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DIY 12" Tannoy Horn

Space, the instruments and the music

[Italian version here]

DIY design: horn loudspeaker for 12" Tannoy Dual Concentric drivers
Author: Chris Templer - TNT-Audio South Africa
Published: July, 2015

Introduction

Once you leave the anemic dynamics of all monkey coffins you are taking on a brave new challenge that few Americans are able to confront...the challenge of authentic music dynamics that only horns can produce at low distortion..and remember there is a great deal of difference between playing loud and dynamic range.

For example...if you use a midrange and tweeter horn and do not use a comparable woofer horn then you will have dynamic discontinuity...but if you use a woofer horn of any significance you will have problems in a normal room (terrible sound) because the room can't absorb the energy. Of course, if you want to use a horn of reasonable length you must fold it and that means a very expensive, heavy and large box....like the Tannoy Westminster's.

Because of the dynamic/low distortion nature of horns every problem in your audio system is exaggerated because it is so obvious....and on the other hand it is a very device for experiencing the gross and subtle differences in components and gear”
Dr Harvey “Gizmo” Rosenberg

“...Once horn speakers get in your blood nothing else will do. They put you IN the music in a way other types of speakers rarely do.”
Paul Eizik, letter to “Readers' Forum”, Sound Practices Vol.2, #1

“Although full-range horn systems are used today only by a small number of enthusiasts, most experts are unanimous in acclaiming their virtues as loudspeaker enclosures, especially their high degree of realism and “presence”.
Jack Dinsdale, "Horn Loudspeaker Design”, Part I, Wireless World, March 1974

The writings of audio luminaries such as Jack Dinsdale, John Crabbe, Paul Klipsch and many others drew me to Horn Speakers and from the first I was hooked. I admit to certain foibles audio wise, love of horns is one, a dislike for small drivers is another and a preference for dynamics and timbre that approaches real live music another.

This rather inevitably leads to the need for very efficient speakers and horns are a way to further this. Unless one is prepared and is solvent enough to spend huge gobs of cash on for example a pair of Tannoy Westminster Royals then DIY with second hand drivers is the only recourse.

Design & driver

The build described here is for my analogue system using either SE 300B (two in parallel) Triode amp or Leak Point One Stereo with an output of 8 to 10 Watts and needing to play back everything from brass band to organ and orchestra in a room of 64 sq M.

First off was to source drivers. I have a preference for Tannoy Dual Concentric drivers and found a pair of 12" Monitor Gold's in rather horrible Lancaster boxes second hand. Of 70's vintage they have the AlNiCo magnets and cloth surrounds and not the degradable Tannoyplast foam.

The Tannoy Monitor Gold is 12" with 93/95 dB free standing sensitivity so a back loaded horn was needed to lift that to closer to 100 dB.

The Dual Concentric Tannoys have two magnets systems (or motors), one for the main paper cone and the other for a HF horn integrated through the pole piece. The associated original crossover and controls for HF gain and roll off are also used.

The design of horn loaded cabinets is not something to undertake lightly so the easiest approach is to find a suitable plan on the internet and "tweak" it to suit which is what I did. Usually a horn loaded cabinet is difficult to build and one look at my big Tannoy Double Autograph drawings, with 65 panels per cabinet decided me against something quite as extreme. I have in the past built the Rectangular GRF cabinets but as they did not perform well I looked further and come up with a cabinet designed by Coral in Japan.

In my research into this cabinet I found some mention of it being suitable for 12" drivers and after a bit of thought a pair were made up with some changes. The hight of the cabinet was increased by 200 mm and the angled section in the cabinet base altered. The mouth of the horn allows the cabinet to extend down a bit further and the changed panel increases the “coupling” to the floor. These changes seem to have worked, more on that later. My sketch follows [click on the pic to enlarge].

In addition all outer panels are 32 mm thick Supawood (dense MDF) and finish is piano black. The crossovers are mounted out of sight inside the back horn mouth and the Controls above them through the wall. As per the above “right” holes have to be made to suit the magnet structure. This is the most difficult part of what is a quite easy build. The speaker driver mounts from the outside. No provision was made for speaker grills and the finish is as per the pictures following.

