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TALKBACK POWER SUPPLY Build a multi-station headset intercom; Pt.2 Last month, we described the circuit of our new multi-station headset intercom & started the substation assembly. In this final article, we complete the substation & give the assembly details for the power supply. By MARQUE CROZMAN At this stage, the substation wiring can be installed as shown last month in Fig.5. This wiring is straightforward but be sure to use a short length of shielded audio cable for the microphone input (4-pin XLR). Pin 2 is the mic signal and pin 1 is ground. It is important to use shielded wire for these connections as the case is not shielded. Light duty hook-up wire can be used for the rest of the connections to the board. Keep all wiring as short as possible. 76 SILICON CHIP The PC board can now be slid into the slots cut into the case (see Fig.6). By slightly bending out the sides of the lower half of the case, the front panel can also now be installed. The rear panel is then slotted in and the top half of the case snapped into place. To be able to wear the substation on your belt, a clip needs to be riveted to the bottom of the case. These are the same type of clips that are used for tape measures, and can be found in some hardware stores. If the substation is being built with the auxiliary input, an extra hole needs to be drilled for the additional pot. An unbalanced phone jack can be used for the input on the rear panel. It will need to be placed between the 4-pin and the 3-pin XLRs. The earth for the phone jack can be taken from pin 1 of the 4-pin XLR. Power supply construction All the parts are mounted on a single PC board coded SC04104921. The component overlay for this board is shown in the diagram of Fig.9. There is provision on the board for a trimpot but this is not necessary- unless you want to set the output voltage to exactly 30V. Begin the board assembly by installing PC stakes at the external wiring points, then install the diodes, resistors and capacitors. The LM317 can now be installed. Its metal tab EARTH OJ1 SOLDER LUG ACTIVE BROWN ' ~ )II <l\l~t\.\.\'S r,~t"- t1'~'" ,j/ ICORD CLAMP GROMMET POWER TRANSFORMER 8S1 ~~POWER Fig.9: assembly details for the power supply. Install the LM317 with its metal face towards the rear panel. should be adjacent to the edge of the board and it should be mounted with its leads at full length so that it can later be bolted to the rear panel (see Fig. 10). Case assembly The case must be modified slightly by cutting away the slots in the base to get the transformer to fit. When this has been done, drill the transformer mounting holes and bolt it into position, as shown in Fig.9. The 3-way mains terminal block can also be mounted at this stage. The rear of the case uses an aluminium panel which doubles as a heatsink for the LM317. The various holes can all be drilled by using the Dynamark® label as a template. This label has crosshairs to indicate the centres for all holes, except the one for the LM317 mounting screw. The position for this is best found with the board temporarily mounted in the case, on two of the pillars. The LM317 is attached to the rear panel using a screw, nut and an insulating kit. Fig.10 shows the mounting details. Looking at our prototype, you will see that we have constructed the 2outlet version and this is also depicted JULY 1992 77 PARTS LIST Substation components 1 PC board, code SC01302921, 90 x 93mm 1 Dynamark front panel label, 90 x40mm 1 Arista UB-1 plastic case, 95 x 34 x 145mm (DSE Cat. H2503, Rod Irving Cat. H10114) 1 3-pin female XLR chassis mount socket 1 3-pin male XLR chassis mount socket 1 4-pin XLR chassis mount socket (male for Jands compatibility; female for Concert-Com compatibility) 1 illuminated momentary contact pushbutton switch (12V lamp); Jaycar Cat. SP-0750 or Altronics Cat. S-1510 1 SPST rocker switch (Altronics Cat. S-3210) 1 small fluted knob (to suit .miniature pot) 1 300mm length of light duty hook-up wire 1 50mm length of shielded audio cable 6 machine screws, nuts & lock washers 13 PC stakes 1 10kQ linear pot Semiconductors 1 LM833 op amp (IC1) 1 LM833 or TL072 op amp (IC2) 2 BC337 NPN transistors (01 ,03) 1 BC327 PNP transistor (02) 3 1N914 signal diodes (D8 ,D9,D1 0) 1 1N4004 rectifier diode (D7) 1 33V 1W zener diode (ZD1) Capacitors 2 470µF 50VW PC electrolytic 2 100µF 25VW PC electrolytic 4 10µF 35VW PC electrolytic 1 1µF 35VW PC electrolytic 2 0.47µF 35VW PC electrolytic 3 0 .1µF MKT polyester 1 .0047µF MKT polyester 1 220pF ceramic 1 100pF ceramic 2 47pF ceramic Resistors (0.25W, 1%) 3 820kQ 2 1kQ 78 SILICON CHIP 1 220kQ 1 100kQ 1 56kQ 1 47kQ 222kQ 810kQ 1 560Q 1 470Q 1W 1 330Q 1W 1 180Q 233Q 1 10Q Power supply components 1 PC board, code SC04104921, 46 x 60mm 1 instrument case, 200 x 160 x 70mm (Altronics Cat. H-0480) 1 black anodised aluminium panel (Altronics Cat. H-0486) 1 Dynamark front panel label, 55 x 190mm 1 Dynamark rear panel label, 55 x 190mm 1 30V 30VA transformer (Altronics Cat. MM-2008) 1 3-core mains cord & moulded 3-pin plug 1 cord grip grommet 1 100mm length of 20mm heatshrink tubing 1 240VAC illuminated rocker switch (Jaycar Cat. SK-0985 or Altronics Cat. S-3217) 1 panel mount 3AG fuseholder 1 3-way chassis-mount mains terminal block 1 3-pin male XLR chassis mount socket (or 2 the for expanded version; see text) 1 3-pin female XLR chassis mount