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February 11, 2009 / consort3

Homage to Tannoy

I have been a fan of the Tannoy co-axial for years but could not afford them, so when Partsexpress offered a 12 in coaxial driver for 50 bucks I thought time to indulge myself. However the shipping cost to the UK for a pair was 135 dollars so I cast around to see what is avaliable in Europe. Coaxial drivers tend towards the PA/Pro market with companies such as Beyma and B&C producing drivers. Ironically a US maker Eminence make drivers which are avaliable in Europe for a reasonable cost. I found the 10CX was a likely contender and then found the German enthusiasts at Klang and Ton had come up with the CT235 project using that driver which roughly met my size and cost goals.

My trademark designs use the Keele alignment which is sixth order with 6dB of filter boost at the box tuning frequency. Using the CT235  measured driver parameters would give a -3db point at 38Hz with a 46 litre box. However their parameters are a little different from the makers so final design wouldl depend on what the actual driver measure at after running in of course. But having difficulties obtaining the 10Cx. Also the consensus seems to be that the 10CX is more of a PA than a high-fi unit. So project on hold for the moment.

The German DIY market seems much more developed than the UK one if the pages of Klang & Ton and the German forums are anything to go by. I was impressed by the innovative enclosure design of the CT235 but the 2 port holes seem too small a diameter. Why not use a proper port tube? The link below is a review of a speaker based on the 10Cx and they made the port tube 10cm diameter.  I guess if you used PVC drain- pipe you could buy a right angle fitting to get the length in.

http://h-audio.de/lautsprecherdaten/eminence/203dell2119-1.pdf

More on this design here:

http://stereointegrity.com/wp-content/uploads/AdireAudioHE10_1MarkIIplans.pdf

Apparently the ‘special’ tweeter was the Eminence APT50

Update: I am now the proud possessor of a pair of  Eminence 8CX + APT50’s so will be devising a design for those bad boys. For someone used to domestic kit this pro gear is built robustly! Measuring the frequency response was interesting. The 8cx had a valley in its response at 4.5k so  the roll-off  beyond 3kHz can be used as part of the crossover. The APT50/8cx combo is a superb transducer between 2 & 4K,  having a plateau there 10db higher than everywhere else.  However I found by putting in a ring with a hollow taper to match the Apt50 flare to the entrance to the 8cx horn flare it got rid of the  large plateau. That still left valleys at 5.4K and 8K which were EQ’d out with the DCX2496.

So the question is can the Eminence combination give the refinement of more sophisticated acoustic designs by electronically EQing the response? My listening tests say a qualified yes.  The treble is not as clear as a good dome tweeter although there are no obvious faults.  One thing I found was that the horn subjectively needed a higher level to get a balance than the measuring instruments indicated. Since I do not want to tie up the DCX2496 in permanent duty with their “Eminences” I will design some EQ based on graphic EQ technology to emulate the DCX EQ. Here is how to do it with analog stuff:

http://focus.ti.com/lit/an/slyt134/slyt134.pdf

Found a 10 page thread on a German forum regarding the best compression driver to use with the 10Cx  Page 9 has the answer!

http://www.hifi-forum.de/viewthread-104-9505-1.html

Discussions on the Tech-talk forum about the high cost of European drivers. Here American made Eminence drivers are a reasonable price and in stock, unlike the US where they are special order!

Update: Discovered the achilles heel of Tannoy type drivers!  How do they seal the loudspeaker?  With conventional designs you have a dustcap. The dustcap on a Tannoy type coaxial is acoustically transparent and will leak air. Found out when I checked the port output which did not make sense. I had a similar problem with a phase plug type driver. You can model this in Unibox by setting the Ql leakage parameter low say 1 or 2.

Others have started to note the leakage with phase plug drivers

http://www.htguide.com/forum/showthread.php4?s=62669434f52a01ddce98aa59fd17c2f2&t=38926

I considered  restoring the missing bass with Volvotreter’s

38Hz double folded Tangband Tapped Horn

However I will probably use  a bass reflex box since it is easier to make. The port design of the CT235 now makes more sense because of the leakage and my criticism can be ignored! Also it may explain why Tannoy cabinets tend to be big since they would be ‘low pressure’ designs

Update 2

After trying and failing to make the compression driver work (OK it works but at 1 to 4KHz ideal for PA use) I have decided to paint the acoustically transparent dustcaps with PVA glue to seal them and make the bass work. Will use a conventional horn instead.

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