jcaut
12-04-2004, 10:41 PM
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
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