Why do my subwoofers rattle like hell ?

Here's my problem :

I can only get decent output from my subs if I maintain a low pass filter setting below 40Hz

At 50Hz low pass they perform ok at moderate volume, but at 60 Hz + LPF they produce a SERIOUS rattle (i.e. the subs sound like they are about to self destruct).

Here is the setup :

1 off Directed 1100D amp
2 off Earthquake Magma subs (3.2 ohm DVC subs) paralleled up to give a nominal 0.8 ohm load.
Common sub enclosure so both subs occupy the same volume. Enclosure volume optimised as per manufacturers recommendations.

Thinking the problem was enclosure resonance, I went to the trouble of fitting various M8 threaded rods to brace the enclosure. This had negligable effect (after trying to convince myself otherwise).

Any thoughts suggestions ? Are the magma's not capable of driving over 50 Hz ?

Thanks,

John

I had problems with my 8" kicker solobaric... the dust cap had worked itself loose. I just carefully peeled it all the way off and then glued it back on with a light layer if glue.

Another thing to check would be to see if one of the voice coils is blown. No sub should give dumpy performance above 50 Hz unless it's over 24", and even then I'm not so sure.

It COULD be the amp, but I doubt it. Check the amp outputs with another speaker to rule it out... then try and check the speakers individually. One blown voice coil will probably make the whole sub sound like crap.

Another thing to check would be to see if one of the voice coils is blown. No sub should give dumpy performance above 50 Hz unless it's over 24", and even then I'm not so sure.

Checked the resistance across all 4 voice coils, i get 1.0 ohms (which is consistent with the reading I got when they were brand new unused). Does this mean the voice coils arent blown ?

[orange}
It COULD be the amp, but I doubt it. Check the amp outputs with another speaker to rule it out... then try and check the speakers individually. One blown voice coil will probably make the whole sub sound like crap{/orange]

Will give those ideas a try. Thanks.

Common sub enclosure so both subs occupy the same volume.

And they're phase-correct, right?

And they're phase-correct, right?

Do you mean : are the pos and neg terminals correctly wired ?

They should be. But mistakes can happen I guess ! Is there an easy way to test without taking the subs out of the enclosure and looking at the wiring ? Multimeter test somehow ? It breaks my back every time I have to fetch these out of my car !

The output at <40Hz is fierce : real stomach churning stuff, so I has assumed phasing to be ok. The amp is single channel, so I made an internal wire to connect everthing up in parallel. By this I mean :

the amp neg terminal is connected to every voice coil neg terminal
the amp pos terminal is connected to every voice coil pos terminal

Just wanted to be clear on that last point ...

paralleled up to give a nominal 0.8 ohm load.

two things come into mind... first, the gain/bassboost might be up way too high causing clipping which is more self-evident at higher frequencies (i actually killed my first sub that way, being the audiophile newbie that i was. i had loudness on my empeg to full, and had my amp bass boost set to max, and gain at halfway... before long, i started to get a noticible rattle, and had to get the driver replaced.).

second, i'm no real expert, but if i recall (and from my own experience) the less resistance the speaker has, the louder it gets but the quality becomes less. i would try to hook the pair in series and put the gain up to compensate and see if it helps w/ the rattling.

I think this should be the next course of action as well. Hopefully you have a friend with an amp that you could somewhat easily get to. Find a long set of speaker wire and park right by each other. Run his speakers or sub from your amp, and your subs from his amp.

I've had an old Rockford Fosgate amp give out a few years back, and it gave that constant clipping to the sub I had it attached to. It ALWAYS sounded bad though, not just high frquencies, so it's not the same problem - could be similar though.

In short, I've seen the amp be the problem before, I'd highly suggest running your sub from a different amp as a test, and ditto your amp to another sub.

I'd highly suggest running your sub from a different amp as a test, and ditto your amp

Fortunately, I have another amp and another set of (mid bass) subs in my car, so I will do some troubleshooting over the weekend.

Thanks for all the suggestions.

The lower the load to the amp, the lower it's damping factor and the worse the sound gets. The damping factor is a number that shows how well the amp can keep the sub 'under control'.
A 0.8 ohm load is rather low and sound will suffer.
Try with 1 sub (2ohm). Chances are that the sound will be much better and nearly just as 'loud'. The 1100D doesn't double its output when the load impedance is decreased from 2 ohms to 1 ohm. I don't think there will be a lot of difference in sound output.

Removed the subs from the enclosure yesterday in order to change the wiring, and managed to snap the tinsel wire of one of the voice coils in the process

Any idea if / how it can be repaired ? Is solder good enough ?

I can't see why not, but it might change the resistance slightly on the voice coil most likely not enough to notice though (soldering it that is).

Update time ...

Got my tinsel wire fixed.

Paralleled up the voice coil on each speaker, then connected both speakers in series (nominal 3.2 ohm load presented to amp)

Result : Much improved. LPF lifted up to 160Hz without any problems.

I have noticed that a slight distortion is present when cold, but dissappears after 30 mins or so in the car (at which point I can crank up the remote gain pot)

Thanks for the help in solving this problem.