Ribbon Tweeter Horn Loudspeaker: Decca London/Stanley Kelly - Efficiency, Size, Filter and Frequency Response.

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Decca London Ribbon Tweeter

A Particular Loudspeaker
Page created September 19th, 2000.

	Do you use the DECCA London Ribbon horn tweeters or super tweeters? Or do you still have a set of the original Stanley Kelly ribbon tweeters? Or, maybe you are just curious about these products? At left the original Decca London Ribbon with the metal horn.
	Read about how these ribbon speakers work and about the possibility to make new ribbons of aluminum household foil and repair these extraordinary ribbon speakers yourself. At left the Decca Super Tweeter
	PRINCIPLE The ribbon loudspeaker consists of a small aluminum ribbon that is placed between two pole plates which are attached to a magnet. A current is fed through the ribbon and thus the ribbon functions like the coil in a dynamic loudspeaker. At the same time the ribbon has the function of a cone (though it does not look like it) because it moves and thus brings the air into motion. It's efficiency (maximum sound level) depends mainly on the strength of the magnet. The first application of the electro-mechanic principle of a ribbon moving in a magnetic field, was not a loudspeaker but a microphone. It was "das Band Mikrophon" and was developped in 1924 by E. Gerlach and (according to Jean Hiraga in his outstandig book "Les Haut-Parleur", Paris) was modified and adapted by W. Schottky, C.A. Hartmann and H.F. Olson. The first commercialisation was by Stanley Kelly.
	MICROPHONE The ribbon loudspeaker is a dynamic loudspeaker. It was derived from the beautiful and at the same time fragile ribbon microphone which is still used in some studios and is loved by many a sound technician. Famous is the LEM ribbon microphone.

	STANLEY KELLY The ribbon loudspeaker as it was developped and commercialised by Stanley Kelly was the Kelly 30 with a relatively small horn the rectangular mouth of it was positioned vertically. Stanley Kelly's design was taken over by DECCA who manufactured the DK 30 (=Decca Kelly 30) as pictured on the left. From there the bigger Decca London Ribbon tweeter was developed.

	HARMONICA The ribbon assembly consists of a frame and a strip that is folded like a harmonica. It is very thin. DECCA used to advertise with the phrase "1/10th of a human hair". POWER HANDLING The ribbon has a very low mass and a very small excursion. The power handling is therefore very low: 30 Watts. Too much power will break the thin aluminum. Without a horn it can be used from 7 kHz (2.2 uF in series) as the DECCA Super Tweeter shows.
	ADAPTATION The ribbon has a very low resistance of about 0.7 Ohm (depending on the mass of the aluminum it varies slightly). In order to make it work a transformer is necessary. The primary windings that connect to the amplifier should have an impedance of 8 or 16 Ohms; those were the values that were used in the nineteen fifties and sixties and DECCA supplied both versions. The secondary coil connects to the ribbon and this coil is wound of very thick and flat wire. Efficiency, frequency response and distortion levels depend on the quality of the transformer, the magnet and the way the ribbon is combined with other units.
	I have owned loudspeaker systems that incorporated the London Ribbon high frequency loudspeaker and the sound reproduction is exceptional because the Decca Ribbon was for a long time one of the two existing high frequency transducers which can reproduce a square wave. The other one is the AUDAX TW8 with aluminum diaphragm well protected by a grid. The maximum power handling is 20 Watt connected via a 2.2 uF capacitor, crossover frequency:8 kHz.(Image and curves taken from the AUDAX catalog of 1973.) Both the Decca and the Audax produce a very precise and correct sound which is achieved nowadays by tweeters which have a metal coated membrane/diaphragm. If compared to a soft dome tweeter, the soft dome is less precise. All depends of course if correct phase is achieved.
	VINTAGE Several years ago I bought a pair. But when a friend and I measured the efficiency we were surprised because the impedance did not measure 16 or 8 Ohms but showed a value far above 20 Ohms. Understandably efficiency was very low: 81 dB. In order to improve the efficiency of the loudspeaker a new transformer should be made. But since that is quite an expensive affair another friend suggested that he would see if the primary coil could be used partially in order to have a value of 8 Ohms. Finally he came up with a value of 4 Ohms and an efficiency of about 91 dB which would make the speaker suitable for working together with modern high efficiency units. (At the time I am using 8" woofers, 2 per enclosures which have an impedance of 4 Ohms.) When we checked the distortion of the modified ribbon speaker we found that it was not higher than before.
	HORN By connecting the ribbon speaker to a horn efficiency, power handling and frequency response will be influenced. The shape and size of the horn determine in what way the sound generated by the ribbon is coupled to the air. You all know the importance of a wide frequency response in audio. Quality preamplifiers should have a bandwidth of about 300.000 Hz. CD's and CD-players actually should have a sampling frequency of 400.000 Hz. in order to have the same resolution as the open reel studio recorder. A wide frequency band is necessary for the reproduction of a square wave. Although square waves do not occur in music they are indicative for a good transient response which you can hear in symphonic music and jazz, music that is played on acoustic instruments. Measurements (left) as given in the original Decca brochure.

