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Cheap’n’Simple 100V Speaker/Line Checker By Ross Tester This Speaker/Line Checker will be a boon to anyone setting up 100V PA systems, especially for temporary installations at sporting events, when you need to do everything quickly before the event and be sure that it is all working. With this tester, you can immediately check each PA speaker and line as it is run. N ecessity, as they say, is the mother of invention. My necessity was something to check both PA speakers and the lines feeding them as they were temporarily placed in position for surf lifesaving carnivals. For many years, I’ve erected temporary PA systems – up in the early morning, down that afternoon. Usually, that’s been a matter of placing perhaps eight horn speakers over a distance of 44  Silicon Chip maybe 600-700m, all fed from a central PA amplifier located where the carnival announcer sits. With eight 30W speakers, a 250W amplifier handles the whole thing quite nicely. But feeding those eight speakers over such a distance demands they not be your usual low impedance (ie, 4, 8 or 16Ω) speakers; to minimise losses they must be so-called “100V” types. What this means is that the out- put from the amplifier is (internally) stepped up by a transformer so that the lines to the speakers are fed by a 100V signal. At the speaker itself, the reverse happens – the 100V is stepped back down again by a similar transformer so that the low impedance speaker driver is presented with just the right level. Why go to all that trouble? The answer is simple: to minimise siliconchip.com.au Fig.1: as circuits go, it’s pretty simple: a 555 timer creates a square wave which is amplified, then fed into a 100V speaker transformer and on to the speaker. It’s capable of delivering a little over 1W but it’s not exactly hifi! losses in the speaker cables. While copper cable is a very good conductor, it does have some resistance. Typically, I use lightweight (14x0.14) Fig.8 cable, which according to the reference books has a resistance of about 16Ω÷100m (ie, 8Ω per side). In a home hifi situation with only a few metres of cable between amplifier and speaker that resistance doesn’t matter too much but when your speakers are up to several hundred metres away from the amplifier, resistance of the cable has a major impact. If, for example, I was to drive an 8Ω speaker 300m away from the amplifier, the speaker line itself is going to act like quite a large resistor in series – about 48 (3 x 16Ω) – and I am going to lose 48÷56 (ie, line resistance divided by line + speaker resistance) or 85% of the signal before it gets to the speaker. In fact, it’s even worse than that because inevitable corrosion in the connectors etc means I’d be lucky to have even 5-10% of my original signal left at the far end. And the further away your speakers are, the worse it gets. With a 100V PA system, the losses are much, much lower. The impedance of a 30W 8 ohm “tap” on a 100V audio transformer is calculated as (100V2÷30) or 333 ohms. So now we have a 48Ω speaker line in series with a 330Ω load. Therefore the loss is reduced to (48÷(48+330)) or about 12% – much more manageable. No impedance problem, either! There is another huge advantage: with multiple speakers, you don’t have siliconchip.com.au to worry about impedance matching. With a 100V line system, all speakers are connected in parallel/in phase and all you need to do is add up the wattage which each speaker is running at (and that simply depends on the tap you use on the speaker’s transformer) and make sure the total doesn’t exceed the rated output of the amplifier. For example, I mentioned before I normally use eight 30W horn speakers (or more correctly, eight speakers connected to their 30W taps). 8 x 30 = 240, nicely inside the rating of my 250W amplifier. Incidentally, if you need to add another speaker or so to fill in a “sound hole” in a 100V system and you’re running close to the amplifier’s maximum power rating, lower the tap on one or more speakers so that when you add the extra(s), you stay within the overall power limit. Simple, eh? Back to the checker As I’ve installed the PA systems, many’s the time I’ve wished for some method of ensuring that the lines and speakers were working properly as I go. “Easy,” you’re thinking. “Just get Inside the box: everything except the transformer, output terminals and batteries mounts on a single PCB. The batteries in their holders can just be seen underneath the board. April 2011  45 S1 A D1 K 1N4004 VR1 10k LOG K 100uF + + A LED1 68k 100k 2.2k V0 100nF 10nF 1 1 220nF someone on the microphone to talk as you go.” Not so easy, especially when the system is installed at a beach at 5AM – with people living all around! For a start, that requires two people to do the installation and I normally do the job by myself. Second, nothing gets residents offside quicker than someone saying “testing 1-2” when they’re enjoying their beauty sleep. So what I wanted was something that would generate a low level tone; just loud enough to ensure that the speaker lines hadn’t been cut (it happens!) or the speaker itself hadn’t developed a mysterious case of silence (ditto!). Then I could test each PA horn and the reels of cable as I went. The circuit The circuit is dead simple; crude even – see Fig.1. Our old friend, the 555 timer, is connected in astable mode so it produces a square wave at about 400Hz or so. It feeds an LM386 power amplifier IC via the volume control pot (VR1). The LM386 gain is set at 20 due to the fact that pins 8 and 1 are left open circuit. Provision is made on the PC board for components to (a) shape the output wave somewhat – effectively in parallel with VR1, and (b) to adjust the gain of the LM386 if required (components