madison dragon 21" - tests and deconstruction - coax?

[geoffire]

Hi everyone. this is my first real post here, but I want to share my findings and get feedback on what I want to do, so here it goes...

I'm building a open baffle speaker of sorts. For the bass I'm going to try the brute force method - a pair of 21" woofers I picked up for a mere $250 shipped! They feature a 5" VC and a rather massive magnet. One weighs about 50lbs!




First thing I noticed is the cone is ultra stiff and barely moved when you push on it. The ultra small spider is to blame for this I believe.


LSBLab shows the FS is 32.45 and the QTS is 0.4852 - looks pretty smooth!

I don’t know if normal people do this, but I decided to take it apart!

Here’s the coil and sloppy glue job on the spider. You can tell the glue actually dripped down onto the 5" coil and they scraped it off.


The motor with the basket removed.

I’m still playing around with lsblab, but I’ll get the MLS and distortion stuff up when I get those tests working. Playing a 40hz tone through the thing was the first time I’ve heard 40hz with authority with an open speaker!

As hinted at in the headline, I have plans to turn this into a coax... more to come...

[tade]

very cool, i am interested!!!
Please keep us posted! How is the efficiency? can you post a link to those drivers? How big was your test baffle?

Are you looking to use a single extra speaker for coax? How high can the woofer play?

Keep us posted

[geoffire]

Heres the numbers I plugged into winisd:

Qes: 0.606
Qms: 2.442
Qts: 0.485
Fs: 32.45
Res: 7.900
Sd: 1926. cm^3 (I' got this from another 21" driver by the same manufacture)
Xmax: 13mm pk-pk -based on the method of here and put a 20hz tone through untill I heard distortion.
Pe: supposedly 1000watts - its a 5" two layer coil of about 20 gauge wire...
Vas: unknown, but I plugged in 15 cu. ft.
winisd estimates an Spl of 95.83db - I have no way of measuing it.


here's the FR I measured unsmoothed, but the setup is all uncalibrated, so don't take any of these numbers and graphs posted here as gospel

[AJinFLA]

Geoff,

I would imagine you will be building a triaxial, perhaps something like this?

If it is indeed a coax, what low pass frequency for that sub? What center mounted driver? I would not use a driver that large pass 200hz or so.
I recalled seeing that sub on sale before http://www.audioasylum.com/forums/hu...es/107976.html
thought it might make a decent OB subwoofer
Did you get it at that sale price?
Do post some pics when building. You're on the right track. If you feel you need some advise, just holler .

cheers,

AJ

[geoffire]

oh yeah, forgot to answer all your questions.

I got these drivers from here , they were on sale on the front page, but I don't see it listed anywhere now, you could give them a call, but expect to leave a message and have them call you back a few days later.

As far as the coax idea goes my first idea was to put a dome mid in there like the Dayton RS52. but crossing to a tweeter with that big of a separation between the mid and the tweeter may be an issue.

Another thought is to machine the pole vent open to say 4" and put a full range driver in there like a fr125, or a high end automotive coax running open baffle as well. Well 3" deep u frame I guess, but the VC's would be aligned even.

I havn't put them in a baffle yet, just a bare woofer.

the FR graph I posted is nearfield btw... I'll try playing with farfield a little later and see if 200hz + cleans up a little.

[geoffire]

yes I got it at the sale price. I was amazed they only charged $50 shipping for two! (it was over 100lbs)

the mid is undecided as of now. I was thinking maybe a rs52 dome mid, so I don't have to worry about the rear wave of the mid, but I don't think I want to run this thing up to 500hz now that I've measured it a little.

AJ - What do you think of machining out the pole vent so the mid mounted within can be open baffle (shallow u frame) as well?

Looking at the infinity perfect VQ as an example removing material from within the motor will raise the qts, which in this case is actually a good thing as the qts now sits at 0.48 - a little low for open baffle use.

How did that triaxial turn out anyways? I've seen that pic before, but not a report on how it sounded.

[Magnetar]

I have a pair of those in parallel per channel for midbass in some large WMTMW open baffles. Four of them is good if you high pass them at 50 cycles and drive them with a beefy power supply 300- 600 watt bipolar SS amp.

Edit - sorry those aren't the 21's I have - mine have a 4" coil and a higher QTS - the Executioner-X21

[AJinFLA]

Ooops, I see that my link to the sales price is defunct, but it looks like you got the Dragon from the same place (I was interested in it for similar use as what you are doing. I can show you another 5" VC for about the same $ also).
As far as machining the pole vent, yes that would raise Qts, by weakening the motor strength. I'm just not sure If I would take such an approach. You may well raise distortion in the driver, etc. by doing so. I refuse to compromise just to go passive.
Obviously you would have to be very careful not to get particles in the gap and if you are going to spend money sending it to a machine shop, buying large, expensive inductors, etc. why not just go the superior way - active.
Drive the sub with a plate amp with eq and go passive with the mid/tweet xo if you must.
Rhythmik sells inexpensive plate amps with add on LT circuits that are cost effective solutions for such an endeavor.
Whats not shown in my pic are the holes(approx 3/4", closely spaced) I drilled around the side (close to the driver) of the phase plug/mid enclosure, allowing some of the rear wave to "leak" and cause side cancellation, to produce more of a cardioid type radiation pattern to (closer) match the dipole woofers directivity.
There was also shielding on the mid to reduce magnetic interference between motors. Neo drivers come in handy here.
The mid/tweeters had aligned VC's, but I moved it well forward of the woofer as not to compromise the mid enclosure, because aligning the VC with the (sub)woofer was not a priority. This can be done electronically.
Plus the need for perfect phase coherence (especially at low frequency/(very)long wavelengths) is debatable. I do it anyway .
How did it sound? I know this may come as a shock to many, but I listened to it very little (unless you consider test tones "listening" - which I do). I was far more interested in the measurements I was seeing. I was testing the platform, not the sound of those particular drivers in that arrangement.
The drivers were just what I had on hand. When I decide on actually building that platform, I will use different drivers. Which I already have of course.
The FR125 is a nice unit, but a) terribly inefficient for passive use b) like all midr....err, fullranges, it needs a tweeter.

cheers,

AJ

[Pano]

Very Cool!
Looking forward to seeing what you come up with.
That ain't your typical MTM, for sure.

[geoffire]

I'm planning on going completely active. handily my roomate is an electronics geek who can help me build whatever circuit I desire.

my reason for wanting to machine the vent is to run the mid mounted within dipole as well.

the pole vent is currently 1.75" Diameter and 2" deep which if left open would act as a port, but if I machined it out to 4.5" the mid will be sitting in a 2" U frame.

The raise in qts is simply a side effect I'm ok with.