Sunday, 23 June 2013

The Tannoy Speaker Design

In the history of loudspeaker development during the first phase cone equipped speakers were primarily developed with an electro magnetic unit (so called field coil speakers). It was not until the end of the second world war that permanent magnets were introduced, due to the fact that materials such as cobalt, nickel, aluminum in greater quantity were available again. So that commonly named alloy "alnico" produced magnets of before unknown strength. So they did resemble the complex (wound coil with iron core and dedicated power supply) and expensive systems from before (cost cutting). At this time the coaxial called loudspeaker systems have been designed by almost every well known speaker company. In this constructions up to three different speakers are usually arranged concentrically in one physical chassis. This had the advantage that it needed only one cut out in the cabinet to make and the speaker did act almost like a point source over the entire frequency range. In the most cases, a strong horn tweeter and a generously sized cone loudspeaker (twelve to fifteen inches) were combined for efficiency-optimized applications. Many manufacturers such Westerrn Electric, later Altec, RCA, James B. Lansing (JBL), Jensen, Stephens and others had been in competition about that sort of speakers for professional demands. They all were quite similiar from that point of view, 15'' low frequency cone and a high frequency horn cut at around 800 hz to the lower frequency with a simple crossover design to prevent efficiency in the days of low power tube amplifiers. But from another point of view the Tannoy design was completely different and superior to all its competitors. The HF-horn uses the shape of the LF-cone as seamless extension for its horn development as the first unique position. The second is the use of the same magnet system for both drivers, which compensates maturity differences perfectly. Have a look at the cut away drawing to understand. Both advantages lead into a much smoother integration of both frequency parts and to a more homogenious sound stage.

The LF-cone completes perfectly the HF-horn development. Both systems use the same magnet at both ends for compensation

All other designs set the HF-horn as a separate unit with own magnet system in the center of the LF-cone without any physical correspondence. I do not actually know if this important difference was regulated by patent regulations made by Tannoy in the very early 1930ties, at the beginning of the companies success. And even later in 1950 when a lot of following companies continued to produce coaxial or duplex designs (University, Goodmans, Altobass, Stentorian, Parmeko, but also later asian replicas from Pioneer or Coral), this ground braking difference never got touched by others.

Casted chassis from 12' Monitor HPD close to the disassembled "Alnico"-magnet showing its construction just before reconing process.
The so called "pepper-pot"-tweeter, the HF-compression chamber with phase compensating elements

The Beginning

The major design of Tannoy speakers started in 1947 with the first "Dual Concentric Speaker" designed by Ronnie H. Rackham from Tannoy. He married the two drivers together so that the flare shape of the 15" bass unit continued the flare rate of the high frequency unit. 
It was designated as the '15" Monitor Black'. It had a power handling of 20 Watts RMS, a voice coil impedance of 15 Ohms and a crossover point of 1 kHz. Magnetic gap fluxes were provided by a cast iron alloy magnet at 12,000 gauss for the low frequency voice coil and 18,000 gauss for the HF coil. (10,000 gauss = 1 Tesla). At 20 Watts power handling in 1947 the unit was very well received and this coupled with a sensitivity close to 92 dB for 1 Watt at 1 meter right up to the highest frequencies, was a milestone for the company in providing high quality speech and music capabilities in an efficient way. With this design Tannoy layed the ground for 30 years of succesfull manufacture of this design before significant changes took place. During that time four generations of "Dual Concentric" speakers were launched which all were based on this incomparable principle design of the first "Monitor Black".

The structure of the casted aluminium chassis can be seen with its typical connectors. The magnet assembly from the side.
Both designs existed without significant change between 1954 and 1978 troughout the Monitor Range of speakers.

The Model Line

In 1954 the "Monitor Silver" replaced the former "Monitor Black" without significant changes in the technical data, accompanied by a scaled down smaller version with a 12'-cone. Both had got a new lighter casted chassis, painted in hammertone silver with a barrel shaped magnet cover of the same color. This new casted chassis marked the way for the next 30 years without change for both sizes. These two sizes spread into the recording industry first in Great Britain, later throughout the Commonwealth till today were you will find them in recording studios world wide.

In 1958 the silver range got to be replaced by the next generation, the "Monitor Red". Now the chassis was painted in darker blueish gray hammertone with new formed pinkred metal magnet covers. Basically the same two speakers with little increased power handling capacity. But they got a small 10'' brother beside, which was mainly designed for the "near field monitoring" needs in the recording industry (III-LZ). This speaker is the only Tannoy design with a 2.5'' voice coil for the LF-cone, were all others are equipped with 2'' coils. This design delivers a perfectly controlled operated LF-cone and its refined resolution of detail up to 1500 hz, a big advantage for near field monitor use.

