ohm resistors pro­vide a path from a 45 volt source to pins 2 and 3 of the microphone input connector.
The resistors and the power source work together to provide 48 ±4 volts, up to a maximum current of 10 milliamperes.
Symptoms of Insufficient Power
A core part of the Model 220’s internal cir­cuitry is a switch-mode power supply that produces +45 volts, +12 volts, +5 volts, and –12 volts. This power supply circuit works very well as long as it is “fed” with sufficient input voltage and current. “Suf­ficient” is defined as a minimum of 24 volts on the IFB input and 20 volts on the exter­nal 24 volt DC input. The necessary cur­rent, 125 milliamperes for the IFB input and 80 milliamperes for the external input, must be supplied over their respective voltage ranges.
It’s worth discussing what will happen if either power source falls below its specified minimum. Typically, if the Model 220 is be­ing powered by an external 24 volt nominal power source, normal operation will con­tinue until the input falls to the 18-20 volt range. As the input voltage drops below this range the Model 220’s internal power supply will have reduced stability, operating in this manner until its low-voltage shut­down circuit halts operation. Note that as the input voltage moves down from 24 volts the input current will rise proportionately to make up for the loss of power.
If an IFB circuit is powering the Model 220, maintaining the required voltage and cur­rent is more critical. Should the voltage or current fall below the specified minimum, the Model 220’s power supply circuit will again become unstable. This will become an issue as noise will be induced into the IFB circuit’s audio signals. The reason is simple: an IFB circuit “multiplexes” 3-conductors so that they carry both power
and audio signals. If sufficient amounts of voltage and current are supplied to the Model 220’s IFB input, the Model 220’s power supply will draw a steady amount of energy. This will not disturb the analog signals on pin 1 (common for DC and au­dio) and pin 2 (DC and channel 1 audio). But if the Model 220’s power supply is not supplied with sufficient energy (volts x amps) it will try to draw what it needs from the IFB circuit, becoming unstable in the process. The IFB circuit’s audio signals will be corrupted by the power supply’s attempt to draw enough power. Instead of nice clean audio there will be squeaks, squeals, and some awfully funky noises added. Again, in a low-voltage or low­current situation, no damage will be done to the Model 220’s circuitry but correct operation will not be possible.
In most cases maintaining the IFB circuit’s required voltage and current shouldn’t be a problem. But issues may arise due to malfunctioning IFB circuit sources or poor interconnect cabling. Typically, ex­cessive cable length won’t be the cause of a problem. Generally, problems will be caused by broken or damaged connec­tor pins, dirty patch points, or damaged (partially open) cable conductors. Measur­ing the IFB circuit’s voltage and current draw directly at the Model 220’s IFB input connector will quickly identify if there’s a power issue.
And now for another shameless “plug” for other Studio Technologies products: Frankly, most devices that supply IFB circuits for broadcast applications use outdated technology that provides medio­cre performance. That’s why Studio Tech­nologies’ developed high-performance IFB interface units. These products do an excellent job of providing power and audio
to connected devices such as the Model
220. However, unlike other products, the
power supplied by these units’ IFB circuits maintain their output voltage all the way to their full rated current. The result is being able to power more devices over longer cable runs. In addition, the audio quality of these units is superior. For further informa­tion please refer to the Studio Technolo­gies website.
LED Colors
As previously described, two LED indica­tors are associated with the main output and are located directly above the main output pushbutton switch. The red LED, located on the left, is lit whenever the main output is muted. The green LED, located on the right, is lit whenever the main output is active. The thought pro­cess behind the color choices was that red would relate to the main output being muted (“stop”) while green would relate to the main output being active (“go”). It’s possible that these color choices may not meet the needs of all users and applica­tions. For example, it’s reported that one European broadcaster typically uses these two colors in the opposite fashion. Their choice is to have the red LED lit whenever the main output is active, warning the tal­ent that they are “on-air.” The green LED is lit whenever the main output is muted, indicating to the talent that it’s “safe” to say whatever they wish, about whomever they wish to say it about!
For consistency, the LEDs associated with the talkback buttons were selected to be green. They light whenever their associ­ated talkback functions are active. It’s pos­sible that some applications may benefit from revising these LED colors too. While red is certainly one possible choice, other
colors are also a possibility, including amber, orange, blue, or white—these days there are lots of choices available.
