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Reasons For underpowered amps blowing speakers?
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audigy
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#1
27th April 2010
Old 27th April 2010
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Reasons For underpowered amps blowing speakers?

I'm sorry if this has been covered many of times before and if it has then please point me in the right directions,

Could someone explain to me the actual reason why an underpowered amp can damage your speakers.

I thought it was due to to the amp clipping and the generated harmonics being feed to the tweeter for long enough would burn it out.

I have since heard (Cant remember where now) that this was a common misconception and is not the case at all.

can someone clarify the reason for me as I'm now a bit confused & unsure on the subject.

Thanks
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27th April 2010
Old 27th April 2010
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Under powered amps will wreck your speakers because people over drive them, when you over drive an amp it obviously clips. Clipping is the most common reason for blown tweeters.
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28th April 2010
Old 28th April 2010
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Quote:
Originally Posted by tampa View Post
Under powered amps will wreck your speakers because people over drive them, when you over drive an amp it obviously clips. Clipping is the most common reason for blown tweeters.
Bingo!
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28th April 2010
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Clipping produces square waves, which destroys tweeters. Get a bigger amp.
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28th April 2010
Old 28th April 2010
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Any amplifier that is driven to the point of clipping can damage any speaker.

In practice I have seen far more woofers or large diaphragm, low freq. drivers ruined than high freq. speakers like tweeters from under-powering.

In the case of under-powering, the driver is blown because the amplifier is driven to the point that it can no longer amplify the signal.
The amp tries to generate the amplified version of the input waveform, but runs out of "headroom" before the full wave is generated.
The result is a square wave.
The sinusoidal version of the waveform is only partially generated and the result is a square wave with a "plateau" on the top.
The "plateau" is pretty much a DC current at that point.
DC in large enough amounts is what burns up speakers.

The driver is only driven part of the way in or out, but it is held in a suspended position at the top of the square wave the amp is generating.
The result is the speaker is in a near DC state and the constant voltage without any corresponding movement causes the windings of the voice coil to heat up.
Eventually the heat is too great for the windings and they burn up.

In practice, the motion of the speaker also pulls in cooler air and forces out the heated air around the voice coil.
With the speaker mostly in the motionless state, but with voltage being applied there is no cooling from the speaker moving in and out.

In the case of too much power, the amplifier is sending more power than the voice coil windings can stand and they burn up, or...
In the case of woofers, the voice coil is bottomed-out against the magnet.
This causes a distortion in the shape of the voice coil.
Once the voice coil is distorted in shape the windings rub against the sides of the gap.
They either wear though or are so thin that power surge burns them out at the weak point caused by wear.

As a general rule it is about best to have twice the wattage available to a given driver.
As an example:
I use a sound system weekly that has a Crown Macrotech 3600 on low end, a Macrotech 2400 on mids and a Macrotech 1200 on the highs.
This rig was originally built with this amplifier configuration for EIGHT 18"s, FOUR 15" mids and FOUR 2" horn drivers.
In this set up the amplifiers are DOUBLE the rated wattage for the driver sets.
Because I am very used to running sound systems I generally run this rig with HALF the drivers (weight issues in the truck.)
So, in my case I have been using this configuration for four years with FOUR TIMES the amplifier wattage in relation to the driver's wattage rating.

Am I going to blow drivers? No....
Can the drivers be blown? Yes, very easily by someone who doesn't know how to set up a system.
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28th April 2010
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I might add that a crappy quality amp with have the same effect on your speakers. I remember selling a guy a pair of KEF 104/2 loudspeakers, I asked him what he was going to use to drive them, he said a 100 watt Technics reciever. I said to him be very careful you don't want to blow the tweeters in these speakers as you have to buy an entire assembly to replace the tweeter. I week later I get the call tweeters are shot, I said well don't say I didn't warn you. Funny part is the speakers must have sounded like hell to be clipping that badly.


