A History on ribbon loudspeakers
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.