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T ALKBACK POWER SUPP LY Build a multi-station headset intercom This multi-station headset intercom is designed to provide clear communication in high noise environments such as at race tracks and rock concerts. It is compatible with existing commercial systems and features low noise circuitry. By MARQUE CROZMAN Rock concerts are renowned for their excessive noise levels. In some instances, even shouting directly into another person's ear will not get your message across. This is especially a problem for production personnel who constantly have to take directions or issue instructions and be heard the first time. This calls for an intercom system with a difference. Race track mechanics and time 16 SILTCON CHIP keepers are also faced with the same problem - to be heard the first time round without rupturing the other person's ear drums. Similar situations occur in helicopters and light aircraft, on construction sites, and in TV and theatre production where the distance between personnel is often the major problem. Our Talkback Headset Intercom system overcomes these problems by us- ing a padded headphone set with a noise cancelling microphone. This drastically reduces the amount of external noise entering the ears and, by placing the mic close to the mouth, can produce high level audio signals that are also substantially free of noise. Each headset is driven by a substation that can be worn on the belt or placed in some other convenient position. Standard 3-pin XLR microphone cables are used to connect the substations together via loop through connectors - see Fig.1. This means that yo.u can connect them together in a simple daisy-chain configuration, distributed along hundreds of metres of cable. This technique allows hands-free, 2-way communication to all substations in the chain and is much easier than connecting every station back to a central point. Each substation is equipped with a / / POWERED POWERED - - - - - - 12SUBSTATIONS - - - - - - Fig.1: the substations are connected together in daisy-chain fashion via "loop-through" connectors. push-to-call switch that flashes a light and/or a buzzer on all stations, to alert operators who have removed their headsets. A volume control sets the headphone level and a microphone switch allows operators to switch off their microphones when not in use , thereby reducing noise input to the system. The buzzer is optional and can be left out of circuit if not required; eg, if the project is for use in aircraft, in a TV studio or for live theatre. Additionally, by connecting the circulating audio line to the line level input of a small amplifier, all communications can be monitored via an external loudspeaker. The substations are powered by a separate power supply unit that may be inserted anywhere in the chain. Up to 12 substations may be connected to a single supply (see Fig.1) but if further stations need to be added, they can be powered from a second supply. In situations where mains power is not available, the system can be powered from a 12V car battery. The only disadvantage here is that the call function ceases to work. a IN ) The substations & ~e power supply are connected together via 3-pin male & female XLR sockets. These are wired according to the industry standard so that they are compatible with commercial units. To add to its versatility, the Talkback Headset Intercom has been made compatible with two popular commercial systems. These are the American Con- r-;r- a aa OUT IN lN OUT OUT IN OUT /PIWERED --7 / / ~WERED, IN OUT IN a""t#"' IN cert-Com system as used by theatres and production companies around the world; and the Australian-made Jands system. IN r~ ) / / rOUT aa 1N OUT IN OUT - - - - - - - - - - - - 12 SUBSTATIONS Fig.2: by adding a toggle switch & an additional pair of XLR sockets to the power supply, it can be used as a master station to switch between two different substation chains. ]UNE 1992 17 Fig.3 (left): the substation circuitry is based on op amps ICl & IC2. ICla functions as a microphone preamplifier, while IClb, Q2 & Q3 form a small audio amplifier that drives the headphones. IC2b functions as a Schmitt trigger oscillator when the CALL button is pressed & drives Ql to flash a lamp & activate a buzzer. ffi •::::: :::, "' a ~ 3:' ~~+ ,..H· ~ .~; C>z N ,_ ao... I• + > .. I· C, O>- + > ;! -0> C>!i1 Cz . ill N == 0 (.) a: w 1- z 1- w en 0 et w J: ~ > ---YMrll· ~ N N !,?Ni :E"' ~ ~~·I ..,, 3:1 "'> Circuit details _.,,M+ . 