m&k knock offs. - AVS Forum
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post #1 of 6 Old 02-23-2006, 12:39 PM - Thread Starter
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I'm thinking off trying to do a knock off of these speakers:

http://www.mksound.com/s150thx.htm

and was wondering if there's anything that needs to be accounted for in the crossover other than the change in impedance from running the drivers in series, parallel, or series/parallel? Is there anything tricky about using 3 of the same driver in one speaker? Should I be thinking of this more as a mini line array?

Any input is greatly appreciated,
Tristan
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post #2 of 6 Old 02-24-2006, 05:25 AM
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For the record, I was not trying "knock off" anything, but decided to use a dual vertical array because it allowed for a dynamic speaker in a compact enclosure (just 400mm high). When I first saw the M&K 150s in the mid 1990's, I didn't believe that they would even image properly - that was until I built my first version and I think they ROCK!

I've used VIFA P17s (I think 6.5" drivers give better midbass over the 5.25") and XT super tweeters (band out to 40K for Hi-Rez) with a 2 way, 2nd order LR network in a sealed enclosure.

I've built three for LC and R and this is actually the fourth set I've built, but the 1st to use the XT tweeter...

I wired the drivers in parallel and the "L Pad" attenuation network in the HP section of the crossover corrects the impedance. The trick to Controlled Vertical Directivity is driver spacing, where the centre to centre from top to bottom is equal to one wave length of the crossover frequency (in my case 1550Hz) and at 10 feet you need to be on axis (vertically) or they do sound dull...

http://i49.photobucket.com/albums/f2...eakers/LCR.jpg

Notice I even had CAVX badges made from them...

Mark

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post #3 of 6 Old 02-25-2006, 05:02 PM
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Here is an earlier version that I finished off in 2PAK. Notice that tweeter's spacing are closer, again to equal one wave length of the crossover frequency which was 2200Hz with this model...
http://i49.photobucket.com/albums/f2...s/DSC01530.jpg
The tweeter plate is made from 6mm black Perspex and was a bit tricky working out how to rebate it into the baffle, but in the end looks quite good...

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post #4 of 6 Old 02-26-2006, 01:12 PM - Thread Starter
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Those look excellent! Very well done, and professional looking, I love the vifa xt tweeters aswell, good look and sound.

I was wondering if it wasn't too much trouble if you could elaborate a bit more on the driver spacing issue a bit more? Like I said this will be my first foray into a speaker with more than 2 drivers... If you could it would be much appreciated!

Thanks,
Tristan
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post #5 of 6 Old 02-26-2006, 01:44 PM - Thread Starter
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I think I got it, I re-read Jim Griffin's white paper on line arrays, so I think I got it. A few questions though(if you don't mind)

1. he says when he's talking about center-to-center spacing of the tweeter line, that with the high frequencies of tweeters in general and the size of the driver makes it impossible to adhere to the one wavelength or the more strict 1/2 wavelength of the driver:
for example:

a tweeter with a highest frequency of say 20Khz, the line would need to be 17.2 mm (.68") for one wavelength, and 8.6mm (.34") for a 1/2 wavelength(basically a straight quote from the paper) When i'm using 1" tweeters with fairly sizeable flanges that becomes virtually impossible.

He suggests, as a compromise, to take one wavelength at 10 khz(which for the example above would give you 1.35") That to me still seems difficult to obtain considering that most tweeters especially the vifa's i've been looking at with flanges would typically be 3.5-4" what did you do to work around this? The woofers seem easy enough, assuming that their highest frequency is the crossover point that shouldn't be difficult to obtain, but I'm still a bit stymied by the tweeters, any suggestions? Is that the reason for the custom faceplates in the second photo?(great finish by the way!)

Thanks,
Tristan

P.S here's the paper if you haven't seen it
http://www.audiodiycentral.com/resource/pdf/nflawp.pdf
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post #6 of 6 Old 02-26-2006, 03:21 PM
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Hi Tristan,

Thanks for the positive feedback :)

Yes centre to centre is correct. There was several test boxes to get both the Vb for the woofers right as well as to get the spacing of the tweeters right. I started researching this many years ago when I was given a flyer with the original B&W THX certified system (CM8? I think the LCRs were called) - 2x 6.5"W + 3x 1"T. It was an all passive system (including the two subs that were wedge shaped that the LCRs sat on as stands. Discontinued now...

Anyway, their tweeter spacing was close, the small rectangle panels touched, but there was a gap between the top tweeter and the flanking woofer and the bottom tweeter and the woofer, and I knew it was calculated.

So I began researching D'Appolito's 3/2 geomertic arrays, and applied the same physics given that these THX speakers were 5/2 geomertic arrays. B&W listed all the specs including Vb and Xc, so from there I could do the math, which I found to be one wave length from centre of the top woofer to centre of the top tweeter, then center of the top tweeter to center of the bottom tweeter, then center of the bottom tweeter to centre of the bottom woofer. So bacically three full wavelengths from top to bottom.

Given that, I then figured that it might be just how M&K did theirs, so I started building test boxes.

VIFA actually released (for a very limited time) three 12ohm 1" tweeters on a single plate, but from my calculations (and I sent these to VIFA), the product was not really suitable below 4.8K crossovers, and given that the tweeters themselves could be crossed over at 3K, that the plate should have been bigger.

Anyway thanks again or the positive feedback, and if there is anything you need to know to get your DIY happening, let me know...

Mark

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