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VC bottoming out? Air leak? need help!

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REIER View Drop Down
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    Posted: 19 July 2019 at 11:20pm
Now I need som serious help, I feel like I have tried everything and I cant figure out the problem. 

The story:

I have a 12v portable PA system. I use a 200ah lifepo4 batterybank, a pioneer mono gmd9601(500w@4ohm) with 2 tham15 (15ps100) for subs and a gmd8604(300w@4ohm x2 bridge) for tops. I blew two drivers last summer bcs someone turned on the bass boost (50hz). They never recovered from that. I played around with amplifier gain on the mono amp afterwards, and toasted them completely. Kind of an experiment as much as an accident. 

I bought two recone kits and a new driver and built a new box. The first reconed sub sounds nice in the new cab, but the second (2) reconed  sounds like its bottoming out. Not like a rub, more like a solid clank when I turn the volume a little bit up. I took it out of the cab, the clank is getting more pronounced, sound like a metallic rattle. I think for my self, maybe it could be the dust cap is off centre. So Im cutting of the dust cap, recentering it, rattle is still on board. 

Here comes the weird part. I open the new 15ps100 . It says on driver it is tested before shipping. Mount it inside tham 15, and rattling/clank is here as well. I take it out of the box and try it open air, it rattles even more like a snake. I try my other amplifier, same sound. I put the brand new 15ps100 in the other cab, same sound. I go back to my first reconed sub, testing both amplifiers, and works like a charm. 

Made a filter setting on minidsp, cut off at 38 and 110 hz. No improvement. Now I'm frustrated and getting creative. I go back to the driver with recentered dustcap, and try to find out if it is a misaligned voicecoil after reconing. I put on a clamp, and add force on frame until I hear rubbing sound, and release, and rubbing sound goes away. Try this again for 3 new angles, and I conclude it is not a rub, more like a bottoming out sound. 

Now I'm out of ideas. The driver is rated for 700w rms and amp 500w, and the sound was there right at the beginning. I cannot remember this sound from before I blew them, but who knows. Maybe I wasn't so critical as I am now. But there is definetely something fishy going on.

Do drivers bottom out in free air always? Is this normal when pushed maybe 100 or 200 watts on music program? I have inspected amps with multimeter, no DC from amp terminal. Resistance on speaker terminals is perfect, so not blown driver. 

I have an event coming up in a couple of days, and I did not expect so much hassle. I am getting quite anxious to say the least. Very new to diy pa, and feel like I have put on too big of a challenge. Now I am asking for help. Could you please point me in the right direction?

Regards

Kristoffer

(posted on another forum aswell)




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studio45 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 22 July 2019 at 2:20pm
If you messed up the recone and got the VC off-centre, there would be a very obvious buzzing sound and you would be able to feel it rub when pushing the cone with your fingers. But from your description that is not what's happening.
With 100 watts of amp power and no high-pass filter, it will be relatively easy to bottom-out a 15PS100 in a THAM15. The tapped horn design isn't good at controlling excursion at the lowest frequencies (below 50Hz) so you always need to use a high-pass to prevent over-excursion. And don't make the mistake of thinking that the driver's "1400 watt" power rating means it can't be driven to full excursion, or even damaged by a 100 watt amp. It depends entirely on the box it's in, and the frequencies being applied to it.
In free air, it's always very easy to bottom-out a speaker with bass frequencies. So that's not unusual at all. When you want to exercise the surround on a brand new driver to get it moving freely, the common technique is to use a small ~20 watt hifi amp and a 20Hz sine wave generator in free air - this is *always* enough to get it moving to the limits of its suspension travel and more, even with a 2500 watt monster 18" driver.
So I think you just need to set a high-pass filter at 50Hz and proceed carefully. Try putting the suspect drivers into a simple sealed or reflex box, drive with a small amp and see if they still make funny noises. It's possible there is some debris in the VC gap, but again you'd expect to be able to feel that when pushing the cone.
Studio45 - Box Builder Commotion Soundsystem -Mobile PA
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