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  #46  
Old 11-06-2007, 07:57 PM
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Zilch Zilch is offline
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Quote:
Originally Posted by Steve O View Post
BTW...Curious how you came to use the Morel DMS-37 HF driver (shielded version of the MDT-37) in a vintage speaker upgraded. Wasn't someone recently pondering the cost/benefit aspects of using a $65 part in an upgrade project?
It was the waveguide, of course!

Well, the element of constant directivity evident in the response curve in comparison to most dome tweeters, and really to determine for myself whether the Morels are "all that." As I understand it, the purpose of domes is max dispersion, which is inconsistent with my view of how HF should be done, i.e., uniform power response.

The whole tweeter thing is daunting -- there are literally hundreds to choose from, and not much in the way of information other the opinions. Seems like Parts Express took down their own measurements of their offerings, which looked very much like the results I get in actual use, i.e., not one heck of a lot like the manufacturers' published curves, which may be the reason, actually.

I chose the shielded version because the alnicos in L77 (and others) are inherently shielded, offering the prospect of use in proximity to CRT TV, which seems an appropriate application. Even though it's all going planar and mooting the issue, I suspect our big old Sonys will be around a bit longer.

With respect to the economics, y'all convinced me, is what. I don't even have any L77s, but my new PE "AR4x project" cabinets are now in, and I'm deciding what load to put in them; the Morels are a top candidate. I'd put them in the L44s they're right now playing so splendidly with, but they won't fit. I'm leaning toward using the little Tang Bands in those.

[The original AR4xs are getting HORNZ! Heh, heh.... ]
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  #47  
Old 11-07-2007, 12:21 PM
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Quote:
Originally Posted by Steve O View Post
Yet another case of "never assume". What I find disturbing is the fact that the response variations introduced by the commercial Lpad imperfections overwhelms the effort I've expended in evening out impedance variation in the HF section of my Carmel project.

I really believe that the switched version of an Lpad is the only practical alternative.
There's documentation of the "backwards" fixed L-Pad I implemented here somewhere on line, but finding it among my bookmarks has thus far been fruitless. I tried both ways, and this produced the better result. Perhaps you have some information on this approach?

You can see I'm now operating around 7 Ohms nominal. If I were willing to give up a bit more sensitivity, I could move that up closer to 8 Ohms by changing R2 to 3.5 or 4 Ohms, and reduce the variability somewhat more. That'd require reworking the filter again, though, so I'm leaving it be for now.

Whether it's 7 Ohms or 8 Ohms is somewhat of an academic issue. With just the driver, it was 6.5 or 6.6 Ohms, depending upon which one of the pair I used....
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  #48  
Old 11-25-2007, 01:15 AM
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L77 Baseline Measurements

Test samples courtesy AK member Mxlews.

1) Gated sinusoidal full-range frequency response, two units, set to Med attenuation.

2) Windowed MLS quasi-anechoic high frequency response of Unit #1, showing the three switched attenuation settings.

3) Same for Unit #2.

4) Unit #2 with driver phase reversal.

LE10A woofers run full range, an apparent sub-optimal arrangement according to these results:

http://manuals.harman.com/JBL/HOM/Te...ancer%20ts.pdf

Useable bass response (-10 dB) to ~40 Hz.

LE20-1 tweeter performance is rolled off above 10 kHz, as expected and documented earlier in this thread.

Foxtex or Morel? Woofer response measurements may dictate the choice....
Attached Images
File Type: jpg Mxlews Baseline FR SIN.jpg (53.2 KB, 44 views)
File Type: jpg Mxlews Baseline Attn #1 MLS.jpg (50.1 KB, 34 views)
File Type: jpg Mxlews Baseline Attn #2 MLS.jpg (51.4 KB, 20 views)
File Type: jpg Mxlews Baseline #2 Phase Rev MLS.jpg (47.7 KB, 31 views)
File Type: jpg Mxlews L77.jpg (17.0 KB, 81 views)
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  #49  
Old 11-28-2007, 01:26 AM
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A Challenge:

Morel DMS-37 acquires a bunch of issues once mounted on the L77 baffle in place of the stock JBL LE20-1. Despite incorporating a "waveguide," being recessed 1" and centered a mere 2.5" from that edge, reflections, diffraction, time-alignment, and phase interference play havoc with the response. First curve shows performance with the stock crossover, Med attenuation. While the HF is extended, the consequences of bandwidth overlap remain prominent.