A qualified technician can easily revise the LED colors to meet an application’s exact needs. The process would begin by disassembling the Model 220’s enclo­sure and detaching the pushbutton/LED printed circuit board assembly. The LEDs would then be unsoldered, removed, and reinstalled (or replaced) in the desired locations. To control the LED current and set the brightness, a 825 ohm 1206-size surface-mount resistor is electrically in series with each LED.
For additional information about changing the LED colors, please contact Studio Technologies’ technical support.
Travel Case
For portable applications it may be desir­able to store and transport each Model 220 in a protective case. After much travel with prototype announcer console units, Studio Technologies’ personnel learned to appreciate the Pelican Model 1450 case. Purchased with the foam interior option, it does an excellent job of holding one Model 220, its associated 24 volt DC power supply, and documentation. Some applications may benefit from selecting a larger case that would also hold a related headset, cables, etc. A larger case could also be selected that would hold multiple Model 220 units. Pelican sells their prod­ucts through a dealer network, many of which can be located via a web search.
Connecting Line-Level Audio to the IFB Input
In “emergency” situations it’s possible to connect line-level audio signals directly to the Model 220’s IFB input connector.
This can be successfully done as long as several limitations are taken into account. The first limitation is that the 10 k ohm input circuit presents an unbalanced load to the source. In most cases this shouldn’t pose a problem. If a balanced interconnection scheme must be maintained an in-line iso­lation transformer can be used. A second limitation is that the audio level presented must not exceed 0 dBu or signal “clipping” may occur. Prepare a 3-pin male XLR con­nector so that the line-level audio source designated as IFB channel 1 is connected with signal high on pin 2 and low/shield on pin 1. The audio source designated as IFB channel 2 should be connected with signal high on pin 3 and low/shield on pin 1. With this connection scheme the nominal input level is –10 dBu, the same as with an IFB circuit. This may require that an external attenuator (“pad”) be used to reduce the level of the connected signal. As expected, powering the Model 220 in this scenario will require an external source of 24 volt DC to be connected.
Additional Connectors
Three spare connector locations are provided on the Model 220’s back panel. From the factory they contain blank plates that can be readily removed and replaced with a variety of XLR connectors. These spare connector locations are specifically included so that a Model 220 can be cus­tomized to meet the many specific needs that arise in broadcast and related audio applications. Expected uses for these locations include adding a 5-, 6-, or 7-pin XLR connector to allow direct connection of a broadcast headset. Other uses include creating “loop through” or “mult” functions for the talkback outputs or IFB input connections.
The spare connector locations are com­patible with the Neutrik DL-series of connectors. For flexibility, versions are available that provide from three to seven contacts. For example, a compatible 3-pin female connector would be Neutrik part number NC3FD-L-1. To support headsets the NC6FDS-L-1 is often used. This is a 6-pin female connector with the unique Switchcraft 6-pin arrangement. The hard­ware that secures the blank plates to the Model 220’s back panel is also intended to secure the replacement connectors.
If connectors are added to the Model 220’s spare connector locations adding labels to those connectors can be helpful. For a great look it is recommended that Brother® P-Touch ¼-inch (6 mm) labels be created. Tape material that prints white text on a black background works out well for the Model 220. The Brother label cassette number TX-3151, white on black, is appropriate for use with many of their printers.
In addition to the spare connector loca­tions on the back panel, provision has been made to allow easy interconnection with the Model 220’s printed-circuit-board­mounted input and output connectors. This was accomplished by including numerous 3-position male “header” con­nectors on the Model 220’s circuit board. These headers, on 0.1-inch centers, are wired in parallel with the Model 220’s con­nectors. This “no solder” solution makes customizing a Model 220 a simple pro­cess. The headers, located on the Model 220’s printed circuit board, are Molex® part number 22-23-2031. They mate with Molex housing number 22-01-3037. To make the inter-connection, separate crimp terminals are attached to loose wires and then “snapped” into the housing. Molex
part number 0850-0114 specifies crimp terminals that are appropriate for wires of 22 to 30 gauge. These parts are available worldwide from sources such as Digi-Key, website www.digikey.com.
To make the process of connecting to the Model 220’s headers a simple task an interface cable kit, part number 31087, is available from Studio Technologies. Each kit includes five cable assemblies and a length of heat-shrinkable tubing. Each cable assembly consists of a mating connector with three color-coded wires attached. These wires, approximately 12 inches in length, allow convenient solder­ing to a connector slated to be installed in a spare location on the Model 220’s back panel. For reference, the wire color for pin 1 is gray, pin 2 is yellow, and pin 3 is blue.
the Model 220 is being customized. For easier access, the pushbutton/LED board assembly can also be easily removed.
