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Old 4th June 2011, 10:30 PM   #1
2litre is offline 2litre  United States
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Default Free is free, right? Just tinkering.

Hi All,

My wife drug home a Radio Shack Optimus SW14 passive subwoofer last weekend. She got it for free and figured I'd like to tinker with it. Cat# 40-4072 if you need to know. RS literature says it's optimized for use with small speakers like the Minimus and Optimus 7's.

So I took the driver out to measure it and get internal dims on the cabinet. Measurements give me a touch over 42L raw. Subtract roughly 1L from the 2.5"x7.75"L port tube I figure I've got a 41L cabinet.
Driver Fs is 28, Qts is .63 (DVC measured in parallel with WT3) and RS spec sheet quotes 88db.

Even after seeing how bad it looked in WinISD I figured I'd just rip out the port, seal it stuff it and run it. I'd power it with a Mission M2Sas plate amp I bought a couple of years ago from MCS. 75w, 18db slope. Oh, and give up another litre or so for the amp.

-But, I also have an @.65 Qts 10" sub driver hanging around, and this got me to thinking, could I use the 10" as the driver and leave the original 12" unpowered as a PR.

So the short answer is "Sure you can Jim, go for it".
But what I'd like to know is how badly doing this will screw up a simple, admittedly not optimal, sealed alignment?

R/
Jim
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Old 4th June 2011, 10:51 PM   #2
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Default tinkersub

Quote:
Originally Posted by 2litre View Post
Hi All,

My wife drug home a Radio Shack Optimus SW14 passive subwoofer last weekend. She got it for free and figured I'd like to tinker with it.-But, I also have an @.65 Qts 10" sub driver hanging around, and this got me to thinking, could I use the 10" as the driver and leave the original 12" unpowered as a PR.So the short answer is "Sure you can Jim, go for it".
But what I'd like to know is how badly doing this will screw up a simple, admittedly not optimal, sealed alignment?R/Jim
Hi there: Somewhere on this foum someone posted a similar concept you may wish to consider: use a driver as a PR and connect a variable resistance across the PR driver for tuning. The author claimed good results. For the cost of a pot (Lpad) may be worth while for your experiment. Hope you post results and listening opservations. ...Regards, Michael
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Old 4th June 2011, 11:23 PM   #3
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Turning a driver into a PR takes a little surgery, cut off the magnet and vc below the spider, then add a plate to the back of the cone, run a bolt through it to attach washers and ta-da... PR
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Old 5th June 2011, 12:31 AM   #4
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Default tinkersub

Hi there: Should you decide to go experimenting with PR, you may consider your first concept with the variable resistance, then should you want to ues a "standard" PR, they are relatively inexpensive at Partsexpress.com (Dayton, 12-inch PR, SD15PR, #295-496 @$30). You may wish to purchase a text book on speakers from Partsexpress, (V. Dickason, #500-035, $34) and read-up on speraker design/PR. It seems like a waste of resources you already have available, to cut-up a usable driver to make a PR for an
experiment. ...regards, Michael
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Old 5th June 2011, 01:03 AM   #5
2litre is offline 2litre  United States
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Thanks for the replies guys.

I have no intention of cutting up the driver. I was just wondering if using it 'as it is' would in effect be a reasonable passive radiator.
Since posting this I did a few searches and read a bunch of stuff trying to find the post about someone who did as I am thinking. No luck.
But I'm getting the impression after all that reading that it's not a good way to go. It looks like a 10" driver should have a lot more cone area than an unused 12" driver could offer as a PR.

So far I've removed the passive XO and made the amp cutout to go over that area. And I've completely removed the port tube assy. I'm working on plugging that area now. I liked the sealed group delay and the @-3db reduction in the high Q, small box response. I'm looking at throwing a full pillows worth of stuffing into it and trying it out for awhile.

I'd still appreciate any why's and why-not's on the whole PR idea though.

R/
Jim
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Old 7th June 2011, 12:14 PM   #6
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Tinkering, TINKERING? Looks like major surgery to me.
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Old 7th June 2011, 01:13 PM   #7
GM is offline GM  United States
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It will work with low Qts drivers in small cabs, but a 0.65 Qts driver requires a relatively huge cab tuned below Fs, so in a small cab it would need to be tuned even lower, ergo a disconnected 28 Hz Fs driver or PR is probably going to create a ~one note boom-box at best, especially since the 12" driver's suspension will be way too stiff for a PR app, resulting in a higher effective tuning. Then again, since the 0.65 Qts driver will probably 'run out of steam' trying to move it without a large cab's coupling air mass to act as a lever it may act as somewhat aperiodic, only one way to know for sure though.

GM
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Old 8th June 2011, 03:46 AM   #8
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After reading your first post I immediately thought that if you wanted to tinker you could start with a better cabinet. I'm not familiar with your sub, but I'm guessing, being from Radio Shack, it uses 1/2" particle board with little or no internal bracing. As long as your cutting a hole in it and removing and sealing up the port, why not just build a sturdier box with 3/4" MDF or birch plywood? You can get just the right Q you want. If you've got WT3 you have all the info you need to design your own sub from scratch!

Also: most drivers are designed either for vented/PR or sealed and don't work well in the "wrong" cabinet type, but you'll see that when you start modeling this sub.

Have fun!
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Old 8th June 2011, 02:09 PM   #9
tb46 is offline tb46  United States
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Hi 2litre,

There is an old thread on the Klipsch Community Forum that tried to go into the direction you are interested in. The almost complete lack of duplication of the OP's intent is quite interesting, but there are a few good parts like the reference to Adire's use of the second coil of a DVC driver:

<meta name="description" content="Klipsch audio systems provide the true audio/video lover a wide variety of high performance loudspeakers and loudspeaker systems for music and home theater entertainment centers, including iPod speakers, multimedia s.

The passive radiator (or drone) substitutes for a long large diameter duct attached to the woofer rear chamber. To make a normal woofer useable as a passive radiator you have to add weight to the moving system to lower the resonance frequency of the passive radiator below the actual usage band of the system, and ideally you'd like this weight to be adjustable so that you can tune the drone (duct) and chamber to the driver. This will involve some kind of permanent modification, which could be done from the front of the active (?) drone. As you are going to use the drone as a generator, no other modifications to the old driver are necessary.

For passive radiator systems I like Jeff Bagby's software, it can help you with getting into the ball park with the numbers. It also provides for simulating the necessary filters for your "Frankenstein":

Loudspeaker Design Software

As a fun learning project, if you have the extra time, why not? As you are implying in your title this is one for tinkering.

Regards,
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