0 .., lE ~3: ... "-o 0--' ' 18 SILICON CHIP The main difference between these two systems is that the Jands system uses a male headset socket instead of a female socket and that's the standard we've adopted here. The pin wiring for both sockets is the same. Both the Jands and Concert-Com systems use a master station and a host of substations. In each case, the master station contains the power supply and a substation, and has the ability to switch between substation chains. Although we didn't put a master station together, this could easily be done by combining the power supply and a substation in one case. Chain switching could then be achieved by adding two extra 3-pin XLRs and switching the audio line (pin 3) between them and the existing XLR socket pair. Alternatively, you can use an existing substation and just add extra XLR sockets to the power supply case (there is room on the rear panel to do this). A switch could then be added to the front panel to switch the audio signal between the two pairs of sockets. Fig.2 shows the basic details of this scheme. In most cases, however, this will be unnecessary unless you want to switch between two substation chains. Generally, the scheme shown in Fig. l will be more than adequate. UJ ww 3:'"' ~ > Refer now to Fig.3 for the circuit details. It can be roughly divided into three basic sections: a microphone preamplifier (ICla); a headphone amplifier (IClb, QZ & Q3); and a call detector and lamp/buzzer driver circuit. Voice signals picked up by the headset microphone are coupled to pin 2 of ICla via a lµF capacitor and 560Q resistor. Alternatively, this stage can be fed with line level signals via potentiometer VRl and its associated components. ICla operates as an inverting amplifier with a gain of approximately 400. Its output appears at pin 1 and is coupled via two lOµF electrolytic capacitors to the microphone on/off switch S2. From there, the signal passes via a voltage divider and 0. lµF capacitor to pin 2 of ICZa. Note that the two lOµF capacitors at the output ofICla are wired back to back to form a bipolar capacitor. This blocks pin 1 of ICla from the DC that appears on the audio line when the CALL button is pressed. IC2a is wired as an inverting op ·amp with a gain of 1:0. This stage provides additional boost for signals from ICla and also serves to boost incoming audio signals from other substations. The l00pF capacitor across the l00kQ feedback resistor rolls the response off above 15kHz, as does the 47pF capacitor across ICla. These capacitors eliminate any tendency for the op amps to exhibit highfrequency instability. The audio output signal from IC2a is AC-coupled to volume control VRZ and then fed to the inverting input (pin 6) of op amp stage IClb in the headphone amplifier. IClb's output (pin 7) drives Q2 and Q3 which operate as a complementary class-B output pair. These two transistors are there to boost the out- This is the view inside one of the finished substations. The PC board is secured by sliding it into slots that are cut into the plastic ribs on both sides of the case bottom (see Fig.6). Take care with the wiring to the XLR sockets. put current capability of the op amp and are included in the feedback loop to minimise distortion. In addition, the crossover distortion has been reduced by using D9 & DlO to provide about 1.2V of bias between the bases of the transistors. The 180Q resistor and .0047µF capacitor at the output form a Zobel network which operates in parallel with the headphones to prevent highfrequency instability. Call detector/driver Fig.4: the supply circuit uses a 30VAC transformer, a bridge rectifier & a 2200µF capacitor to feed an LM317 adjustable 3-terminal regulator. The 120n and 2.7k.Q resistors at the output of the LM317 set the supply rail to a nominal +30V. F1 25DmA ICZb is wired as a Schmitt trigger oscillator and functions as a call detector/ driver stage. It is activated for a short time whenever the CALL button (S3) is pressed and drives a 12V lamp r---7 Por,ER I 11 and the optional buzzer via transistor Ql . Let's take a closer look at how this circuit works. Pressing S3 pulls the audio line to +Vcc so that all substations respond to the call signal. When this happens, a lO0µF capacitor (in each substation) charges via a 22kQ resistor and this applies a fixed +15V (ie, ½Vee) bias to pin 5 of ICZb. ICZb now oscillates at a frequency of about lHz, as set by the 47kQ feedback resistor and the l0µF timing capacitor connected between pin 6 and the positive supply rail. Diode DB and the lOkQ resistor in ICZb's feedback network set the duty cycle at IC2b's output (pin 7) to about 5:1. This is because the l0µF timing capacitor discharges only via the 47kQ 05 1N4002 12 * A~ 240VAC I I I +3DV 1200 E • EXTRA SOCKETS If REQUIRED 2200 5DVW 06 1N40D2 10 35VW + POWER + - *FEMALE XLR POWERED LED1 *MALEXLR UNPOWEREO DV .,. TALKBACK POWER SUPPLY ·-~- jl *5EE TEXT OUT JUNE 1992 19 feedback resistor and then quickly discharges via DB and the lOkQ resistor when pin 7 of IC2b swings low. The +30V rail from the supply enters the substation on pin 2 of the 3pin XLR socket. The lOQ resistor acts as a fusible link and D7 protects against reverse polarity connection. The +30V rail, shown on the headset intercom circuit (Fig.3) as V+, drives the CALL lamp and the output stage of the headphone amplifier. Because the LM833 op amps have a maximum supply voltage of 36V, the V+ rail is fed via a 330Q resistor to a 33V zener which provides protection in the event of the supply rail rising. This zener-protected rail is shown as Vee on the intercom circuit. This rail also feeds a voltage divider network consisting of two lOkQ resistors which provides the Vcc/2 rail (nominally +15V) and acts as a bias supply for the op amps. zero tolerance; ie, exactly the value