New MJK Baffle Article

[TerryO]

Originally Posted by chrisb

Jim - to be clear, she did more than her fair share with the wheelbarrow - and for her loads were only a hundred pounds or so at a time. All I got to do was the spreading, compacting & screeding of the 2 substrates, and laying the 1800 + bricks on 2 sloping grades.

actually, let me rethink that first sentence ....

and we're still 14 years from celebrating our 50th anniversary - probably by that time it's "hey, how was your day sweetie?" "fine, is Survivor on tonight?" (I'm sure it will be )

What Chris won't tell you, is that when it comes to wielding a wreaking bar, there is nobody, and I mean nobody, that can surpass him.
I'm certain that his wife feels privileged to work alongside her man.
There's just something about International Super Star Status that really attracts women. I've sometimes wondered if the reason Chris got married in the first place was just to get away from the hoards of "groupies."

Best Regards,
TerryO

[Aengus]

Originally Posted by chrisb

All I got to do was the spreading, compacting & screeding of the 2 substrates, and laying the 1800 + bricks on 2 sloping grades.

Hey, be fair - I laid at least 4 of those 1800... Hmm, let's see: 4/1800 = 1/450 = ~0.222%... OK, never mind.

Regards.

Aengus

[Jim Shearer]

Executive Summary:
Up and running!
WOW! WOW! WOW! WOW! WOW! WOW! WOW! WOW! WOW!

Review:
I am using a plate amp to provide the XO and drive the woofers; I may not have the settings just right yet. And the plate amp has a hum which was beneath notice when it was used with an inefficient sub, but which is bothersome with the 95 dB sensitive Alphas. My brother listened to them and pronounced them too bright. I applied a small amount of treble cut (via tone controls on the Yamaha CR-1000) and agree that helps. I will decide later if I want to buy the rest of the XO parts so I could eliminate the plate amp.

What I love about Magnapan and electrostatics is all there. In fact, most of what I like about Martin’s Lowther PM2A’s is there. I was thinking that I would have to save up to buy a pair of Lowthers--but no more! OK, they aren’t Lowthers, but it’s close enough for me.

Open, spacious, clear, and detailed. Not quite the holographic projection I’ve heard from Martin’s PM2A’s (both in MLTL and OB.) Still, much closer to what I want than the Fonkens and the Metronomes (both F120A and FE108eSigma.)

Jazz and vocals are excellent. Good sound stage, dept, imaging.

They can do justice to large scale orchestral works which strain the smaller speakers. And curiously, they are the first speakers I have heard which make Paavo Jarvi’s recording of Britten’s The Young Person’s Guide to the Orchestra (Telarc CD-80660) sound decent. On any other system the bass drum is loose, flabby, boomy--makes me wanna puke. Makes me sorry I bought the CD. On these OB’s, it’s OK--not quite right, but listenable. (But I still wonder WTH the recording engineer was using for monitors.)

The low freq extension seems to be enough that I am not motivated to add a sub at this point. I will make some measurements later.

Pics to follow....

Chrisb:
Stop what you are doing. Forget about the yard. You need to build the FF85k OB NOW! If you think the FF85k sounds good in a box, you’ve GOT to hear what it does OB. (Assuming you like the dipole sound.)

Martin:
Thank You! Thank You! Thank You! Thank You! Thank You! Thank You! Thank You! Thank You! Thank You!

[otto88]

"Below Fequal, response is - 6 dB/oct drop in output, that's directly dependent on the size of the baffle . .
At Fequal, SPL of the baffled driver will be the same as its response in an infinite baffle.
Above Fequal, response rises to a 6 dB peak at Fpeak (approximately equal to 3*Fequal); and at higher frequencies, the response will depend largely on the shape of the baffle."

If the baffle width is increased, can we increase Fpeak - so that the efficiency of the FF85k is increased?

[Jim Shearer]

Pic below shows the OB with the F120a Metronome behind it and the FE108eSigma Met to the right. Plate amp is sitting on the little sub (which is not used w/ the OB.)

[chrisb]

Originally Posted by Jim Shearer

Chrisb:
Stop what you are doing. Forget about the yard.

After this past weekend, I appreciate that advice perhaps more than you do Martin's project papers , but fat chance, until it's finished

You need to build the FF85k OB NOW! If you think the FF85k sounds good in a box, you’ve GOT to hear what it does OB. (Assuming you like the dipole sound.)

Well actually, that's exactly how I first heard this amazing little driver- while Dave was crunching the numbers and drawings for "boxes", I rear mounted the raw drivers into a small MDF panel, and ran them in for several dozens of hours. I may yet conduct a reality check on my testosterone levels and build a pair of the OBs for the TV room that my wife seldom inhabits. My dedicated man-cave is a narrow, L-shaped former bedroom, and I fear would not be the best environ for a full sized OB.

Martin:
Thank You! Thank You! Thank You! Thank You! Thank You! Thank You! Thank You! Thank You! Thank You!

so that's 9 wows and thank-yous - what happens when you get really excited?

[chrisb]

Jim, et al;

Any anecdotes regarding the efficacy (if any) of slight angle to the side panel/support legs to the OB design?

Jim, the aesthetic ratio of your build, including the beveled edges of the keystone shaped baffle, is very appealing; I just wonder about any sonic advantage of angled side panels ( say around the same 5deg as the tilt-back)?

[Rudolf]

Originally Posted by chrisb

I just wonder about any sonic advantage of angled side panels ( say around the same 5deg as the tilt-back)?

Try to keep the sides of the baffle (front AND back) free from major protrusions at the height of the FF85K. The big supporting brace near the Fostex in Jims baffle looks less than optimal to me.
If you look at the right picture beyond you can see what kind of brace I use in this case:



Having side panels (AND the top brace) around the woofer is OK -especially if it is just the 5 deg of the tilt. The (small) resonance of that U-frame should be well outside the woofer passband. And the wings will pretend a wider baffle to the woofer (added depth = added width at both sides).

Rudolf

[MJK]

Jim,

You are very welcome. I take it you are happy with the results.

The speaker build looks great, just like the others you have built, and having heard both drivers I know the combination is a real winner.

[Jim Shearer]

Chrisb,

If I get any more excited, it might be hazardous to my health! With each build I have kept looking to get closer to the characteristics I find appealing in Lowthers, Maggies, Martin Logans, etc. These OBs are delivering on that. I know that these qualities are not universally appealing, as my brother found them to be just OK.

As to the design specifics: I have no way to quantify the effects of the beveled edge (it should reduce diffraction,) the side panels (which do follow the angle of the edge's slope and should have some effect similar to a shallow 'U' baffle,) or the 2x4 bracing (which should improve the quality of the bass.) I have never listened to this MJK OB in it's Rev 1.0 style. When building I just go on what little knowledge I have and a gut feeling for what might be helpful. It would be very interesting to compare my build with the original version. And if there is anyone out there in the upstate NY region in possession of the rectangular baffle style, I would be eager to get together and compare.

Cheers, Jim