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In Reply to: RE: Active crossover for ET-LFE8b posted by neolith on October 11, 2015 at 09:36:47
ET may be an unusual case, as it purposely uses very simple cross over design with no "tricks."
The manual actually discusses active cross over as an option and recommends contacting dealers for help. I don't think a typical dealer would be much of one. However, Bruce T. will probably be very helpful - he was with me, discussing power requirements for his speaker in detail.
My problem with active is that requires you to build a cross over, and even a passive one is quite a bit of work. Then you get the choices - what brand of "special" resistor and cap to use - the highly rated ones are far from cheap. Finally, you have to rip up the internals of the speaker and eliminate/bypass the built-in circuit. This is not particularly convenient to do with many designs, and I think ET may be on of them.
I plan to replace a bypass cap on the midrange/tweeter cross over. I will let you know how difficult it is to get at it once I open the speaker by removing the woofer.
Follow Ups:
I agree that the choices of caps for a PLLXO can be overwhelming and the design takes a little work. FWIW I like tin foil REL caps and PRP resistors. I actually have a spreadsheet that will assist in the calculations both for single-ended or balanced designs. The construction is quite simple and could be done even by a novice in a few hours on a breadboard. I can give you that values for the caps and resistors if you provide the input impedances of your amps and output impedance of the preamp.
As far as tearing out the insides, it would be no more difficult than replacing the by-pass cap and a big advantage would be the elimination of that large electrolytic which I am sure is no longer to spec. Just run a jumper wire in place of the 47 uF cap, essentially by-passing the mid driver caps. Use the woofer and mid-treble input terminals.
The LFE does have a simple XO which makes design of a PLLXO or ALLXO pretty easy but the idea of multiamping is valid for any speaker. The more complicated networks with Zobels, etc are that way because of the problems with speaker level XOs and higher order filters are still easily reproduced with active analog filters or DSP solutions.
47uF capacitor? On the schematic I'm looking at it shows a 470uF capacitor. That's approximately a 30Hz cutoff frequency.
I guess that capacitor is essentially just for DC blocking since the driver itself provides the high-pass acoustic roll-off.Also, your simulation in the first post indicates an inverted polarity on the midrange driver. The LFT-8b schematic shows all drivers connected with the same polarity. That may be a misprint on Bruce's schematic.
Dave.
Edits: 10/12/15
Actually for the schematic to work the woofer and tweeter have to be inverted relative to the mid. On the schematic, the woofer is shown inverted (look carefully at the markings on the speaker pins) but not the tweeter. I did the necessary inversions to get a proper Bode. Whether the tweeter and woofer are normal with an inverted mid or the reverse situation is immaterial to my ears but some may claim they can here a difference (absolute polarity nuts).
Yeah, I see what you're referring to on the pictorial below (page 22), but the schematic itself doesn't agree with that. As I said, I think it's a misprint.
Dave.
I hadn't read the REG review of these speakers, but it seems he noticed the apparent polarity issue as well.
Second paragraph:
http://www.theabsolutesound.com/articles/eminent-technology-lft-8b-loudspeaker/?page=3
Dave.
I used to think as you did about polarity but I was experimenting with the Etymotics and sure enough, it did. The thing is, and I'm assuming I'm not imagining things, I'm not sure whether this has to do with something the ear is doing or with the asymmetrical nature of the driver/earbud system (single balanced armature, so no XO phase distortion).
I've wondered the same thing about Maggies, which, after all, have a single-ended nonlinear B field meaning that the direction the driver moves in matters.
You're coupling pretty much directly to your ears with those so any asymmetry of the transducers would tend to accentuate absolute polarity differences.
It still doesn't help a listener to know which way is "correct"....assuming there even is a correct way... for a particular recording. It's a complete crapshoot. :)Dave.
Edits: 10/12/15
I was astonished not only that I could tell which way was right with each recording, but could do so quickly and consistently. I would call the difference subtle but obvious, which is to say I think I could ABX it. (These were all audiophile recordings rather than multimiked ones with botched phase -- I was experimenting with the intriguing Isone VST plug in, an HRTF emulator for headphones.)
The only way I can think of to test whether it's the device or the ear can sense polarity would be to use a symmetrical transducer like an ESL in free air conditions. I think it's unlikely that the ear is sensing the polarity directly -- it's basically a rectifying device -- but I can't rule it out, since the ear is after all sensitive to static pressure. But can those nerves distinguish between low and high relative pressure? If so, it seems possible to me that it could sense something like a drum thwack -- after all, you can feel that in your gut if it's loud enough. Then too, the ear is itself an asymmetrical device and perhaps the nonlinear behavior alters the sound?
Slight typo, it should have read 4.7 uF cap (the by-pass cap on the 470 mF electrolytic). Also I should have mentioned jumping the inductor on the woofer - again, just a simple replacement of the inductor with a wire.BTW, the manual suggests using a 180 Hz HP filter (not 30 Hz) for biamping. I suspect this is because the "balancing" of inherent woofer inductance is no longer needed. Any thoughts?
Edits: 10/12/15
Yes, I noticed the 180Hz recommendation......obviously completely different than the stock crossover.
There are a couple of questionable directions/statements in the LFT-8b owners manual regarding bi-amping.
See my edited post above for the issue regarding midrange polarity.
Dave.
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