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You vintage guys remember the old "Is it live or memorex" ads right? So I'm being a little silly here. I've made a few posts here and at lansing heritage.org about my JBL L-88 and desires to improve etc. over the past 8 months or so.
So far I replaced the original 8mF caps with new Solen caps. I have added about an 1 1/2 of fiberglass to the back, top and bottom. Some improvements. Now I have replaced the LE25 tweeters with 035Ti titinium domes. I was suggested that I plug the port as well. So now I have a temp plug in the port and have added polyfill to the top half of the box down to the entrance of the angled port. I only plugged the port a few days ago.
Since we were going to take my father-in-law to the Cleveland Orch concert on Sunday I did some intense listening to a Berlin Phil Cd of Stravinsky's Symphony in 3 movement which was to be performed on Sunday. I listened before and after the plugging. I did notice that the bass was less "boomy" The parts of the piano were more pronounced and I could more clearly tell there was a bass drum and tymphony.
So of the concert we go.
Of course nothing can compare to a live performance in a well built hall. But I must say the L-88T's, as I now call them, hold up remarkable well to the comparison. True an A-B test of several hours delay between the A and the B is not much of a test.
Now comes the real question. With the original configuration the 123a woofer was allowed to just roll off. Its reasonable flat to almost 6K according to testing done by Troelsgravesen.dk The original L88 was suppose to cross at 3K but my calculation show this 8ohm tweeter to cross at 2500 with the 8mF cap. Now the 035 is, according to JBL lit and some testing done by? (can't find the link) is a 4ohm driver. This makes it cross as 5K! if my calculations are correct. So do I add another cap in parallel to bring it up to 10ohm for a 4k cross or to 13.25 for a 3k? I also could add a choke to the woofer.
Follow Ups:
Seems your weekend-EE calculatons are using Textbook learning-tool reactance formulae for fixed resistances. However, speaker voice coils have impedance that varies with frequency (nominal impedance). Those resistance reactance formula cannot determine the blended acoustic cross point for all drivers. IOW your calculations do not equate to the system crossover point.
OK, that makes sense. I know the Res varies but I really don't have the equipment to measure it myself under all the variable conditions. I have an Ohm meter and that's about it and some literature I've found on various forums. The 123a woofer is pretty well documented however the 035 is very difficult to find any hard facts on. JBL lit. like the L80T, l20T, 4312 etc all list the 035 as having a DCR of 3.5 to 4.2 ohms. The 2way L60t crosses at 2500 HZ. The oldest and simplest is the 4312 3way. It has a 104H3 mid 2213 woofer(replacement for the 123a) It crosses at 1.5khz and 7khz. using a 10uF and .01 caps to the mid and L-pad and 3mf
and.01 mf with L-pad to the tweeter and lets the woofer roll off just like the L100. The L100 uses the 8mF cap to the Mid and 3mf to the tweeter both with L-pads. crossing at 1500 and 6000 repectively.
Just thought of something.... The L-pad is 8 Ohms in all of these. How does that effect things? They appear to be wired in series so the resistance is what?... 16 for the old LE25 (8+8) and 12 for the new? (4+8)? Does this make sense?
Nope, the combo [parallel R with driver Z] summed with series R always = 8R. BTW, it is called an L-pad because (no matter whether variable or fixed) there will be a series R & a parallel R which looks like an upside-down letter L when drawn schematically.
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