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| Date: |
August 09, 2003 at 22:19:05 |
| From: |
Adrian Mack, [dialup-196-97.wasp.net.au] |
| Subject: |
Small vs Large powered amplifiers |
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URL: My website, Peerless XLS 12 Dual PR Sub Project, 18LW1400 vented sub, Horns, etc..... |
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Hi Richard!
I'm beginning to doubt that the clipped component is DC too, I think from now on my take will be that the clipped portion is not DC.
But the statement "You won't protect your speakers by using a larger amplifier" is not entirely true. I'm going to aim this post at the high frequency components - the tweeters, and not the sub-drivers as we've already discussed, they are much, less prone to damage than the rest of the components are in a multi-way system from clipping problems. Except maybe a 50 cent radioshack woofer :P [grin].
First off, we have the issue of energy distribution in the program material. Say we have a 50W speaker driven by a 50W amplifier, large 20db peaks can be damaging thermally and also by driver excursion-limitations depending on the nature of the program material. But I guess with music, this is a bit of a non-issue as the energy distribution in the treble region is typically 10 to 20db less than bass and midrange freqeuncies. So that means in our multi-way system, the tweeter is allowed to have a much lower power handling than the woofer component must handle.
But back to the subject of real clipping; the power output of an amplifier is not set in rock. A full turn of the volume control isn't a true indicator of its output capabilities. Manufactures spec their amplifiers to a certain THD level - and this may even be at a half turn of the volume knob, where power output may be 100W at 0.5% THD. In this case, THD could be overdriven to produce 200W of power output to the speakers and so on. When this happens, the distorted output could very well be in the treble region which can kill the tweeters.
This content is rich in harmonics (distortion). These are one of the main killers of tweeters, as these harmonics are damaging. They are of higher frequency multiples of the original signal, and therefore the high frequency component of a loudspeaker system must bear the burnt distortion - even if the original signal may have been generated by a bass guitar.
As an example, above is a picture of a pure sine wave signal with no overtones or distorted harmonics displayed on an oscilloscope. It can be reproduced safely by the amplifier, as visible the top and bottom are nicely rounded contours. The average power output is one-half the peak power output. But when the amplifier is overdriven, the contours are "clipped off" and results in a near-square wave, which means it has flat areas at the top and bottom limits.
Above shows the top and bottom clipped off and the resulting square wave signal. When this happens, average power approaches the peak power, and this will cause the amplifier to deliver up to twice its rated output to the high freqeuncy driver, which may not be able to handle the abnormal load. To quote JBL: "A higher powered amplifier however, can generate the required power levels without clipping". I think the quote is relevant, because the "main meat" of this post was material sourced from the JBL tech notes. This will allow the loudspeaker system to recieve program material containing a normal distribution of energy levels. Under these conditions with the higher-powered amplifier, it is unlikely damage to the high frequency driver will occur than with the smaller amplifier and its clipped signal.
So theres a few things we can do in this situation. First off, just dont drive the amplifier into clipping. That means, keeping the volume knob down to safe levels. Second, is to purchase an amplifier capable of producing more power than you will need. Remember that a loudspeaker can require additionally up to 10 times the average power level for transients. This can mean, getting an amplifier with large reserve power which will reproduce these transients clearly. When an amplifier runs out of undistorted power, it is forced to exceed its design capabilities, producing dangerous power levels rich in high frequency distortion, which is a major killer of tweeter components.
But of course one thing runs sort of counterproductive - to quote my last post, "you can damage the speakers from too much power that they can safely tolerate, unless your fingers wont ever be tempted to nudge that volume control any further". Thats the only reason the larger amp will kill a high frequency component. But if we got down to measuring a low-powered amplifier outputing a clipped signal, and larger-powered amplifier outputing a clean signal, and then measuring each ones power output so that each one is sending the same voltage signal to the tweeter that it can safely tolerate - the larger amplifier is less likely to damage the component.
Adrian
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