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The PortaPAL Fitting a Wireless Microphone Back in the February and March issues, we described an all-new portable PA System, the PortaPAL. It was always our intention to add a wireless microphone to the system for extra versatility . . . T O SAY THAT the PortaPAL PA Amplifier has stirred up a lot of interest is something of an understatement. We’ve fielded a lot of enquiries here at SILICON CHIP – not the least of which was “is there a kit available.” If we might digress for a moment and answer that question, Altronics (1300 797 007) have produced “short form” kits for both the amplifier and the power supply/charger. By short form, they mean that the PC boards and all the electronics are supplied but not the “hardware”, although the heatsink and a screened front panel/chassis are included in the main amplifier kit. The main amplifier kit (Cat. K5360) sells for $179.95, while the power supply/charger kit (Cat K1695) sells for $19.95. So for less than $200 (not much less, but less!) you get the bulk of the project. Compare this with $575 for Altronics nearest “built up” equivalent and it represents very good value for money. What don’t you get? You have to supply all of the timber/woodwork, box hardware (corner protectors/top hat, handle, etc) and the speaker carpet. Importantly, the amplifier kit also does not include the speaker itself. The power supply kit contains only the PC board and on-board components – it does not include the 12V battery or the AC plugpack. Altronics General Manager Brian Sorensen told us that they had decided to go the “short form” route to make the kit as versatile as possible. “Many people want to make variations to the basic design,” he said, “and our kits make this easy.” Yes, you’re still going to have to build the box. But we showed you how 64  Silicon Chip siliconchip.com.au Revisited: to do that in the March issue. OK, so that’s the kit out of the way. Now let’s get back to the purpose of this article: adding a wireless microphone. But first, we might have a quick look at wireless microphones in general. VHF vs UHF Until fairly recently (the last couple of years or so), by far the majority of wireless microphones used in Australia were VHF models – especially for non-professional and semi-professional applications. Most transmitted in the band between about 150MHz and 210MHz, which is predominantly occupied by television stations. By judicious choice of frequencies according to your area, you could obtain a wireless mic that didn’t suffer interference from either a TV audio or video signal. (Given the dramatically higher transmission levels of a TV station compared to the wireless mic – a hundred thousand watts or more compared to a few milliwatts – the TV station always won any battle!) But even that cosy situation changed when the government (in their wisdom) decided to introduce VHF digital TV services in the “gaps” between analog TV services. So if you take Sydney for example, TV channels 6, 8, 11 and 12 became “no go” zones for wireless microphones. The vast majority of those who already had wireless mics on these frequencies (and there were literally thousands upon thousands of them) simply had to replace them. There was a flurry of activity on Ebay and similar auction sites as those “in the know” unloaded what were about to become paperweights. Some very fortunate people, especially outside the capital cities, were able to buy cheap VHF wireless mics which were, and arguably forever will be, perfectly usable in their locations because there was no VHF station, analog or digital, using that frequency in their area (most non-metro digital signals are or will be UHF). Part 3: by ROSS TESTER But most people do live in metropolitan areas – state capitals, mainly, which by and large all have the same 2/7/9/10 VHF TV bandplan. Ergo, the same 6,8,11,12 digital bandplan. It’s true that there remains a very small window between TV channels 9 and 10 which is (thus far) unallocated and therefore remains usable for VHF wireless microphones. But this space has become somewhat crowded and it’s not unusual for a sports announcer to have the local gym instructor belting out her movements on the same channel. (Trust me, I speak from experience here . . .) So what is the alternative? It’s quite difficult to buy a VHF wireless microphone these days. To avoid the digital TV problem, most have gone into the UHF band – up around the 800MHz mark or even higher. The frequency collisions of VHF aside, UHF does have some significant advantages for users. First, antenna lengths are less – much less. A typi- The “Redback” 16-Channel Mini Wireless Microphone System from Altronics. It operates on the UHF band, away from digital TV interference. In the receiver pack, you get the diversity receiver, 12V plugpack supply, 6.5mm to 6.5mm audio cable, “screwdriver” (for adjusting squelch level), the 2-part mounting bracket and the two screws to fix it to the receiver. The microphone is sold separately. siliconchip.com.au August 2003  65 Here’s that two-part mounting