Thursday, 29 August 2013

A Tannoy 15'' Enclosure for the Enthusiast – Part I

Hello to all interested people into Tannoy enclosures,

after I've already reported about the Autographs and even when the final contribution unfortunately is still pending, I would like to start a multi-part series on DIY Tannoy enclosures. For twenty years I have been building housings for these great vintage speakers, i.e. for 12'' and 15'' chassis. Such speakers with its powerful drive moments, their large mass and their strong magnetic drives need other housing parameters than most speakers today or what haunts the so-called literature since the speaker DIY boom of the 1970s. La Maison L'Audiophile, Auditorium 23, and many people from that circles, recently such as the Wolf von Langa have been busy about cabinet design principles of historic professional designs. It can not be said enough how important the conservation of the historic proofed knowledge is, if you want a quality product with exceptional abilities. Todays consumers don't need anymore loudspeakers, they are happy with earphones as lifestyle accessories.

So today, even internationally, the small group of people with high quality demands and exceptional high quality tube amplification chains concentrate on Western Electric products or its later successors of good availability from the Altec era. Asian copies of WE equipment give this incomparable products a wider availability again. One of my major interest was, when I did start this blog, to give the classic Tannoy speakers a auditorium of interests and exchange of informations. These speakers are unique in their design and quality, their typical sound is different from any other coaxial speaker made. The older vintage 16 ohm hard edge chassis are exquisite in terms of finesse and listenability, I think better than any other duplex speaker made. So they are much sought after in the asian markets (alone in Japan there are three companies existing with a huge production line of enclosures for Tannoy), but Tannoy don't have a wide acceptance in Europe, where they came from. One reason might be that the most people only have real practice with the latest models, the HPD line, which which was the worst incarnation of this speaker line. The typical Tannoy enclosures are made for private home use, since the company Lockwood made the professional cabinets for the use in recording studios, which was the main business profile and success. Since the death of Tannoys founder in 1977, when the company has been sold, japanese customers have bought Tannoy speakers in Europe for the export to Japan. A edition of 500 Autograph speakers have been made solely for the japanese market in the late 1970ties.

The most people I know with this speakers use standard original hifi-cabinets, which are generally not ideal. They are furniture products to fit the british home, instead of being serious music reproducers. They are made from particle wood boards with nice verneers, nothing to take care about. So it makes sense to improve on cabinet designs to work out the full potential of this exceptional speakers.

A lot Tannoy users are in search of optimized cabinets for years and try to do it themselves, mainly with the wrong concepts based on theories of small scale highly filtered loudspeaker concepts of the 1970ties or the diagrams published by Tannoy itself in this period. But big chassis with hard edged cones need completely different concepts work best. It does not matter wether this search is for the 12-inch or 15-inch chassis, the starting point is for most people the original cabinets to be modernized. The early cabinets from the 1950ties are so called vented port designs, but they are made from quite good materials, since they were developed through listening tests. The later ones from the 1970ties followed the reflex philosophy of Thiele&Small and are extremely rigid, damped to dead in terms of resonances and made from mdf-boards. Often due to their dissemination you will see Lancasters or their successors the Berkley cabinets, both generations have a rough volume of 120 liters. Many have chosen this cabinets because of their size as compromise for their living rooms. Both generations have had a bigger cabinet as brother, so the Lancaster had the Amesbury and the Berkley had the quite common Arden (image). Both had a rough calculated volume around 220 liters, a much better value than the smaller versions. But if you look at the qt-parameters for hard edged paper surrounded 15'' Tannoy chassis like the Golds, Reds or Silvers, you will see a closed 360 liter cabinet will do the job of a quite more linear low frequency response much better. Anything smaller sets compromises as a major fact, but who wants really such big cabinets?The Onken cabinets from Japan published in the 1970ties are a first step forward, but are still quit big. Or it needs to look for other ideas of compromise like open baffle?

