Does a clipping amp damage speakers? |
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infrasound
Old Croc Joined: 13 May 2011 Location: Brizzle Status: Offline Points: 2276 |
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It's still a DC component within an AC waveform. Think about the time domain. |
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audiomik
Old Croc Joined: 06 April 2010 Location: Bath, UK Status: Offline Points: 2962 |
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Re: "There is not DC present in a clipped signal,"
unfortunately not necessarily true as there will be a DC offset created by an asymmetric signal driven into clip within an Amplifier, even after High Pass filtering. No DC component only occurs for a perfectly symmetrical signal but with Music where most signals are asymmetric then......... If you understand the Fourier Transform Maths then this offset is relatively easy to calculate as a Value..... The time related duration of the Lower Frequency components of a complex waveform, as in Music, and ultimately DC for the full series, is a function of the Current or Voltage offset created by the positive and negative half cycles of the waveform not being equal. (over simplified) This Link to Fourier Transform - Wikipedia will be informative. Another method of calculation is to integrate the waveform half cycles above and below the zero level and sum them..... This is what the 'Speaker coil does with the Amplifier Output signal fed to it so if the Areas under each half cycle of the waveform aren't equal then the coil doesn't 'return to zero'. Plenty of pages from a Search for "Asymmetric signal" will also provide information. My apology if this is technically somewhat beyond the Forum this Thread is located in, but it's a well known issue in Engineering circles Mik |
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Warning! May contain Nuts
plus springs, washers, screws, etc, etc. |
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Liteworks
Registered User Joined: 29 April 2008 Location: Cambs / Luton Status: Offline Points: 383 |
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Here's QSC's take on it: http://forum.qscaudio.com/forum/viewtopic.php?f=29&t=2736&sid=180d50fa225a6596013908b3a0e8f76b Similar to BFM's explanation, from a imo more respected source. -matt |
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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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"Is that the general jist of it?"
You've got it! "It's still a DC component within an AC waveform. Think about the time domain." I've heard less intelligent remarks, I just can't remember when or where at the moment. Here's QSC's take on it: Finally, something sensible. |
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djk
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Grubbah
Old Croc Joined: 14 July 2011 Location: Portsmouth Status: Offline Points: 2014 |
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Newbie discussion huh ?
I think this thread needs to be moved!
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GEB
Old Croc Joined: 13 November 2009 Location: East Midlands Status: Offline Points: 1993 |
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It'd be useful I think for people starting out to know the facts from the start, instead of being fed utter rubbish from some and spouting it out as fact for the rest of time for more people to be mislead. Some of the more long standing members should know better really! What else gets stated as fact to the unsuspecting!? Sticky maybe?
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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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RaneNote 128 © 1991 Rane Corporation
INTRODUCTION Power amplifier clipping is quite common. This note examines the clipping phenomenon which allegedly damages loudspeakers. We suggest that this form of distortion is not the cause. Rather, we show that amplitude compression of the audio spectrum is the culprit. Rane limiters provide a solution to amplitude compression, thus preventing loudspeaker failure. Monty Ross Rane Corporation Power Amplifier Clipping and its Effects on Loudspeaker Reliability • Harmonic Theory • Power Ratings • Blown Tweeters • Clipping and its Indicators • Limiter Protection RaneNote POWER AMPLIFIER CLIPPING AND ITS EFFECTS ON LOUDSPEAKER RELIABILITY WHY DO LOUDSPEAKERS NEED PROTECTING? All loudspeaker drivers have power handling limits. Once exceeded, damage occurs. There are several ways a loudspeaker suffers power damage. A couple of these warrant explanation. The first is over-excursion of the diaphragm. The diaphragm of a loudspeaker is the radiating surface that moves in response to an electrical signal. This surface may be conical, domed or flat in shape, and it creates sound by physically pushing and pulling the air in the room. The laws of physics say that in order to play louder or to reproduce lower frequencies, the diaphragm must move further toward its mechanical limits. If it is asked to move still farther, it experiences over-excursion. This most often occurs in woofers but can affect midranges or tweeters if low frequencies are not limited. If the loudspeaker cannot handle over-excursion, mechanical destruction of the driver is likely the result. Another enemy of loudspeakers is heat, generated bypower losses in the voice coils. No device is 100% efficient. For loudspeakers, 1 watt of input power does not produce 1 watt of acoustic energy. In