Does a clipping amp damage speakers? |
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GEB
Old Croc Joined: 13 November 2009 Location: East Midlands Status: Offline Points: 1993 |
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Posted: 13 May 2012 at 8:17pm |
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As per the title really, just wanted the 'pro's' opinions really?
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AJordan
Young Croc Joined: 06 July 2007 Status: Offline Points: 1013 |
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Cant claim to be a pro but yes seems to be the general opinion. The occasional clip probably not but sustained clipping yes.
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Mircea Bartic
Old Croc Joined: 18 February 2005 Location: Romania Status: Offline Points: 2581 |
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every time
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general manager & head designer at nexus-acoustics research
http://www.facebook.com/nexus.acoustics.research Ex Nexus_3 |
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cyte
Young Croc Joined: 15 September 2008 Location: UK Status: Offline Points: 513 |
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Yes.
Clipping introduces a DC element to the signal (flat peaks and troughs). DC kills drivers because when a current is flowing through the voice coil, it relies on the movement of the coil and air around it to cool it. With DC, there is no movement, and therefore no cooling so the temperature just increases until the coil burns up and shorts out. |
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AJordan
Young Croc Joined: 06 July 2007 Status: Offline Points: 1013 |
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I dont think thats true. There is no dc. even thought the clipped peaks of the waveform might look like dc. I think its just the much greater energy content of a clipped signal that heats the coils.
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GEB
Old Croc Joined: 13 November 2009 Location: East Midlands Status: Offline Points: 1993 |
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So this from Bill Fitzmaurice is wrong then? Confused!
Amplifier clipping, and it's respective causes and effects, is perhaps one of the most misunderstood concepts amongst audio circles. There is a whirlwind of myths surrounding this topic that seems to exceed all other topics I have come across. Now is your chance to learn the truth about clipping. How Do Speakers Become Damaged There are only two ways that a speaker can be damaged, both of which occur from too much input power. 1. Mechanically 2. Thermally Every speaker has an excursion limit (often measured in mm), or how far the speaker can move forward or rearward before damage occurs. This is the mechanical limit of the speaker. This limit remains the same regardless of the use of the speaker, but the power required to reach this limit changes dependent on the enclosure. If you exceed this limit, the speaker will suffer mechanical damage, whether it's ripping your spider, bottoming out on the back plate, or any other mechanical damage. The second type, thermal damage, occurs when you exceed the thermal power handling limits of the voice coil itself. Voice coils are simple pieces of metal that will melt if too much power is applied. This limit is pretty much constant, ie. if a voice coil will be damaged at 1 kw, it will be damaged at 1kw regardless. There are two final myths to cover here. Despite the tireless efforts of some, there are still many people that believe that underpowering a speaker will damage it or that clipping will damage a speaker. Please remember that these last two thoughts are entirely UNTRUE! And now we will find out why. Where Does This Power Come From? Let's first understand the power potential of an amplifier when clipping. The power created is largely determined by the rail voltages. Let's compare two amps, each one connected to a 4 ohm speaker rated at 75 watts rms. Amp 1: 50 watt amp 50 Watt amp means this amp can cleanly deliver a sinewave of 50watts into a 4 ohm load. This means (Vrms)^2/4 = 50W Vrms = 14.14V Vpeak = Vrms*(1.414) Vpeak = 19.99V The rail voltages of this amp must be a bit higher, to prevent output stage distortion at this power level. In this case, the Rail voltage would have to be +/- 20 Volts. Amp 2: 75 watt amp (Vrms)^2/4 = 75W Vrms = 17.32V Vpeak = Vrms*(1.414) Vpeak = 24.49V In the example, the 75 watt amp is delivering 75 watts as it is not distorting at all. The 50 watt amp is in hard clipping, as and such, is delivering a fair bit more power. P = Vrms^2/R = (19.99V)^2/4 ohm = 100 watts. It is quite obvious that there is potential for an amplifier that is clipping to deliver substantially more power than you would expect. Keep in mind that this is only a way to determine peak voltage potential. Average Power Now we can get into how a speaker really gets hurt. The key issue is average power over time. Let's get to the nitty gritty. The first key is understanding Crest Factor. "Crest Factor" is the difference between the average level of the signal and its peak level. A pure sine wave has a "crest factor" of 3dB, meaning that it's peak level is 3dB higher than its average level. We all know that 3dB represents a difference in power by a factor of 2. Another way to look at it is that the peak power of the signal is twice that of its average level. If we were to play a sine wave on our 50 watt amplifier, just below its clipping level, the average power over time the speaker would need to dissipate is 25 watts. On the other hand, a square wave has a crest factor of 0dB. In other words, its average power and peak power levels are equal. Our same 50 watt amplifier playing a square wave into our speaker requires the speaker to dissipate 50 watts. Keep in mind that this refers to sine and square waves only. Music has a much higher crest factor. Most widely available recordings have a crest factor of approximately 10dB. Looking at this in terms of power, the peak power is 10 times greater than the average power. If we were to play one of these recordings with our 50 watt amplifier when not clipping, the speaker needs to dissipate a mere 5 watts of average power over time. When the amplifier begins clipping, the peak level/power does not increase. BUT, the average power DOES increase. If we were to turn the volume up 6dB higher than the clipping level of our recording, we have reduced our crest factor to 4dB. Guess what? We are now needing the speaker to dissipate 20watts. This is four times the average power and four times the heat when measured over time. As you can see here, it is not the distortion or the waveform or anything along those lines that is killing your speaker; there is simply more average power over time. However, if the average power of time is still below what your speaker can handle, it doesn't matter if it's clipping or