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Old 12th July 2002, 02:21 PM   #1
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Question Subwoofer overload protection?

Hi all.

The other day I was listening to music and had my system and sub playing pretty loudly. I hadn't heard the song before, but thought to myself "this is pretty bassy!". *Then* the *real* bassline came in and all my sub had to say was "rATtTttleEE!" as I scrambled to find the remote and press the mute button. This happens more than I'd like to admit. It conjured up a memory of seeing PA speakers in high school that once they were driven beyond a certain level, you could see light coming out of the ports, which I figured to be a protection mechanism that diverted the excess power to a light bulb of some sort. Is there a simple DIY method to achieve something similar? I've read on the site about speaker fusing, etc, but I still want the speaker to play at it's maximum volume, just not bottom out if it is being fed more than it can handle, sort of like a compressor. Thanks!

Phil
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Old 12th July 2002, 02:28 PM   #2
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Hi Axeman (sounds quite martial

Is this a ported sub or a closed-box one ? Is it active (with built-in power amp) and if yes how is it fed, via high-level or low level inputs? Or is it driven by a separate amp ?

Regards

Charles
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Old 12th July 2002, 02:36 PM   #3
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Hi Phase_accurate,

My sub is a 15" car audio driver(8ohms) in a sealed box. It is being powered by a car amplifier, which is fed the line level sub-out from my receiver.

The name is back from the quake 1 days- when I would often be stuck with just the axe.
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Old 12th July 2002, 03:31 PM   #4
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Hi Axeman

Do I guess it right that this subwoofer is in a large box or your amp is bloody powerful or even both ?

The solution to your problem could be a circuit with a VCF (voltage controlled filter) and/or a compressor circuit. The VCF thing is nowadays used quite heavily for PA applications though the first application of such a thing I ever saw was the dynamic equalizer circuit used by KLH (to sqeeze the maximum amont of bass out of a small two-way speaker).
I only have the JAES article describing its working principle but maybe somebody out there has the whole circuit diagram.

It works by modelling the LF excusrion behaviour of the driver by the use of a lowpass filter followed by a level detector which in turn controls the Q and cutoff-frequency of a 2nd order highpass filter.

Maybe you could use this circuit and tweak a little (i.e. mainly the transfer functions of the modelling filter and the VCF and ). If the original circuit isn't available, it would be possible to back-engineer it from the aforementioned article.
Another possibility would be to start from scratch to develop such a circuit which might take a little longer.

Regards

Charles
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Old 13th July 2002, 03:32 AM   #5
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If this happened to me I'd first want to figure out if it was the sub or the amp crapping out. Or, perhaps, the power supply for the car amp is giving up the ghost.

Knowing where the problem is you might want to consider more subs, a stronger power supply or a better amp.

If you're willing to put up with what a compressor will do to the sound you might as well just put a light bulb in series with the woofer!
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Old 13th July 2002, 05:01 AM   #6
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The rattling can be a few reasons. One, the box is not well braced. The second reason, is that the woofer is bottoming out. Third like you said it could be the amplifier.

Mostly it is always the woofer bottoming out than the other two. Sometimes a few designers use nails for the box when they should use screws. To correct this problem. Connect a capacitor or a combination of capacitor or inductor in series with the woofer. This circuit is known as a sub-sonic filter or in simpler terms a high-pass filter.

Using a bulb will not do anything unless you know the exact frequency to compress.
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Old 13th July 2002, 05:50 AM   #7
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Axeman:

Tip o' the day:

Do NOT listen to Electro. Judging by his reply, his is extremely ill-informed about a great many things.
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Old 13th July 2002, 07:22 AM   #8
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Hi Axeman

When I suggested the use of some kind of compressor (or better said a limiter) I assumed that your woofer is in a RIGID box of apropriate size (can you give figures. i.e. TS parameters of your driver and the box volume ?).
A limiter might of course deteriorate your sound but it would be something like a live insurance and it would be only active when it comes to the dangerous situation.

Regards

Charles
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Old 14th July 2002, 12:10 PM   #9
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A positec thermistor will provide protection and limiting cheaply and easily.

Regards, Eric.
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Old 14th July 2002, 09:51 PM   #10
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Using a compressor won't do anything. It minimizes the problem not correct it. Bose uses a compressor but it never helps them. Their woofers always bottoms out.

Sub-sonic filter is a high-pass filter cross over at around 15 hertz to 40 hertz. I used a 1st order filter around 40 to 45 hertz on my subwoofer it works. It still reaches down to around 20 to 30 hertz because of room acoustics. Also my subwoofer plays cleaner.

Quote:
A positec thermistor will provide protection and limiting cheaply and easily.
An thermistor won't do anything. If used, the woofer will be above damage point before it limits the audio. It is great for temperature control for amplifiers, fans, computers, etc.

Quote:
15" car audio driver(8ohms) in a sealed box
Car woofers are not designed for home audio. Many 15 inch car woofers have low Xmax that can not produce high SPLs. Vega woofers are an exception.
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