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An improved tweeter horn for the Majestic loudspeaker system When we published the Majestic loudspeaker system in the June 2014 issue, it created a great deal of interest; more than even we had hoped for. We thought that readers might think it a huge throwback to the 1950s but evidently it has been a hit. So much so that the originally specified Etone tweeter horn has now become unavailable. Fortunately though, we have been able to source a Celestion horn which looks and sounds even better. By Allan Linton-Smith & Leo Simpson This photo shows the front and rear views of the Celestion T5134 diecast aluminium horn, together with the mating Celestion CDX1-1730 compression driver. Note the two rubber inserts in the horn on its longer axis. These evidently are for damping as the horn itself is quite ‘dead’. Note also that only two mounting holes are provided for attaching the horn to the compression driver. 86  Silicon Chip siliconchip.com.au This photo shows two views of the assembled compression driver with the diecast aluminium horn. The horn has four holes for attaching it to the baffle and pan head screws should be used. Majestic Speaker Frequency Response Etone vs Celestion Horn <at>1 watt / 1 metre 07/04/14 19:19:16 +50 +40 W siliconchip.com.au +30 +20 +10 dBr A E WERE PEEVED, to say the least, when the Etone tweeter horn used in the Majestic speaker system became unavailable shortly after the June issue went on sale. The news was even worse when Etone advised us that the Asian manufacturer of the horn had gone out of business. We had to do some frantic searching to find a substitute horn which would perform at least as well as the Etone horn. Ultimately, we found that Celestion themselves had a suitable diecast aluminium horn which looked to be even better than the larger Celestion H1-9040P composite/plastic horn which we originally tried before rejecting it in favour of the Etone. We then had to source the horns from Celestion in the UK and had to wait weeks before they arrived. Then they had to be fitted to the Majestic enclosures, and listening comparisons made between the Celestion and Etone horns. Our initial impressions were that the Celestion horns gave a smoother overall response and their vertical dispersion was somewhat better than the Etone unit. Then the whole system had to be precisely measured with a calibrated microphone etc. Fortunately, our listening impressions were confirmed and the Celestion compression driver and horn com- 0 -10 -20 -30 -40 -50 20 50 100 200 500 1k 2k 5k 10k 20k Frequency (Hz) Sweep Trace Colour Line Style Thick Data Axis Comment 2 3 4 5 1 1 1 1 Red Mauve Blue Cyan Solid Solid Solid Solid 15 15 15 15 Anlr.Level A Anlr.Level A Anlr.Level A Anlr.Level A Left Left Left Left Etone Horn Etone Horn Treble Boost Celestion Horn Celestion Horn Boosted Fig.1: these four sweeps show the frequency response of the Etone and Celestion horns when fitted to the Celestion CDX1-1730 compression driver. As can be seen, the sweeps with the Celestion horn are smoother overall. bination is clearly superior. It would have been great if we had known about this horn right from the start! Frequency & distortion The Audio Precision graph in Fig.1 gives a comparison between the Majestic fitted with the now defunct Etone horn and the new Celestion horn. There are four frequency sweeps, numbered 2, 3, 4 & 5. Sweeps 2 & 3 are for the Etone horn, with sweep 3 showing the effect of the treble boost above 10kHz, produced by the peaking circuit in the crossover network. Both sweeps produced a pronounc­ ed dip at 2.5kHz which is not audible but was caused by a standing wave September 2014  87 We made this adaptor plate out of 4.8mm thick Masonite to cover the large hole in the baffle for the Etone horn. We made the slightly elliptical cutout with a circle cutter set for a diameter of 130mm and then used a rasp to slightly chamfer both sides to suit the wider axes of the Celestion horn. It was finished with a matt epoxy-based paint to match the finish of the diecast horn. from the floor reflecting into the test microphone. However, using exactly the same measuring set-up with the Celestion compression driver and aluminium horn gave no trough at this frequency. In this case, sweep 5 includes the boost above 10kHz while sweep 4 is without the boost. Overall, the Celestion horn had a much smoother response which was a confirmation of our initial listening tests. More importantly, the harmonic dis- Etone vs Celestion Horn THD+N 1 Watt (500kHz bandwidth) 07/04/14 19:42:56 100 50 20 10 5 2 1 THD + N (%) 0.5 0.2 0.1 0.05 0.02 0.01 0.005 0.002 0.001 0.0005 0.0002 0.0001 1k 2k 3k 4k 5k 6k 7k 8k 9k 10k 20k Frequency (Hz) Sweep Trace Colour Line Style Thick Data Axis Comment 2 3 4 5 1 1 1 1 Red Mauve Blue Cyan Solid Solid Solid Solid 15 15 15 15 Anlr.Level A Anlr.Level A Anlr.Level A Anlr.Level A Left Left Left Left Etone Horn Etone Horn Treble Boost Celestion Horn Celestion Horn Boosted Fig.2: these four sweeps show the THD performance of the Etone and Celestion horns when fitted to the Celestion driver. These figures have been taken with a 500kHz measurement bandwidth, resulting in higher overall figures than those shown in the June 2014 issue. Overall, the Celestion horn is the better performer. 