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JBL M2 for The Poors
JBL M2 for The Poors
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Old 6th December 2013, 12:23 AM   #1
Patrick Bateman is offline Patrick Bateman  United States
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Default JBL M2 for The Poors

This afternoon I had an idea for a JBL M2 for those that don't want to drop $10,000.

But first, some background on the M2.

Click the image to open in full size.

First off, JBL M2 is a two way monitor which uses a similar format as my reference monitors. (Gedlee Summa.) There's a 15" waveguide atop a 15" woofer.

Click the image to open in full size.
The compression driver in the M2 is very interesting. It's a dual ring radiator, very similar to the BMS compression drivers, except with two diaphragms instead of one. BMS sells some coaxial compression drivers, but the D2430K is different. In the D2430K the diaphragms face each other. (Not 100% sure if it's push-pull or push-push.)

Click the image to open in full size.
Click the image to open in full size.
Here's a pic of a BMS compression driver that Paul Spencer posted to his blog. Basically it's a ring radiator, that mylar ring is the diaphragm.

Click the image to open in full size.
Click the image to open in full size.
In this cutaway of another BMS, you can see how the sound radiates, basically the output from the ring radiates toward the throat. One of the reasons that the BMS has such good bandwidth is because every point on the ring is equidistant to the throat. (IE, draw a straight line from the throat to the ring, and that distance will be the same no matter where you draw it.)

So the idea I had was to replace the ring radiator with an array of very small drivers.

Click the image to open in full size.

The 1/2" tweeters used in the Keele CBT array are a great candidate.

More details in the next post...
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Old 6th December 2013, 12:31 AM   #2
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.

Here's a pic of what the ring shaped array would look like.
There are eight of the Parts Express tweeters in the array.
In this configuration, the ring is under five centimeters in diameter. (To be exact, it's 4.57cm in diameter.)

The sound radiates the exact same way as the BMS and the JBL, the only difference is that we're using $20 worth of computer speakers instead of $4000 worth of compression drivers.

Click the image to open in full size.
Here's a pic of what it would look like with a waveguide in front of it, in this case the QSC PL-000446GP. ($35 at PE.)

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The array of 1/2" tweeters radiates into the horn exactly like this ^^^

QSC PL-000446GP Replacement Waveguide Horn for HPR152i | 245-625
1/2" Dome Tweeter Element 10 Pcs. | 275-010

Eight of the tweeters and one of the waveguides cost a whopping $60.
By comparison's sake, the D2430K doesn't have a listed price that I can find, though they can be found on eBay for about $450 used

The BMS 4550 is $150
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Old 6th December 2013, 12:40 AM   #3
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.

Here's a comparison of a B&C DE25 on a 15" waveguide, versus the same waveguide driver by eight of the Dayton 1/2" tweeters.

The output of the B&C is way higher, of course, because the B&C has a sensitivity of nearly 110dB while the Dayton has a sensitivity of 65(!) dB

In the sim the Dayton array is getting 20 watts, while the B&C is getting one.

Yes, a conventional compression driver will get louder, but for the home, I don't need to hit 120dB. Plus, this is different and cheap!
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Old 6th December 2013, 12:52 AM   #4
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.
JBL M2 for The Poors

Click the image to open in full size.

If the output level is too low, there's a bunch of ways to make it higher:

1) You can double the number of drivers in the array by making it push-push or push-pull. Basically the tweeters will be face to face in a ring, exactly like the JBL 2430K

2) You can make the ring bigger. The ring in the BMS 4550 is 4.4cm. The ring in my second post, using the Dayton tweeters, is about the same size. The ring in the D2430K is much larger, 7.6cm in diameter. The downside to using a big ring is that you need a bigger throat; the JBL M2 uses a 3.8cm throat. A bigger throat will allow for more output, but it's also tricky to find waveguides with a 1.5" throat. (Note that there's a cone shaped phase plug in the center of the throat.)

3) You can do both. If you do push-push rings and you increase the diameter to the same size as the JBL M2, you can squeeze twenty six tweeters on each side. Cost goes up too, up to $63 per side, but it's still a small fraction of a BMS or a JBL.
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Old 6th December 2013, 01:56 AM   #5
nc535 is offline nc535
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Easier to make a line of those tweeters and use them to replace your paraline
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Old 6th December 2013, 12:41 PM   #6
1audiohack is offline 1audiohack  United States
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So it would make noise when fed signal, beyond that what is comparable to the M2?

I'm starting to think your crazy.