A modern cabinet is more an instrument than a dead piece of furniture

A good cabinet is made from grown wooden materials, which will have a tremendous influence on the sound qualities, in particular if you handle such big motor units. For Altec users with separated HF-horns the "Voice of the theater A7" is a enclosure, which is a very good base of size to frequency response ratio (roughly 410 liters volume). These cabinets are known to be working very well, but are still a lot to big for the most of us and will not match the HF-horn opening for the Tannoy. The most of us would go further to accept some restrictions to shrink the volumes of a speakers to match their living room aesthetics. The well known wife acceptance factor WAF is the most compromising aspect to match a speaker to a living room as furniture. The most people I know want cabinets for their Altec/Tannoy/Vitavox/Stephens/University/etc. 15'' speakers, which will fit to such preferences, ideally not bigger than 200 liters with a furniture friendly design. Since we are not the typical aficionados or are living in Japan and accept rooms to be filled with hifi equipment, were you cannot open the window anymore. All others seem to have enough space/money to incorporate special listening rooms for their WE15 setups with two 18'' field coil in a suited baffle per side. I do not.

In this series I will try to show the detailed development of a 200 liter enclosure with integrated front loaded horn from the very beginning. Lot of the construction aspects are based on informations coming from the traditional music instrument designs. Uncountable tests about energy transmission of wood types, their resonant properties and construction secrets from guitars or violins, as well pianos, different conditions of dryness of the wood have been taken into account before the final wood will be ordered. 
For the beginning a test set up will be very helpful. From earlier tests with WE-compatible speaker designs I had a so called "dipole-enclosure" for a 15-inch chassis lying around with a quite suitable size for the first tests. It has ugly proportions with a 0,75x0,85m baffle size. Since its two identical parts were originally faced to each other before to form the H-dipole, one half is now only 0,30m deep. It is made from perfect materials, 25mm northern hemisphere coniferous plywood for the boards and stripes as strives from the same boards for the reinforcements. This enclosure has been originally made to act like a low frequency sub-200hz-cabinet with a 15'' speaker. For a hefty full range unit of the same size, I rather would use thinner materials like 16 mm plywood of the same type. From my experience the thinner plywood works better with the important transmitted 1000 hz frequencies, so later on I would do the real enclosure with thin materials. Other people tell me the same experience from the original "VOT A2-cabinets", which are made as well from 16mm plywood comparing these with modern versions.

I definitely wanted a front loaded horn to extend the lower end of the high frequency unit, like it is used in the Westminster enclosure or similar at the Autograph. I did use such a front loaded horn since two years in in front of my berkley enclosures. When I started to test the effect, I did make the funnels from gray card board of 2mm thickness. Since almost 20 years I wanted to test the loading effect at my speakers, but I never did that. When I realized the funnels two years ago, I realized that this was my biggest mistake I made with Tannoy speakers during all that time not to go for "front loaded". It has an amazing positive effect and completes the unique design principle of the Tannoy speakers in a perfect way.

Berkley cabinet made from medium density particle boards with card board funnel for testing.
On top the new made wooden funnels for the new cabinets to be made soon.
at the cut out for the crossover the little switch can be seen were it can be switched to full range mode.

Were the cone completes the HF-horn, the funnel completes the cone further on. But this construction needs the customizing of the crossover, otherwise the effect does not do the trick. The funnel works like a mechanical amplifier for the distributed frequencies and it extends the high frequency horn for a lower cut off. I have never made measurements, but it is clearly noticeable, that the frequency between 400 hz to 1000 hz will be distributed. The speaker sounds  a whole lot more naturally detailed with a much better HF-dispersion. The illusion of space is wider and deeper, so the amount of listenable air in the recording grows dramatic. For me this effect is always a sign for a remarkable quality upgrade. I copied the funnel of the Westminster design. Both, the Westminster as well the Autograph have customized crossover designs in order to integrate the extra energy of the middle frequencies. Both designs open up the frequency cut off around 1000 hz to higher cross over points. 

Test cabinets with 220 liter volume for a 15'' Tannoy speaker with front loaded horn extension just screwed to the baffle
I did work out through the years something completely different. Coming from my personal preference of the smaller 12'' units of the Monitor Red and Silvers, where Tannoy coupled the cone as full range units, I tried to do the same with the 15'' Red recently. In combination with a front loaded horn the speaker just did struck me down by its shear energy distribution and much more colorful tonal spectrum of the middle frequencies. I use my 15'' Monitor Red as full range unit, while the HF-unit gets cut below 1000 hz with a soft 6db filter. This filter is made with oil paper caps and copper foil inductors with wax isolation. I still have the classic 1000 hz filter for the LF-unit built in, I did it switchable, so I can test it always in different enclosures, but for me the full range set up beats all other options. At least my favorable sound option of the 12''-chassis was just the full range option of the cone and did not have anything to do with the size of it or any better sound qualities arising.

Read on soon about my first impressions, Volker

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