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Progress on MiniPseudoLaScala


WMcD

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I'd promised updates. The theory is that a 66% near copy of a LaScala could be used for a center and surround. Naturally, there will be reduced bass response.

A problem was building a midrange. I built a 400 Fc midrange. The top and bottom surfaces are flat but angled. Then the sides are filled with strips of wood and trimmed.

The bass unit is 16 x 16 x 16 and I've used a strip in front and a smaller chamber.

I a few weeks I will mount drivers and give you some test results.

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Thanks Al.,

The file names shown have no spaces, even though the uploads do. After some fooling around I got the upload to work.

I have some Eminence Beta 10 A for the bass and 1824 and 1829 for the mid. One will get my remaining T-35.

Selenium has an adapter to put a screw on driver on a 1.3 throat. It is to go up against the round plywood throat. I have to trim and fill with putty. I'm of the mind that 10% error with rough putty at the throat is not going to make too much difference. Perhaps over-optimistic. However, they work well enough on the other three beasts.

The real test will be with the LMS.

I understand from your posts you got a measuring system. In my experience, the bad news is that it can show up a lot of irregularities. On the other hand, it is good to have the real story and work on solutions.

Gil

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Gil,

Yes, I got a Mighty Mike II calibrated mike from Old Colony Sound Lab and a

used Wavetek 188 sweep / function generator from eBay. I'm using a AC

voltmeter to look at the output of the mike. This lets me do CW testing, full

or partial sweeps and a warble tone of sorts. About all I can seem to do is

verify the +-5 dB specks of the drivers. Attempts to measure absolute

sensitivies and efficiencies is questionable. I can get in the ball park but

I think my room response is messing things up. An anechoic chamber might

help! I haven't got the gumption to haul the speakers outside.

Al K.

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Boy! Those wooden horns are exciting! I'll be really interested in how they sound. It looks like you've done a great job so far. I think you should try to make the throat as perfect as possible. Getting the wave front started out right seems like the most critical thing. Could you use some sort of router bit to help you transition from round to square, or does the adapter do that?

How do you plan to finish them? Cherry and hand rubbed oil would have to look rich. I could imagine one and a T-35 on top of my C7 covering the upper ranges.

John

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Thanks for the encouragement. Sometimes it is needed to keep up enthusiasm.

Three units already have a walnut veneer applied to some surfaces. I'm planning on mahogany for the other two units.

Okay, okay, I'll do a good job on the throat.

Elmer's wood putty with some careful filing has been doing okay for me.

I started the project thinking they would turn out with the quality of a fine violin. But that was unrealistic. At least they may reproduce like a fine violin.

Gil

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Nice work William. Keep us updated on your progress. I am very interested in your results because I am working on a La Scala based system too. It will be a 2-way full size version. I know how much you guys like to look at DIY stuff so here are some photos of my version of the 300Hz Edgar salad bowl tractrix horns. And wife never thought I would get any use out of that lathe. I showed her! The driver that is attached to them is a vintage Altec 288C. Have a good look. When I get closer to the end I will post a new topic and have a show and tell session. Later guys

See the photos at the following address http://www.klipsch.com/ubb/uploads/288saladbowl.jpg and http://www.klipsch.com/ubb/uploads/288saladbowl2.jpg

JLHasqui

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quote:

Originally posted by JLHasqui:

Nice work William. Keep us updated on your progress. I am very interested in your results because I am working on a La Scala based system too. It will be a 2-way full size version. I know how much you guys like to look at DIY stuff so here are some photos of my version of the 300Hz Edgar salad bowl tractrix horns. And wife never thought I would get any use out of that lathe. I showed her! The driver that is attached to them is a vintage Altec 288C. Have a good look. When I get closer to the end I will post a new topic and have a show and tell session. Later guys

See the photos at the following address
and

JLHasqui

Your horn looks great.Of course it is not an Altec 288C.With the ceramic magnet and Tangerine phase plug it is much newer.Looks like a 288K but it could be a 299 or 291.Any of these are better than a 288C.If the sticker is missing off the back cover get the number off the diaphragm and we will be able to determine the model and impedance for you.

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djk,

You're right it's a 288K 8 ohm driver. I was still half asleep when I submitted that post. Anyway it is not my driver, just my horn. I plan on using the B & C speakers model DE-75P 2" driver on a 300Hz round tractrix horn with the La Scala bass horn. Should make for a killer 2-way system crossed at 500Hz. Yeah, I know the stock La Scala will only go up to 400Hz, but mine will have corner reflectors at the last bend to help out the high end. Do a search for "corner reflector" there is a thread that Tony Reed started that explains this mod. Details forth coming.

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I'm also working on La Scalas for 10" drivers and some 300Hz Edgar midrange horns.

My goal is to enhance my existing system. It consist from a (selfmade) Klipschorn subwoofer, mains (selfmade): Eminence 8" bass, old Aduax 6.5" midrange (the one Bruce Edgar mentioned in his "Edgar Midrange Horn Article"), and Fostex FT66H tweeter. The center is a Magnat Vintage Center 5.

The mini La Scalas will replace the mains.

Right now, I just hooked the horns in front of the audax to see (hear) what they do.

