Howard Dawson Audio
What is the HRL-1 Ribbon Tweeter?

A bit of history (skip)

It is now roughly 50 years since the first ribbon tweeter was introduced to the mass market by Quad. It heralded a quantum leap in high-frequency precision over the crude cone and dome devices which were then the accepted norm. The secret lay in the almost non-existent inertia of the wafer-thin current-carrying aluminium diaphragm and the fact that the diaphragm was driven equally and in-phase over its entire surface, unlike coil/dome units where the force is applied only to the circumference of the moving diaphragm.

A very large magnet was necessary for efficiency reasons, since the "voice coil" of the ribbon speaker effectively had only one turn. The units were very expensive indeed at the time and were only an option for the wealthy connoisseur audiophile.

This situation held for a few years until the advent of the Kelly ribbon tweeter (later to become the Decca DK30), when some further engineering on the part of Stanley Kelly (one of the most respected "bastions" of the British audio industry) led to a cost-effective design, resulting in a large D.I.Y. ownership base and some O.E.M.'s using it in their top-end models.

Although less expensive than the Quad, the Kelly/Decca units were still substantially more expensive than top-end cone or dome tweeters, and were very popular with the D.I.Y. fraternity where the high cost of the units could be offset by labour savings. A great many of these are still in use today, with only one inexpensive component able to suffer the effects of long-term use or accidental overload. This is the aluminium element itself and therefore it was designed to be easily replaceable by the owner as a simple D.I.Y. operation. This was done for a fraction of the cost of the whole unit, and would restore them to "new" specification every time, giving them an indefinite lifespan - truly an "investment for life". Few or no alternative technologies could boast such longevity.

Unfortunately availability ceased in the mid 1980's, probably due to the escalating cost of the (necessarily) large cobalt magnet and the difficulties of mass-producing the delicate ribbon diaphragm assembly. Second-hand acquisition is nigh on impossible since they are (quite rightly) dearly loved by their owners. Anyone selling these units on is invariably doing so for financial reasons and not in a search for that elusive "upgrade".

Since then there have been many and varied "alternatives" which have attempted to lower the cost of manufacture and to "firm-up" the delicate ribbon against abuse and accidents. This has usually resulted in the use of smaller/cheaper magnets and multi-turn ribbons using parallel conductors on an insulating substrate. The substrate adds mass to the moving diaphragm while contributing nothing to the motor force. The resulting compromised performance usually varied between similar and rather poor, as the underlying principles of the original designs, i.e. lowest inertia and highest efficiency, had been partly or even completely lost. In many cases the membrane cannot be replaced, and accidents or abuse are often followed by the painful purchase of complete new units. Even today, those early Quad and Kelly designs have few peers when it comes to performance or longevity.

To help keep these fine old units in service, we are able to supply ribbon assemblies and transformers. The latter can be specified for any sensible terminal impedance.

Enter the HRL-1 Ribbon Loudspeaker

The HRL-1 is a hand-made thoroughbred ribbon tweeter/supertweeter of the highest quality intended to fill the void left by the demise of such units as the Decca/Kelly. The traditional criteria for a high-performance ribbon tweeter have been reinstated. With a massive "Alcomax-III" cobalt-based magnet assembly and a minimum-inertia aluminium diaphragm weighing only 0.01gm, the response extends to well beyond 30kHz with useful output being detectable at frequencies beyond 50kHz. This leads to an impressive command of the octave from 10kHz to 20kHz, where most conventional tweeters are beginning to "fall over". Certain instruments have significant acoustic output in this octave, the most prevalent being metallic percussion, piano, and many other non-amplified acoustic instruments, even the human voice. No longer need these instruments suffer the muddling effects of a tweeter which only just "makes it" to 20kHz or thereabouts, but instead metallic percussion, for example, is reproduced with a spine-tingling precision and prescence which is impossible to describe in words. This is down to the lightning-fast transient response, quite free from harshness, resonance, sibilance, colouration or "listener fatigue", allowing complex high-frequency wavefronts to be effortlessly transcribed without distortion products or phase errors.

The ribbon circuit has a very low impedance, and so the unit carries an integral matching transformer. This is a ferrite device with low leakage inductance (to minimise high-frequency insertion loss), and is intended for operation down to 3kHz. The tweeter section of the crossover must be at 3kHz or higher for thermal reasons, and the slope must be of 12dB/octave to prevent bass frequencies ever reaching the delicate ribbon. The unit's impedance is predominantly resistive, unlike many coil drivers, and is defined by the turns ratio of the transformer, allowing configuration for any desired impedance, facilitating straightforward crossover network design, and providing an "easy" load for the fussiest of amplifiers. This is a valuable asset, particularly with valve amplifiers where impedance matching is fundamentally important for maximum power transfer. The unit's impedance is nominally 8ohms, but any sensible impedance may be ordered. If it is later desired to change this, then it is a simple D.I.Y. operation taking only ten minutes or so.

The delicate ribbon element assembly of the HRL-1 has been designed with low cost of ownership in mind, and as such represents only a small fraction of the cost of the whole unit. Barring abuse or the proverbial "accident", the delicate element will never need replacing as a result of normal use, but nevertheless a prospective owner can be happy in the knowledge that it can be easily replaced as a simple D.I.Y. operation should the unthinkable ever happen, with new specification being restored on completion.

Mechanical construction of the HRL-1 is very sturdy indeed, being based on a pair of substantial aluminium castings, with the matching transformer mounted on a stout glassfibre printed-circuit board which also carries the 6mm input terminals. Weight of the unit is 2.7kg (5.9lb). These other parts are designed to withstand the rigours of a lifetime of listening pleasure.

The units are supplied in pairs with baffle-cutting templates and applications manual covering crossover design, list of F.A.Q.'s, mechanical drawings etc.. The HRL-1 may be incorporated into new designs or used to significantly enhance the upper-octave performance of existing two or three-way systems. Our ultimate aim is a delighted customer - nothing less will suffice.

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HRL-1 specifications

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price list

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hand-crafted loudspeaker units for music lovers