|Typical Tannoy Lancaster cabinet (sealed version for the 12'' driver),|
these generation cabinets are well made from lumber core plywood, excellent material.
There are the well known designs of similar speaker cabinets made in the past: some Altec designs like the VotT-line, the japanese Onkens, JBL, EV, Jensen and some other professional small scale pa-addressed designs for 15'' diaphragm sizes. They all have one thing in common, to be originally designed for a different speaker type in mind than the Tannoy Dual Concentric. The uncomparable design of the Tannoy offers opportunities other speakers don't show and these should be respected to find a perfect matching solution. And finally I did want to share my experiences with other Tannoy addicts, because those don't have a lot of choices of cabinets in our culture (may be in Asia, foremostly in Japan there are options for after market enclosures).
|Tannoy Amesbury with 15'' Monitor Golds|
|The biggest enclosure of all vintage Tannoy back loaded horn designs, the Tannoy Autograph Professional with two 15'' drivers in parallel, very poor material choice: chipboard|
|Inside the Tannoy Autograph, the complex development of a back loaded corner horn shown at a set of japanese after market copies (plywood).|
Ideally I want a performance with realistic and fast dynamic transients, a huge and holographic soundstage, with refined and detailed smooth Hf-response (without any phase shifts and resonant knots), a similiar performance like a natural wooden instrument of comparable size. A horn design at it best comes very close but will be extremely huge and room dominant. The other possibilty to leave the limitations off a boxed speaker is the folded baffle principle. To make it work, it needs a lot of acceptance as new approach of understanding of the role of cabinet (new was meant to be connected with Tannoy speakers – Western Electric and Klangfilm did incorporate such design principles about 70 to 80 years ago).
|Chinese copies of the Westminster for 12'' and 15" drivers. They do copy everything.|
I definitely search a solution to use with the superior hard edged chassis from the Monitor Gold, Red or Silver line in conjunction with low power tube amplification. These hard edged drivers need naturally a bigger enclosure to work superior to their later derivates from the HPD line. The limitations of the early cones depend hardly to the physical parameters shown at their low Qt of 0,26, but their sonic advantages are a close mirror of these facts. The nominal 15 ohm (at 1000 Hz) voice coil (Red, Silver and Black) was the preferred VC-impedance when tube amplification was the standard. This impedance matches the plate resistance preference of a pair of tubes and even more of a DHT-Triode better, the different coild at the output transformer to match are only a little help, since impedance shifts over the total frequency spectrum of the speaker can not be covered here. In particular in the lower frequencies the hard edge surrounds have helped tube amps a lot to control the cone movement. All this results is a increase of fine dynamic details and a better resolution of finest overtone harmonies. The light weight paper cones have a much lower mass to be driven and can therefor much better controlled by a given tube power amp, resulting in a improved sensitivity. The heavy HPD cone (double weight through the stiffing back support brackets and the three layer high power voice coil 85W) does not only make the double power output possible, it needs it it to be adequately driven.When the Monitor Gold was changed already for the establishing transistor amplifiers, the highly maneouvrable HPD cones are a clear reminiscence of the adaption to the high power output transistor amplifiers of their time. When used with tube power amplification, in particular with low power DHT tubes, the HPD drivers show mirrored characterics of the former chassis. All the live, color and speed in lower mid frequencies is gone, to me it sounds quite uninspiring and retarded comparing it with the earlier drivers. Within this drivers the front loaded funnel cannot add any extra quality to the soundstage.
The well known front loaded funnels from the Autograph and Westminster cabinets are a perfect extension for the 15'' dual concentric driver. Such a design helps fundamentally to conduct the cone radiation to the air of the listening room. In theory it works like a pantograph with a adjusted transmission ratio. The finer physical resolution of the stiff light weight cones completes the full potential of this design feature. A clearly listenable quality advantage of wider spread tonality and much improved micro dynamic response are the obvious benefits. In a horn design principle the cone almost works at its physical optimum, since it is not retarded by the air cushion of a closed cabinet. A further fine adjustment of the crossover design will complete this positive effects.
My personal experience was once initialised by the classic 12'' chassis (Monitor Red and Silver), where Ronnie H. Rackham set the Lf-cone as a fullrange unit. Later with the Monitor Gold line (1966) Tannoy limited the 12'' inch chassis performance with a 12 dB filter cutting above 1500 Hz. This intervention brought a flat measured response in the 1000 Hz region, but it can be easily approved, that it takes out a lot of micro dynamic detail. To my taste it sounds slower and a bit uninspiring, so I do like the more lively presentation in full range mode a lot better, even if there might be some little uneven inaccuracies in the measured response. Starting from this experiences and accustomed over the years with every day use, I missed the benefits of the full range driver when I restarted to listen to the 15'' chassis (Gold, Red and Silver) some years ago. Together with my first test of the front loaded funnel I changed here the crossover into full range mode. This intervention is a basic principle for the following incorporation of the front loaded funnel with greatly improved success. The Westminster and Autograph have as well verified (lower) crossover points in conjunction to the front horn opening. I tested both filters as well in my set up and decided to stay with the unfiltered full range mode for my 15'' Monitor Red drivers as to be a superior solution.
With the rejection of the filter for the Lf-cone even with the bigger 15'' units, two important advantages come together. First the sensitivity gets a better ratio. I am sure that the cone gets into trouble at the upper end of its physical ability. It will show a good portion of deformation in the measured response curve, this starts around 4000 Hz and peaks around 8000 hertz with already 6 db or even more loss, so it does not have a fundamental effect to the overall sound. On the other hand the uncontrolled cone plays so much better in terms of dynamics and speed in the important 1000-2000 Hz area, that I can accept the effect as almost completely positive.
As already said in other articles, the concentric high frequency horn is cut at 1000 hz with a 6db filter in the ordinary crossovers. Listening with the front loaded horn funnel shows a dramatic better distribution and dispersion of fine detail and micro dynamics in this most important tonal range around 1000 Hz, due to its acoustic coupling from 400 Hz on to the higher frequencies. It gives a 100% better front radiation of the cone and helps the high frequencies to be supported into the lower registers. Here almost every instrument or any voice will be covered with finer micro dynamic abilities. The overall soundstage is dramatically improved in its illusion of width and depts and the impression of air is noticeably extended, so this change has a fundamental impact with extremely positive effects for my new design.
Another word about measurements. They can help to find some problems, they can shorten evaluation proceeds, but their graphics doesn't say anything fundamental about sound qualities and their harmonic structure. If you look to graphs of measured natural instruments, they show a tremendous deviation from the idealized flat response. It is almost a opposite fact, the most colorful tonal presentation with the best harmonics in overtones, show the most degraded graphs. I don't think that the publishing of graphs shows any content about musical response, so I don't show them.
It is now a different loudspeaker to go. Try yourself, the intervention into the network, its easy to be done, just bridge the capacitor and the coil at the Lf-rail with a crocodile clip wire, – done. For the funnel to test, I can recommend a simple version made from 2mm grey card board as a first stage. This will show almost all the benefits with your existing speaker enclosure. If you have Lancaster cabinets, remove additionally the back panels and position the speakers almost 1 m (4 ft) as minimum distance in front of the backing wall. I swear, you never have heard such a good sound from your Tannoy speakers, if your chain is able to supports it. Don't fear this step, it will be reversible in 5 minutes... And if you do, please ... let me know what you think about!
Read on soon about constructing the new open baffle enclosures in the Christmas holidays, Volker
All photographs shown from Google with respect to their unknown original resource.