The 3-position headers on the Model 220’s main circuit board assembly are lo­cated close to their related input or output connectors. The following list provides the printed circuit board reference numbers and associated functions:
P4: Headphone Output, pin 1 common, pin 2 tip (left), pin 3 ring (right).
P5: Microphone Input, pin 1 shield, pin 2 high, pin 3 low. Follows back-panel 3-pin female XLR pin assignment.
P6: Main Output, pin 1 shield, pin 2 high, pin 3 low. Careful! Back-panel 3-pin male XLR has pin 1 floating, pin 2 high, pin 3 low.
The heat-shrinkable tubing is provided so that the connector solder cups can be in­sulated from each other. It will also provide some strain relief to the solder joints. Be certain to slip the desired length of tubing over the wire prior to soldering a connec­tion! (If the writer had a dollar for every time he forgot to put tubing on a wire (or slip on a connector shell) before making a solder connection…)
The Model 220’s enclosure must be disas­sembled prior to installing connectors in the spare locations. Four hex-head ma­chine screws, two on the bottom front of the enclosure and two on the back panel, must be removed. A 5/64-inch hex driver is required. The cover can then be care­fully separated from the chassis, remain­ing attached by means of a flexible cable assembly. This “flexible” assembly links the main printed circuit board assembly with the board assembly that contains the pushbuttons and LED indicators. Ensure that the flex cable is not damaged while
P7: Talkback Output 1, pin 1 shield, pin 2 high, pin 3 low. Careful! Back-panel 3-pin male XLR has pin 1 floating, pin 2 high, pin 3 low.
P8: Talkback Output 2, pin 1 shield, pin 2 high, pin 3 low. Careful! Back-panel 3-pin male XLR has pin 1 floating, pin 2 high, pin 3 low.
P11: IFB Input, pin 1 common, pin 2 DC with channel 1 audio, pin 3 channel 2 audio. Follows back-panel 3-pin female XLR pin assignment.
P13: External 24 Volt DC Input, pin 1 common, pin 2 +24 volts, pin 3 not used. Back-panel 2.1 x 5.5 mm jack has +24 volts on center pin. Header P12 is used by the back-panel 24VDC jack assembly and is electrically in parallel with P13.
P14: Line Input 1, pin 1 common, pin 2 high, pin 3 low.
P15: Line Input 2, pin 1 common, pin 2 high, pin 3 low.
Additional functions can be accessed using these 3-position headers:
P9: Auxiliary Relay Contact B, pin 1 normally closed, pin 2 common, pin 3 normally open.
P10: Auxiliary Relay Contact A, pin 1 normally closed, pin 2 common, pin 3 normally open.
P16: Internal Power Supply Rails, pin 1 common, pin 2 +12 Vdc, pin 3 -12 Vdc.
P17: Remote Switch Connections, pin 1 common, pin 2 talkback 1 button, pin 3 talkback 2 button.
P18: Remote Switch Connections, pin 1 common, pin 2 main out button, pin 3 auxiliary (currently not supported in software).
P20: Remote Data Interface, pin 1 common, pin 2 data +, pin 3 data -.
P21: Internal Power Supply Rails, pin 1 common, pin 2 +12 Vdc, pin 3 -12 Vdc.
Pushbutton Backlighting
For special applications, provision has been made to allow LED illumination (“backlighting”) of the three pushbutton switches. This may prove useful for appli­cations where adequate room lighting is not available. It can also serve in custom Model 220 configurations. It’s important to note that the button lighting is not intend­ed to serve tally applications. It is strictly intended to provide a moderate amount of illumination of the button’s clear lens and associated labeling.
From the factory, LED lamps are not installed in the pushbutton housings. The socket in each housing was designed for inserting a pluggable T-1 bi-pin incan­descent type but we find that the more
modern leaded T1 LEDs work very well. The Model 220’s 5 volt DC power supply is connected to the socket in each push­button housing by way of 2.2 k (2200 ohm) resistors. With this implementation, along with the recommended LED, only 1 milliampere per LED is consumed.
