Klangfilm KL-L405 project
It was here where the project started: Klangfilm KL-L405 in their original mobile cabinets.
This is how the speaker looks now (in this pic the top plate is missing).
And as a stereo pair.
Well, before things got this far, it was necessary to develop some knowledge about the unit. The KL-L405 is a 14" semi-full-range 8 ohm driver from the 1940s - 50s with a typical Alnico magnet system.
The units were listened to, of course, but measured too. The measurements were done not in anybody's living room, but in a professional environmental with professional equipment so that chances are good that the results are sufficiently reliable (thanks Jorma). The first graph is measured from a very close distance:
And the next one from a distance of roughly one meter: as one can see, the driver appears to require a tweeter even to reach the 10K benchmark. On the other hand, sensitivity is well beyond 100dB so that there's quite a bit energy still at 10K even without a tweeter. If perfectly balanced with the bass, the driver can be tonally fully acceptable even without a tweeter, despite the fact that most of us would want to add one. The hole around 1000Hz may need some EQ'ing.
So a tweeter was added, a Klangfilm cone tweeter.
A very mild crossover was build by Veikko Luukkanen for the tweeter taking over around 7-9kHz. The response of the KL-L405 was not filtered.
And here's how the speaker looks from behind. A granite plate was placed on top of the speaker to reduce resonances.
The speakers were driven by a pair of QUAD II mono blocks with no lack of power whatsoever.
The sound's not perfect yet but highly promising. There's great potential in these drivers for a super fast and transparent sound. It won't be easy to conjure a useful tight bass out of the baffles but once that goal is achieved the speaker will provide its owner with many many hours of happy listening. Congratulations.