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28th April 2010
Old 28th April 2010
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What Danny told you is spot on. In professional sound reinforcement it is common practice to overpower speakers to avoid clipping. Headroom is your friend.
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audigy
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28th April 2010
Old 28th April 2010
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Thanks for all the clarification guys.

this really clears it up for me!
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28th April 2010
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Quote:
Originally Posted by dbbubba View Post
As a general rule it is about best to have twice the wattage available to a given driver.
As an example:
I use a sound system weekly that has a Crown Macrotech 3600 on low end, a Macrotech 2400 on mids and a Macrotech 1200 on the highs.
This rig was originally built with this amplifier configuration for EIGHT 18"s, FOUR 15" mids and FOUR 2" horn drivers.
In this set up the amplifiers are DOUBLE the rated wattage for the driver sets.
Because I am very used to running sound systems I generally run this rig with HALF the drivers (weight issues in the truck.)
So, in my case I have been using this configuration for four years with FOUR TIMES the amplifier wattage in relation to the driver's wattage rating.

Am I going to blow drivers? No....
Can the drivers be blown? Yes, very easily by someone who doesn't know how to set up a system.
This is a dumb question but how exactly does one accomplish managing power like that? Basically how do you NOT feed four times the rated wattage into the speakers (unless you meant it in a different way)?
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28th April 2010
Old 28th April 2010
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Driving an amp wont produce a square wave, the top of the wave will be cut and it will look more like a square wave but when someone can create a circuit that puts out a squarewave when clipped he should immediately start making an analogue synth pedal for bass/guitar (people will pay thousands).

There are two ways in which underpowering can cause speaker damage:

1. You overdrive the amp which makes it clip and produce extra harmonics which results in a higher output (usually most profound in the highs) which can blow the speaker (or tweeter that is not designed for a lot of highs).

2. You You overdrive the amp which makes it put out (way) more power than it's rated for. This can make the amp give so much power that it will overpower the speaker and thus blow it.

The two usually go hand in hand and result in speakers getting blow because of too much power (speakers are sometimes rated rather positively which doesn't help).

If clipping a waveform could destroy a speaker there would be thousands and thousands of guitarists and bassists buying new speakers every week. It is just completely not true.

The only way to prevent blowing speakers is listening to how it sounds and when you hear anything clip or flub or fart or similar you should turn down. Also buying speakers that can shred your ears before they shred the cones help .

In conclusion speakers don't blow because of underpowering but because of pushing amps past there limit (thinking your safe because it is rated lower) which puts out way more power than it's rated and overpowers the speaker. Clipping the amp will not blow the speaker if the amp is not overpowering the speaker.
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28th April 2010
Old 28th April 2010
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Quote:
Originally Posted by dbbubba View Post
Any amplifier that is driven to the point of clipping can damage any speaker.

In practice I have seen far more woofers or large diaphragm, low freq. drivers ruined than high freq. speakers like tweeters from under-powering.

In the case of under-powering, the driver is blown because the amplifier is driven to the point that it can no longer amplify the signal.
The amp tries to generate the amplified version of the input waveform, but runs out of "headroom" before the full wave is generated.
The result is a square wave.
The sinusoidal version of the waveform is only partially generated and the result is a square wave with a "plateau" on the top.
The "plateau" is pretty much a DC current at that point.
DC in large enough amounts is what burns up speakers.

The driver is only driven part of the way in or out, but it is held in a suspended position at the top of the square wave the amp is generating.
The result is the speaker is in a near DC state and the constant voltage without any corresponding movement causes the windings of the voice coil to heat up.
Eventually the heat is too great for the windings and they burn up.