It’s intended that high-efficiency white LEDs be installed. They use very little DC current and have essentially infinite life. The recommended LED is the Kingbright WP7104QWC/D which as of the time of this writing is available from sources such as Digi-Key (www.digikey.com) and Mouser Electronics (www. mouser.com).
The mating socket in each pushbutton as­sembly is accessed by removing the but­ton’s lens caps, graphic label, and back frosted lens. Once this is done installing the LED should be very simple, requiring only lead trimming and careful insertion. Begin by removing the top clear lens, la­bel, and frosted under-lens in each push­button. Trim the LED leads to 0.25 inches in length and insert into the sockets with the cathode end located nearest the front of the Model 220. Power up the Model 220 and observe if the LEDs light. The LEDs are polarity sensitive and if one or more of them do not light, remove the offending LED(s), rotate them 180 degrees, and then re-insert.
Remote Control Connections
Provision has been made on the Model 220’s printed circuit board assembly to allow external switches or contact closures to control the main and talkback output functions. Two 3-position headers, labeled P17 and P18, provide access to the cir­cuitry associated with the functions.
P17: Pin 1 is connected to the common point of the Model 220’s circuitry. Pin 2 is connected to the circuitry associated with the talkback 1 button. Pin 3 is connected to the circuitry associated with the talk­back 2 button.
P18: Pin 1 is connected to the common point of the Model 220’s circuitry. Pin 2 is connected to the circuitry associated with the main out button. Pin 3 is connected to the circuitry associated with the auxiliary pushbutton input. (This function is not cur­rently supported in software.)
The input circuitry is “active low,” with a 10 k ohm resistor connected to +5 volts acting as a pull up. A combination of resis­tors and capacitors provide ESD protec­tion. A qualified technician can use these inputs for special applications. Contact the factory for additional details.
Compressor Circuit
In this section some general information about the Model 220’s compressor circuit will be provided. As previously discussed, the output of the microphone preampli­fier circuit is connected to a studio-qual­ity compressor circuit. The output of the compressor is used by the talkback out­puts and, if configured, the main output. (In most cases the main output will utilize the signal coming directly out of the micro­phone preamplifier.) The gain element in the compressor circuit is a laser-trimmed voltage-controlled-amplifier integrated circuit. It provides accurate, low-noise, low-distortion performance. The threshold of the compressor circuit is 2 dB above the Model 220’s nominal internal operating level of –2 dBu. A 5:1 compression ratio is implemented and, like the threshold level, is not field adjustable. The threshold and
ratio settings were selected so that excellent talkback audio would be provided. By controlling the dynamic range, intelligibility can be improved and overloading of con­nected devices can be avoided. An LED indicator lights whenever the compressor’s threshold has been reached and the circuit is actively controlling the dynamic range. This LED is provided as an aid when setting the gain of the microphone preamplifier.
Auxiliary Relay
The Model 220 provides an auxiliary relay for use in specialized applications. Some “head scratching” or “brainstorming” should lead to a number of interesting ways to take advantage of this unique resource. To implement any auxiliary relay applica­tion does require the services of a qualified technician. This is because the Model 220’s enclosure must be disassembled and the desired wiring scheme implemented. The relay operates under software control, fol­lowing the configuration selected using two of the DIP switches. Four operating modes are available: relay disabled, relay follows main output status, relay follows talkback output 1 button status, and relay follows talkback output 2 button status. These choices were previously discussed in this user guide and should be reviewed.
The relay provides two “form-C” contacts, each consisting of a common, normally open (not shorted), and normally closed (shorted) connection. Obviously the two form-C contacts change state in unison; two independent relay functions are not provided. These relay contacts are accessible on the Model 220’s main printed circuit board assembly by way of 3-position header connectors. The con­tacts are titled A and B, but there is no significance between the two. The header
labeled P10 provides access to auxiliary relay contact A. Header P9 provides ac­cess to auxiliary relay contact B. With both headers pin 1 is normally closed, pin 2 is common, and pin 3 is normally open. For additional details on connecting to the 3-position headers please refer to the Additional Connectors section in the Technical Notes area of this user guide.