socket (or 2 for the expanded version; see text) 9 machine screws, nuts and lock washers 2 self-tapping screws 2 solder lugs 4 PC stakes 1 300mm length of light duty hook-up wire 1 50mm length of shielded audio cable Semiconductors 1 LM317 voltage regulator 6 1N4004 diodes (D1-D6) Capacitors 1 2200µF 50VW PC electrolytic 2 10µF 35VW PC electrolytic Resistors (0.5W, 1%) 1 2.?kQ 1 120Q This aviation headset from Altronics (Cat. C-9070) is suitable for use with the Headset Intercom but note that the standard aircraft jacks fitted to the unit will have to be replaced with XLRjacks. in the wiring diagram of Fig.9. Don't drill and cut holes for four XLR sockets on the rear panel unless you are going to build the 4-outlet version. All exposed metal on the case needs to be earthed; ie, the rear panel and the transformer mounting screws. The rear panel is earthed by a solder lug secured just above the LM317, while the transformer is earthed via one of its mounting screws. Make sure that both solder lugs are in contact with bared metal to ensure a good earth connection. The mains switch and fuseholder terminals should be sleeved with heatshrink tubing to prevent any chance of accidental contact with the 240VAC mains. The rest of the wiring details are shown in Fig.9. Double check all wiring and the PC board assembly before assembling the case. The 250mA fast blow fuse can then be installed and the unit switched on with nothing connected. Use your multimeter to check the voltage between pins 1 & 2 of the XLR sockets it should be close to 30V. If not, switch off and check the parts layout on the board and the wiring. If this doesn't locate the fault, check the voltage across the 2200µF filter capacitor (ie, at the output of the bridge rectifier)- it should be close to 42V. If this is OK but the output of the LM317 regulator is incorrect, check the 120Q and 2.7kQ resistors on its ADJ terminal. If the output voltage is correct but This inside view shows the completed power supply unit. Keep all mains wiring neat & tidy & be sure to sleeve the switch & fuse terminals in heatshrink tubing to prevent the possibility of accidental electric shock. the power LED fails to light, check the orientation of the LED. If everything is OK, connect up a substation and headset and have a listen. If there is any buzz, check the earth connections and the proximity of AC cables to audio lines. The unit can now be fully tested by connecting a second substation and checking that the CALL function works ' ~ <V 17 2'. ~ -~-- . . : ., .., Headsets The choice of headset will of course depend on your particular application and budget. The most popular headset with professionals, in the theatre and film industries, is the Beyer DT109. It can be worn comfortably for long periods, without reducing the diameter of your head. They are most commonly seen on the heads of TV camera operators. There is a drawback though; they are not cheap! A cheaper version from Beyer is available but other companies such as Telex (model 500) and AKG also make reputable sets, that _are half the XLR WIRING STANDARDS ,. l ~ ~ "" and that speech can be transmitted back and forth between them. Pin No. . 3-pin XLRs 4-pin XLRs 1 Gnd Mic Gnd 2 30-36V Mic +ve 3 Audio Line Headphone +ve 4 . Headphone Gnd t This close-up shot shows the mounting arrangement for the power supply PC board & the LM317 regulator (see also Fig.9). ]UL Y 1992 79 TALKBACK POWER SUPPLY + + + + PIN1: GND PIN2: PWR POWERED IJl;iuqa TALKBACK SUBSTATION MICA INSULATING WASHER w,~jl SCREW { ll1lllll(3 1 10220 DEVICE Fig.10: mounting details for the LM317 3-terminal regulator. Smear all mating surfaces with heatsink compound & check that the rear panel mounting area is smooth before bolting the assembly together. Finally, use your multimeter to confirm that the metal tab has been correctly isolated from the case. price of the DT109. Unfortunately, they do have a tendency to induce fatigue if worn for long periods. For extremely high noise environments, such as in helicopters and light aircraft, the only way to go is to use aviation h eadsets. There are a few SILICON CHIP + + MIN - - CASE 80 PIN3: AUDIO UNPOWERED MIC + MAX LEVEL CALL Fig.11: here are the full-size front &.rear panel artworks for the power supply & substation. brands to choose from, David Clark being the most sought after, but you will have to change the plugs, as the aircraft standard is 6.5mm phone plugs. A very reasonably priced pair is available from Altronics (Cat. C9070) for $189.00. Note that if you build the Talkback Intercom for use in aircraft, you will be able to run it from the standard 28V DC supply. However, you would be wise to spray the interior of the case with a product such as Electrolube® nickel screening to provide shielding as aircraft are usually plagued with a lot of electrical noise (as well as acoustic noise). Alterna- tively, you may decide to build the Talkback Intercom into a metal case for superior shielding. If your system is for amateur use, headphone/microphone sets are available from most electronic stores starting at about $39. Most of these do not use enclosed headphones and hence let in external noise. But if your noise levels are not excessive, these are quite adequate. Acknowledgment Special thanks to Tony Moffat for suggestions and to Lindsay Hall Productions for the use of their headsets during the design of this project. SC