specified). Also shown on the circuit of Fig.4 is power indicator LED 1 and a 2.7kQ resistor feeding it. These components are optional and may be omitted if you use the specified illuminated mains switch. The circuit shows the output connected to two XLR sockets but you may want four sockets, depending on your requirements. The 4-outlet version allows the supply to be placed anywhere in the chain and will power up to a dozen substations. The two powered outlets provide in and out connections. A further two outlets are unpowered to allow the connection of additional substations which are fed from an-• other power supply. These sockets are only wired to pins 1 and 3, to separate the supplies. The 2-outlet version only has one powered and one unpowered outlet. CAPACITOR CODES (10%) Power supply The power supply is shown in Fig.4. It employs a 30VAC transformer, a bridge rectifier and 2200µF capacitor feeding an LM317 adjustable 3-terminal regulator. The 120Q and 2.7kQ resistors at the output of the LM317 set the supply rail to a nominal +30V (actually 29.51Vifall components are 0 0 0 0 0 0 Value 0.1µF .0047µF 220pF 100pF 47pF IEC Code 100n 47n 220p 100p 47p EIA Code 104K 472K 221K 101 K 47K RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 20 No. Value 4-Band Code (1%) 5-Band Code (1%) 3 1 1 1 820kQ 220kQ 100kQ 56kQ 47kQ 22kQ 10kQ 2.?kQ 1kQ 560Q 470Q 330Q 180Q 120Q 33Q 10Q grey red yellow brown red red yellow brown brown black yellow brown green blue orange brown yellow violet orange brown red red orange brown brown black orange brown red violet red brown brown black red brown green blue brown brown yellow violet brown brown orange orange brown brown brown grey brown brown brown red brown brown orange orange black brown brown black black brown grey red black orange brown red red black orange brown brown black black orange brown green blue black red brown yellow violet black red brown red red black red brown brown black black red brown red violet black brown brown brown black black brown brown green blue black black brow11 yellow violet black black brown orange orange black black brown brown grey black black brown brown red black black brown orange orange black gold brown brown black black gold brown 2 8 1 2 2 1 SILICON CHIP ~ r AC SC04104921 0 Fig.8: full-size etching pattern for the power supply PC board. Fig.6: here's how to modify the plastic case to accept the substation PC board. You will also need to remove several integral pillars before the board will fit. This means that the supply has to be placed at one end of the chain. Furthermore, if more than a dozen stations are to be used, the next power supply has to be connected after the twelfth station - see Fig.1. Construction The Talkback Headset Intercom is housed in a compact plastic case made by Arista. The PC board, coded SC01302921 and measuring 90 x 93mm, is made especially for this case, although other cases could also be used. Fig.5 shows how the parts are installed on the board and how everything fits inside the specified case. Before you start assembly, check the board carefully for breaks and shorts in the tracks. If there are any, they need to be corrected now. Install the wire links first , followed by the PC stakes and the resistors. This done, install the diodes and the transistors but make sure that you orient them correctly. The ICs can now be installed. IC2 can be an LM833 or a TL072 but IC1 does need to be an LM833 if a good signal to noise ratio is to be obtained. Fig,7: this is the full-size etching pattern for the substation PC board. The ceramic and MKT polyester capacitors may be inserted either way round but make sure that the electro1ytic capacitors are correctly oriented. The miniature potentiometer is the last item to be installed. Case assembly Installing the completed board requires some modification to the plastic case. It has several pillars and mounting brackets that need to be removed before the board will fit. Using a Dremel®tool or file, these can be cut out or filed down. Next, slots need to be cut into the strengthening webs to secure the board, as shown in the diagram of Fig.6. The front and back panels can now be drilled. You can use the Dynamark® label as a drilling template for the front panel. The hole for the microphone switch needs to be rectangular and is best done with a file or nibbler. Once drilled, you can mount the two switches and pot (already secured to the board) on the front panel. The 3-pin XLR sockets can be mounted on the rear panel with screws, nuts and lockwashers or with pop rivets. If you are going to install the buzzer, it can be glued to the base of the case, between the XLR sockets. Light duty hook-up wire connects it in parallel to the CALL light. To avoid any problems with back-EMF from the buzzer, a 1N4004 diode needs to be wired across the terminals (see Dl 1 on the circuit, Fig.3). Any miniature 12V buzzer is suitable, provided that it will fit into the restricted space inside the case. That's all we have space for this month. Next month, we shall complete the substation construction and give the full assembly details for the power supply. SC JUNE 1992 21