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This is, of course, impossible! High performance Dipole Loudspeaker Design by Allan Linton-Smith This loudspeaker, intended for smaller homes and apartments, will really challenge your assumptions about speaker cabinet design. In fact, it has no cabinet – and yet it is a wide-range design with a frequency response from 20Hz to 18kHz. To pinch a phrase from the Hitchhiker’s Guide to the Galaxy, “This is, of course, impossible!” But as Arthur Dent and Ford Prefect found, even the impossible can work, albeit (in this case) with the aid of electronic skull-duggery (and perhaps the ultimate answer: 42#)!* 40 Silicon Chip Celebrating 30 Years siliconchip.com.au M ore than a year ago, when this project was first mooted for SILICON CHIP, we were very disparaging, making comments like, “Stupid, impractical idea!”, “It could not possibly produce any useful bass!” and some others which cannot be published. Since then, there have been a number of prototypes produced and the electronic skull-duggery has been refined. Now the results are quite impressive: real, wide-range high quality sound from a loudspeaker system with no cabinet at all, as can be seen in the photos. But why would anyone want to build a loudspeaker system without a cabinet? Well, why not? Cabinets can be hard to make and they can have unwanted resonances. It turns out that there are a number of manufacturers around the world who do produce some weird and wonderful dipole loudspeaker systems. But to purchase them you’ll need to pay top dollar – we have seen prices from $10,000 to, wait for it, $4,000,000! Our Dipole Loudspeaker costs dramatically less than even the lower figure – and can be built by any electronic enthusiast. You don’t even need the wood-working skills a normal speaker cabinet would demand. In essence, it is just a large timber baffle with three loudspeakers mounted on it: a Celestion horn tweeter and two 10-inch woofers from Altronics. And the electronic skull-duggery? Well, this is an active (powered) loudspeaker system, with three power amplifiers and the SILICON CHIP 3-way active crossover featured in the September & October 2017 issues (siliconchip.com.au/Series/318). The suggested SILICON CHIP power amplifiers are the SC200 (135W into 8Ω) amplifier (described in January, February & March 2017 – siliconchip.com.au/Series/308) and the Tiny Tim 10W per channel stereo amplifier (described in October & December 2013 – siliconchip.com.au/Series/131). Why such a huge disparity in the power outputs? We use the Active Crossover to provide lots of bass boost to the woofer, hence the 135W amplifier, while the midrange and horn tweeter can be driven to more than adequate levels with both channels of the Tiny Tim stereo amplifier. That is a brief description of the concept but let’s now discuss the development of the Dipole Loudspeaker. Development The designer, Allan Linton-Smith, has been developing the dipole loudspeaker concept over a number of years and had produced some reasonably promising systems but these had always been rejected by SILICON CHIP staff as being way too “thin” in the bass department. Bass is the real stumbling block, of course. After all with no cabinet, there is nothing to prevent the out-of-phase output from the rear of the woofer from cancelling the output from the front of the woofer. In fact, the overall bass response will be mainly determined by the dimensions of of the baf* Above all, DON’T PANIC! Only those who have heard the original BBC radio play “The Hitchhiker’s Guide to the Galaxy”, by Douglas Adams, or read the book, or even seen the TV series or the movie (all with the same name) will have the slightest idea what these obscure references are all about, especially 42! Hitchhiker’s Guide graphic on opposite page courtesy http://hitchhikers.wikia.com/wiki/Earth siliconchip.com.au If money is no object (!), you could consider a pair of these Australian-made “Kyron Gaia” dipole speakers. Each one weighs about 200kg and is made to order by Adelaide-based Kyron Audio. They might look pretty cool. . . but be prepared to fork out around a quarter of a million dollars, though! (If that’s a bit too much for you, their 3-way “Kyron Kronos” model will set you back just $121,000). Or if you really wanted to go all out, try the Swedish “Transmission Audio Ultra” system. They’re reputed to be one of the most expensive speakers in the world – at about $2 million Australian EACH! fle board on which the woofer is mounted. Some extremely expensive Dipole Loudspeakers attempt to overcome this problem by using multiple woofers and huge bass drive power but in the end, there are definite limits. Allan Linton-Smith soon found that bass boosters, such as the Bass Extender from the April 2005 issue (siliconchip. com.au/Article/3034) only made things worse by creating reasonable bass but a very distorted lower mid-range. Not only was their performance simply not up to scratch but these experimental systems used some very expensive drivers with very impressive specs. These would have been largely unobtainable and/or beyond the budget of average DIY speaker builder. Then he tried a commercial 3-way active crossover