	Technical specifications and filter design as given in the original Decca brochure.
EXTENDED Loudspeaker systems perform more naturally if the tweeter has an extended frequency band. The DECCA literature of the nineteen seventies mentions a response that is practically flat up to 40.000 Hz. after which the response weakens a few dB and continues to 80.000 Hz. The dimensions of the horn determine the lower cutoff frequency. The mouth opening of the DECCA horn measures 14.9 by 28.1 cm (the surface of the mouth opening is 418 cm2) and makes it possible to use the London Ribbon from 1000 Hz. on. So essentially it is suitable for use in a 2 way system. But generally a higher crossover frequency of about 3000 or even higher is recommended to avoid coloration.
SOURCES In the days of valve amplifiers with low output power the power handling of 20 Watt and the efficiency of 87 dB were no exceptional values. Although many designers strive for a higher efficiency this route is being abandoned nowadays by many manufacturers of high end loudspeaker systems like Avalon in the US That is possible because of the nature of PCM. Digital sources have strong dynamics and designers work on "slimming" the reproduction in order to avoid a heavy sound reproduction. If one strives for an efficiency of around 86 dB the Decca London Ribbon can be used. But the ratio of 91 dB for my modified ribbon speaker is welcome of course.
	Mordaunt Short MS 737 and filter.
MORDAUNT SHORT The qualities of the Decca London Ribbon made it an interesting tweeter to be combined with other transducers. The engineers of Mordaunt Short designed a two-way system using Kef B200 units and named their systems MS 737 which was manufactured from approximately 1974 to 1976. The cabinet measures 33"x17"x12" (84 x 43 x 30 cm.), and has a 1.75" plinth (4.5 cm). The weight of the cabinet was 62 lb. (about 28 kg.). Earlier a two-way system was designed by Decca with their Decca low frequency loudspeaker. And there was the Decca Volt loudspeaker system. The DIY version was designed by David Lyth. (The original filter is l copyright David Lyth.) If the Decca London Ribbon is part of a 2-way system, it is important to choose the woofer and the volume of the woofer cabinet (loading) in such a way as to bring about a firm and detailed mid band. Otherwise it is possible to have a midband that does not connect well to the London Ribbon and the sound becomes rather weak. But that is not all. Careful damping of the enclosure is a must. David Lyth pointed out that the woofer should have a dispersion pattern (lobe) akin to the Decca London Ribbon. That is why he used the Volt BM-250. The filter he designed has a crossover frequency of 2.8 kHz. He states in his article in HI-FI NEWS from November 1982, that the efficiency of the complete system is 90 dB. His filter and the other filters on this page may give you some ideas when and if you plan to incorporate Decca London Ribbons or Kelly DK30 transducers in a two or three way loudspeakeers system. If you need more details about several designs, just mail me.
PASSIVE FILTERING Power handling was 45 Watts. The crossover has 18 dB slopes for the Decca and 12 dB for the Kef units. The crossover frequency is 3500 Hz. The price was £216.76 in 1975. (Thanks to Colin Royle from Great Britain for sending the images of the MS 737 and of the diagram of the crossover network, and for the details.)

EFFECTIVE COMBINATION
You can build a high performance loudspeaker system with this ribbon speaker, but only if you use the ribbon effectively. Since the efficiency is rather low, you best alter the impedance in order to raise the efficiency by several dB. The efficiency will be kept high by using classic filter sections of 12 or 18 dB which are straightforward. Do not add corrective circuits. They are really not necessary. They will lower the efficiency.
It is important to optimize the volumes of the units for the low frequency register and the midrange band in relation to the performance of the ribbon speaker. Too large volumes of woofer and midrange will make the high frequencies soft and sometimes weak. If the woofer has a Q of 0.71 and the midrange the same value or above, chances are that the tweeter will add to the realism of the music with good transients.
ENGASOUND 3KF
A Dutch loudspeaker system was designed with this ribbon and became very famous.
It was built by the late Jan Endenburg from Haarlem in The Netherlands. He introduced his original system (which bore the name 3KF) at the Firato Audio Fair of 1965. He then used the smaller, vertical Kelly Ribbon tweeter.
For the low frequencies up to 600 Hz. he had chosen a 12" Fane woofer which was followed by a 5" (10 cm) unit of the same manufacturer.
During the years he improved the 3KF and in the mid nineteen seventies he used for woofer the Fane 17BF122/17GLR which was followed by the Fane 501 (5") midrange unit.
The London Ribbon had its crossover frequency around 3200 Hz. Until the end of the nineteen seventies Jan Endenburg manufactured and optimized each and every pair that was ordered by music lovers and audiophiles alike. The sound quality was then what you hear nowadays with modern 87 dB high-end loudspeaker system which cost a fortune, like the Avalon systems, but then with an extended frequency band.
The reproduction was even more phenomenal in the nineteen seventies when he added two Janis W1 Subwoofers and two Janis Interphase Cross Overs. See also the Active System page.

