The Lii Sound F-15 Coaxial driver must be the most perplexing driver I've ever come across.
It's sold as a full range driver, and the ears and measurement confirm it's doing a reasonable attempt at that, but its seriously rolled off both in the lower bass and upper treble.
But despite that it has a extraordinary ability to sound quite interesting with exciting dynamics that makes the sound literally jump out of the drivers.
It sounds much more efficient than its rated dB . Its just an excited driver, extracting a lot more information than you'd expect from a 15" driver.
BUT, part of that excitement comes from a far from flat frequency response. It's nothing like the response graph provided by the factory, which is pretty much flat.
The F-15 has a huge boost and plateau from 6-8Khz , and a dip around 2Khz.
This is hardly what you'd call hi-fi, although I suspect it might be something approaching the inverse of what a lot of older guys with limited hearing loss have.
Maybe it is a quite clever marketing ploy by Lii Song - give all the old hi fi guys a driver that restores their old ears , a sort of giant hearing aid?
Is that too skeptical ? 🤫
Nevertheless, despite it being quite interesting for a while, the brain/ear interface is too clever to be fooled for long , and the F-15 soon starts to sound a bit too detailed and eventually quite shrill.
It's too inaccurate to be called even remotely "hi-fi" , and I'd call it a novelty driver not to be taken that seriously.
Still I did recognise something in it that deserved a bit of work to see if it could be brought to the fore.
I was using a JBL 2226 15" bass driver under the F-15 for bass enforcement , but the problem of the brash upper mid/treble had to be solved.
First thing I tried was a notch filter , but that was mostly ineffective- the plateau is just too broad and high to be tamed by a notch filter , and I've never liked using them, as they can often do more harm than good .
Then I tried a low pass filter around 6Khz to see if would generate a reasonably flat response - it worked better than the notch filter, but still not the smooth response I was after.
Looking at the raw response, it really is quite hopeless and it was then I decided I had to totally remove everything above 2Khz , and use another driver to fill in the 2Khz-20Khz response.
I eventually settled on a 2Khz 2nd order low pass filter that worked satisfactorily , if not perhaps perfectly.
There's still a bit of a boost at 6Khz - it really needs a 24dB 4th order crossover, which I will build.
Interestingly the values I used - 0.82mH and 9.4 uF are nowhere near what the crossover calculators suggest, but measurement confirmed they are reasonably close in achieving the desired roll-off.
Now I had the problem of what to use for the 2Khz to 20Khz range, and more importantly would it work with the difficult F-15.
I had some horn tweeters I could try but decided to try a pair of RAAL 70-20XR ribbon tweeters.
I was worried that the directivity of the ribbons would clash with the F-15 , especially as I was using the F-15 off axis , to avoid some undesirable beaming.
The RAAL 70-20XR has a very good reputation , and I've used it very successfully in other speaker designs.
Its expensive at over $1000 each , but I believe it lives up to the manufacturer's sales blurb:
This model is somewhat different from other models. The ribbon of 70-20XR is wider in comparison to the others, which is indicated by its label. Ribbon is 70mm long and 19mm wide. Housing is made out of 8mm thick steel. At the bottom of the chamber there is an adapter transformer, which is completely covered with resin. Net volume of the chamber in between the transformer and the magnet is reduced with wooden tiles to the required level and that space is very precisely filled with sound absorbing material. In this ribbon, the filling is made of finest medical cotton wool from pure cotton. The strong neodymium magnets are the same as those installed in all other models except for the 64-10.
The transformer in this model is special, also wound by hand and completely handmade. The primary is wound with a copper wire with a round section and high purity, while the secondary is being wound with copper foil of a 0,1mm thickness in three layers. The transformer is special because it has 3 outputs: 6,3 ohms, 9 ohms, and 12,6 ohms. In this way, the user can choose which sensitivity fits the best. The outputs of the transformer can have other values which can be chosen by the user himself on special request.
The next feature that sets this ribbon apart from the others is that the ribbon is made like a sandwich. It’s made out of two layers of aluminum foil with a thickness of 8 microns, which is ironed and stiffened by a very demanding process. In between the two layers of aluminum foil a special grease is extremely precisely measured and applied, which has the role of soothing the ribbon in an even shorter time after the impulse stops. This ribbon doesn’t have carrier foil made out of MYLAR, it’s in direct touch with air. This provides the smallest possible moving mass, several hundreds of times smaller than any classic DOME tweeter and far smaller than RIBBONS with a MYLAR carrier foil.
The result is shockingly rich, detailed, the sound image is unencumbered by any colorations.
The ribbon itself is fixated on the ends to the supporting frame via tensioners. After installation, the ribbon is being precisely tensioned in order to achieve optimal performance. In this model, the ribbon is the same length as in the 70-10 model. In all models, the first number indicates the effective length of the ribbon, and the second number indicates the width of the ribbon. Model 70-20 is user serviceable, which means that the user can replace the ribbon himself if necessary.
Like every other model, this one is characterized by exceptional openness, detail, spaciousness and depth of the sound image. The sound is fluffy, colorless, it will not hide or mask even the finest information read from the source.
However, I didn't really expect it to work optimally with the F-15, as they are such different drivers.
To say I was pleasantly surprised is an understatement; I was shocked at how well they integrated .
To be honest this is the most natural, immediate, and vivid vocals I've ever heard, and I've had half a century playing around with electrostatic, horns , planar and just about every speaker you can imagine.
It has the positive attributes of all these speaker types , without any negative attributes .
Excellent imaging and soundstaging , utilising the best of well mastered recordings.
I would never have thought that the combination of a Chinese 15" co-axial driver and a Serbian ribbon tweeter would make such a grand musical sound.
The F-15 does a superb job restricted to upper bass/midrange , and the Raal 70-20 XR fills out the treble in a splendid fashion.
I do think that the Loftin White 6B4G/2A3 SE amp driving the Raal 70-20XR would be contributing to the beautiful, smooth sound , as it is a gorgeous sounding amp.
Technical description: a Supratek Grange preamplifier with dual adjustable level outputs - one output for low bass and the other for upper bass, mid and treble.
JBL 2226 15" bass driver used in open baffle configuration. Graham Maynard T-Bass OB EQ. Minidsp digital crossover 10hz 48dB high pass, 180Hz 24dB low pass. Carver solid state amplifier.
Lii Song F-15 15" co-axial driver. Passive low pass passive crossover 12dB 2Khz , reversed polarity. 300B/6B4G SE amplifier.
Raal ribbon tweeter. 10uF capacitor in conjunction with inductance of internal transformer forms a 12dB 2Khz high pass crossover , with 24dB acoustical crossover- see Raal website for more info. 6B4G Loftin White SE amplifier.
The dual outputs of preamplifier allows me to make fine adjustment of bass to mid/treble balance of each recording.
There is no perfect setting of this balance as every recording is influenced by the quality of the recording , the mastering engineers perception , and various other influences.
I like being able to make fine and subtle adjustments to suit my system, room and ears, rather than being stuck with one "suits all" option.
It's amazing how much difference just a slight change to the bass balance can make to the overall sound.
So a very surprising and satisfactory result, that I'm enjoying very much.
Always something to learn in this peculiar hobby!
JBL 2226 Bass response
JBL 2226 Bass, F-15, Raal 70-20XR