system and that enabled some reasonable progress to be made. This involved using one 10-inch woofer from Altronics (Cat C3026) to cover the low bass frequencies at 100Hz and below. A second, identical woofer was then used to cover the range from 100Hz to 2kHz. Finally, he used the well-tried Celestion CDX1-1730 and Celestion T5134 horn to cover frequencies from 2kHz upwards. This is where the first problem arose because there is a huge disparity between the efficiencies of the Celestion tweeter and the Altronics woofer: 110dB/1W<at>1m compared with 93dB/1W<at>1m. Clearly the power amplifier for the tweeter only needs to provide one or two watts. In fact, the tweeter’s output needs to be attenuated by about 17dB to match the sensitivity of the Altronics 10-inch driver used as the midrange unit and it can also be driven to reasonable levels from a 10W amplifier such as the Tiny Tim. But to get bass response to match the levels from the midrange and attenuated tweeter, the second 10-inch Altronics driver needs to be boosted by a whopping 25dB. To do that you need a big amplifier and an active crossover with attenuation slops of 24dB/octave – which the SILICON CHIP Active Crossover is designed to provide. OK, leaving aside the need for all this electronic augmentation, are there any advantages in a dipole system? Celebrating 30 Years November 2017  41 Front and rear views of the final Dipole Loudspeakers. As you can see, the woofer and midrange (actually the same drivers – Altronics/Redback C3026) are mounted to the “baffle” in the conventional way, while the tweeter, a Celestion CDX1-1730 matched to a T5134 horn, is simply screwed to the top of the same sheet of timber. While the woofer and midrange are identical. they handle different frequency ranges, fed to them via the three sets of terminals at the back (one pair feeding the tweeter), all under the control of the SILICON CHIP Active Crossover (September/ October 2017 – siliconchip.com. au/Series/318). The side supports and base were finished with a matte black spray paint. Note: if you’d prefer not to mar the front panel with screws, you could screw’n’glue the base, side panels and front panel with appropriately placed cleats. Some people apparently like them because of their subjectively “light, airy and smooth sound”. (Does that mean weak bass?) Another stated advantage is that dipole loudspeakers are “neighbour friendly” because high bass pressures are not created in a listening room and therefore therefore heavy bass signals are not transmitted next door or through the floor – a distinct advantage in flats or apartments where thumping bass can be a major cause of complaint! So does this mean that no real bass is apparent the listening room? What you will tend to notice that there are no pronounced standing waves in the room, compared to the sound from a bass-reflex or sealed enclosure. And don’t think that because there is no cabinet, just a straight baffle, that most of the sound will be cancelled. If Parts List - for EACH speaker 2 1 1 1 250mm (10”) woofers (Altronics/Redback C-3026) Tweeter driver (Celestion CDX1-1730) Tweeter horn (Celestion T5134)    580x864x18mm baffle, material and finish your choice (we used Kaboodle [Bunnings] blind corner base panels) 1 18mm plywood sheet 610 x 1220mm (or one sheet 2400 x 1200 customwood, etc – does two) 8 12mm x 8g round head screws [for mounting woofers] (or 30mm M4 screws, nuts and washers – see text) 8 30mm x 8g csk head wood screws [for side panels] 14 12mm x 8g stainless csk head wood screws [tweeter/hinges] 2 85mm stainless hinges 3 polarised binding posts (eg Altronics P9257A) 2.5m heavy duty figure-8 cable (with polarity trace/colour) cable ties, black spray paint etc 42 Silicon Chip that were true, there would be virtually no sound produced by a loudspeaker suspended in air with no baffle. Clearly, that is not the case and you can easily verify that for yourself. But the bass will be weak. So bass frequencies below about 100Hz will need a significant boost in power, as noted above. Actual performance As you can see from the frequency response diagrams, this Dipole Loudspeaker system is good from 20Hz to 18kHz ±5dB which is really remarkable when you realise that we are using off-the-shelf drivers! We should state though that it cannot deliver this very good bass response in large rooms – the amount of power needed would simply overload and burn out the woofer. In fact, we are relying on the Bass Limiter in the SILICON CHIP Active Crossovers to prevent the power amplifier and woofer from being over-driven on loud music passages. Overall, we were very surprised that a speaker which is virtually “boxless” can produce such impressive bass and which sounded so smooth! We listened in a furnished room of 5 x 5.4 metres at a distance of about three metres and the bass proved quite substantial. But when we stepped outside the direct listening area, the bass was noticeably reduced. And of course, there is an advantage when using an active crossover, in that the listener can play around with both the crossover frequencies and the amplitudes of the signals fed to the drivers. Everyone had a slightly different opinion of how it should sound