The cabinets are mounted on casters but spikes or small feet could be substituted. They are needed if the angled panel is extended out as I did. The panels behind the driver have a 12mm layer of felt to stop back reflection through the cone. No other damping is needed!

Above, cabinets upside down without backs, [middle] showing the hole for the gain and roll off controls (note the wood thickness) and [left] polishing in progress.

In their final form and including the caster hight they measure: 1.140 High X 0.470 Wide x 0.450 Front to Back, that measurement does not take into account the lip sticking out at the bottom. The idea of the lip was cribbed from the Living Voice Air Partner which is most probably one of the nicest speakers available.

There are ring covers for the driver mounts the screws of which still need to be blackened and the light colour wooden cabinet surround is of Jacaranda wood.

In the background is one of a pair of NOS Goodmans Axiom 201 in a Reflex cabinet which will soon be housed in another set of these horns.

Listening impressions

The equipment used is as follows. Shure V15 V with Gyger stylus, EMT 948 turntable with EMT 929 Tone Arm, Jolida Phono Stage, Valve pre-amp and Parallel 300B SET Mono Blocks built by Alan Hobkirk here in South Africa. Maximum power is around 8 - 12 Watts. Interconnects are from LM 240LF coax. Good for up to 5 GHz with a loss of 5 dB per/100 meters at 30MHz so at the low frequency that we use in audio there is no loss worth speaking about. Speaker leads are from twin lead (rip cord), conductor 2 mm sq.

Probably the salient point here is that due no doubt to Tannoys drivers, the sound is as one would expect tonally. In terms of sensitivity they are at least 100 dB/1 Watt/1 Meter for the main cone. Another way of putting it is that the horn cabinet gives at least 6 dB and probably more. The downside is that the HF driver now needs help as from the gain point the HF is too soft. This can be overcome by the addition of a suitable extra driver that can run from 1 kHz (the crossover point) upwards.

I have done this with two EMIT ribbons per side in separate horns although other less exotic cone drivers could be used as in the original drawing. This gain imbalance when horn loading the main cone also had to be addressed in my Double Autographs.

The cabinet needs no sub-woofer as it very easily plays the bottom note of a Double Bass at 41 Hz or Bottom C 16' (32 Hz) in an organ at a volume equal to that of higher notes.

The 32" octave of an organ gets softer as one would expect but for all normal music there is ample bass and as it's done by a horn - fast and clean with no bloat and power aplenty and on a recording of the West Point organ, the low bass is both heard and felt. Bass drum beats are visceral and fast. Tonally the horns are clean with no peaks and very, very dynamic. I mentioned earlier my preference for large drivers for up to at least 1,000 Hz as this gives body to the sound and comparing these cabinets to my main system which has two 15" Dual Concentrics per side I am more than happy with the result.

These Coral designed horns also give the scale that monkey coffins, in my experience, can't give and do very well in replicating the recording space. Imaging is pin point and the cabinets totally disappear on any decent recording. One remarkable recording in particular on a FAS label from 1975 is of the organ at St Mary's Church, Stoke D'Abernon every nuance of each note played is beautifully reproduced and as the organ only has one 16' stop on 2 1/4" of water pressure it is easy to judge every separate note for both timbre and volume.

The cabinets reproduced this stop evenly note to note with no apparent differences in loudness. With something a bit more adventuress, "The Glory of Gabrieli" (Columbia MS7071) in original surroundings, (even if it did mean E Power Biggs had to uproot not only choruses from America, recording equipment from Switzerland, brass players from Germany and a pipe organ from Austria!) the wonderful sonority of the space shone through, almost as if one was there!

A "Digital Classics" (Philips 416-159-1), if that's not an oxymoron label for an LP, of Daniel Chorzempa playing the Cadet Chapel, West Point organ. A huge beast of an instrument with multiple 32' (16 Hz) stops and huge resources and came through clearly with enough power to feel the bass against your skin (and ribs).

The bass on this LP is well recorded and many years ago I blew up a friends Bose 501 speakers with it, in fact the cones tore loose from the surrounds. Not so the Tannoys in the horn cabinets. Even played too loudly for comfort the main cones just tremble, so little in fact the movement was a bit difficult to see. This is where a well designed and built horn comes into it's own. Low power (very little stress) for lots of volume which therefore gives very little distortion and concomitant to that, the sound produced is very clean with no distortion from the speaker system that one can hear.