bracket which makes it so easy. One part screws to the back of the receiver while the other attaches to the surface on which it is to sit. Push one part inside the other and . . . presto! It’s just as hard to get a decent photo inside the completed PortaPAL case as it is to get enough room to install the mounting bracket! This shot looks down into the well, the bracket on the left (the black disc on the right side is the top hat). cal whip antenna at 200MHz is about 300mm long, although loading within the circuit might reduce this a bit. At 800MHz, that has shrunk to about 90mm (or less). Second, when using a “diversity” system of reception, the two antennas do not have to be anything like as far apart as on VHF (the distance apart is a function of the wavelength). Diversity, by the way, refers to a system of reception where two separate antennas and two separate receivers are used, with the system determining which has the best signal and automat- ically selecting that one. It can (and constantly does) switch back and forth as signal levels change, particularly if the radio signal source (eg, a wireless mic!) is moving around. This is done in a way which is completely transparent to the user. There are some disadvantages of UHF. The main one is that UHF signals are more readily blocked by the body, as anyone who has used a modern wireless mic will attest. One solution is to always face the receiver/antenna – but this is not always practical. Overall range, too, is theoretically reduced on Selecting the operating frequency is as easy as setting DIP switches on both the microphone (transmitter) and receiver. But make sure both are the same, or you won’t hear a thing. Two “AA” cells power the microphone/transmitter. 66  Silicon Chip UHF – but in practice they tend to be much the same. And all that brings us back to our selection of a wireless microphone for the PortaPAL. Size does matter! There is not a great deal of room left inside the PortaPAL case. And most of the wireless microphone systems around are built into 19-inch rack mount cases – not because they need to be but because that’s what the professionals demand. At first, we thought that we might have to mount the Wireless Microphone Receiver either on top of the case or on one side. Apart from spoiling the aesthetics of the PortaPAL, that would also place the receiver in a position where it (or more particularly its antennas) could be damaged (and that’s easily done!). Incidentally, you might wonder why we did not look at building our own wireless mic system. The reason is threefold: (a) wireless microphones are radio transmitters and as such have to be “type approved”. It is not economic or even practical to try to obtain approval for a DIY UHF system; (b) speaking of economics, it would be difficult to build a system for what you can buy them for and (c) building anything for UHF requires rather specialised componentry, techniques and most importantly test equipment that the average hobbyist probably would not possess. So a commercial system it had to be. Another thought that crossed our minds was to “gut” a commercial receiver and just put the “works” inside the PortaPAL. It’s messy and would siliconchip.com.au certainly void any warranty. That’s when we spotted a couple of much smaller receivers from our old friends, Altronics. Sold under the “Redback” brand, one was described as “half rack” size (or about 240mm wide) while another was even smaller – a tiny 130mm wide (for the record, 130 x 90 x 35mm, plus antennas). Both were 16-channel units, the operating frequency being selected by DIP switches on both the receiver and transmitter. They could be used in conjunction with up to five other transmitter/receivers at the same location without interfering with each other (in case you wanted to use them for multiple performers, actors, etc). In addition, both had either handheld or belt-pack transmitters available. We imagine most users would prefer the hand-held mic (which we chose) but the belt-pack also has its uses – it allows the use of a headset mic, for example, freeing up the user’s hands. Given the limited space inside the PortaPAL case, we chose the smaller of the two, the Redback C8866. And that proved a wise decision – it fits inside the case beautifully. The smaller unit also had a price advantage – almost $250 less than the slightly larger model and $330 less than the rack-mounting model. The inbuilt antennas are both an advantage and a disadvantage. They cannot be removed to allow higher gain antennas to be fitted but they’re nice and small so there is less risk of damage. They can also swivel to ensure they are always vertical, regardless of whether the receiver sits horizontally or vertically. Of course, the rack-mounting model is more a professional type, with increased specs and performance. The main disadvantage of the Mini Receiver compared to its big brothers is that the very small (70mm high) one-piece antennas do limit the range and cannot be removed. In typical (professional-type) use, if you are after longest range, you would normally use