88  Silicon Chip tortion was also reduced, as shown in Fig.2. This shows harmonic distortion taken with a measurement bandwidth of 500kHz for both horns, over the range from 1kHz to 20kHz. Again, sweep 2 is for the Etone without treble boost and sweep 3 is with the boost. Sweep 4 is for the Celestion without treble boost and sweep 5 is with boost. Overall, the Celestion compression driver/horn combination seldom exceeded 0.6% THD+N, even when boosted, whereas with the Etone horn it often exceeded 1%. Note that these THD figures are significantly higher than our original published sweeps which used an 80kHz measurement bandwidth. This tends to include a lot more harmonics and high-frequency noise, but of course anything above 20kHz is simply not audible. Incidentally, some readers have noted that the CDX1-1730 driver is only rated at 45 watts on the Celestion website but we have the actual printed specification sheet which clearly states that it is rated at 75 watts RMS (AES standard) which we believe is the correct rating. Physical dimensions As can be seen from the photos, the Celestion diecast aluminium horn has a smooth hyperbolic flare and an elliptical mouth, somewhat wider than it is higher, to give better horizontal dispersion. Even so, it also turns out to have improved vertical dispersion than the Etone horn and that means that the “sweet spot” for listening is somewhat higher – you don’t have to be seated to get the best treble response; it is more widely dispersed. The horn also has rubber inserts along its wider axis and we assume that these have been included to damp any resonances. Certainly the horn itself is quite “dead”, with no tendency to ring. The horn is attached to the Celestion CDX1-1730 compression driver using two M6 x 20mm bolts, with a washer under each bolt head. These bolts are quite tricky to fit as they are little too long to be easily poked through the two mounting holes of the horn. To fit them, you need to slightly distort the rubber inserts, to temporarily provide clearance for the bolt heads. The bolts should not be over-tightened. The new Celestion diecast aluminium horn is more compact than the Etone horn and will require a siliconchip.com.au Fig.3: this diagram of the Majestic cabinet has been modified to show the smaller baffle cutout for the Celestion horn. If you have already made the larger cutout for the now unavailable Etone horn, you will need to make an adaptor plate (see text). Note: the original article stated that the 1.5mm gap between the angled panel and rear baffle resulted in a 63cm2 port whereas the correct size is actually 6.3cm2. Also, the acoustic wadding used (from Jaycar) is made from acrylic rather than bondedacetate fibre (BAF). smaller cutout in the front baffle of the enclosure. The cutout can be in the form of a rectangle measuring 140mm wide by 100mm high. Fig.3 shows the dimensional diagram of the Majestic cabinet, amended with the smaller baffle hole for the Celestion horn. If you have already made the larger cutout for the Etone horn, you will need to make an adaptor plate because the new tweeter horn will not cover the larger hole. We made our adaptor plate from 4.8mm thick Masonite. siliconchip.com.au We made two Masonite adaptor plates, one for each enclosure. These plates each measure 255 x 205mm. Instead of cutting a rectangular hole in each for the horns we used a circle cutter set for a diameter of 130mm. We then used a circular rasp to chamfer both sides of the hole to allow the elliptical tweeter horn to sit squarely on it. After lightly sanding it with very fine emery paper, we painted it with a matt-finish black epoxy-based paint which resulted in a finish quite similar to that on the Celestion horn. The horn requires four mounting screws and we used 8G x 25mm pan head stainless steel screws for this task although we would have preferred screws with a black oxide coating, to make them less obtrusive. Finally, the Celestion T5134 “NoBell” horn is available from Elfa (www. elfa.com.au) for $35 plus p&p (order code 28/H1-7050). The Celestion CDX1-1730 compression driver is also SC available from Elfa for $189.00. September 2014  89