not. At higher power levels, the fact that a clipped signal carries more average power over time can result in damage. DC in Clipping One of the most famous myths regarding clipping is that it produces DC. The assumption is made because of the flat tops and bottoms to a square wave. It's incorrect to think of a squarewave as made up of positive and negative dc components. The only way for a it to be DC would be if there was a non-zero average value over long periods of time. If the polarity changes at all within the time frame that you are looking at, it is simply not DC. What are these flat portions of the signal? It is simply a combination of the fundamental frequency and all of it's higher order harmonics in sine wave form. For example, if you were to play a 20hz tone while clipping, there would be the fundamental frequency (ie. 20hz) and the second (40hz), third (80hz), and 4th (160hz) order harmonics. The sum of these frequencies creates what appears as a squarewave. There are two ways to test this for yourself; one is quite easy, the other is a little more advanced. The first way is simple if you have a variable crossover and an oscilloscope handy. Pass a low frequency square wave. You will notice the square shape on the oscilloscope. Now turn your crossover's low pass filter on. Slowly lower the setting as you approach the fundamental frequency. You will notice the waveform on your oscilloscope slowly rounding off into a typical sinewave. Once you have reached the fundamental frequency, your oscilloscope will show a perfect sinewave. The second way is for your math guys (or for those who like to use Matlab). If you look in the frequency domain using a Fast Fourier Transform, you will see the fundamental frequency and its higher order harmonics only. There will be absolutely no DC present. Clipping and the still voice coil The final myth is that of the still voice coil. It is perhaps the most believed myth regarding clipping. The idea is that because of the square wave, the coil is not moving during the flat portions of the signal. This is simply not true for a variety of reasons. The speaker does exhibit mechanical damping and remains in constant motion. Assuming the same voltage and excursion xmax, the cooling at any given frequency will remain the same, whether the signal is clipped or unclipped. Here is a great article to read on the cooling effects and how a driver fails under various waveforms: http://forum.carstereos.org/f-car-audio ... ults-47441 Summary To provide a final review of all that we have discussed on this topic, there are only two ways to damage a speaker: Mechanically and Thermally. The only way to do this is by applying too much input power in a given enclosure (mechanically) or too much average power over time (thermally). There is no DC in a clipped signal; the coil does not stand still; air passing over the coil (and thus cooling) is the same regardless of the waveform; and clipping is acceptable provided that the average power over time is lower than the speaker's limits. The next time you hear those famed words "your speakers died because of clipping", remember what you have learned, and above all, keep searching for the truth. It's out there somewhere. Edited by GEB - 13 May 2012 at 8:46pm |
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Conanski
Old Croc Joined: 26 January 2006 Location: Ottawa, Canada Status: Offline Points: 2515 |
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The answer is yes but like everything audio related the answer comes with qualifiers.
Delicate high frequency drivers are easier to blow with amp clipping than big hairy woofers but it also comes down to the total power developed when an amp is driven into clipping.. it's got nothing to do with the distortion that is developed. If distortion killled speakers then every guitar amp ever made would instantly vaporise the second a distortion peddle was plugged in, but of course that does not happen. SO.. it's overpowering that melts voice coils and it's pretty easy to double an amps rated power output with heavy clipping so the bigger the amp the more careful you have to be. It also depends how close the two devices are in rated power, so for example a 50w amp could easily blow a 100w CD but connected to a 250w woofer the amp would die before the driver is harmed.
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Conanski
Old Croc Joined: 26 January 2006 Location: Ottawa, Canada Status: Offline Points: 2515 |
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Blaize110
Young Croc Joined: 11 May 2010 Location: Gloucestershire Status: Offline Points: 673 |
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The text you copied out is basically how I understand the situation.
Yes clipping is not the direct cause of speakers breaking, and you can drive a speaker with a completely clipping amp all day, as long as the speaker is rated high enough to cope with the power. But quite often, if the amp and speaker are of similar power ratings, then when the amp does start clipping, the inherent increase in power that comes with clipping, causes the speaker to melt from thermal overload. So in this situation, you could probably justify using the phrase. "Clipping broke the speaker". Ok, it was actually too much power causing a thermal overload, but that was caused by the clipping.
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infrasound
Old Croc Joined: 13 May 2011 Location: Brizzle Status: Offline Points: 2276 |
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Yes, it is DC.
It does not "freeze" the woofer cone due to the restorative forces of the suspension, but it does attempt to hold it there. I haven't got a bloody clue what this Bill is on about with his harmonics. The fact is that when you overload a transistor it holds up against the voltage rails until the input falls within the voltage limits.
Edited by infrasound - 13 May 2012 at 9:06pm |
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Conanski
Old Croc Joined: 26 January 2006 Location: Ottawa, Canada Status: Offline Points: 2515 |
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Edited by Conanski - 13 May 2012 at 9:15pm |
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cyte
Young Croc Joined: 15 September 2008 Location: UK Status: Offline Points: 513 |
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Play a low frequency square wave through a driver with no crossover/filter and the driver will stop moving during the peaks and troughs.
This is also true. Regardless of the mechanism (increased RMS power in a square wave or DC) the fact is clipping amplifiers DOES damage speakers. As usual Bill Fitzmaurice is pretending he knows more than professional designers in order sell more of his plans... And let's face it, using a car audio forum as a reference in an electronics discussion is not really a good look Edited by cyte - 13 May 2012 at 9:30pm |
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