external (gain?) antennas, often mounted up high, usually with a booster. But with this receiver, you cannot do that. Given the majority of uses for the PortaPAL, we don’t see that as a major problem. The antennas can swivel through 360° horizontal and nearly 180° vertical so can handle vertical, horizontal or even angled mounting. (It’s quite important to have the antennas oriented the same direction as the microphone – ie, vertical). On the plus side (if you’ll pardon the pun), it was designed to operate from a 12V DC plugpack. We already had a perfectly good 12V supply inside the PortaPAL, so would be able to operate You can just see the receiver on the side wall of the PortaPAL – the two antennas are aligned vertically as this gives the same polarity as the (normally) vertical microphone. As yet the power cable and audio output cable haven’t been installed. siliconchip.com.au August 2003  67 be used with similar wireless mics to avoid interference between them). Still nothing? Ummm. . . have you remembered to put two “AA” cells into the microphone? Now check to see what setting of the receiver “volume” control is the most appropriate. If the fault LED lights, it’s clipping and is too high. Mounting it The prototype PortaPAL was made with an open well at the bottom, alongside the battery compartment. The main reason for this was to store the Here’s how to permanently connect the audio output from the wireless mic plugpack but could also store receiver into the guitar input. Connection can be made to either the tracks under microphone leads, etc. the PC board or to the pins of the 6.5mm socket – it doesn’t matter which. Note the It was also the perfect place tracks underneath the socket which need to be cut to stop it shorting. for mounting the wireless mic receiver. It could mount from that – completely portable, if we 6.5mm phone socket (a 6.5 to 6.5mm vertically on the side wall with room wanted to. lead is included). We wouldn’t need for the two antennas to also be posiOutput level is only stated as “line to open the receiver case at all! tioned vertically. Being craftwood, the level” (that usually means somewhere Finally, the mounting of the receiver PortaPAL case would not substantially between 100mV and 1V) but as there is delightfully simple. A miniature block the RF signal from the microwas also an inbuilt “volume” control 2-part mounting bracket is supplied: phone transmitter. on the receiver, we didn’t see that as one half screws to the receiver, the Well, maybe it was not the perfect a problem. other half to the mounting location place – ideally, we would have liked to – and the receiver simply drops into We had always intended to use the mount the receiver higher to give it the “guitar” input on the PortaPAL (16mV place. Easy! best possible range. But we’re pretty min, 1.9V overload) as the input for happy with the range we achieved, Test it out first! our wireless mic so if there was any along with the simplicity of receiver overload problem, it would simply be Assuming your PortaPAL is already mounting. a matter of winding down the wick! constructed (or at least the electronics Having said it was simple, mounting (We could have just as easily used the is complete) it’s a good idea to connect the receiver was probably the hardest “line in” socket but we often use this the receiver (to the guitar input socket part, because we were doing it after input for a cassette or CD player – and via the 6.5mm to 6.5mm lead), plug the case was finished – after the horse my guitar playing is about as good as in the plugpack and check that the had bolted, so to speak. It would be my quantum mechanics). system works! much easier done during the case Of course, we could have also adYou should find that both receiver construction because you would have justed the feedback resistors around and transmitter are set to channel 1 more room to work with. the guitar input op amp (IC3) to de- as supplied from the factory but if the How do you get the right spot for the crease the sensitivity – but didn’t find DIP switches are different on each you bracket? You could measure carefully that necessary. If we ever do need a won’t hear a thing! (By the way, the and hope for the best, or you could do guitar input, it’s still there! wireless mic receiver is supplied with what we did: cheat! Output from the receiver is via a a chart showing which channels can We used the photographer’s best friend – Blu-tak™ (being a photographer helped – I had some on hand)! Parts List – PortaPAL + Wireless Microphone All you have to do is “sticky up” some Blu-tak™ by briskly rubbing and 1 PortaPAL PA Amplifier (see SILICON CHIP, February & March 2003) kneading it between your hands, then 1 “Redback” UHF diversity Mini Receiver (Altronics C8866) sticking this onto the back of the rear 1 “Redback” UHF Wireless Microphone (Altronics C8872 or C8875) half of the bracket (the bit that is to 1 300mm length of shielded audio lead be screwed in) with the two halves of 1 300mm length light duty polarised figure-8 cable the bracket assembled on the receiver. 