In 1967 the reds were upgraded to the new "Monitor Gold" line. The chassis got a grey paint with golden plastic mold magnet covers. Now the first important changes were introduced. The crossovers got transformer operated controls for hf-energy and for cut off frequency. The speakers had to follow the market changes. Now transistor amplifiers were used with higher power priority. As a result, the impedance was reduced to 8 ohms to match the transformerless output stages of the new power amplifiers. As well the increased power handling capacity with corresponding heavier voice coil was a requirement for the new line.

Two 12'' chassis from two generations showing their design progress, the 1958 Monitor Red on the left beside the 1974 Monitor HPD frame needing cone and diaphragm to complete (without plasic cover).

1974 the new "Monitor HPD"- Line got introduced with 15'' (HPD 385), 12'' (HPD 315) and 10'' (HPD 295) versions. These speakers are sold in Canada as Monitor Royal line with blue magnet covers. The new chassis had golden paint with gray plastic covers. They (HPD = High Power Drive) followed its predessors with hardly improved power handling capacity. For this reason the paper cones got redesigned. The cones have been equipped with agile foam surrounds instead of the former hard paper edges (the foam surrounds show typical aging after 20 years been exposed to uv-light and need replacing). As second treatment for increased power handling capacity the cones got stiffened out through 8 paper bars ("Girdacustic" called) glued to the back of the cones and a reinforced voice coil with better temperature abilities. This changes make a wider cone deflection possible to high power input, resulting in less deformations of the cone structure and a smoother response as resulting benefit.

Exceptions of the production line between 1947 and 1978

Tannoy is the only speaker company of this age I know which completely concentrated on the dual concentric design principle. Some rare exceptions from this continuation of model history are known. During the Monitor Silver range Tannoy released a 12'' direct radiator speaker (without the HF-unit) based on the standard chassis with green colored magnet. It was completed by a HF-unit based on the standard magnet assembly extended with a metal horn of the size and shape of the 12'' paper cone.
The Monitor Blue called speaker which is based on the HPD series as direct radiator chassis without HF-unit available in 12'' and 15'' to extend the standard series.
Through out a long period (I do not have any precise knowledge about that product and any advice and help would be appreciated), Tannoy is most famous for its eight inch (?) direct radiator units made for the british railroad stations. These speakers have been spread widely common for announcements made for departures/arrivals in lots of stations in the UK. This use has made the company name Tannoy so popular that these sorts of announcements are populary named: a tannoy. (to be continued).

The crossovers

The crossovers of the different generations did start very simple with a 6 db filter for the 1000 hz cut frequency at the beginning. Tannoy used huge paper capacitors and waxed impregnated copper coils for the Monitor Black. With higher production numbers of the Monitor Silvers they got a specially made paper capacitor with three units in one can supplied. With Monitor Reds Tannoy started to use electrolytics for the crossovers (cost cutting). The Silver and Red 12'' versions did drive the cone in full range mode, just the tweeter got cut to be protected for the low frequencies. All the crossover designs kept efficiency in the foremost stage. With Monitor Gold and the new lower impedances of 8 ohms the crossovers needed to be redesigned completely. Tannoy used the chance to change to more complex filters with the possiibility to cut or damp the HF-horn in four steps each for all three sizes. This new luxury brought the opportunity for room compensation of the cabinets. Thanks to the believe in quality Tannoy made these attenuators with switched auto transformers, the by far superior way to do so (expensive). And they supplied compensating filters to improve measured linearity over the entire spectrum. All supplied "improvements" were accepted as further degradation of efficiency as tolerable side effect.

Over the years the speakers have been treated with numerous improvements, mostly in order to improve power handling capabilities for high output transistor devices. The company tried to follow the tremendous increase of power output in amplifier design, right to a point were it found a simple physical limitation. Seen from today the treatments did not always improve the general performance soundwise, in particular if you do use them with low powered tube amps. It is more like permanent stepping backward. At the end the lightweight cone with the 30 watts voice coil of the early designs, added up to the double weight with the HPD range. These differences together with more complex crossovers lead to clearly listenable degradations, foremost in terms of lost dynamics and a decrease of fine details. If you ever have compared a Monitor Silver to a Monitor HPD you might know what I am talking about.
In this logic a lot of products did not benefit from the advantage of its "market advance"; i.e. means have fallen down their staircase of development. This is fairly common observation and a poor truth at the end of modernism and industrialized mass production with almost any of its products.

When Guy R. Fountain, the founder of the Tannoy company died in 1977, the company was sold. This chapter ended the "West Norwood"-history and the company was moved to Scotland by its new owners in order to cut production costs. From 1978 the "Dual Concentric"-speaker got completely redesigned with ferrite magnets for both frequency parts. New materials got applied, as well a new hf-driver design was developed to cut production costs. Only within the Prestige-Series some of the old magnets ("alcomax 3" named) get installed in countable amounts to enclosures at tremendous price ranges. The other dual concentric designs got a horn amplified dome tweeter design ("tulip wave guide" marketing speech) with hefty coils wound, able to play 300 watts of power, as a tribute to modern demands of audio fidelity.

Read on soon, Volker

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