An example of a High End system of the nineteen sixties:
(1) AR turntable, the first floating (suspended) sub chassis turntable constructed along the principle
devised by Mitch Cotter, (2) Rogers Master preamplifier, (3) Rogers Master main amplifier, and
(4) the first version of the 3KF.

The earliest 3KF loudspeaker systems had the Decca Kelly Ribbon vertical horn. Then the "sizzling" Ionofane tweeter was incorporated. As this was a more expensive solution the 3 KF was also available with the Decca London Ribbon. In order to keep the efficiency high, the filter was very simple and straightforward and its functionality was assisted by a very effective and meticulously applied damping of the extremely sturdy cabinet, filled with Australian sheeps wool. Phase coherence was established by placing the small midrange under an angle of about 35 degrees. The woofer's excursion was controlled by a mechanical brake on the left and right. This brake also influenced the directivity in a positive way.

The frequency characteristic let you decide what the best crossover frequency can be. The measurement was done with the microphone at 1 meter on axis (in the heart of the horn) and 20 kHz. as upper measurement limit. Although the frequency band extends to 40.000 Hz. at about -3dB, the overall characteristic extends to 80.000 Hz. at - 10 dB. (Measurements from the documentation of the late Jan Th. Endenburg, engineer, creator of the Engasound 3 KF.)

See the overall curve

The polar diagram shows the measurement of 2.000, 5.000, 10.000, 15.000 and 20.000 Hz. The dispersion at 2 kHz. and 5 kHz. is more than 60 degrees and is excellent. At 10 kHz. and 15 kHz. it is about 40 degrees. Only at 20 kHz. the dispersion is narrowed down to 30 degrees. The microphone was positioned at 1 meter in front of the Ribbon Tweeter, on axis. (Measurements from the documentation of the late Jan Th. Endenburg, engineer, creator of the Engasound 3 KF.)

See the larger image

Engasound loudspeaker system with the smaller Kelly Ribbon Tweeter which was later named DK 30. This is a version from around 1965 using a Fane crossover for woofer and midrange and the standard Decca crossover for midrange and ribbon. The volumes for woofer and mid range were calculated in a way that differed from the volumes advised by Fane. The 3KF loudspeaker systems from the nineteen seventies were available for some time with Ionofane plasma tweeter and later only with Decca London Ribbon High Frequency units. They had Fane 17BF122 or 17GLR woofers, Fane 501 or 601 mid-range units and Decca London Ribbon.

At left a drawing of the front of the 1974 version of the 3KF loudspeaker system.

Typical features are:

1. heavy, non resonant cabinet construction,
2. mid range unit leaning back for optimum phase,
3. part of the woofer cone is covered by a wooden panel with a "slot" which gives directivity to the frequencies up to about 600 Hz. while at the same time restricting the cone excursion (X max) to avoid lumpy bass rendition.
4. damping material used is Dr. Baily's, not stacked away but loosely sewed by hand into sheets which hang from the ceiling of the woofer compartment and fill the entire space.

This translates itself in controlled and natural bass, a firm yet limpid mid range (Fane 501/601), and neat high frequencies (Decca London Ribbon).

Crossover frequencies were approx. 600 Hz. (later to be changed in 450 Hz.) and 3000 Hz.

Because the appropriate woofers are no longer available it is possible to fit a 12" woofer like Kef B 300 B into the 100 liter cpompartment (87 dB efficiency) instead of the Fane unit. Or find an equivalent with parameters that resemble the Fane unit even more. Some hobbyists just screw units from a different make on the front baffle and keep the more or less original filter connected. These re-constructed systems do of course not deliver the original high end sound as the designer intended it to be. Moreso if the mid range unit and the ribbon high frequency unit are no longer incorporated and have been replaced by units and dome tweeters with different frequency characteristics and impedance curves, the sound is not at all what it should be.