Pink Floyd or Jacques Loussier or Harry Mortimer's brass band, they all play well, so too does Leon Berry on an large Wurlitzer Theater Organ (Audio Fidelity AFLP 1828) issued in 1958, one of the very first Stereo recordings to market.

Digression

In talking about the sound from a system I find I need to use a reference otherwise it's all too easy to get carried away! Because of my interest in the organ I have been lucky enough to have one at hand when setting up my first Tannoy Autograph and then the Double Autograph hence my reliance on the organ for illustration of tone, harmonics and bass response and how well a speaker cabinet does duty as a true reproducer of sound.

This picture above is more for interest and illustration than part of the article on the small Coral derived horn and is meant only to represent where my ideas of sound reproduction derive from.

The organ, no longer with me, had around 2,500 pipes, five 16' ranks of pipes including the stopped wooden sub-bass pipes on the left. Note also on the left my original Tannoy Autograph.

At Left. 32" Open Diapason pipes, Midmer Losh, Atlantic City a bit of a problem to reproduce with tiny drivers and the usual one note subs!

The cabinets [above left] were for the three 32" Stops (30 Hz down to 16 Hz) and the large port needed to tune to that frequency. Each cabinet was 600 liters and the speaker 15" with separate 250 Watt Mosfet amps. Cabinet walls were for the bottom two 32 mm thick with lots of bracing. One speaker cabinet could not play multiple stops at the same time due to interference between the tones hence the need for discrete cabinets. There was also a Duo-Art Piano in the room being re-built at the time.

Conclusion

Reproducing instruments recognizably is not difficult, most speakers can do that. Getting the actual timbre, scale and space information is a different thing altogether. Keeping the different instruments separate and being able to hear each one as a unit as one can do in a live setting, is the mark of a great speaker.

The combination of Tannoy driver and Coral horn does this and I believe when another pair of cabinets are made up for the Goodmans they will do equally as well. My prejudice against small drivers used for anything below 1 kHz stems from physics dictating that a loudspeaker's effective radiating diameter should increase with wavelength of sound to provide low distortion, broad dynamic range and uniform power distribution. The following graph is a visible representation of this.

A Lowther driver has a cone diameter of 7.9" which gives an Sd = 201 cm2, a Fostex FF225WK an Sd = 220cm2.

“A loudspeaker's radiating area must increase in relation to the size of the wavelength reproduced to have uniform power distribution linear output efficiency and dynamic range will increase as loud speaker piston area and motor size is increased. For a given sound pressure level, distortion will decrease as piston area and motor size is increased.
Transient response characteristics usually improve as piston area and motor size is increased. (For you skeptics who still believe that an 8" woofer is “quicker” than a 12" woofer, remember this is a volume velocity issue.”

The picture is of a finished cabinet with a Coral Beta 10 speaker fitted.

On program material such as brass band with drums or rock music they can thump you in the chest while retaining the delicacy needed to hear the sticks on the drum. Because of the very small cone movement there is no tendency to become congested with complex material and cone breakup never happens. The crossover point is 1 kHz so most of the music comes from the main cone but as the HF is also horn loaded so all the positives apply there too albeit a bit soft. Adding extra HF drive by means of additional tweeters brings everything into balance without detracting from the general sound and seemingly without destroying the “Point Source” which is probably due to the bulk of the HF sound coming from the Tannoy. They can also play very loud if wanted with no audible distortion - a horn feature!

The addition of 200 mm to the cabinet length and tongue seems to have been worthwhile and I say “seems” only because I have not heard the original. I will be doing it again with my Goodmans Axiom 201's in the near future. My hope that it would couple to the floor better has proved out in that bass response and extension is remarkable from such a small cabinet and far, far better than what one gets from a sub-woofer. Plus there is no need to apply corrective equalization to keep things accurate, it works out of the box as it were.....!

A very strong upside is not needing to use a sub woofer as there would be real problems trying to match the speed of the response plus extra cost.

Probably the most impressive feature is that the cabinets totally disappear and all that remains is the space, the instruments and the music - what more could one want?

Part II of this article features a similar design, with a 12" Goodmans Axiom driver instead.

© Copyright 2015 Chris Templer - chris@tnt-audio.com - www.tnt-audio.com

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