DC Clip, Underated amplifier, Watts as a variable? - diyAudio
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Old 8th October 2007, 04:39 PM   #1
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Default DC Clip, Underated amplifier, Watts as a variable?

Please bare with me (sorry)...

If Watts are a measurement of energy then surely Watts are considered a variable amount.

If I have a subwoofer rated at 1000W but only take it to 50% capacity, then the subwoofer will only be emitting 500W of dissipated energy.

The reason for this question is that I've been told about DC clipping in such a way that if I try to run an amplifier in any way smaller than the rated wattage, I can expect to do long term irreparable damage to the speakers and run the risk of seeing the cones tear off and land on the floor.

I can understand that it requires more energy to create wavelengths of several metres as opposed to shorter ones which explains why subwoofers and bass amplifiers are often rated at much higher wattages.

What I don't understand is why I must have so much head room in available power to the speaker if I am not going to use it?

Can I run an amplifier rated less than the speaker is rated without fear of damaging the speaker?

Thanks in advance
~groover
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Old 8th October 2007, 05:32 PM   #2
ppia600 is offline ppia600  United States
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You can run an amplifier that produces less than the rated RMS power of the woofer, but if you push the amp into clipping there may be problems. If the amp can produced a clipped signal at more than the rated woofer power the woofer has a good chance of being damaged. The woofer will operate at lower power levels as long as the power input is a clean ac wave. For example::

Woofer 1 has a rating of 50 to 500 watts rms - so you should use an amp that can produce at least 50 watts rms clean and up or more than 500 watts rms clean (you can always adjust amp gain levels or deck eq to avoid going over the maximum rating of the woofer)

Using an amp that only puts out 150 watts rms for example, will sound very nice until you start pushing the amp way past the 150 watts it is capable of. The amp might be able to produce 450 watts of clipped signal which will be dangerous for the woofer even though the woofer may be rated at 500 watts rms.
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Old 8th October 2007, 05:47 PM   #3
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Is there a way I can know how many watts I am sending? How is it that an amplifier can produce more watts that it is rated for?

I understand the effect of clipping and the changes to the waveform but I don't understand how a 2L jug can hold 3L of water.
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Old 8th October 2007, 05:55 PM   #4
ppia600 is offline ppia600  United States
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An amp can produce more distorted power than rated clean power. Usually manufacturers list their rms power at a low (non audible) distortion level.
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Old 8th October 2007, 06:03 PM   #5
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If Watts are a measurement of energy then surely Watts are considered a variable amount.

**** Watts are the units for 'power', not energy. Energy has a 'time' component (i.e. watt-seconds).



If I have a subwoofer rated at 1000W but only take it to 50% capacity, then the subwoofer will only be emitting 500W of dissipated energy.

**** The voice coil will be dissipating nearly all of the power. The speaker will convert a tiny fraction of that into audio.



The reason for this question is that I've been told about DC clipping in such a way that if I try to run an amplifier in any way smaller than the rated wattage, I can expect to do long term irreparable damage to the speakers and run the risk of seeing the cones tear off and land on the floor.

**** Get your information from a better source. The power handling of a speaker has a lot of variables. You should also realize that many of the power ratings are totally useless. If you drive a badly clipped signal into a woofer, the woofer will not instantly fail. If the clipped signal produces significantly less than the power handling of the speaker, the clipped signal will do no harm. If you drive a clean signal into the speaker that's higher than the power rating, there will not necessarily be any damage to the speaker. The length time that the speaker is being abused will also determine if the speaker survives.


What I don't understand is why I must have so much head room in available power to the speaker if I am not going to use it?

**** You don't have to have much headroom. If you don't plan on using 1000 watts of power on a speaker, you should probably buy a speaker rated for less power. Many times, a speaker that's rated for more power handling is less efficient. You should select a woofer that's best suited to your needs.



Can I run an amplifier rated less than the speaker is rated without fear of damaging the speaker?

**** Yes. If you're abusive (with too little or too much power), you can damage the woofer. If you're not abusive, you won't damage the speaker. If you have a woofer that's truly capable of handling 1000 watts, you would never damage it with a 200 watt amp no matter how badly distorted the signal is. With a 500 watt amp, you may be able to damage it if you drove the amp into hard clipping for an extended period of time.
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Old 8th October 2007, 06:07 PM   #6
AndrewT is online now AndrewT  Scotland
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Hi,
an amplifier sending out maximum sinewave voltage into the load will not produce any more voltage than it's rated power (unless the manufacturer is playing safe to avoid litigation).

If you increase the input signal so that the signal clips the output voltage cannot rise any higher.

Let's take an example.
Output power of 500W into 2r0 is equivalent to 31.6Vac and 15.8Aac into that 2r0 load.
The peak voltage is 44.7Vpk and current is 22.4Apk.
If the absolute maximum output is increased to a square wave rather than a sinewave the peak voltage cannot be exceeded and the resulting peak squarewave power is double the sinewave power so this 500W amp will output 1000W into 2r0 when driven to square wave output.
The amp should run a little cooler when doing this. The output devices are alternately switched off or fully saturated. But the PSU will seriously overheat trying to supply the continuous output current alternately to the two supply rails.

If the amp were to send a transient signal to the 2r0 load then it can develop slightly more peak voltage than the continuous power quoted earlier. Similarly if the load impedance is raised then again the amp can deliver a higher maximum peak voltage both before clipping and in the fully clipped mode. Non regulated PSU are particularly good at this dynamic headroom trick, but only if the PSU is underspecified. I reckon that a goodness factor for unregulated PSU and power amplifier combination is how low the dynamic overhead is. Whereas manufacturers actually turn this around and quote how high their product's dynamic overhead is.

BTW, most speakers are very inefficient at converting electrical input to sound output.
Your 500W amp and 500W speaker may give out as little as 2W to 10W of acoustic power, the remaining 490 to 498W is lost as heat in the cables and voice coil.
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Old 8th October 2007, 07:12 PM   #7
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WOW thanks for the detailed information. Quite a bit to absorb but makes a lot more logical sense to me. Thanks.

Perry, excellent site for basic car audio electronics.

As for the subwoofer, I'm a graphic artist and do product photography. I designed a heap of packaging for Option Audio and took the product shots in my custom made acrylic lightbox. The short of it is I got to keep the 15" as a bonus for doing a good job!

I would've been better off without it as it has wasted countless hours of diversions from the work I should be doing.

I attached a shot of the top of one of the boxes. I didn't do their site but I did do the photos, bling, logo and styling that their site is based on.
Attached Images
File Type: jpg pro sub top.jpg (87.1 KB, 52 views)
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Old 8th October 2007, 08:11 PM   #8
ppia600 is offline ppia600  United States
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All the expanations of how the amps magically stop producing more wattage after their rated RMS power output somehow manage to skip over the fact that in most instances of woofer failure, the woofer will be destroyed by the DC power which it is NOT rated to handle.
-ALL woofers MUST be moving to cool themselves and handle their rated power, they will NOT move sufficiently to do this with straight clipped DC VOLTAGE. THAT is why the majority of woofers fail in poorly designed systems. Because the owner skimped on the amp and used one that provided too little AC voltage and too much clipped DC. The woofer will not conduct the heat away from the voice coil when recieving highly distorted input and the coil will basically overheat and seperate from the bobbin.
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