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Old 11th March 2011, 01:10 PM   #1
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Default PM's Bodging thread, becoming nice monitors, actually.

Well, the Interstial Project is up and running, but as I suspected, and others predicted, it really is a bit of a compromise. Sounds very nice, but only in about a 1 square metre sweet spot. Still, it'll do for now.

So, Interstitial MkII is born. This was triggered by acquiring three custom designed prototype 15" drivers by Ciare. Very nice - neo magnets, well built and high sensitivity 97dB, but Fs at 54Hz was a bit high for bottom end use at home. So, I decided to do some modifications, and throw away a little of the sensitivity by mass loading the cones so I could get them to go lower.

Firstly, I chopped out the dustcone to see if I could attach the added mass to the VC, (much the best option), but it was all a bit congested in there. So, some lead was cut, weighed, and glued to the back of the cone. Testing then showed I'd got the Fs down to 40Hz, but sensitivity had dropped to 90dB, still very useable however.

Below are a couple of pics of the bodging prototype, the dustcap glued back together, andf the lead sheeting fixed to the back of the cone:
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File Type: jpg repaired dustcap.jpg (93.5 KB, 100 views)
File Type: jpg added lead.jpg (66.7 KB, 99 views)
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Old 11th March 2011, 01:19 PM   #2
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Next up, mods to the drivers I'll use in the final project. I cut 20 8g discs from code 1 lead flashing, and fixed ten evenly around each cone, using plenty of glue to avoid any buzzes or rattles. Once the glue has set up I'll run them in for a bit and post some SE measurements.
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File Type: jpg driver.jpg (75.4 KB, 96 views)
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Old 11th March 2011, 01:33 PM   #3
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And now for the mids. Stuck on a shelf at work for years has been a pair of Beyma 6Mi80's. Much as I would have loved to get some new production Audax PM170s, budget would not allow, so I made an offer for the Beymas, and they are now running in prior to testing. As you do, I stuck some audio through them whilst they were sitting loose on the bench, and they sounded very clear and open - I think this could work.
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File Type: jpg IMG_0544.jpg (89.9 KB, 95 views)
File Type: jpg IMG_0545.jpg (99.7 KB, 95 views)
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Old 11th March 2011, 03:24 PM   #4
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Rather than the lead, you might consider cone coatings. You can get much cleaner performance that way, at the cost of high frequency extension.
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Old 11th March 2011, 06:58 PM   #5
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Hi BM, I didn't think of that! I suppose you just keep coating and measuring 'till you get the results you want. Oh well, next time.

I've done impedance measurements to extract the TS parameters for the modified bass and mid drivers, (I'll post them later, they're on the laptop ATM), but will have to wait 'till tomorrow to measure FR and distortion, as I'll need to make a new test baffle. I'm hoping for a crossover point at about 200-250Hz, but we'll see how that pans out tomorrow.
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Old 22nd March 2011, 07:29 PM   #6
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Default Playing with the tablesaw

Teaser pics.
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File Type: jpg IMG_0553.JPG (79.9 KB, 24 views)
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Old 25th March 2011, 03:01 PM   #7
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Default Woodwork complete

Well, the first box is done, and the other is just setting up, so a few pics just in case anyone is still watching this thread.

In the first and second pics you can see the "nautiloid" type back enclosure for the mids. Sized for 1/3rd wavelength at 250Hz crossover frequency, (the longest I could get in the box), it will be stuffed with long hair wool. No idea if this'll work, but hey, it's worth a punt. I can always block it off if need be.

The last pic shows the back, with a rear mounted port, and chamber for the amps and crossovers. Because I decided to go active at the last minute, my cutting list went awry, so I didn't quite have enough ply. Still, a little MDF won't hurt, and the end grain is hidden so no difficult finishing issues. Everything has been given a coat of sanding sealer for the time being. I'll decide on final finish at a later date.
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File Type: jpg IMG_0554.jpg (109.2 KB, 8 views)
File Type: jpg IMG_0555.jpg (102.3 KB, 7 views)
File Type: jpg IMG_0556.jpg (95.4 KB, 5 views)
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Old 25th March 2011, 03:23 PM   #8
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Next up, test fitting the drivers. This has changed a tad, with the Beymas going back, they really didn't do the job, with nasty breakup modes over 1K. So I blew the dust off the wallet, and ordered a pair of SB 17NRXC35. This was a difficult call, as I wanted to get something reasonably priced, yet sensitive, with low distortion and a good bandwidth so they would go up to match the XT25s I'm using as tweets. They also had to be 4Ohm to match, as one idea was passive crossover for mid/high, with active on the bottom.

However, I do have some chunky heatsinks, so I can get some decent drive on these puppies. Looking at what I have in stock, I first thought of the five AAK GB Symasym modules waiting for a good home. Now, I could either use a pair bridged for the 15" with another doing mid-highs, or separate drivers for mid and high and build something else for the bass. Or stick with the bridged pair on the bottom and build something like the BabyAKSAs. Decisions, decisions...
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File Type: jpg IMG_0558.jpg (102.7 KB, 9 views)
File Type: jpg IMG_0560.jpg (105.8 KB, 10 views)
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Old 25th March 2011, 03:47 PM   #9
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Quote:
Originally Posted by badman View Post
Rather than the lead, you might consider cone coatings. You can get much cleaner performance that way, at the cost of high frequency extension.
That is the way i'd do it. It gives equal weight distribution, controls cone flex (instead of making it more complex as the weights will), reduces cone noise (and makes the cone more opaque to time smeared signals from inside the box), and makes the cones shiny (just a cosmetic benefit)

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