fact most loudspeakers are typically well under 10% efficient [1] [3]. These losses convert to heat that builds up in the voice coils, causing mechanical deformation, like melting of the voice coil former. It causes weakening of the structure by charring the voice coil former, which later shakes apart. The heat causes the glues to bubble up, fill the air gap and glue the voice coil solidly in the gap. Often the voice coil wire melts like a fuse link, resulting in an open driver. Obviously we wish to prevent this.Music power-handling capability for multi-way loud-speakers always presents a problem to the loudspeaker user and designer. Users who must replace blown tweeters often feel they didn’t do anything wrong, because their amplifier only put out 50 watts and their speaker had a 200 watt rating.Yet, the tweeter blew up. This recurring problem motivated engineers to find out why this happens. Many opinions developed. Some of these have been scientifically verified— others remain theory. CONFLICTING “FACTS” Studies show the typical spectral energy for different types of music have high frequency energy considerably lower in level than low frequency energy [2]. This knowledge has further complicated the studies of how tweeters get destroyed. It seems that woofers should blow rather than tweeters if the high frequencies are lower in amplitude.Loudspeaker manufacturers use this knowledge about theenergy distribution of music when they design their products. This knowledge allows them to make better sounding tweeters because they can use lighter moving structures. Smaller wire in the voice coils can be used because there is significantly less power in the high frequency ranges. Since smaller wire is lighter, it takes less energy to move. For a speaker system rated to handle a given number of watts, the tweeter by itself can probably handle less than one-tenth that amount. From all this came a theory that spread quickly through the industry. Since there is more musical energy at low frequencies than high frequencies, there is not enough high frequency power to blow out tweeters. Therefore, high frequencies loud enough to burn out tweeters must come from somewhere else. Where do they come from? Well, it was reasoned, if there is enough low frequency energy to clip the amplifier, then it perhaps would produce enough high frequency distortion products (as a result of clipping) to blow up the tweeter. This theory convinced many in the early 70’s and slowly evolved into “fact”. While doing research into the reliability and protection of power amplifiers, I had to study how the)rewoPsuoenatnatsnI(sttaW001=Bd0,evaWerauqSzH001afosedutilpmAcinomraH.1elbaTcinomraHedutilpmABdnisttaWycneuqerF110001zH00120-80zH00233/145.9-21.11zH00340-80zH00455/189.31-4zH00560-80zH00677/19.61-40.2zH00780-80zH00899/11.91-32.1zH009010-80zHk11111/18.02-38.0zHk1.1210-80zHk2.13131/13.22-985.0zHk3.1:::::Amp Clipping-3typical consumer used amplifiers and speakers. I found that clipping is a common occurrence and is not as audible as most people think. I also found that the operation of many clipping indicators is very slow and does not always show actual clipping. (Many manufacturers slow them down, using their own rule of thumb for how much clipping can occur until it lights the indicator.) Newer and better sounding amplifiers, including amplifiers with soft clipping circuits, still blew tweeters. But amplifiers with higher power were having fewer incidences of blown tweeters. This appeared to reinforce the theory that clipping caused tweeter blowouts. One thing was clear, when clipping occurred, tweeters blew. If you’re getting the idea I don’t believe in the clipping/harmonic theory, you’re right. So let’s investigate the phenomena further. WHEN SINE WAVES CLIP When sine waves clip severely they resemble squarewaves in shape, introducing massive distortion. In the extreme case, a perfect square wave has the highest level of harmonic components (See Figure 1). A less clipped sine wave has components at the same frequencies but at lower levels. Let’s look at the square wave example shown in Table 1(at left). Fourier analysis shows the harmonic structure.As you can see, the total amount of instananeous powerleft to make it through an ideal 1kHz crossover (and on to the tweeter) is less than two watts (0.83 + 0.589 = 1.419W). Hardly a problem. And remember, this simulates severe overdrive of a 100 watt amplifier with a sine wave to make an ideal square wave. Driving it harder will not increase the harmonics.This analysis shows if a small tweeter that only handles 5or 10 watts is used in a 100 watt speaker system it would not blow out, even under square wave conditions. Yet it does. It takes a lot more than this to cause major failure. So what’s happening? Compression is what’s happening [3]. Today’s newer higher quality amplifiers have greaterdynamic range and sound better when clipped with musical transients than older amplifier designs. So it is more likely for a user to overdrive and clip newer amplifiers on low frequency dynamic peaks because of lower audible distortion. This results in compression of the dynamics of the music. The Figure 1. Harmonic Comnponents of a 100 Hz Squarewave vs. a 100 Hz Sinewave. Figure 2. Low Level, High Frequency Sinewave Mixed with a High Level, Low Frequency Sinewave Burst. Figure 3. 100 W Amp with 3 dB Overdrive Figure 4. 