between pins 8 and 1). A 10µF capacitor and series resistor will set the gain, from 20 up to 200, depending on the resistor value (open circuit = 20, short circuit = 200). While we made this provision, we were happy with both the tone and the gain, so these pads are left empty. You can also adjust the frequency from the 555 by varying the 150kΩ re46  Silicon Chip + + + + OUTPUT TO TRANSFORMER 47uF 47uF 1 K 47nF IC1 555 IC2 LM386 V21+ A Fig.2: the component overlay and matching photo below. Note that the capacitors are all laid over so there’s enough room underneath the case lid. The empty holes in the PC board are for adjustment to the 555 output waveform (left holes) and the LM386 gain (centre/right holes), as explained in the text. sistor or the 10nF capacitor. For example, 68kΩ and 10nF gives about 1kHz. Normally, the output of the LM386 at pin 5 would drive an 8Ω speaker via the electrolytic capacitor (the Zobel network of a 470nF capacitor and 10Ω resistor to ground at pin 5 helps prevent supersonic oscillation). But in our case, instead of driving a speaker, we drive the primary (ie, 8Ω winding) of a 100V speaker transformer. The secondary is taken to a pair of binding posts which can connect to speaker cables. But because my speakers and cables are all wired with XLR plugs and sockets for quick connection, I’ve included a male XLR socket as well. That makes checking fitted leads really quick and easy – just plug ’em in! To make it truly portable, power is supplied by eight AA cells, giving a 12V rail. This connects via a silicon diode (D1) to protect against polarity reversal and thence to an on-board power switch. A LED pokes through the front panel to show that power is applied, with a large volume control knob alongside. Just in case you’re wondering why we didn’t simply connect the LM386 to oscillate and produce a square wave (which it can do easily) we wanted to make the level variable – and it’s just as easy to do that with a $1 555. Construction With the exception of the 100V Fig. 3: the top trace shows the output of the amplifier while the lower (green) trace shows the (unloaded) output from the 100V transformer. OK, it’s not exactly a textbook square wave – but I find the distortion actually makes the sound a little more distinctive. siliconchip.com.au Parts List – 100V Speaker/Line Checker 1 UB-1 Jiffy Box, 158 x 95 x 55mm 1 PC board, coded 04104111, 100 x 60mm 2 4 x AA cell holders 1 8Ω to 100V 5W speaker transformer (T1) (eg, Altronics M1112 or equivalent) 1 SPST slide switch (eg, Jaycar SS-0812 [DPDT] or equivalent) 2 binding post terminals 1 chassis-mounting male XLR socket [optional] 1 knob to suit potentiomenter 1 pack 4 rubber feet, self adhesive 4 25mm threaded pillars 4 12mm threaded pillars 8 10mm x M3 screws 4 20mm x M3 screws 4 M3 nuts 7 PC pins Aluminium sheet (for battery clamp) Semiconductors 1 555 timer IC (IC1) 1 LM386 Audio amplifier IC (IC2) 1 1N4004 silicon power diode (D1) 1 5mm LED (LED1) The two 4 x AA battery holders are clamped in place by a scrap of aluminium. In this shot you can also see the mounting pillars on which the PCB sits, along with the output terminals and optional XLR socket. output transformer, binding posts/XLR socket and battery packs, everything is mounted on one PCB, coded 04104111 and measuring 100 x 60mm. First step, then, after checking the PCB for defects, is to mount the components. Start with the seven PC pins (two for power, two for output and three for potentiometer) then the resistors, low profile capacitors, diode and then the electrolytic capacitors. Note that the electros are all mounted “laid over” so their height does not interfere with the front panel. When mounting the LED, it should sit about 5mm above the PC board surface so it can just poke through the panel. The slide switch mounts hard down on the PC board, which makes it just the right height to emerge through the panel without being too proud of it. We deliberately selected this type of switch so it would be harder to knock on when bouncing around in the gear bag! Note that it will almost certainly be a DPDT type as SPST are not easy to find! siliconchip.com.au Before mounting the 10kΩ log pot, it would be wise to cut off the excess shaft, to the length required for the knob you choose. The pot itself mounts flat onto the PCB so its three terminals can solder to the three PC pins. You will note a couple of holes in the board alongside the pot – these are for a length of tinned copper wire which goes over the top of the pot to ensure it stays in place. We soldered the wire to the pot body, after scratching away some of the passivation on the body (it won’t solder otherwise). Finally, solder in the two ICs – making sure you get them in the right spot and oriented the right way. Checking Before mounting the PCB in its box, it should be checked. It’s so simple it should work first off. Connect 12V DC to the power terminals (watch the polarity) and turn on the switch. Ensure that the LED lights. If it doesn’t, you either have a dead power Capacitors 1 100µF 16V electrolytic 2 47µF 16V electrolytic 1 220nF MKT or monolithic 1 100nF MKT or monolithic 1 47nF MKT or monolithic 1 10nF MKT Resistors 1 150kΩ 1 100kΩ 1 2.2kΩ 1 10Ω 1 10kΩ log pot, 24mm (VR1) supply or it is connected back to front (or perhaps you’ve put the LED in back to front). Wind the pot down to minimum and connect virtually any normal (ie, low impedance) speaker to the output pins on the PC board. As you wind the pot up you should be rewarded with a raspy tone which increases in volume. If you don’t, switch off and check your soldering – especially for dags between the IC pins and for dry joints – and also your component placement and, if applicable, polarity. Final construction Using the photos as a