Direct Microphone Output
The Model 220’s main output is intended to be used as the on-air, stadium an­nouncement, or other primary audio feed. The path from the unit’s microphone input connector to the main output connector is by way of active circuitry. A low noise, low distortion microphone preamplifier stage, “clickless” audio switching, and a high­quality transformer-coupled output circuit work together in achieving excellent per­formance. The nominal level of the main output is –2 dBu, allowing interconnection with related devices at a near-standard “line” level. However, there may be ap­plications where an output that directly re­lates to the connected microphone would be beneficial. The Model 220 has made provision to accomplish this by using the optional direct microphone output card. This card, available from Studio Tech­nologies and purchased separately (part number 31058), is mounted in a spare connector location available on the Model 220’s back panel. With this addition both an “active” main output and a “passive” direct microphone output will be available. Of course, both outputs provide quiet noise-free on/off (“muting”) control.
The primary application for the direct microphone output is when the Model 220 is integrated into a system that in­cludes a high-performance microphone preamplifier. This preamplifier may be part
of a stand-alone voice-processor unit, or a microphone input associated with an audio console. Engineering personnel may prefer the sonic characteristics of the external microphone preamp over that of the one integrated into the Model 220.
It’s important to note that the direct mi­crophone output card provides a direct “metallic” path from the microphone input to the external preamplifier circuitry. It is not simply an audio “pad” (attenuator) that reduces the level of the main output from “line” to “mic.” The Model 220’s auxiliary relay contact, along with circuitry on the direct microphone output card, provide click-free muting of the signal.
The direct microphone output card contains only passive circuitry. A 3-pin male XLR connector is provided for the output signal. A resistor and large electro­lytic capacitor form the muting circuit. This arrangement allows muting to take place without disturbing phantom power that may be enabled to support a con­denser microphone. The direct micro­phone output card also contacts two 3-position header connectors. These interface with “headers” on the Model 220’s main printed circuit board assembly, providing access to the microphone input connector and auxiliary relay contact.
The direct microphone output card kit contains a printed circuit board assembly, two interconnecting cables, and mounting hardware. Installing the kit is very simple. The card’s 3-pin male XLR connector is mounted into one of the three spare connector locations on the Model 220’s back panel. Specifically, the connector location that is closest to the microphone input connector should be used. The two screws, with associated locking nuts, are used to secure the connector and
associated printed circuit board to the enclosure. The interconnecting cables are then used to link the card with the Model 220’s main printed circuit board assem­bly. One end of the first interconnecting cable is plugged into the card’s 3-position header that is labeled MIC. The other end of this cable is plugged into the 3-position header associated with the microphone input connector, labeled P5, located on the main printed circuit board assembly. One end of the second interconnecting cable is plugged into the card’s header that is labeled RELAY. The other end of this cable is plugged into the header associated with the auxiliary relay, labeled P9, located on the main printed circuit board.
After the direct microphone output card has been installed, one configuration step must also be performed. Using the con­figuration switches, located on the bottom of the Model 220’s enclosure, the auxil­iary relay control mode must be set to the “follows main output status” position. This provides the on/off (muting) control of the direct microphone output signal. Should the auxiliary relay’s configuration be left in the “relay disabled” position, the direct microphone output will always be in the off (muted) state. It’s interesting to note that the recommended auxiliary relay configura­tion assumes that the direct microphone output will be used in place of the Model 220’s main output. However, for other applications there is certainly no reason why the direct microphone output can’t be configured to follow the status of one of the talkback buttons. Special situations may benefit from having a microphone signal that is active only during “talkback.”
Using the direct microphone output is essentially the same as connecting directly to a microphone. An interface cable
should be wired so that signal high (+ or hot) is connected to pin 2, signal low (– or cold) is connected to pin 3, and shield is connected to pin 1. When connecting a condenser microphone it’s recommended that the Model 220 provide the source of phantom power. In this way the micro­phone will stay active whenever the Model 220 is operating, even if the connection made to the direct microphone output is broken. By ensuring that the microphone remains active, the talkback functions will continue to operate correctly.
Two slight differences between connecting to a stand-alone microphone and connect­ing to the Model 220’s direct microphone output should be noted. The first is that pin 1 on the direct microphone output’s 3-pin male XLR connector is electrically isolated from pin 1 on the Model 220’s 3-pin female XLR microphone input con­nector, as well as the Model 220’s signal common/chassis connection. This is in­tended to minimize the chance of “ground loops” being created. Contact the factory should assistance in this area be required.