COMBINATIONS FROM FRANCE
In most loudspeaker systems the Deccas were used in a passive configuration like in the systems designed by Jean-Claude Fourrière from France. He designed by Jean-Claude Fourrière two systems in 1976 when he had his firm Valois Distribution. (Later he was the designer of Audio Référence loudspeakers and finally developed the ALH full range plasma loudspeakers.) Fourrière used the Decca London Ribbon from 2000 Hz. on. Decca Special Products in England authorized him to use the denomination DECCA and he named his creation Decca AL2000. It was a 3-way system with complementing units from Audax (HD24 and HIF13). He also designed a 2-way system at the time: DECCA AL1500 with crossover frequency of 1500 Hz. The panels of the cabinets were heavily damped with sheets of lead.

USING AN ACTIVE CROSSOVER
An internationally known high-end combination was Mark Levinson's HQD-system. Each transducer consisted of two Quad ESL electrostatic loudspeaker panels and a Decca London Ribbon in a d'Apollito configuration. The Hartley subwoofer (not shown) completed the system. Instead of the Hartley subwoofer many audiophiles used the Janis W1 subwoofeplus Janis Interphase Crosover crossover with amplifier.
When using the Decca Ribbon speakers or any other relative fragile high frequency transducer (like a piezo electric unit) in an active configuration, it is common practice to insert a capacitor in series to avoid damage of the ribbon or membrane. A single capacitor keeps correct phase as the slope is 6 dB/octave. The value of the capacitor has to be determined in relation to the chosen crossover frequency of the active filter. The -3 dB point should be lower than the frequency of the active x-over. Some experimentation will let you determine the best value to protect the tweeter and keep the sound coherent. If a 12 dB slope (capacitor and coil) is the choice, than also experiments will guide you to the best configuration.

The original London Ribbon was a development of DECCA Special Products. Up till 1977 the horns were of aluminum, but from then on were made of plastic.

It is possible to repair the ribbons.

Repairing the ribbon assembly takes skill and probably a few trials. Here is how to go about it. If you are not handy and wish to order new ribbons, just go to Howard Dawson's Page.

1. Split the frame into 2 layers. Do this very carefully so you do not break the halves. If this is not possible you have to have duplicates made. If you need new frames you can contact me.

On the left the old frames. On the right new frames made by Hugh Salt in his Amsterdam workshop.

2. Take out the old ribbons. Keep the ends to which you have to solder or glue the new ribbons. Clean the ends carefully.

3. Make 2 molds of thin carton, the gray kind you find at the back of a writing pad can be useful. Cut out with a sharp knife (a Japanese paper knife) pieces of carton of about 4 by 4 cm. You glue these together in such a way that you get a sort of staircase with steps of about. Make two of these and make them as long as the length of the ribbon you are going to need which is about 28 steps.

4. Carefully cut from aluminum foil 2 long strips. The width of the strips has to be the same as that of the old ribbon. If you do not succeed the first time, just try again, and again. It is always handy to make more strips so you can get experience in repairing the tweeters.

5. Carefully put this long strip on one of the molds (matrixes) and very slowly put on the other mold beginning at one side. Try again and again until you have an aluminum accordion with the steps perpendicular to the long side.

6. Now comes the difficult part: the metal connecting part (which you have saved from the old ribbon) has to be glued or soldered to one end of the ribbon. Put this one end in the frame and see (measure) how long the ribbon has to be. Without being stretched it has to be a little shorter than the distance between the top and bottom inside the frames.

7. Glue or solder the other end of the ribbon to the other connecting part. The soldering should be very well done without damaging the ribbon. Furthermore the connection should be conductive otherwise the ribbon does not work. Remember it is the coil and the diaphragm at the same time.

8. Now you can put the ribbon in between the frames. You have to glue the frames together with a glue which permits you to open the frames again later.

You will find out that it is not easy to do this repair. And you probably have to try several times. Do not despair. If you yourself are not handy or technical, you can ask a workshop to do it for you.

Take the thinnest aluminum foil that you can get. And if it is too difficult to handle, take a less fragile kind. Depending on the thickness of the aluminum the impedance can differ. I remember that there is a difference between the 16 Ohm and 8 Ohm ribbons. But I would not be troubled about this information. The impedance on both sides of the transformer is more important. Since my ribbons did not have enough efficiency a friend of mine made new contacts in the transformer so that the impedance on the side to be connected to the amplifier is now 4 Ohms and the efficiency is now 91 dB instead of 87/88 dB. If you do not succeed you always can search with Google and try to find a manufacturer in England who sells ready replacements. Good luck.

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