LE10A sensitivity is 90.41 dB at 1W, and it's allowed to run full range in L77 (no filter on the LF,) probably not optimum, and certainly a consideration in any upgrade. A 90 dB tweeter's going to have enough sensitivity, particularly if we put a lowpass on the woofer. Would I like a little more to work with? Well, sure, but I like the Fostex triangulate form, given the space constraints of the driver/passive layout of L77. I'd expect DC to make a little paper cutout model to see how it'll fit before actually buying a pair.
I like the low Fs of both the Tang Band and the Fostex. I ended up throwing away the extended LF response of the TB because of a major notch in the response; it's crossed above 2 kHz in the AR4x project. I've stopped tweaking that until I transfer the drivers to their new boxes for mirror-imaging. I don't know how low I can run the Fostex until I get it in hand, do some measurements, and listen. Madisound recommends 3 kHz; I'm hoping it'll play well a bit lower than that. The Fostex suggested filters seem to be in that range, but I don't see a minimum recommended crossover frequency/slope in the factory spec.
I've never measured LE20-1 standing alone, and will do that, but if its performance in other systems I have here is an indication, yes, it's true HF extension is the desired improvement for L77, and I'm confident the measurements will demonstrate that to be substantial. Sonically, there's no contest; tweeters have moved well ahead during the intervening 40 years, and I have little doubt that one or more of the others you suggest might be suitable, or a better choice, even. Not my job, tho; compression drivers and horns are my HF thing.
Footnote: I categorically deny being a "Pro." I'm a DIY hacker with some gear and a willingness to try stuff, is all.... :
I like the low Fs of both the Tang Band and the Fostex. I ended up throwing away the extended LF response of the TB because of a major notch in the response; it's crossed above 2 kHz in the AR4x project. I've stopped tweaking that until I transfer the drivers to their new boxes for mirror-imaging. I don't know how low I can run the Fostex until I get it in hand, do some measurements, and listen. Madisound recommends 3 kHz; I'm hoping it'll play well a bit lower than that. The Fostex suggested filters seem to be in that range, but I don't see a minimum recommended crossover frequency/slope in the factory spec.
I've never measured LE20-1 standing alone, and will do that, but if its performance in other systems I have here is an indication, yes, it's true HF extension is the desired improvement for L77, and I'm confident the measurements will demonstrate that to be substantial. Sonically, there's no contest; tweeters have moved well ahead during the intervening 40 years, and I have little doubt that one or more of the others you suggest might be suitable, or a better choice, even. Not my job, tho; compression drivers and horns are my HF thing.
Footnote: I categorically deny being a "Pro." I'm a DIY hacker with some gear and a willingness to try stuff, is all.... :