How did I blow a driver?

Discussion in 'Archived Threads 2001-2004' started by TomL, Apr 10, 2002.

  1. TomL

    TomL Extra

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    Can a driver "blow" due to something other than clipping? I've had my ceramic Norh 4.0 speakers for about 4 months now, and the midrange stopped working on one of them. I removed the driver and it reads as "open" to an ohm meter. Norh told me that this type of damage is not covered by warranty, since it is caused by clipping. But I have never driven these speakers at high volume levels (95 db max).

    So what went wrong (I don't want to blow another one!)? I was originally driving them with an old JVC RX905 receiver (near their top of the line in 1992) which I've used for 10 years without problems (never blew a driver even at high levels) - does the amp matter? What about the source - I originally played some less-than-perfect MP3 files?
     
  2. Cees Alons

    Cees Alons Moderator
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    Tom,

    Yes a bad waveform can in principle cause the same effect as clipping: if your waves are almost square (meaning they "fill the area" a lot, in other words: generate a much energy = power = heat), they can blow your driver.

    Normally, sound waves are closer to sinus wave forms (or even smaller: spikes), so a loud sound doesn't have to be equal to lots of electric power. The more square a wave is, the more power it represents, even at low sound volumes.

    Cees
     
  3. JohnnyHK

    JohnnyHK Stunt Coordinator

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    ...but where would a damaging waveform like that come from besides a clipped signal?
     
  4. Richard Greene

    Richard Greene Stunt Coordinator

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    Virtually all driver damage is caused by turning up the volume too loud.
    The blame goes to the person who last touched the volume control.
    Clipping doesn't destroy drivers -- that's an old wives tale that ought to be thrown away.
    That fact that an amplifier may be frequenctly clipping just before a voice coil is destroyed does NOT prove clipping is the primary cause of voice coil damage.
    It's not the primary cause -- it's a minor cause.
    In fact, clipping can alert an audiophile that the SPL is very high and speaker damage may occur -- harsh distortion on bass transients and compression of bass SPL causing the treble to sound (relatively) bright is a warning.
    But a high power amplifier can destroy a driver with no clipping at all -- the only warning (other than the music being very loud) will be the smell of voice coil adhesives melting or or voice coil wires burning or just silence from the driver.
    Clipping is usually brief moments of distortion caused by bass transients. Clipped bass transients do generate some higher frequency harmonics that some people claim will blow the tweeters ... but these clipping harmonics typically account for one or two watts of power going to the tweeter
    -- that's a very small fraction of the total power needed to destroy a typical dome tweeter voice coil.
    I'm not sure why people talk about square waves (and sometimes DC) when the subject of clipping comes up.
    I suppose a clipped sine wave test tone would look something like a square wave. But real music is much
    more complex than sine waves -- clipping just means the amplifier is unable to reproduce some transient peaks and this does not look like a square wave.
    The low-power-amplifiers-are-more-dangerous-than-high-power amplifiers-because-clipping-distortion-kills-tweeters- advocates become mute when I take their incorrect theories to the ultimate conclusion:
    - If clipping is so dangerous, than driving my subwoofer with a 1wpc amplifier clipping 100% of the time would be much more dangerous than driving the subwoofer with a 10,000 wpc amplifier that never clips.
    - Of couse a 1 wpc amplifier (or even a 50wpc amplifier) can not damage a subwoofer driver designed to handle 100's of watts continuously even at full power ... while a 10,000 wpc amplifier could easily destroy the subwoofer voice coil with no clipping at all.
    If you don't want to believe me, there is a good article on the subject at Rane.com that explains how minor the effect of clipping is on speakers.
    http://www.rane.com/pdf/note128.pdf
     
  5. Cees Alons

    Cees Alons Moderator
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  6. Phil Iturralde

    Phil Iturralde Screenwriter

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    FYI: Danger: Low Power: Information on one of the most common causes of speaker failure. (from JBL PRO website)
    (partial excerpts)
     
  7. Richard Greene

    Richard Greene Stunt Coordinator

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    I read the JBL article many years ago (and many others) about how dangerous low power amplifiers can be.

    Any article that implies low power amplifiers are more dangerous than high power amplifiers is not correct.

    Doesn't matter if "JBL" posted it on their website,

    it's still wrong. The person who turns up the volume control is dangerous ... and that danger increases with higher power amplifiers.

    I think you ought to read the Rane article whose link I provided.

    High power amplifiers are more dangerous than low power amplifiers because they can destroy a voice coil faster than a low power amplifier.