guide, drill the nine holes required in the Jiffy box – four for the PCB mounts, two April 2011  47 for the transformer, one for the battery holders (all in the base) and two for the terminals on the end (plus, if you wish to use an XLR socket, a larger hole [usually 18mm]). The PC board itself sits 37mm above the bottom of the case on suitable pillars. We used a combination of a 25mm and 12mm threaded pillars to make up the distance with four 20mm screws holding them in place from underneath and a 10mm screw holding the PCB onto the pillars. If you have difficulties finding the right length pillars, one cheap trick we have used in the past is to use plastic wall plugs as pillars – they’re easy to obtain and easy to cut to the appropriate length with a sharp (hobby) knife. The dark blue ones (10mm) make a nice secure “platform” and you can use small self-tapping screws. When in place, the PC board sits hard up against one end of the case and actually slightly overlaps the transformer, with a clearance of perhaps 2mm or so. So if you wish the transformer could be mounted further under the PC board, as long as none of your soldered joints under the board can short to it. To do this, though, you will first need to disconnect and remove the 2-way terminal block on top (we’d done this anyway because we needed a terminal block for another project and this one is redundant!). We mounted the XLR socket between the two terminals, with pins 1 and 3 connected to the terminals (ie, in parallel). There’s not a great deal of room between the XLR socket and the transformer – in fact, we had to cut the ends off the XLR socket solder pins to give us enough room for the transformer and PC board. Solder wires from the common and 5W transformer taps to the output terminals. Most transformers have flying leads on their primaries; solder these to the output terminals on the PC board (if no flying leads, look for the “primary” or “8 ohm” labels). Because it is not easy to buy suitable and put on your pot knob (we didn’t worry about a pot nut). Finally, screw in the four lid screws and you’re done! How to use it Fitting the XLR socket required some minor surgery to the back of the pins to allow it to fit in – but there’s still plenty of meat to solder to. Note that the 2-way terminal block has been removed from the top of the transformer – it’s redundant because the output leads solder straight to the appropriate taps. chassis-mounting 4 x AA holders (although Altronics has one) we used two ordinary 4-cell “AA” battery holders which go alongside each other under the PC board. These were connected in series (one red to one black wire) with the other red wire going to the + terminal on the PC board and the black wire, obviously from the other battery holder, to the – terminal. To hold them in place we used a scrap of aluminium as a clamp and a single screw and nut coming up through the bottom of the case onto the clamp to hold the battery holders firmly in place. If you can find chassis-mounting 4 x AA holders, connect them the same way but secure them to the bottom of the case alongside each other using suitable screws and nuts. To avoid scratching the boss’s desk (he’s got a thing about that), we placed four small self-adhesive rubber feet in the corners of the case. You’re almost finished! Drill the front panel for the pot shaft (10mm hole), LED (5mm hole) and the slot for the on/off switch (4mm wide x ~10mm long). Place the lid on, making sure the LED, switch and pot shaft all come through where they are supposed to Resistor Colour Codes o o o o No. Value 1 150kΩ 1 100kΩ 1 2.2kΩ 1 10Ω 4-Band Code (1%) brown green yellow brown brown black yellow brown red red red brown brown black black brown 48  Silicon Chip 5-Band Code (1%) brown green black orange brown brown black black orange brown red red black brown brown brown black black gold brown I’m sure everyone who puts together temporary PA systems has their own way of working – but this will give you an idea of how I do it – especially now I can check the installation as I go. Usually, I erect all the horn speakers where I want them first, then go back and roll out the cables which connect them together. There is a reason for this: I know where each area of the carnival is to be set up so provide speaker coverage for those areas. The speakers are “daisy chained” one to the next – not in series but as I mentioned earlier, all in parallel. Each of my reels of cable has two XLR male sockets on the reel itself and an XLR female plug on the other end. A short female-to-female patch lead connects the reel to the speaker, while the second XLR socket on the reel is ready to accept the female plug on the next reel, going off to the next speaker. All are wired the same way, using pins 1 and 3 of the XLR plugs, so I never have a problem with phasing. With this gadget, I don’t need an amplifier connected (which probably won’t even have power available at that time of day) nor do I need a second person. I simply go to the furthest speaker, plug it into the checker and make sure it’s OK. Then I plug in the patch lead and if it tests OK, I plug it into the still-rolled-up cable and plug the checker into the opposite end – again, the tone tells me if it is good. I then roll out that cable back to the previous speaker and repeat the procedure. So at each speaker I’m checking it, the patch lead and the cable reel. You’d be amazed the number of times a reel of cable tests no-go – so I can substitute another roll right then and there. It saves having to come back later to swap it all out (and also having to roll SC out the cable twice). Capacitor Codes Value µF value IEC Code EIA Code 220nF 0.22µF 220n 224 100nF 0.1µF 100n 104  47nF 0.047µF 47n 473  10nF 0.010µF 10n 103 siliconchip.com.au