Using the 2-pole crossover developed above for this Morel improves the situation somewhat, though the LE10A woofer evidences a 5 dB notch at 1.4 kHz, and the tweeter now has one at 3.2 kHz. Second curve. Even with these anomalies, the system sounds much better than the original. Note that installing this lowpass filter on the woofer has flattened its rising response, substaintially improving the bass and the overall balance.

The crossover region is still a puzzling mix of cancellations and reinforcements; the drivers are sometimes summing and sometimes nulling. So, I move to a 3-pole crossover to bring order to the chaos, third curve. The drivers are summing properly. The sound is similar, but less sensitive to listener position.

For anyone interested in comparing the behaviors two crossovers, they are superimposed in the fourth curve.

Finally, fifth curve, frequency response of Mxlews's two LE10A (alnico magnet) woofers and a pair of LE10H-1s (ferrite,) the factory recommended replacement in L77, which would exhibit even more bandwidth overlap with the tweeter. Both driver types have rising response, 5 dB between 100 Hz and 1 kHz.

Bonus curve, last, 3-Pole filter works fine with LE10H-1, too. Note again orderly summing of driver responses in the crossover region. The crossover has assumed control of L77, and it sounds mighty fine with this Morel tweeter.

Schematic here:

http://www.jblproservice.com/pdf/Con...rol%20128W.pdf

As built here:

http://audioheritage.org/vbulletin/s...250#post116250

Can it be tweaked better for L77? Yeah, later, maybe.

On to Fostex, next....
Attached Images
File Type: jpg DMS-37 L77 Stock SIN.jpg (54.0 KB, 24 views)
File Type: jpg DMS-37 LE10A 2-Pole.jpg (54.6 KB, 20 views)
File Type: jpg DMS-37 LE10A 3-Pole.jpg (54.3 KB, 19 views)
File Type: jpg DMS-37 LE10A 2- & 3-Pole.jpg (57.8 KB, 18 views)
File Type: jpg L77 LE10A & LE10H-1.jpg (56.6 KB, 24 views)
File Type: jpg DMS-37 LE10H-1 3-Pole.jpg (53.6 KB, 27 views)
File Type: jpg DMS-37 in L77.jpg (25.5 KB, 58 views)
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  #50  
Old 11-29-2007, 02:03 AM
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Trifid - Another Round:

Triangulate flange soft dome Fostex FT207D is also a mechanical drop-in to the existing L77 LE20-1 tweeter cutout. I was able to locate it an additional 1/4" away from the cabinet edge/lip.

It plays better than the original tweeter using the stock crossover, first curve. Driver bandwidth overlap generates now familar phase interference issues, but the overall system response is, well, "listenable."

Fostex specifies a recommended 2-Pole network, the performance of which is shown in the second curve. The phase issues are somewhat mitigated, as is the woofer's rising response, to a degree, by the addition of a lowpass filter on the woofer, but it's clear that adjustments of that filter specific to LE10A might improve the crossover.

Said adjustments made and the results shown in the third curve, providing the best measuring and best sounding system response of these attempts. Rising woofer response is further mitigated making the unique passive radiator bass rich and full. Drivers are summing predictably in the crossover region. If these L77s were mine, they'd be keepers with this tweeter and filter combination.

Finally, a 3-Pole filter shows promise for producing an even flatter response, but this Fostex tweeter is out of headroom for response shaping; it'd need at least 3 dB greater sensitivity for this approach to work optimally.

In summary, then, tweeter upgrades are not a "plug-and-play" proposition. Manufacturer's specifications and published response curves may best be viewed as comparative guidelines only; when it comes to actual performance in an existing system, other factors prevail, not the least of which are driver, baffle and network variables.

Yes, any of scores of choices will work as replacements, and modern tweeters can certainly upgrade vintage system performance, but achieving an optimum outcome may require comprehensive system redesign....

Edit: I've put some additional effort into tweaking the highpass filter Q to improve the system response more, but what I have seems to be the best compromise without complicating the circuit with notch filters. Circuit, sim, and system frequency response, last three images.

Sim says I'm crossed electrically at 2.55 kHz; acoustically, pick your pleasure.