    In fact, some very low power amplifiers can not destroy a voice coil at all even at full volume with constant clipping ... while some very high power amplifiers are capable of destroying the voice coil in virtually every driver made ... with no clipping at all.

    Do you really believe a 20wpc amplifier can destroy a subwoofer driver rated for 500 watts of power handling

    because "small amplifiers are dangerous"?

    If you do, think again.

    If a low power amplifier is really more dangerous than a high power amplifier, then the most dangerous amplifier in the world is probably the 1 watt amplifier that drives the 3" speaker in my cheap TV set! Of course the truth is that a one-watt amplifier can not damage any modern driver even when clipping 100% of the time!

    These old wives tales have been repeated again and again over many decades. The Rane article link I provided has a technical explanation of my point -- clipping distortion is a minor factor in driver damage -- it does add some harmonics that do reach the tweeter through its high-pass filter ... but a few watts of harmonics are only a small fraction of the total power needed to destroy a typical tweeter that is rated to handle 40 watts RMS or more:

    (Examples: Vifa D25AG-35-06 1" aluminum dome tweeter rated for 100 watts RMS power handling for $29.50 ... and even the cheapest tweeter I could find in the Parts Express catalog -- the $4.35 Pioneer 2.5" cone tweeter -- was rated for 40 watts RMS power handling!).

    The voice coil does not care whether it is playing music too loud ... or playing distortion too loud ...

    (or playing some modern music that sounds like distortion to my ears!) -- when the power input to a voice coil exceeds its power handling ability, damage will eventually occur.

    The fact that clipping distortion sounds bad to our ears

    compared with music does not mean loud music with audible clipping distortion is dangerous to drivers ...

    but loud music (with no clipping) is not dangerous.

    I suspect the "small amplifiers are dangerous" myth helps audio stores sell more powerful amplifiers (or is used by audiophiles to justify their purchases of powerful amplifiers).

    But the fact is many amplifiers can produce enough power to damage drivers and the cause is turning the volume control too high -- it barely matters if the amplifier is clipping or not.
     
  8. Cees Alons

    Cees Alons Moderator
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    Richard,
    You're absolutely wrong. Voice coils are not destroyed by the potential power of the amplifier, but by the actual ("effective") power that is feeded into the driver at a given moment. That amount depends heavily on the waveform (hence the notion of "music power").
    Now whether or not you believe and understand this is one thing, but if you use the term "old wives tales" once again in relation to any sound (pun intended) theory a co-member (including myself) of this forum is stating here, I will act according to the rules of this forum.
    This is the second time you say that in a direct reply and I won't have it!
    And do not construe this warning as a limitation we impose on your own venting of theories. It's not: it's about the form and tone.
     
  9. TomL

    TomL Extra

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    Gosh - I didn't mean to start a fight with this question! But maybe what I have learned from all of your replies is that I need to be a bit more carefull with the volume and the source (now I've re-encoded everthing using EAC and lame). Perhaps my new Norh 4.0's are not as robust as my old Infinity SM120's (but they sure sound a LOT better!). My new SR7200 should also help - probably a better amp than the old JVC. I suppose it's possible to be sending some harmful signals to the speakers well before I actually hear distortion or clipping! These speakers held up well to a friend's demo, using his Adcom 200W amp and good source material at levels much higher than I have attempted.
     
  10. John Royster

    John Royster Screenwriter

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    Tom,

    that's the best lesson you can learn. if it sounds bad or distorted/compressed then turn it down. I push my gear very hard all the time but when I do I'm always careful and if I hear a touch of distortion I turn it down a notch.

    I've had blown drivers and melted crossovers before but that was caused by a "friend" jamming van halen at 2:00 on the volume dial. His only excuse? "well it sure was loud":b
     
  11. Charles Gurganus

    Charles Gurganus Supporting Actor

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    Certainly there is merit to both underpowered AND overpowered amps(in relation to the speakers) destroying speakers. My guess is that more speakers are blown by driving underpowered amps to the max. There are probably far more speakers being underpowered than overpowered, IMO, and folks with MORE than ample power (like me, I suppose) are more likely to be aware of the danger of overdriving speakers with insane amounts of power.
     
  12. Richard Greene

    Richard Greene Stunt Coordinator

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    To Cees:

    I apologize for using the term "old wive's tale".

    I was not attempting to insult anyone and I hope no one else was insulted. These were only three words in a long post.

    I hope that disagreements are allowed at this website because debates lead to learning. There has never been complete agreement on audio topics.

