Sunday, 19 January 2014

A Tannoy 15'' Enclosure for the Enthusiast – The Realisation II

Hello to everybody interested to follow on my realisation designing new enclosures for the classic Tannoy Dual Concentric speakers. In short summarized the first part, I want to find a living room friendly size for the 15'' Tannoy speakers which will be a incomparable upgrade for the common aficionado not having a restored farmhouse in Cornwall for the second or third hifi setup.

"James Bond" like phantasies of Tannoy sales managers in a product catalogue, were the woman, the whiskey, the furniture, the car and the audio components will match the same decorative "taste" of empty chauvinistic representation.  Imagine a car outside? But there are definitely 200 better choices of malt whiskey in Scotland...

As ended in the last article my first listening to a Western Electric horn array extended completely my idea of audio representation to a new dimension. Their unfolded horn designs (foremost made to transduce the middle to high frequencies of the audio spectrum from 300 to 12000 hz) showed me a unknown dynamic spread scale of musical informations, –  its inherent captured air, speed and a true naturalism of representation.

A winter tale...

This particular experience brought me back to my long lived dream about a front loaded horn funnel as an improvement of the classic Tannoy speaker, knowing that such a feature will help the Tannoy a lot to overcome its typical "archilles' heel".  In the mid 1990ties I was interested to introduce it to a common sized cabinet and discarded the plan because of complications in the technical realization (bending wood). In theory I was always convinced that the funnel will open up the important medium frequencies with a widely improved response and now I wanted to practice this idea. I built a first pair in 2010 from card board and introduced them to my cabinets  –  I could not believe what acoustically happened even with a such simple design. I directly realized, that I had failed a absolute important feature for almost 20 years of use. In original it got introduced with the Autograph cabinet to bring the middle and high frequencies to the same level as the mechanical amplified low frequencies from the backloaded horn. As I now know myself, with the Autograph cabinet it is very difficult to find a almost invisible natural audio representation. The size and materials of the cabinets, so the position in the room corners make them…,  let's say, to prominent transducers, were the holographic presentation in the middle frequencies is not the foremost quality.

The new enclosure

In other applications the front loading horn can help very well to overcome Tannoys most problematic attribute, the frequency cut around the 1000 hz, where Tannoy married the low and the high frequency diaphragms. The 1000 hz point is the most problematic area in the whole audio spectrum. Almost every instrument and voice covers this part of the frequency, which means, in the audible reproduction any mistake will be shown with a tremendous general effect. With the support of the funnel the Tannoy "achilles' heel" gets highly improved. I would go as far to say, it completes the speakers to a new design, when the modified crossover will support that task.

and 2/3 out...
The interior cabinet almost complete in

The interior cabinet is sealed with a felt lip for tight and smooth extraction

The distribution of the medium frequencies are in a different class and remind me a lot to the qualities I did notice first time from a big Western Electric medium frequency horn. As well here now a dramatical improvement in the coherence of the middle frequency response is noticeable. A lot more air between every instrument is present as a result of mechanical improved high frequency dispersion. Together with much better transient response and a dramatically bettered homogeneity, the most significant benefits are shortlisted. The vintage Tannoy Monitor speakers are already known and valued for their homogenous refined sound quality as a single point source (mostly in Japan, to me it looks that more vintage Tannoy speakers are in use there, than ever been produced in the West Norwood factory). Being a 15'' driver of enormous dynamic abilities, it combines two almost contradictive properties hardly found with other drivers. With the customized front loaded horn it reaches new claims of extended natural musicality and tonal refinement.
But one thing needs to be said at this point about the amplifying electronics to be used with such a horn to everybody who is on a hunt for a cheap shortcut to the wonderlands of life. It needs the finest tube amplification at every stage of the chain, able to deliver the best refined musical signal without any sort of phase degradation or other typical electronic deformation in the response. When such improved signal quality will be obtainable, a front loaded horn can be a fortune (similarity with the huge WE horns). 
With some mediocre shop hifi products or even worse some chinese "bling bling tube amps" such a audio transducer of this ability will show straight the inherent limitations of the poor profit orientated designs and their inferior used parts like seen with a magnification glass. It needs a perfectly matched chain of single components built from well chosen parts to a higher order of quality. The necessary refinements will rarely be found in commercial made hifi components due to its limitations of cost cutting values and sale strategies. With highest grade parts built diy projects might be able trealize a different stage, even when the budget will limit the plans. Or similar concepts are consequently made and matched together by idealists like Shindo, Kondo, AN and some others. With superior DHT amplification today such a quality is realized like no other by Thomas Mayer.
The advantage of the open baffle principle for the cabinets combines the perfect conditions for a driver in a horn design (the decrease of the braking air cushion behind the diaphragm and therefor the increase in linearity and speed of its radiant ability) of a comprehensible sized cabinet. As well the open baffle adds the backward radiated energy to the common front radiation (in anti phase mode), which can be  noticeable as increase of the low frequency radiated energy. Here the position of the cabinets and the size of the surrounding room gets into account, since the lowest radiated frequency will be limited by the room itself and the total length of the from front rim of the lf-driver to the back rim.
A typical open baffle speaker radiates into both directions, which means that a fullrange driver radiates the same energy into both directions. The backward part will be reflected quite soon from the backing walls of the room. The distance between speaker and backing wall defines the time delay between both contra phasing signals. If they would reach the listener in time, a whole part of the audio spectrum will  distinguish itself. So its is quite important to find a ideal position for the speaker between wall and listening position in a given room. Generally it can be said that a position in the center of a room will give the best undistorted (longest delay of reflexions) radiation. But the most people don't want to sit in the middle of a room with the huge speakers 1.5 m distance in front of their face. Here a compromizing position has to be evaluated. I have my speakers positioned with 1 m distance from the back walls and sit with three meters distance in front of their 3 meter base.

With 1 m behind it starts that the backside signal adds with a positive effect and enough delay comparing to the frontside radiation. The funnel directs the front radiation to the listener, so the typical negative effect of all fullrange baffle speakers, namely a decrease of space illusion in the soundstage by wandering reflections is not audible. The reflections in the room normally decrease the precise definition of the stage, thanks to the physical benefits of the funnel the illusion of space is extremely good. At this point all goodies come together. The unique Tannoy Dual Concentric principle works acoustically like a precisely defined point source for both frequency units. The funnel completes this design to a wider overlapping of both parts, operating the enclosure similar like a full range instrument of defined position. As result the speakers are able to produce a perfect illusion of space (like a lot of small drivers are known for this habit), a holographic dimension were other duplex designs of this weight class are showing limitations and produce some problems (phase and time delays between their units) to be corrected. Ask the most people driving a huge WE installation about time delay (some of them can write books about these facts), you will be astonished how much technic will be needed to get it  corrected. A integration of a analogue to digital conversion, digital time delay shift, followed by a digital to analogue converter before power amplification, is one of the simple operations to get it corrected. This is one reason, which protects me against such highly complicated systems, even if they are massive virulent sound wise.

Read on soon about the variable options with the inner cabinets, Volker

footnote: the image from the Tannoy product catalogue is shown with respect to the original intended use

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