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Topic: WATT theory? (Read 17976 times) previous topic - next topic
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WATT theory?

Reply #25 –
This is why you can't beat premade systems.  You don't have to worry about what speakers to by with what amp.  Makes life easier

WATT theory?

Reply #26 –
Quote from: DigitalMan on
Quote from: Gecko on

I don't really buy into this "your amp needs to be much more powerfull than your speakers". Just as speakers can take transients with much more power than their RMS rating, an amplifier can deliver transients much stronger than its rated RMS power without much distortion. There's nothing wrong with some headroom to steer clear of clipping but you don't have to overdo it.


The problem here is that amplifier ratings are relatively (!) accurate vs. speaker power ratings.  Speaker power ratings are a general guideline - many tweeters can only take 25W, but because the majority of the music spectrum is below about 3kHz, in a multi-way system (midrange, woofer, etc.) you could likely get away with 100W no problem on typical music.  Unless you send a 100W sine wave at 20kHz which will pass through the crossover a toast the tweeter pretty quickly.

Speaker power rating is a very ambiguous, general guideline whereas an amplifier output can be unambiguously measured and verified.  Speaker power handling will depend on the spectrum of the signal, duration of the high amplitudes , crossover design and potentially how heated the electromagnetics are.

I would never fear hooking a 1,200 watt amplifier to any speaker because I could easily tell when the speaker was being overdriven.  My 25W receiver example (from the poor school days) would make me leary of underpowering.


Maybe I'm being naive here, but the thing that kills the speaker is the total power absorbed, regardless of frequency, right?  As in if you put X amount of power in for Y seconds, it burns up or blows out or whatever?

If so, couldn't you do a frequency sweep at constant voltage, measure current and have a very good idea of the safe limits at each frequency?

WATT theory?

Reply #27 –
Quote from: Mike Giacomelli on
Quote from: DigitalMan on

Quote from: Gecko on

I don't really buy into this "your amp needs to be much more powerfull than your speakers". Just as speakers can take transients with much more power than their RMS rating, an amplifier can deliver transients much stronger than its rated RMS power without much distortion. There's nothing wrong with some headroom to steer clear of clipping but you don't have to overdo it.


The problem here is that amplifier ratings are relatively (!) accurate vs. speaker power ratings.  Speaker power ratings are a general guideline - many tweeters can only take 25W, but because the majority of the music spectrum is below about 3kHz, in a multi-way system (midrange, woofer, etc.) you could likely get away with 100W no problem on typical music.  Unless you send a 100W sine wave at 20kHz which will pass through the crossover a toast the tweeter pretty quickly.

Speaker power rating is a very ambiguous, general guideline whereas an amplifier output can be unambiguously measured and verified.  Speaker power handling will depend on the spectrum of the signal, duration of the high amplitudes , crossover design and potentially how heated the electromagnetics are.

I would never fear hooking a 1,200 watt amplifier to any speaker because I could easily tell when the speaker was being overdriven.  My 25W receiver example (from the poor school days) would make me leary of underpowering.


Maybe I'm being naive here, but the thing that kills the speaker is the total power absorbed, regardless of frequency, right?  As in if you put X amount of power in for Y seconds, it burns up or blows out or whatever?

If so, couldn't you do a frequency sweep at constant voltage, measure current and have a very good idea of the safe limits at each frequency?


No, that wouldn't tell you much because impedance varies highly with frequency. 

The power limits of the typical cone/dome speaker are usually the thermal dissipation in the voice coil or the physical excursion extremes of the voice coil in the magnet structure.  Speaker maximum power will be highly frequency dependent because of the different properties of drivers and the absorption of power in the crossover.  A speaker might absorb 100W all day long at 100Hz and only 15W at 15kHz for a few seconds.

For example, I've seen burnt out voice coils (wire melted) in tweeters and bent voice coils on woofers due to the voice coil hitting the magnet during extreme signals.
Was that a 1 or a 0?

WATT theory?