    I will test the ability of the "management" of this website to tolerate polite disagreements ... by disagreeing with something you wrote: "You're absolutely wrong. Voice coils are not destroyed by the potential power of the amplifier, but by the actual ("effective") power that is feeded into the driver at a given moment. That amount depends heavily on the waveform (hence the notion of "music power")."

    RG replies:

    I did not say voice coils were destroyed by the potential power of the amplifier and don't even understand what you mean by that. I was discussing whether a low power amplifier was potentially more dangerous to speakers

    than a high power amplifier. Most people believe a low power amplifier is potentially more dangerous.

    I disagreed, explained my points as clearly as I could,

    and posted a link to a technical article at the Rane.com website. that supported my points. It does not appear to me that anyone has read that article or cares to refute it. So be it.

    Voice coils are damaged by excessive power.

    The obvious symptom of excessive power is REALLY LOUD MUSIC.

    The blame is almost entirely on the person who last

    touched the volume control -- not on the amplifier.

    Clipping harmonics are a minor cause of voice coil damage

    and this is explained well in the Rane.com article.

    High-power amplifiers are potentially more dangerous

    to voice coils than low-power amplifiers.

    I'm sorry if you don't agree with what I've written

    or what was published in the Rane.com article.

    Your disagreement does not mean my post was incorrect.
     
  13. Ron Shaw

    Ron Shaw Stunt Coordinator

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    Richard,

    You must understand power ratings for tweeters. When a manufacturer states 20 watts, 40 watts, or whatever, what is really being said is that the driver will handle it share of the power (meaning within its lowest rated crossover point) from an amplifier putting out xx watts, unclipped. This does not at all mean the driver will handle 20, or 40 or whatever watts. In a post a few months back, I quoted the spec 'sine wave power' of a driver, and was promptly chastised for it, since 'no one actually plays sine waves'. Some manufacturers used to quote driver power ratings in 'continuous sine wave' power, meaning just that. Put a sine wave into them, and see what they can handle. It was typical in those days to see power ratings for woofers in the 20-30 watt range, and for tweeters, about 3-5 watts. This is still the case today. That 40 watt tweeter you see advertised will handle maybe 5 watts real power, if you are lucky. 40 watts into 8 ohms is almost 2 ¼ amps. To get a delicate voice coil to handle that on a continuous basis isn't going to happen. I think we should go back to sine wave ratings, as this is the only way to get a real rating. Most woofers wont even handle a true 40wRMS for long term.

    The reason people mention square waves, is because a clipped waveform looks very much like a square wave when you see it on a scope. The top is flat. The spectrum of a square wave contains many frequencies, and always includes high frequencies. Clip an amplifier, and I will guarantee you will have more than a couple of watts in the high frequency portion of the spectrum. Clipped amplifiers do indeed damage a lot of loudspeakers. Bact to Toms problem, I would be surprised to find a clipped amp damaged a midrange driver before it damaged a tweeter (unless the tweeter is a piezoelectric device, or has some sort of protection circuit). Did you check your crossover unit? It may be possible that the series capacitor in the midrange portion shorted, sending low frequencies to the midrange. If that's the case, then its back to the manufacturer. Measure the cap and see if its shorted. If it isn't, on your ohm meter, you will see resistance first drop to a low value, then rise higher and higher. If its shorted, it should drop to a low value and stay there. It condo also be that the manufacturer crossed the mid over too low, or at a slower slope than it should have been. This will be difficult to determine unless you know who made the midrange driver and can get specs on it.

