Subwoofer overload protection?

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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!

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. :D
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.


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!
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.
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.


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.

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.

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.
My professional speakers have a 12volt 1156 turn signal bulb in series with the drivers in them and if they get overdriven they overheat the filaments in the bulb causing the filament to break saving the driver from any damage. Most Yorkville Sound speakers use this as the fuse but i don't think that it will keep the driver from bottoming out just keep it from blowing up.
Hello Electro,

"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."

"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."

You are talking through your hat again Electro.

A compressor/limiter will do a fine job of limiting bass levels, as will a POSITEC PTC purpose made for protecting loudspeakers and available in a range of power ratings.


Thanks for the comment about electro. Someone has to help me to discourage input from "know littles." Alas, it probably won't work. One of the moderators emailed me and suggested that I back off with my hostility toward jerks. Fat chance! If I had more time I'd follow electro around and stay tightly in his face.

So where did Axeman disappear to?
Enduring Idiots

Hi Bill, Yes idiots like mr electro keep propagating disingenuous drivel, and like you I can find it irritating, though I don't take it personally.

My thoughts have turned to seperating this forum into at least two sections - Idiot/Newbie for those without formal training or experience, and - Expert Class for those who are professionally involved with electronics and have formal training and experience.
Or maybe an age delineation so that the older folks can talk on similar wavelengths without the jive/hip talk of some of the impestuous youngsters making their presence felt.

This forum I regret to say has deteriorated due to the influx of those with little real knowledge or relevant experience, hence the tide of basic questions being asked, and the corresponding misinformed replies, and some really inflamatory snipes at those giving serious knowledge and observations by those without.

There is now far too much asking of simple questions without 'Searching the Forum First' !, or searching the net.
Anybody notice that the likes of Jocko and Harry are not around so frequently to give of their experience and knowledge ?.
Maybe they don't like to suffer fools gladly either.

To mr electro, an ages old saying is "an empty vessel makes the most din" - please take note.


Well, I do take exception to "Idiot/Newbie" for obvious reasons.

And, I'm not sure age delineation is a good idea. There are plenty of young people with Masters and plenty of older people with Zip. There has got to be a better solution.


Yes, any device that alters the linear (hopefully) characteristics of a sound system will effect quality. When a woofer sounds like it's about to fly out the window, you don't duct tape it to the floor. I mentioned a light bulb, as an alternative to the proper fix, to axeman because it's cheap, readily available and easy to use. No serious person will entertain the use of and kind of limiter or compressor in their system.
I have seen this SOLUTION

It uses a diode bridge like in PSU rectifiers.
connected to input terminals of Woofer.

Connected within the bridge is voltage divider
in form of resistors or/and zenerdiodes

This shunt have a POWER transistor
at its limiting point, that can handle
the heavy currents, that will be shortcircuited.

If you can avoid it, do not use circuits/components
that would eventually compromise the soundquality.
Better to get a woofer, that can cope with
the power your require in the output
of your SoundSystem.

uses power
but within limits
My knowledge in electronics is very vast. I'm being doing it for about 12 years. I been doing audio for about 4 years. I haven't have the excessive amounts of time and money like you guys have. Computers and programming also took my time. Anybody can get their masters. Many classes don't even do hands on assignments. Mostly they do simulations.

I already know how the bulb method works. People have used bulbs from the beginning of electronics. Bulbs are cheap so is capacitors, resistors, transistors, diodes, relays, etc.. I don't think Axeman has tried either one. Either a bulb and a capacitor will work.

For many years hobbyist and companies have been trying to correct thermistor because of its non-linearity. If anybody measures the thermistor you will see a log graph. It is very hard to make a near perfect linear from log. These type of circuits rely on linearity. If a thermistor is used it is best to shove it into the vent hole of the woofer. This is where heat develops. A decrease in power handling and increase in friction will result. I rather rely on copper coils as temperature gauges. Much more linear.

Loudspeaker protection has been tried and failed to protect the loudspeaker with out adding more components. The only true way of protection is keeping a finger on the volume control. People that want massive amounts of bass should look for a woofer that has high Xmax (12mm to infinite one way) and a large diameter cone.

I do agree with gromanswe. It is best to use a woofer that can withstand abuse from excessive amounts of bass. I disagree using transistors, mosfets, and triacs to switch off current hogging devices like a speaker driver. Relays are much more better to turn off speaker drivers remotely.

People here like to jump on very, very advanced projects. Yes, they will get there, but the question is when. Creating a challenge is good, but it is good to take baby steps to the goal. Instead of trying leaping millions of miles or kilometers to the goal. People here flame other people too much. Also they get off topic.

Everybody that came here, came from search engines or they saw the address on business cards and t-shirts.

I have seen several lame answers from mrfeedback on many forums.

BTW, Axeman, have you ever inspect the woofer. To me this is common sense, so this is why I didn't post it in previous replies.
Dear all

I do of course agree that the best solution would be an appropriate amp/driver/box combination. There are some very rugged drivers around that are indeed very hard to get bottoming even when their lin x-max is heavily exceeded (although it would be better to never reach lin x-max at all!).
I once tested a Dynaudio 20W75 (admittedly a little off-topic when it comes to subwoofers) in free air and couldn't get it bottoming with an amplifier rated at 100 watts (i.e. one can assume that the progressive restoring force of their spiders is actually working and not only sales-talk), of course the sound became heavily blurred.

But you can't always get what you want and then you might use some sort of trick to ease your problem a bit.
That's where the kind of limiter that I suggested comes in.
The principle I suggested is a voltage controlled highpass filter whose f3 and maybe Q as well is electronically controlled.It has to be connected between the active crossover and the power amp. It can be regarded as a combination of both a subsonic filter and the famous "hand on the volume knob" (although in an automated form). There would ABSOLUTELY no regulation be in action for normal listening volume but it will protect the driver in case of abuse.
This is heavily used within modern P.A. systems. But even with the most sophisticated technology it would never be possible to make anything idiot-proof since idiots are much too inventive !!! But to conclude the principle wouldn't work at all, just because B%*! can't make it work ........... ?!

I do not know anyone else's taste but I prefer the sound of a system that sounds a little compressed during times of abuse (that's what we are talking of, I assume !) against the sound of a bottoming transducer.

A circuit like gromanswe suggested is normally used to protect tweeters in P.A. applications (please try it only with passive crossovers, your amp(s) will be grateful ! Otherwise you'd rather use a limiter before the amp or use a circuit with a relay, I can supply a circuit, if anybody is interested) where a higher THD in the upper frequency range every now and then would be definitely less audible than a burned tweeter!

Kind regards

(working in electronics for 25 years ;) )

P.S.: Non-compromise audio-systems exist only in user's dreams, salespepoples mouth and on high-gloss brochures !!

Yes, you are right!
I thought of this after I had posted.
My circuit has to be combined
with an amplifer that is protected for shortcircuit,
or have some kind of current-limiting
device in output-stage.
Or you can have a lot of fuses at home.

A relay to disconnect load/subwoofer
is much to prefer as Electro say.

Still you need to introduce some messuring device.

A coilsensor circuit, with appropriate amplifying of
its input, has almost no effect on
the signal, so that would be best.
Can of cource be combined with some filter,
if you need another trigger value,
than just peak current over whole f-spectrum.

But it is not so easy for all of us to design/build
such a circuit.
Even an experienced engineer will most probably
spend more than a day before it will work alright.

A very small current sensing resistor in Supply-Line
messured by a diffential op-amp,
is an alternative. With a relay as breaker to Speaker.

many road leads to goal
the goal is not everything
once you're at it
you miss the road
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