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Old 3rd February 2009, 02:44 AM   #31
CLS is online now CLS  Taiwan
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Default Some more info and updates

I made some more mods on the drivers last night. I re-sealed the slot-cutting on surround by glue and made 3 fan-shape cuts on the spider.

Click the image to open in full size.

Click the image to open in full size.


I've not yet auditioned the result, but some quick impedance scans were made. Together with the previous measurements, here are the comparisons:

This is the raw driver with dust cap removed:
Click the image to open in full size.


This is with slot-cutting on the surround:
Click the image to open in full size.
(the impedance peak are lowered and split)


This is with surround cut and PVA treatment on cone:
Click the image to open in full size.
(the 2 impedance peaks are further suppressed)


This is with re-seal of surround's cut and cutting on spider:
Click the image to open in full size.


For your reference.
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Old 4th February 2009, 08:54 PM   #32
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What was the listening result of cutting the spider?

I've got some big, cheap 12inchers at home that I eventually look to push as high as possible, so I'm following along here with interest.
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Old 5th February 2009, 12:48 AM   #33
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Hi,

On this driver, I found the overall stiffness of suspension is dominated by the paper surround, which is the same piece of material of the cone. While the spider is relatively quite soft.

So the cuts on spider didn't afftect the overall stiffness very much. Together with the cut and glue-sealed surround, they lowered the fs a little anyway.

The current status is:
1. De-capped and PVA coating on cone
2. Surround is cut and re-sealed by glue
3. Spider is cut

I found the most significant change is brought by the glue-treatment on surround. It sealed the cut slots and saved the loss on lower mid and also suppress the peaks on mid high very much.

Previously this driver sounds somewhat edgy. With PVA coating and cuts on surround, the mid high peaks were lowered by some extent, but still sort of 'snapy' and needs some EQ to be reasonably flat. With glue on surround, the sound turned into mellow and warm.

Funny, I think I overdid the damping. Even with re-adjustment on EQ (now it's almost flat in the range by itself), the original senses of energetic and vivid are largely muffled. I'll need to do something to bring them back in the near future.....

And I also found the side effect of cutting on the spider -- the cone movement is not so 'straight' as before. I mean, when being pushed on the edge, the cone used to move back and forth as a whole (well, mostly). Now it would tilt somewhat and the opposite side of cone which is not pushed moves very little. Pushing it further, eventually the voice coil would rub on the gap.

Maybe, in the frequency range I'm using it, coil rocking is not much to worry. But I guess the 'proportion' of compliances of spider and surround changes somehow. Now the surround provides even more resistance and damping, so the cone might 'distort' earlier, thus changes the whole sound character....

More to come (maybe)....
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Old 5th February 2009, 01:30 AM   #34
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I think you shouldn't have cut the spider. that new wiggle at 600hz is probably caused by spider's nonlinearity. I've seen scanspeak woofer mod, where spider was replaced with thread, woven around speakers frame and cone (can't find webpage now)
since spider is already ruined, you may try that thread mod too, if it fails you can always convert this woofer to passive radiator
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Old 5th February 2009, 03:07 AM   #35
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Passive radiator? No!

It's too stiff and too short in travel anyway.

Luckily, the 600Hz bump in impedance plot shows little effect in the frequency response. That slightly increased and broaden 2kHz peak is also absent on both RTA and listening. Strange but true.

I will see what I can do with the spider. I've seen 'string' spider mod, too. Fixing points and the adjustment of tension might be the problem....
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Old 5th February 2009, 04:48 AM   #36
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Quote:
Originally posted by CLS

I've seen 'string' spider mod, too. Fixing points and the adjustment of tension might be the problem....

Its no problem if you mount strings/threads so that they just touches the voicecoil...and apply glue at the voice coil as the last move
With proper and equal tension it will last a very long time
Mount a little wood at the chassis, fore the threads

If dustcap is removed you can stick paper between voicecoil and poleplate
If you dont remove dustcap its possible to stick thinn metal threads through the dustcap and into magnet gap
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Old 5th February 2009, 04:52 AM   #37
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With large drivers such as 12" running very high, how do you accept/how audible is the beaming?

Let's say a 11" effective diameter, it will start to beam at about 700Hz ?
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Old 5th February 2009, 05:10 AM   #38
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Hi tinitus,

Thanks for the good tips. Will do.

But I can't follow the 2 methods about dust cap. The caps on my drivers are removed anyway. And I inserted a phase plug in the hole, so it seems no space for that...


Hi gainphile,

It's actually not that bad. I often listen to them far off axis -- almost at the side. It seems not a good idea for a pair of OB speaker, but it's no problem maintaining very good tonal balance In such listing position, only the quantity of bass is slightly reduced, no big deal. The sound is always filling the room and evenly spread everywhere in the home -- no that kind of "boom in next room" effect thanks to dipole.

I guess 2 things help: 1 is the cone is far from ideal piston, so the 'effective' area would be far smaller (and the phase plug might help a little here). 2 is on the open baffle, the 'beamwidth' of mid to lower mid is also relatively narrow, so the beaming at higher frequency might not be so obvious.
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Old 5th February 2009, 05:16 AM   #39
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Quote:
Originally posted by CLS

Thanks for the good tips. Will do.

But I can't follow the 2 methods about dust cap.

.

With the spider removed it should be possible to insert paper into magnet gap, from behind the cone...just makes sure you bend the pieces of paper to secure it, and being out of the way when mounting threads
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Old 5th February 2009, 05:22 AM   #40
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Oh I see, those are shims for center-positioning the voice coil ! Of course!

My phase plug is easily removable, so I'll do it from the front. A piece of bizcard cut into 4 stripes maybe good for this job
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