    Good luck…
     
  14. Cees Alons

    Cees Alons Moderator
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    Richard,
    No, problem, actually. I agree with most of your reasoning, just not all of it.
    Yes, voice-coils are destroyed by power. Yes, setting it waaay to loud is a potentional risk. Yes, the person who did set it too loud, actually "did it in".
    But here's where I disagree: when you say clipping cannot be a cause at all. And when you deny that a underpowered amplifier can be a risk.
    This is how it goes: A powerful amplifier needs to have a large amplification factor. That's because in a given speaker (with a given impedance) more power can only be achieved by a higher voltage across that driver's voice coil. Now, if the amp is big enough for a certain room and environment (number of people, etc.), the person at the volume control will set it according to what is needed - and the amp can deliver that without distortion. The actual heat in the voice-coil is determined by power x time, and this is not very high for normal music (because most of the music consists of sinus type waves and spikes). Especially the midrange and higher range speaker will be spared.
    Of course we have to assume that the speaker is up to it and can promptly deliver the required sound.
    Now a smaller amp (one that's too small for the circumstances). Smaller amps have a smaller amplification factor (see above), so the person at the volume control tries to crank up the sound, but the amp actually fails. It's not loud enough to his ears, and he cranks up the amp further - but it won't go louder and even starts to bottom out: clipping.
    If you hear that, you quickly turn it down again, saying "sorry not enough music today" to the crowd, but someone who doesn't notice this, leaves it at that position. The maximum voltage that is sent to the driver isn't enough, but now the time factor kicks in: the wave forms are much more "square" as a result of the clipping and the time x power factor is HIGHER than in the previous example, although the sound isn't necessarily much louder to the ear! More heat is dissipated in the voice-coil, and that may be allright if the driver can still handle it. But then again, it may not.
    Note that a 2 times higher max. voltage and max. current aren't as easily to achieve as a two times higher "filling" of the wave.
    So this is why some people say: a smaller amp may be more dangerous under certain circumstances. A higher powered amp may have a bigger potential to deliver power, but mostly, the setting is determined by someone's ears, setting it to "bearable". A smaller amp may be driven into dissipating more heat in the voice-coil than a larger amp would have been, while the volume sounds less!
    Of course, you're totally right when you say that a very smallish amplifier won't probably blow the speaker, but beware: a purely square wave can generate lots of heat quickly!
    So, what we're saying is: you're not wrong, but neither are we.
    Cees
     
  15. Richard Greene

    Richard Greene Stunt Coordinator

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    Richard,

    No, problem, actually. I agree with most of your reasoning, just not all of it. Yes, voice-coils are destroyed by power. Yes, setting it waaay to loud is a potentional risk. Yes, the person who did set it too loud, actually "did it in". But here's where I disagree: when you say clipping cannot be a cause at all. And when you deny that a underpowered amplifier can be a risk.

    ************************************************** ****

    I don't expect you to agre with everything I say

    -- I don't even agree with everything I say!

    I'm going to have to be more careful in future posts because I must not be communicating my thoughts well.

    I didn't say clipping is NOT a cause of driver failures.

    I said clipping was a minor cause of driver failures and explained that the typical clipping of bass transients does generate clipping harmonics ... and a watt or two of those harmonics will reach the tweeter through its high-pass filter (the Rane Corp. article provides further details).

    One or two more watts from clipping harmonics could be just enough to burn up a tweeter = a clipping low power amplifier is potentially slightly more dangerous to tweeters than a non-clipping high power amplifier.

    On the other hand woofers are more at risk with high power amplifiers. And if the amplifier is very low power (rare these days) it may pose no danger to any drivers, whether clipping or not.

    So in summary I was trying to say that almost all

    amplifiers are potentially dangerous to drivers,

    whether low powered or high powered, if the owner

    turns up the volume too high. I think low-power amplifiers get too much blame for tweeter damage when the blame belongs mainly on the audiophile who turned up the volume so high!

    YOU WROTE:

    Now a smaller amp (one that's too small for the circumstances). Smaller amps have a smaller amplification factor (see above), so the person at the volume control tries to crank up the sound, but the amp actually fails. It's not loud enough to his ears, and he cranks up the amp further - but it won't go louder and even starts to bottom out: clipping. If you hear that, you quickly turn it down again, saying "sorry not enough music today" to the crowd

    **** RG replies:

    Since clipping off the bass transient peaks changes the character of the music (harsh transients, compression and a relatively brighter sound as the volume control is increased but the bass can't get any louder) I'd expect that sound quality change would serve as a warning -- I would notice it and turn down the volume. Maybe most people would turn UP the volume?

    With a high power amplifier a voice coil can be destroyed with little or no early warning from a deterioration of the sound quality.

    I believe we agree about most of this subject

    and you'll notice I did not use the phrase:

    "old wife's tale" (or is it: "old wife's tail"?
     
  16. Cees Alons

    Cees Alons Moderator
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    Richard,
    Agreed! Yes, if "old wives tale" appears to be a compliment... (in the vein of "grandma's soup"), then we didn't have to quarrel in the first place [​IMG].
    Cees
     
  17. Martin G

    Martin G Second Unit

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    I should mention that on a fairly recent concert I played a piece that required the 'destruction' of two speakers (there was a background track that was supposed to be played at a volume loud enough that you could audibly hear the sound start to break up and finally fail). At first we tried using a powerfull (350 watt I believe) amp. It was loud and annoying, but the speakers never did die on us. eventually we tried and old pioneer stereo reciever that had 30 watts a channel. the sound was incredibly distorted after turning it up all the way. In fact we had to turn the reciever down a little so that the speakers would die slow enough.

    BTW we used KLH 9912's as the speaker. (no point in spending more then we had to)
     

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