The second difference is that while the circuitry between the microphone input and direct microphone output is entirely passive, it will still impact the microphone signal. The impact is benign but is still worthy of description. The circuitry asso­ciated with the Model 220’s microphone preamplifier and phantom power supply is always connected across (“bridged onto”) the microphone input. This adds a 2 k ohm essentially resistive load to the microphone, something that should have no sonic impact. In some cases this may reduce the microphone signal level by less than one dB. Also, two 150 ohm resis­tors are electrically connected in series between the microphone input connector
the direct microphone output connector. These resistors provide isolation, allowing the direct microphone output to be muted while still maintaining microphone audio on the Model 220’s preamplifier input. This audio is necessary so that the talkback functions can continue to operate. The series resistors will have minimal impact, simply raising the microphone’s apparent source impedance as “seen” by the exter­nal preamplifier.
Remote Data Interface
For future applications the Model 220’s main circuit board provides a bi-directional data interface connection. It complies with the RS-485 standard and operates at a data rate of 115.2 kbps. It is expected that the data interface will link the Model 220’s microcontroller integrated circuit with a remote switch/status display panel. Con­tact Studio Technologies to find out the implementation status of this function.
Specifications
General Audio: Frequency Response:
in/main out
Distortion (THD+N): 0.008%, measured at 1 kHz, mic in/main out
S/N Ratio: 86 dB, referenced to –42 dBu mic in/ –2 dBu main out
Dynamic Range (A-weighted): 108 dB
Connectors: Mic In, IFB In, Line In 1 & 2: 3-pin female XLR
Main Out, Talkback Out 1 & 2: 3-pin male XLR
Headphone Out: ¼-inch 3-conductor phone jack
24 Vdc Power In: coaxial power jack, 2.1 x 5.5 mm,
locking bushing, compatible with Switchcraft S760K plug
Spare Connector Locations: 3 Allows up to three Neutrik NC*D-L-1 connectors
to be installed (*=3F, 3M, 5F, 5M, 6F, 6FS, etc.)
Microphone Input/Preamplifier: Type:
electronically balanced
Input Impedance: 2 k ohms, nominal
CMRR: >80 dB, 20 Hz-20 kHz, 40 dB gain
Gain Range: 20 to 60 dB, nominal, adjustable in
10 dB steps
Compatibility: dynamic or phantom-powered mics
Phantom Power: 48 Vdc, nominal, meets IEC 1938
P48 standard
IFB Input: Type:
2-channel, unbalanced (pin 1 common; pin 2
DC with channel 1 audio; pin 3 channel 2 audio)
Impedance: 10 k ohms, nominal
Nominal Level: –10 dBu
10 Hz-20 kHz, ±0.2 dB, mic
Main Output: Type: balanced, transformer-coupled
Nominal Level: –2 dBu
Maximum Level: +20 dBu into 2 k ohms
Impedance: 100 ohms, nominal
Talkback Outputs: Type: transformer-coupled with series capacitors
and isolation resistors
Impedance: 600 ohms, nominal
Nominal Level: +4 dBu
Maximum Level: +11 dBu (compressor restricts
maximum)
Headphone Output: 1, stereo Compatibility: intended for connection to mono
or stereo headphones or headsets with nominal impedance of 100 ohms or greater
Type: voltage driver
Maximum Output Voltage: 12 Vpp, 150 ohm load
Auxiliary Relay: Function: software configurable
Contacts: 2, form C (Common, Normally Closed,
Normally Open)
Rating: 1 A, 30 W (resistive)
Access: requires user-implemented connector
scheme
Power Sources: IFB: 24-32 Vdc, 125 mA
External: 24 Vdc nominal, 80 mA @ 24 Vdc;
acceptable range 20-30 Vdc. Each unit shipped with a universal input/24 Vdc output power supply.
Dimensions (Overall):
8.1 inches wide (20.6 cm)
3.3 inches high (8.4 cm)
8.5 inches deep (22.4 cm)
2
Line Inputs: 2 Type: balanced, transformer-coupled
Impedance: 10 k ohms, nominal
Nominal Level: –12 dBV to +6 dBu, adjustable
Compressor: Threshold: 2 dB above nominal level
Attack/Release Time: 2 mSec/100 mSec, nominal
Slope: 5:1, nominal
Status LED: compressor active
Weight:
Specifications and information contained in this User Guide subject to change without notice.
4.5 pounds (2.1 kg)

Appendix A

Attached to the bottom of the unit is a security plate with text that provides a summary of the configurable parameters and related information.
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