Barry.
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Old 6th December 2013, 03:05 PM   #7
Patrick Bateman is offline Patrick Bateman  United States
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Quote:
Originally Posted by 1audiohack View Post
So it would make noise when fed signal, beyond that what is comparable to the M2?

I'm starting to think your crazy.

Barry.
Oh I'm definitely nuts, it's the whole reason I chose my pseudonym
I obsess about audio projects the same way that some people obsess about their favorite sports team
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Old 6th December 2013, 04:07 PM   #8
weltersys is offline weltersys  United States
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Quote:
Originally Posted by Patrick Bateman View Post
In the sim the Dayton array is getting 20 watts, while the B&C is getting one.

Yes, a conventional compression driver will get louder, but for the home, I don't need to hit 120dB. Plus, this is different and cheap!
Patrick,

So at near full power (20 watts compared to a maximum of 32) the 8 driver array can hit considerably less than a conversational sound level in the high frequency range (no chance of equalizing 20 dB back, that would require 2000 watts, poof), and the wide throat will result in ratty high frequency response.

This idea has as much in common with the JBL M2 as a wooden dog sled has with a Mercedes.

How much is crack cocaine going for where you live ?

Art
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Old 6th December 2013, 05:45 PM   #9
Patrick Bateman is offline Patrick Bateman  United States
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I'm a huge fan of smaller compression drivers; I've used BMS 4540ND and Celestion CDX1-1425 almost exclusively in all of my diy projects for about half a decade now.

My reference monitors use B&C, and in the top octave, the smaller compression drivers sound 'airier.'

I know a lot of people prefer the BMS 4550 to the 4540ND; the former goes lower. This is because the 4550 has a 1.75" voice coil and a larger diaphragm than the 4540ND which uses 1.5".

Here's a quick look at the lineage of some of these drivers, based on some data from audioheritage and avsforum.

Click the image to open in full size.
1) First came the BMS. This is a 4540ND, with a 1.5" voice coil. Output extends past 20khz and it's recommended to use a 1.9khz xover.

Click the image to open in full size.
2) Then came the JBL 2407H, which looks suspiciously similar to the BMS 4540ND. Is it the same driver? See Ring Radiator Comparisons - Page 7

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
3) Next came the JBL 2408H. This one seems to be an evolution of the 2407H, but with a much different phase plug. Note how the area of the phase plug is much smaller than in the 2407H and the 4540ND.
Click the image to open in full size.
Here's my 4540ND taken apart; see how the phase plug length is about the same, but the size is smaller?

An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.

Click the image to open in full size.
4) After the 2408H came the 2408H-1. The phase plug in the 2408H-1 is *completely* different from the phase plug in the 2408H. (2408H is similar to the 2407H and 4540ND, but still a bit different.)
As far as I know, this is the first appearance of the new square phase plug documented in http://d27vj430nutdmd.cloudfront.net...8/99548-32.pdf. According to the literature, "The phasing plug’s slots have a unique pattern arranged diagonally with respect to the ring-diaphragm to provide averaging of the acoustic energy at varying positions to reduce chamber resonances."

Providing further evidence that the D2430K is an evolution of the JBL 2408H-1 which is an evolution of the JBL 2408H which is an evolution of the BMS 4540ND, the patent for the phase plug used in the 2408H is from the same designer that did the D2430K, Alexander Voishvillo, senior manager of transducer engineering at JBL.

Here's the patent on the 2408H, from six years ago:
Patent US20070147647 - Phasing plug for a compression driver - Google Patents

And the D2430K, from a year ago:
https://www.google.com/patents/US8280091
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Old 6th December 2013, 06:04 PM   #10
Patrick Bateman is offline Patrick Bateman  United States
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Quote:
Originally Posted by weltersys View Post
Patrick,

So at near full power (20 watts compared to a maximum of 32) the 8 driver array can hit considerably less than a conversational sound level in the high frequency range (no chance of equalizing 20 dB back, that would require 2000 watts, poof), and the wide throat will result in ratty high frequency response.

This idea has as much in common with the JBL M2 as a wooden dog sled has with a Mercedes.

How much is crack cocaine going for where you live ?

Art
Smoking crack would probably be a cheaper habit than audio for me.
Just placed an order for the following:

1) 20 of the 1/2" tweeters from Dayton ($50)
2) One JBL 2408H-1 ($109)

On the upside, the JBL is surprisingly affordable, a bit cheaper than a B&C DE250:

JBL 2408H-1 Compression Driver 365011-001X | Speaker ExchangeSpeaker Exchange
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