and this pretty well depended on the program material. Construction We’ll look at the base and side supports first of all, be- Celebrating 30 Years siliconchip.com.au Overall “room response” of the speaker, measured with a microphone placed directly in front. It’s remarkably flat, with just a slight dip above 12kHz; there is no detectable bass roll-off. Ampltiude of signals being delivered to each driver with a constant amplitude sweep tone fed into the active crossover. Note the large difference between the peak woofer and tweeter power. cause these are less obvious and can be cut from a variety of timber. We used marine ply – because we had some – but you could use just about any 18mm thick ply, MDF, chipboard, etc. (Don’t use anything thinner because it will not support the baffle properly). If you don’t happen to have suitable timber lying around, there is an advantage in buying a single 2400 x 1200 x 18mm sheet of “craftwood” (or whatever brand it’s called!) because this gives you much more timber than what you need and we found it to be significantly cheaper than two sheets of 1220 x 620 x 18mm ply ($33.00 vs $37.00 each at Bunnings!) – and you’ll have plenty left over for another project. A cutting diagram is shown for 1220 x 620mm but this would be easily transcribed to the larger sheet. Even though there are only a few pieces to cut, accuracy is important because you need the pieces to fit together well. If you are a competent woodworker with a saw bench you should have no problem cutting the pieces but if you doubt your skills we recommend having it cut professionally (most kitchen cabinet makers will do this for a reason- able cost) and this will make assembly much easier. If doing it yourself, cut out one speaker at a time to save you correcting any errors twice! The side supports are screwed to the base using M4 countersunk head (CSK) stainless steel woodscrews. The side supports can be centred on lines 100mm in from each edge, (see drilling diagram overleaf). Four screws are used for each side support with the base countersunk to suit. A narrow brace (380mm x 50mm) is cut and screwed between the two side panels to support the three stereo binding posts. As mentioned elsewhere, we sprayed the side supports, base and brace with matte black spray paint, just for appearance. The Baffle Like many of our earlier speakers, we used a 560 x 864 x 18mm “Kaboodle” blind corner panel (from Bunnings) as a baffle. These are available in a variety of colours and finishes and save us having to paint or otherwise prepare and finish the baffle. Cutting diagram for the base and sides from a sheet of 18mm ply. Further investigation suggests a 2400 x 1200mm sheet of craftwood will cost significantly less and you’ll have a lot left over for other projects! siliconchip.com.au Celebrating 30 Years There are only four holes to drill in the base – these accommodate the side panels shown at right. November 2017  43 However, be careful with the surface: they mark very easily if you aren’t careful. Leave the protective plastic on the panels until you are finished working with them. Unlike our earlier speakers, we only need two of these panels because there are no box sides, tops, bottoms or backs to worry about. Carefully cut the woofer and midrange holes (233mm) as shown in our baffle diagram – a circle-cutting router makes the neatest cut but if you have to, you could use a jig saw, or drill a series of small holes (say 6mm) inside the required areas and finish off with a rasp, wood file, etc. Cutting (and drilling) from behind results in less chipping on the front. And remember that old adage: measure twice, cut once! While you’re about it, drill the 4mm holes through the baffle which will hold your side panels in place. The drilling guide (at right)shows their position. But as an afterthought, we imagine many people would prefer not to have the side panel screws going through the polished wooden baffle. You could instead use small (say 20 x 20mm) cleats screwed and glued to the panels on the rear side to hold them in place. These would be placed on the inside of the side panels. If you’re going to paint the side panels and base (we sprayed them matte black) now is a good time to do it so that the paint can dry. Hinges hold it together Rather than screw the base to the baffle we simply used a pair of 85mm stainless steel hinges to join them. This also allows the correct angle (8°) between the pair when the two side panels are fitted. You will need an extra pair of hands to hold the baffle against the base when marking the hole positions. Drill all 4mm holes (6 per hinge) to, say, 12mm deep – use the old trick of putting some masking tape around the drill bit (12mm from the pointy end) to ensure you don’t go too deep. Again using an extra pair of hands, align a side panel with the holes you drilled in the base and the baffle and once happy with the location, screw it in position – then repeat for the other side panel. Placing the speaker drivers in position If you are happy with the way your dipole speaker “box” looks, you can mount the three speaker drivers. Start with the two 10-inch drivers. These mount from the front of the baffle. Whether you