Elliot Thompson wrote on Mon, 12 April 2010 21:46 |
Art,
It seems JBL purposely marketed the JBL 2268H with a high xmax however, never explained it is based on xmax damage.
In order for the JBL 2268H to offer a 23 mm xmax, the efficiency would be very low since all of the coil will be sitting outside the magnetic gap. Yet, it offers an no.% of 2.8
The numbers are based directly from the engineer’s specifications. I would imagine this is before marketing got their pretty little hands on it. There is no xmax rating on the engineer’s sheet. However it clearly states, “Xmax, damage 23 mm peak before the spider bottoms on gap sleeve.” It also states, “Special Notes: JBL 2241H Replacement in Neo Differential Drive line of transducers.”
It seems the JBL 2241 offers the mathematical xmax. As we know, manufactures were more conservative on their specifications when the “2241” was classed JBL’s best decades ago. Today, that 7.62 mm measurement would be marketed as 9 mm taking distortion into account.
Best Regards,
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Elliott,
Your JBL engineering note quote does sound convincing that the 2268H does not have substantially more Xmax than other drivers, in which case the lesser power compression due to double the voice coil surface area (and lighter weight) would be the main advantage.
One correction to a previous post of mine, the 2268H is only 800 watts(PE), the "Ultra Long Excursion" 2269H is 1200 watts (PE).
I found no Xmech figures for JBL differential drive speakers other than on the VT4880A spec sheet, which states an 89 mm (3.5”) maximum peak to peak excursion, in other words a 44.5 Xmech (or Xlim, whichever flavor you prefer) for the 2269H, which is only rated 19 mm Xmax.
The 2269H has only a fraction of the efficiency of the 2268H or 2241H, what one would expect from extra long (and heavy) coils that extend well beyond the magnetic gap.
This would correspond to the engineering note you mentioned, I’m inclined to agree the 2268H is basically a differential drive dual coil version of the 2241H.
In which case a pair of decent Xmax drivers like the Eminence Lab 12, the 4018LF or the B&C18TBX100 will certainly walk over a single 2268H.
The Lab 12 has an Xmax of 13 mm, 659 cc Vd.
Vd is the amount of air that can be linearly displaced by the speaker, just like in car engines, the more displacement, the more ultimate power.
The 4018LF has an Xmax of 7.9 mm, 939 cc Vd.
The B&C 18TBX100 has an Xmax of 10 mm, 1188 cc Vd.
The 2268H most likely lies right between the two 18”, if it has the same ratio of Xmax to Xlim as the 2269H, it would have an Xmax of 9.82 mm, about 1167 cc Vd.
Before laying out the big cash for 2268H speakers, rather than the others, I’d be doing a quick and simple Xmax check to verify what is real, and what is a print mistake.
Art Welter