Projects

jcaut

Seems I've been rather busy this past year with fixing up a house and moving, in addition to the normal work load. I've finally recently been able to muster some "spare" time for audio stuff in general and speaker-building in particular. (Still don't have a proper 2ch or HT rig set up in the new place, however.)

Anyway..

Last year at the TX gathering, RuSsMan was kind enough to send me back home with a trunk-load of goodies. (Thanks, Russ!) Included in the package was a pair of Paradigm MiniMonitors in working condition, but pretty beat-up cosmetically. I finally got around to working on them, and I just thought I'd report on my progress.

After listening to them for a while, and measuring an impedance curve in order to discern the cabinet tuning frequency (about 58Hz, and despite what they say, they look like solid 4 Ohm speakers to me), I began to disassemble them and take note of the cabinet construction, enclosure volume etc. I noticed that the rubber woofer surrounds had some cracks, and I repaired these with black silicone. This pair must have been used in some rough environmental conditions, which might also explain some of the cabinet damage. It struck me kind of funny that the cabinets were 100% stuffed with polyfill. I mean it was packed in there tight! I'm surprised that the port was effective with that much stuffing. Nothing else special about the original cabinets. They were constructed from 5/8" and 3/4" particle board with a full-length vertical shelf-brace. Internal volume was about .4 cu.ft., double-flared port on the back.

With the drivers out, I measured the free-air impedance of the woofer, in order to determine the T/S parameters. Turns out the original enclosure was significantly smaller than the "ideal", as predicted by the box building programs I use. That probably explains the abundance of polyfill.

So now it was decision time: Time to decide whether to stick pretty close to the original design or go out on my own. I had already decided that I was not going to monkey with the crossover. Though I've been able to work up a few that I'm not ashamed of, I seriously doubted that I could do better than Paradigm did originally. The cabinet size bothered me a bit though. Just a marginal increase in cabinet volume to .5cuft. appeared to flatten and extend the bass response of this woofer, reducing the midbass peak that the model predicted with the original cabinet size. And since I was going to be building a cabinet anyway....

Fact: I'm NOT a good cabinet-builder. I have to keep it simple or things get out of hand real quick. The drivers were recessed into the original baffle, and the tweeter faceplate is molded in a curve to match the woofer. To make matters worse, the grille frame (which I wanted to reuse-- I hate making grille frames) was cut out to fit around the protruding part of the tweeter, to help reduce diffraction effects. So, I needed to keep the baffle layout the same, and make the overall size close enough that the grille wouldn't look out of place. By making the cabinet about 1/2 inch bigger in each dimension, internally, and modifying the bracing inside, I was able to get close to .5 cu. ft. internal volume with a 3/4" MDF cabinet that seems more "dead" than the original. Free-hand routing of the odd shape for the tweeter recess was fun...

I still haven't applied a finish. I'm thinking I'll probably just wrap these in a wood-look vinyl and paint the baffles semi-flat black, or with the black truck bed coating. I hooked them up to listen and experiment with the port tuning. For now, I've just placed a piece of convoluted foam inside on the back wall and on the bottom. I think I'll tune a little lower-- say 50 Hz. I believe they're going to sound real nice, once I get the other one together so I can hear them as a pair.

Pics attached of primer-gray enclosure with unfinished baffle. Sorry I don't have pictures of the original enclosure to compare right now. If you made it this far, thanks for reading!

Jason

jcaut

..with some Polks and a couple of other speakers I'm working on.

RuSsMaN

I'm STILL waiting to purchase a custom Jason design. I thought I was golden on the Tang-Band micro - but......

Good work, looking forward to more thoughts and pictures.

On the P-dimes, I dont remember any surround blems, but I never really looked that close. I just found em at a pawn-o-matic for $25, walked out with them for $10.

Regardless, if you want, pull em, send em to me, I'll have them re-done at Freeman Tuell and send them back to you.

Cheers,
Russ

VR3

Those are awesome looking dude!!!

I wish I had more money to throw at my DIY projects I have been doing...hehe

gatemplin

Nice work Jason, beautiful job recessing the tweeter faceplate. That is hard to do well. If you have the T/S specs could you send them my way?

I have the Paradigm Monitor 3 v2 (same motor as the 6 1/2" but larger diaphragm) and the Mini Monitor V3 and the CC370 V3. I rebuilt all the cabinets using MDF and better bracing. Because I was missing the the T/S specs I took my best guess at picking the volume and I made them sealed. I flush mounted the drivers (didn't turn out as well as yours but I have gotten better at it) and veneered the cabs in red oak and finished in polyurethane. They are crossed over at 100 Hz so the low end extension that is lost by going sealed isn't that important.

It is good to see another speaker builder here. I am starting a new project that will take me a while to complete. I started a thread over in the 2 ch area about them. Eventually I plan to get a mic and build the jig so I can use Speaker Workshop to design my own crossovers like you.

It looks like you have a Vifa two way in that pic and I've seen those aluminum drivers in the MTM somewhere. What are they?

Edit: If you want to get rid of them sometime in the future let me know. I'd be interested in the crossovers and drivers for spare parts.

HBombToo

Is that a 55 series book shelf Jason? It is beautifull! Great job and I loved that speaker.

Henry

VR3

I see...

RT5, RT55i, a kit from PE, and 2 other speakers??

RuSsMaN

That's a 55i Henry, Jason has 'our' orignal '55' (non i) crossovers in his stash now.

Cheers,
Russ

jcaut

Thanks for the kind remarks, guys.

Russ: Those T-B... I never did get those sounding like I wanted. I haven't given up on them, but I did put them on the shelf for a while. I thought they would be so easy to work with..! And really they should be.. I'm not sure what I'm doing wrong. Last try, I ended up with third order electrical to tame a huge measured spike at 6.5Khz in the woofer response. The x-overs wouldn't fit in the box! I'm finding it hard to make that woofer play nice with any tweeter i've tried. I'll get back on that one soon. And on the P'digm surrounds: The cracks were VERY small and hard to notice. Unless they decide to crack further, I don't think it's a problem. My silicone job seems to be holding!

In the bottom pic, left to right: MTM with Dayton aluminum 5.25/Dayton silk dome tweeter. PE sells this as a kit, however I didn't buy it that way. I had picked up a set of the finished cabinets with the intention of using them for something else. Had the silkie laying around from a DOTD purchase. When PE came out with the kit, the only parts I DIDN'T have were the woofers. It looked like a good easy project. I didn't follow the kit exactly. My box is tuned a little higher, port placement is different slightly, and the tweeter padding was adjusted to my liking. It's a very nice-sounding speaker, especially with acoustic instruments and vocals. The only drawback is that the sensitivity is still relatively low, despite being an MTM.

RT55i-- I really like these, too.

Vifa two-way I'm working on currently. The woofers are 7" Vifa PL18's and the tweeter is the chambered D25AG35 aluminum dome. Cabinets again are the pre-finished PE variety (did I mention I'm no good at building cabinets?) These have strong bass output down to about 45 Hz, and still sound very tight and controlled, for a ported design. With my current crossover, they are a little less forward in the upper midrange, compared to the RT55i's. Vocals are a little more subdued. To my ears, they sound surprisingly similar to the LSi7's, with stronger bass. Thanks to PE Deals of the Day, I have about $30 in drivers, per speaker.

The Mini Monitor

Polk RT3's-- My first Polks!