1 6.5mm mono phono plug Find the spot you want to mount the 1 2.5mm DC power plug receiver, then press it firmly against 2 “piggyback” (or double adaptor) spade lugs the wall. Hopefully, the Blu-tak™ 2 woodscrews, 12mm long (for mounting bracket) will stick to the surface and you can 68  Silicon Chip siliconchip.com.au Here’s where we connected one of the power leads. We found it easiest to connect the +12V to a “double adaptor” spade lug on the “fused” side of the fuse, as shown in the above photo . . . . . . and 0V to another “double adaptor” lug on the “–” connection to the power supply PC board. Only the relevant section of the board is shown at right. Make sure you pick the correct spade lug to fit the double adaptor to! carefully slide the receiver off with the back half of the bracket exactly where you want it. It worked for us! As there are only had two screws to install it didn’t take too long (there were a couple of barked knuckles and a few undeleted expletives but we won’t go into that). Anyway, the receiver mounting plate was secured and the receiver (with the other half of the bracket already affixed) was simply dropped into place. This placed all of the receiver’s external connectors – power, output (and also the screwdriver socket to adjust the squelch level, if required) on the top of the receiver where they could be most easily got at. The “volume” control was at the bottom of the case but could still (just) be reached and adjusted if needed. Once the right level was set, though, it could basically be forgotten. the prototype) is a switching type, which effectively shorts out the input if nothing is plugged in. (The reason for this is to minimise noise from the unused input). But that short creates a minor problem after soldering the wireless microphone output in parallel with the socket: no audio signal! Fortunately, the solution is very simple – cut the two PC board tracks shown (underneath the guitar input socket) and the short disappears. Of course, you could fit a non-switching/ shorting socket instead . . . While we had the supplied 6.5 to 6.5mm lead and could have simply chopped it off at a suitable length, we were reticent to cut up a perfectly good cable – one that might come in handy for something else! So we made up another short cable from a 6.5mm plug and a 300mm length of shielded audio cable and soldered it to the “input” PC board, where the guitar input socket connects. We similarly made up a power lead using a 2.5mm DC plug and a length of figure-8 cable. Note that the centre of the socket is the positive – we don’t know if there is any protection diode inside the receiver (no, we didn’t even lift the lid!) so don’t take a chance. The opposite ends of the power lead connect, naturally enough, to appropriate +12V and 0V points. The +12V wire needs to connect to a switched supply line. The easiest source for this is right after the fuse. We used a “double adaptor” spade lug directly on the fuse terminal (on the “fused” line to the PC board). The 0V supply can go to a variety of places, again using a double adaptor Wire-in or plug-in? There’s a choice here. You could simply connect the 6.5mm lead to the output of the receiver, out the back of the PortaPAL and into the guitar input on the panel and you would then have a Wireless Mic system which could be removed at will. It’s easy, but looks just a bit messy. Or you could wire the receiver in permanently. We chose the latter path because we didn’t intend ever separating the receiver and PortaPAL once finished. And even if we did, it wasn’t too difficult to remove anyway. Woops. . . It shorts out! However, there is a “little” problem here: the 6.5mm socket shown on the circuit diagram (and used in siliconchip.com.au space lug. We used the 0V supply connection point on the power supply PC board. In use There really isn’t much to tell here. It works and that’s that! The instruction booklet supplied with the receiver gives you several troubleshooting steps, just in case. The range of your wireless microphone can be significantly extended by mounting the PortaPAL up high – that’s why the top hat connector was included. You should be able to easily achieve the stated 30m minimum and most of the time, it should be 50m or better. We mentioned before that the receiving antennas should be aligned the same way as the microphone is held. The reason for this is to achieve maximum range. It’s good practice to hold the microphone nearly vertical if that’s the way the antennas are set because once you depart from transmitter/receiver antenna alignment, the signal loss becomes significant. (Besides, for both the microphone’s sake and for personal hygiene it’s also good practice to speak/sing across a mic rather than directly into it, as many performers are prone to do)! Finally, wireless microphones are no different to any other microphone when it comes to acoustic feedback. Avoid getting too close to the speaker – especially at high volume levels. Remember that the back of the unit is not sealed so feedback can occur SC from both front and back. August 2003  69