Could the "issue" with the FAL C-60 driver be a result of your crossover design?
AKAIK, the C-60 was designed to run full range with no crossover in front of it.
Are you using the silver or copper wire version?
The problem is not with the crossover. We are able to reproduce the phenomenon with the crossover and without, in free air or mounted in a cabinet. And I seriously doubt the wire version could have any impact with respect to this issue. I think the problem is mechanical in nature.
Of the eight test drivers we have received so far, one driver did not exhibit the phenomenon and performed flawlessly. New test units have just been shipped from Japan and may perform flawlessly as well, but only time will tell.
For 98% percent of the music we have played on the speaker, there is no problem. But we ran across three musical selections that caused the driver to go into an oscillation of some sort.
I don't know exactly what is happening, but here is my theory...
In order to keep the sensitivity up, the voice coil is positioned in the middle of a relatively tight magnet gap. At a certain frequency and drive level, the cone starts to flex causing the wires of the voice coil to vibrate and come into direct contact with the magnets themselves. Upon close inspection, you can actually see this happen. This produces a very irritating and startling sound.
If you widen the gap, you could probably eliminate this. But the sensitivity of the driver would suffer.
In order to address this, FAL produced test units where they re-positioned the magnets and moved them slightly closer to the center of the cone where the distance the voice coil could travel during cone flexing was reduced. The driver that did not fail was one of these. But, unfortunately, it was only one out of four with this new configuration. So that did not completely address the situation.
When we first noticed this issue, we tried isolating the cause of the problem. We ran test tones and found that this oscillation always occurred between 1100 and 1200 Hz - sometimes 1125Hz, sometimes 1175Hz, etc. We were able to induce the oscillation with every driver tested except one.
This issue does not present itself, even at relatively high volume levels, in normal use except in those very rare cases where there is a saturation of energy in that frequency band. We have only found three musical cuts so far that excite the driver.
But we certainly would not sell a speaker that can play every musical selection except three. If it can't play everything we throw at it, we simply can't use it.
I should point out that FAL has been very responsive and has gone to great lengths to resolve the issue. They have been a pleasure to work with and are totally committed. So we hope the results of their efforts will be fruitful and we can look forward to a long and happy relationship.
The good news is we will either have FAL drivers that have addressed this issue or we will switch to another driver that, in tests so far, appears to equal (and in some ways surpass) the performance of the FAL. So we are covered either way and will finally put the design to bed shortly.
- Jim