I'm thinking of tuning the back chamber of the cabinet so that the system resonates at the cut-off of the horn flare frequency approx. 95Hz (to extend the response as far as possible downwards)? I don't know if it's worth the extra effort?

The driver will be a WESTRA 10" with fs=40Hz, BL=6.8, Qes=0.62, 4ohm, 92dB/W.

My mayor concern in this driver: it's weak efficency.

I'll give a report when I got the whole thing together.

Erik

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  • 5 weeks later...

John,

Thanks for the inquiry. Unfortunately, Ive bitten off more than I can chew, again, considering time and talent limitations. Ive been working on the three units for my high school. A lot of time has been taken up working on veneer and finish before installing the hardware. So, acoustic testing is just ramping up.

In the interest of science, and the journey, and advice to other builders, Ill give you a long post, as follows:

Mechanically, the installation of the bass driver and motor board was made complicated by my decision to close off much of the back chamber. You see this in the early photos. It was difficult to get the washers and nuts on the far side mounting carriage bolts. No room for a wrench or hand. That was solved with wing nuts. I should have seen this coming but did not. (Self delivered head

slap.) None the less, see below that reduced back chamber volume is an issue.

I got to play with the LMS and the almost complete units over the weekend for about a half-hour. Real solid measurements and tweeks will take days. But a few things are evident.

The midrange horns (Fc = 400 Hz) are working very well. The midrange horns are so classic that they cant go wrong. But the driver is a source of problems.

The quick test was with an EV-1824 driver. I tried that driver on my similar plywood variation of a K-400 years ago and the results were poor. Eventually I rigged up a plane wave tube and found the 1824 driver ragged in 1 kHz to 2 kHz range and rolled off sharply at 3 kHz. Naturally, the overall

horn response suffered. This recent test on the mini horn showed about the same.

The 1828 tested much better on the plane wave tube years ago. Also, anechoic measurements of the big midrange horn with the 1828 were very favorable. So, I think the smaller midrange horns will be okay with a better driver. Even with the 1824, they have good response down to 500 Hz. So

Id say the geometry is correct. Yes, Im waiting for the new Atlas driver to come out.

The mini pseudo LaScalas bass units need adjustments. Even with the reduced volume of the back chamber, the driver system is resonating at 85 Hz and Id like to see it at 100 Hz. The response from 200 to 500 Hz is about level and even goes to 1 kHz, at 102 to 104 dB at 1 watt/ 1 meter. Therefore, I believe the choice of the Eminence Beta 10-A was a good one.

The problem is that there is a big dip in the 100 to 200 Hz region (I was not hoping for anything under 100 Hz). One consideration is that this measurement was near a floor-wall intersection and

not a corner. However, I believe the response will be better if the back chamber volume can be reduced to get system resonance up to 100 Hz.

I tried doing that by putting some scrap wood and spray paint cans in the back chamber, all that came to hand. However, response fell overall and I believe I failed to get a good seal at the hatch.

This is consistent with an air leak. Then I ran out of time. Rats.

Id like to do some serious testing on the effect of back chamber volume and bass response. A hazy explanation is that reactance annulling requires the system resonance to be at the frequency where

there is the big hit of mass reactance from the finite horn around cut off frequency. An old paper by the Jensen people implies this. Ive found that pushing system resonance up to that frequency is

non linear with back chamber volume. I.e., Ive reduced back chamber volume but it doesnt force up the resonance as much as is expected because it is getting into an area of more mass reaction, requiring even more spring in the back chamber.

Horn design continues to be a good hobby. Im reminded of Journey to the Center of the Earth. Arnie Sarknuson (sp???) left a description of his journey and some trail blazing marks. Yet explorers have to find their own way. It is an interesting trip.

Regards to all, Ill keep you posted.

Gil

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Yeah, Arny didn't come back did he? smile.gif

I wish I had your tools and wood working skills. I would like to have a wooden K-400. I could finish it easily enough, but making it isn't goung to happen around here. I've got a line on an aluminum K-400 and driver; maybe I can figure out how to make it attractive.

John

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  • 2 weeks later...

quote:

Originally posted by William F. Gil McDermott:

John,

The response from 200 to 500 Hz is about level and even goes to 1 kHz, at 102 to 104 dB at 1 watt/ 1 meter. Therefore, I believe the choice of the Eminence Beta 10-A was a good one.

The problem is that there is a big dip in the 100 to 200 Hz region (I was not hoping for anything under 100 Hz).Gil

Gil,

I found someting similar with my mini La Scalas. The response I got is at level from 200Hz to approx. 1KHz. Below 200Hz it drops -20dB at 100Hz. As you know, mine driver is more like a HiFi driver: low fs and a little to high Qes, low BxL. The back chambers are perfecly air tight. So, that could not be the reason.

Currently I am trying to get my measuring system to work (sticking around with a mic. pre amp). Then I will give more detailed results.

But Gil, how deep is the dip in your La Scalas response?

Erik

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Sorry to not be able to give you any info from here. It will be the first of the year before I can get back to the project.

I did some work on cutting plywood inserts to decrease the back chamber volume. I want to force up the resonance and get some more reactance annuling.

I'd like to be able to show some experimental results showing frequency response versus resonance. That will take a lot of experimentation and measuring.

Gil

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  • 1 month later...

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