In practice, the motion of the speaker also pulls in cooler air and forces out the heated air around the voice coil.
With the speaker mostly in the motionless state, but with voltage being applied there is no cooling from the speaker moving in and out.
Sorry for the double post but wanted to point out that this is a myth. When the waveform gets chopped off it won't be completely flat and to get somewhere close to a flat top of the waveform you should be getting the worst screeching distortion you have ever heard. Also a squarewave will not produce DC current and the coils won't stop moving for any significant amount of time (else you wouldn't hear a square wave would you?). The clipping will make the signal more powerful and thus produce more watts but it has nothing to do with squarewaves and DC current.

This doesn't mean you can start clipping amps. Pushing an amp that far (when it's not build for it like guitar amps) will make it produce way more wattage for it's rating. It's best to just get speakers that can handle how loud you want them to go and get an amp that has enough headroom (nothing wrong with getting an amp rated higher than the speakers if you are careful with the volume).

btw. This is not meant as an attack on the poster (this myth is actually pretty common "knowledge" in audio world and there are loads of people who believe it's true). There is also nothing wrong with the advice to not ever clip an amp which is not meant to clip. It's just that the theory is not correct.

Edit: Right now I'm playing a squarewave through my laptop speakers. I'll report back when they start smoking
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28th April 2010
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Bootzilla:
There is a HUGE difference between say clipping the amp(Square wave) and sending a square wave into an amp BELOW clipping the amp..
Plus ever see a square wave thru a tube Gtr amp???
And then look at a square wave thru a GOOD solid state amp...very different..
The output transformer on the tube amp changes that square wave, the rising edge is not the same..a solid state amp will/can send a DC signal into the speaker, a tube amp can not...
You also can have HF oscillations that you can't hear BUT that tweeter WILL respond to..have seen this several times..
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28th April 2010
Old 28th April 2010
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Quote:
Originally Posted by The Real MC View Post
Clipping produces square waves, which destroys tweeters. Get a bigger amp.
No, this is not true with all amps and all tweeters all of the time.
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28th April 2010
Old 28th April 2010
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Quote:
Originally Posted by Bootzilla View Post
Sorry for the double post but wanted to point out that this is a myth. When the waveform gets chopped off it won't be completely flat and to get somewhere close to a flat top of the waveform you should be getting the worst screeching distortion you have ever heard. Also a squarewave will not produce DC current and the coils won't stop moving for any significant amount of time (else you wouldn't hear a square wave would you?). The clipping will make the signal more powerful and thus produce more watts but it has nothing to do with squarewaves and DC current.

This doesn't mean you can start clipping amps. Pushing an amp that far (when it's not build for it like guitar amps) will make it produce way more wattage for it's rating. It's best to just get speakers that can handle how loud you want them to go and get an amp that has enough headroom (nothing wrong with getting an amp rated higher than the speakers if you are careful with the volume).

btw. This is not meant as an attack on the poster (this myth is actually pretty common "knowledge" in audio world and there are loads of people who believe it's true). There is also nothing wrong with the advice to not ever clip an amp which is not meant to clip. It's just that the theory is not correct.

Edit: Right now I'm playing a squarewave through my laptop speakers. I'll report back when they start smoking
How can you produce more power (watts) when the power rails drop?

Unless the speaker impedance drops (which is another effect and one that also can cause amps to clip if they can't supply sufficient current), a reduced voltage rail will result in less current through the load and therefore less power. Surely?
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28th April 2010
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And isn't the tweeter thing because the squaring wave of big LF signals (on an underpowered amp) creates additional harmonics that (some) make it through the crossovers and fry the tweeters due to overcurrent/overheating
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3rd May 2010
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Quote:
Originally Posted by tampa View Post
Under powered amps will wreck your speakers because people over drive them, when you over drive an amp it obviously clips. Clipping is the most common reason for blown tweeters.
i have a problem with teenagers blowing my speakers and i'm about to use a solution that is common to club houses around here:
to put a passive volume knob after the master volume to not allow people go to highest volume of the amp.

But does this mean that i'm making it easier to clip, hence easier to generate square waves and blowing the speaker?

thanks.
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