Circuit is six components, and tweeter polarity is inverted. Leave out the L-Pad if you like the sound of the measured response, +/- 4 dB from 40 - 22+ kHz....
Attached Images
File Type: jpg FTD207D L77 Stock.jpg (57.1 KB, 50 views)
File Type: jpg FTD207D Fostex XO.jpg (54.5 KB, 25 views)
File Type: jpg FTD207D Fostex XO Mod LF.jpg (55.1 KB, 32 views)
File Type: jpg FT207D L77 3-Pole.jpg (53.4 KB, 29 views)
File Type: jpg Fostex and Filter.jpg (44.4 KB, 205 views)
File Type: jpg FT207D in L77 Circuit.jpg (15.1 KB, 332 views)
File Type: jpg FT207D in L77 Sim.jpg (40.3 KB, 25 views)
File Type: jpg FT207D 2-Pole.jpg (51.5 KB, 46 views)
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  #51  
Old 02-12-2008, 07:49 AM
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Quote:
Originally Posted by Zilch View Post
I have three "loose" LE20-1s here; all say "8 Ohms" on their foilcals.

Here's the results of quick runs on WT2. I'll do more definitive curves using CLIO in comparison to the Fostex once they arrive.

Re = 3.5, 4.0, 4.0 Ohms
Fs = 1450, 1200, 1300 Hz
Z 2kHz = 5.0, 4.7, 4.8 Ohms
Z 2.5kHz = 4.4, 4.8, 4.9 Ohms

Body diameter = 2.65"
Flange O.D. = 3-13/16"
Mounting hole cutout in S99 = 3"

I'm not making it up when I say manufacturers played fast and loose with "nominal" impedance specs in the olden days.

The L77 filter is 6 uF and 0.4 mH. What's that calc as frequency @ 5 Ohms?
i finally removed the tweeters from my L77's. the darned things have an 16 ohm rating. would that mean that the fostex build that this thread pertains to would not be a go in the pair i have. by the way one tweet id doa. i was watching one on ebay and noticed the differance and it got me wondering.
thanks.
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  #52  
Old 02-12-2008, 09:29 AM
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Quote:
Originally Posted by redspell View Post
i finally removed the tweeters from my L77's. the darned things have an 16 ohm rating. would that mean that the fostex build that this thread pertains to would not be a go in the pair i have. by the way one tweet id doa. i was watching one on ebay and noticed the differance and it got me wondering.
thanks.
To the best of my knowledge, all LE20s are nominal 8 ohm, actual ~5 ohm regardless of what the label says. If your L77s are nominal 8 ohm (I've never seen a 16 ohm version but I haven't seen everything...) the Fostex discussion applies to your situation.
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  #53  
Old 02-12-2008, 01:02 PM
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Quote:
Originally Posted by redspell View Post
i finally removed the tweeters from my L77's. the darned things have an 16 ohm rating. would that mean that the fostex build that this thread pertains to would not be a go in the pair i have?
Nope, you're swapping out the tweeters and building new crossovers. They're all nominal 8-Ohms, anyway, what Steve O said. Measure their DCR to confirm.

Quote:
Originally Posted by redspell View Post
Byy the way one tweet id doa. i was watching one on ebay and noticed the difference and it got me wondering.
thanks.
If you're refurbing for resale, you want the correct LE20-1s in there. If you want mighty nice-sounding speakers, do the Fostex thing, above....
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  #54  
Old 02-15-2008, 08:00 AM
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16 ohm LE20-1's

Quote:
Originally Posted by Steve O View Post
To the best of my knowledge, all LE20s are nominal 8 ohm, actual ~5 ohm regardless of what the label says. If your L77s are nominal 8 ohm (I've never seen a 16 ohm version but I haven't seen everything...) the Fostex discussion applies to your situation.
you are correct about the 5 ohm thing on a supposed 8 ohm speaker, also about 5 ohms on a 16 ohm speaker. got a couple of pics here for you. if i can get the system to work. thanks.
Attached Images
File Type: jpg 2008_0215jbl0001.JPG (117.5 KB, 69 views)
File Type: jpg 2008_0215jbl0003.JPG (119.0 KB, 64 views)
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  #55  
Old 07-24-2010, 12:00 PM
gbburkhardt gbburkhardt is offline
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used your design, results are great!

Thanks so much for this detailed post. I have a set of Lancer 77's I bought in the early seventies. I had the woofers refoamed, built a crossover from your schematic, and installed Fostex tweeters. The sound is better than ever, clear and crisp in the midrange. There are several recordings I have that have choral music, and I can now understand the words they're singing. Makes a huge difference.

The real kudos come from my teenage son, who takes great pleasure in showing his friends how great they sound. They are all impressed.

Thanks again for the schematic - wouldn't have know where to start without it.
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  #56  
Old 07-24-2010, 01:33 PM
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Quote:
Originally Posted by gbburkhardt View Post
Thanks again for the schematic - wouldn't have know where to start without it.
You are very welcome.

I recall it being a substantial upgrade to the system performance....
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