100 W Amp with 10 dB Overdrive Amp Clipping-403813 8-97high frequencies get louder but the low frequencies can’t. This may be heard as an increase in brightness of the sound. Some may simply interpret it as louder with no change in tonal balance. For example, in a 100 watt amplifier, as you turn up the level, the low frequency components will limit (clip) at 100 watts. Meanwhile the high frequency components continue to increase until they (the high frequencies) approach the 100 watt clipping point. The graphs in Figures 2, 3 & 4 are scaled in volts. With an8 ohm load the 100 watt level corresponds to 40 volts peak. Below clipping, the low frequencies reach 100 watts (40 volts peak) but the high frequencies are only 5 or 10 watts (9 to 13 volts peak).Let’s assume a musical signal with low and high frequency components driving a 100 watt (8 ohms) amplifier. We use a low level/high frequency sinewave mixed with a high level/low frequency sinewave burst. (See Figure 2). The high frequencies reproduced by the tweeter are at least 10 dB lower in level than the low frequencies. Now as we turn up the amplifier to clip the signal (3 dB overdrive—See Figure 3). Notice that only the low frequency burst portion of the waveform clips but the high frequency portion increases in level. The clipping, of course, produces harmonics but not nearly as much as the square waves discussed earlier. The amplitude of the high frequencies went up by 3 dB in relation to the low frequency fundamental. (3 dB compression). If you overdrive the amplifier by 10 dB, the high frequency amplitude goes up by 10 dB. This goes on dB for dB as you turn up the volume, until the high frequency reaches the 100 watt level. Meanwhile the peak level of the low frequency portion can not increase above 100 watts (See Figure 4). This now represents nearly 100% compression (no difference between HF amplitude and LF amplitude).Now it is easy to see how the high frequency portion exceeds the 5 or 10 watts tweeter rating. Sure, clipping is producing extra harmonics but they never approach the levels of the amplified high frequency source signals. It may be argued that the signal’s distortion would be intolerable. Don’t fool yourself. It really surprises people how much clipping they tolerate before they cannot listen anymore. Just disconnect the clipping indicator on a power amplifier and see how loud someone drives it. Watch the amplifier output with an oscilloscope. There will be a surprising level of clipping. 10 dB clipped off the top of low frequency transients is not an uncommon occurrence when the purpose is to impress your neighbors. ©Rane Corporation 10802 47th Ave. W., Mukilteo WA 98275-5098 USA TEL (425)355-6000 FAX (425)347-7757 WEB http://www.rane.com REFERENCES 1. K. Blair Benson, Audio Engineering Handbook,(McGraw-Hill, New York, 1988)2. R. A. Greiner and Jeff Eggars, “The Spectral AmplitudeDistribution of Selected Compact Discs,” Journal of the Audio Engineering Society, vol. 37, pp.346-275 (April, 1989)3. Carlo Zuccatti, “Thermal Parameters and Power Ratings ofLoudspeakers” Journal of the Audio Engineering Society, vol. 38, pp.34-39 (Jan-Feb, 1990) |
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djk
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SBC
Registered User Joined: 25 November 2010 Status: Offline Points: 1266 |
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F.T.S.S.
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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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To summarize, clipping does no harm unless:
a) the long-term average power of the driver is exceeded b) mechanical damage occurs |
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djk
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GEB
Old Croc Joined: 13 November 2009 Location: East Midlands Status: Offline Points: 1993 |
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Just a few points from QSC, although I'm sure some know better!
Clipping is bad for loudspeakers because it really heats up the voice coils. Myth. Clipping at any power level is capable of damaging your loudspeakers. Myth. Clipping is bad for loudspeakers because it is DC. Myth. Clipping is bad for loudspeakers because the sharp corners rip up speaker drivers. Myth. Clipping is bad for loudspeakers because the amp may put out more power than you expect. True. There you go folks!
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Earplug
Old Croc Joined: 03 January 2012 Location: Europe Status: Offline Points: 7199 |
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I really don´t think this one can be dismissed so easily. Clipping = greater average power = more heat = greater stress on the glues used in the driver = less life for the driver. Keeping the VC as cool as possible through proper venting and/or heatsinks isn´t done for nothing. |
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Earplugs Are For Wimps!
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duck1
Registered User Joined: 22 February 2008 Status: Offline Points: 173 |
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I would agree with the cause of failure being excessive average power.
At the end of the day, the voice coil is just a piece of wire which will burn out when it gets too hot. I think what Bill Fitz' was suggesting is that square waves, like the sort a synthesizer is capable of producing, are made up of every harmonic- this is why a square wave bass tone still has a buzz or sharpness to the sound. But, this is not the same as an amplifier output stage in clip which would be dumping DC from the rails into the speaker, and therefore subjecting the voice coil to much greater average power. |
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