use 12mm woodscrews or 20mm screws with nuts (ie, right through the baffle) is entirely up to you. You can see the heads of either from the front – so black screwheads will look the best. In either case, place the driver in its 233mm hole and mark the mounting hole positions (eg, with a felt-tipped pen). Remove the driver (don’t be tempted to drill the holes in situ!) and then drill either 3mm pilot holes (for woodscrews) or 4mm mounting holes (for screws and nuts). Repeat for the other driver before mounting the speakers and (carefully!) placing and tightening the screws (or screws and nuts). Attach the tweeter driver to its horn, place the assembly 44 Silicon Chip against on the top edge of the baffle and mark the screw holes. Again, it’s your choice whether you use woodscrews or screws and nuts but as it’s only held in with two screws, we’d be more inclined to use the latter – say 4mm x 20mm. Remove the tweeter/horn before drilling the two mounting holes, then screw the tweeter/horn firmly into position. We used some weather stripping between the horn and baffle to damp any possible vibration; you could also use silicone or a strip of rubber – anything that will break the metal to panel connection. Wiring it up Before screwing the terminal mounting bar to the side hinges (where positioning isn’t at all critical), drill the holes required to accommodate the three polarised binding posts. Each requires a pair of 13mm holes at 19mm centres – again, while overall positioning isn’t important, it will look much better to have one on the centre line and the other two equidistant apart – say about 75mm each. Once done, and with the binding posts firmly mounted, screw the bar to the two side panels, as shown above. We’ve specified 2.5m of heavy duty figure-8 cable per assembled speaker. Realistically, only the woofer needs heavy duty cable (remember we’re going to pump up to 135W into it!) but there’s not too much point in using different cables for the other two drivers. By the way, we’re not specifying “monster cable” or other marketing cons – just polarised, garden-variety figure-8. You’ll only need about 400mm to wire in the woofer, about 600mm for the midrange and about 1000mm for the tweeter. This is being generous, allowing the three cables to be neatly laced together with small cable ties. When wiring, watch the polarity: the red terminal should always go to the + speaker terminals and black for - (regardless of what colours your wires actually are!). While any terminal can be wired to any speaker, it makes sense to go in the same order as the drivers: left to the woofer, centre for the midrange and right to the tweeter. Celebrating 30 Years siliconchip.com.au The three diagrams above and opposite, along with the photograph above, should assist you in both drilling and assembling the Dipole Speakers. It’s relatively straightforward – just make sure the panel cuts are true and they mate well with each other. While we didn’t find it necessary, you could use some wood glue between the base and side panels. That way, there won’t be any errors when you connect your amplifier. (You could also attach some small labels). Lace together the three cables with some small cable ties – you might also wish to anchor the cable sets to a cabinet side panel but that’s up to you. Sanity check Just to make sure nothing is amiss, give your finished speaker the once over: make sure the wiring is correct; that there are no rattles or movement etc. Pick the whole thing up (with two people?) and give it a good shake! If it all checks out OK, repeat the above steps to put together the second speaker. Checking it out Even without three amplifiers and the Active Crossover, you can check out that there are no resonances or buzzes etc, by feeding the woofer with a reasonably beefy amplifier (at least 50W or so), playing some good, bassy music (did someone mention Toccata and Fugue in D Minor on a pipe organ?) or even a signal generator feeding it 20-100Hz. And finally . . . Final wiring up is simple: connect your music source to the Active Crossover, take the three active crossover outputs (bass, midrange and treble) to your SC200 (bass) and Tiny Tim (tweeter and treble) amplifiers and take their outputs to the appropriate Dipole Speaker terminals, as shown in the connection diagram below. Some adjustment of the Active Crossover controls to suit your particular tastes may be required – but having total control is what it’s all about! As a start, aim for: Lower crossover: 100Hz; Upper crossover: 2kHz Woofer gain: 0dB (ie, maximum); Midrange gain: 25dB below woofer Tweeter gain: 42dB below woofer # What has 42 (the answer to the ultimate question of life, the universe and everything) got to do with anything? Tweeter sensitivity is 42dB below the woofer, of course! SC Wiring the Dipole Speaker pair requires a little more attention to detail than a “normal” loudspeaker setup. Here we’re using two “Tiny Tim” stereo amplifiers to drive the midranges and tweeters and two (mono) 135W SC200 amplifiers to drive the woofers. Note the unusual connections to the Tiny Tims – we’re actually using them as two mono amplifiers. siliconchip.com.au Celebrating 30 Years November 2017  45