Reply #28 –
Quote from: Mike Giacomelli on
Maybe I'm being naive here, but the thing that kills the speaker is the total power absorbed, regardless of frequency, right?  As in if you put X amount of power in for Y seconds, it burns up or blows out or whatever?

If so, couldn't you do a frequency sweep at constant voltage, measure current and have a very good idea of the safe limits at each frequency?


Close, but you have to deal with the speaker at the component level:  the thing that kills a *driver* is overheating (which should be somewhat proportional to the energy of the signal) as well as a driver's over-excursion beyond the in/out limit set by the designer.  Most audiophile speakers, however, use two or more drivers and, as has been explained above, each driver is rated for a certain amount of energy *within a certain frequency range*, which is handled by the cross-over network and the impedences of the speakers at various frequencies.

If your amp is putting out 100W at 12,000KHz, it'll probably overdrive the tweeters on your 200W-rated speakers and cause them to blow (either overheat and die or over-excurt and fall apart), since the tweeter itself generally isn't rated to handle the majority of the sound energy.

The speakers in front of me have five drivers.  And, they also have fuses in back, which probably wouldn't help in this situation. 

EDIT: yet again, my post is redundant.  I really should learn to finish reading a thread before responding.  Thanks DigitalMan.

-brendan

WATT theory?

Reply #29 –
On that 'overloading the speakers' theory, perhaps the sane thing to do is:

1/ Buy (loan?) the amp
2/ Hook it up.
3/ Get a good mic
4/ Feed Mic and one channel (perhaps right after the preamp) into an oscilloscope
5/ Play
6/ Increase volume until Input != Output
7/ tape off everything above current output.

Or am I missing something?

WATT theory?

Reply #30 –
Quote from: puntloos on
...
6/ Increase volume until Input != Output
7/ tape off everything above current output.
Or am I missing something?
Yes. For a start, all speakers introduce a certain amount of distortion and, unlike amps, they don't suddenly increase in distortion towards their top power rating. Your procedure wouldn't really be able to measure the maximum safe amount of power to give the speakers. It would, however, be able to tell when the amplifier is clipping, but you could tell that by plugging a scope straight into the amp outputs.

This isn't the sort of thing that can be measured easily. Speakers (especially ones with more than a single driver) tend to defy easy specification. For example, my speakers will play a 50W 3kHz sine happily - but the power at 15kHz would smoke the tweeters instantly. I think the best way to do this is to apply some common sense. Don't use your good HiFi for parties (get some second hand PA gear, for example) and don't abuse it too much.

It's a bit like driving a car - it doesn't say in the manual what the max rated cornering speed is, but you can generally tell when you are going too fast.

WATT theory?

Reply #31 –
Quote from: DigitalMan on
Quote from: Mike Giacomelli on

Quote from: DigitalMan on

Quote from: Gecko on

I don't really buy into this "your amp needs to be much more powerfull than your speakers". Just as speakers can take transients with much more power than their RMS rating, an amplifier can deliver transients much stronger than its rated RMS power without much distortion. There's nothing wrong with some headroom to steer clear of clipping but you don't have to overdo it.


The problem here is that amplifier ratings are relatively (!) accurate vs. speaker power ratings.  Speaker power ratings are a general guideline - many tweeters can only take 25W, but because the majority of the music spectrum is below about 3kHz, in a multi-way system (midrange, woofer, etc.) you could likely get away with 100W no problem on typical music.  Unless you send a 100W sine wave at 20kHz which will pass through the crossover a toast the tweeter pretty quickly.

Speaker power rating is a very ambiguous, general guideline whereas an amplifier output can be unambiguously measured and verified.  Speaker power handling will depend on the spectrum of the signal, duration of the high amplitudes , crossover design and potentially how heated the electromagnetics are.

I would never fear hooking a 1,200 watt amplifier to any speaker because I could easily tell when the speaker was being overdriven.  My 25W receiver example (from the poor school days) would make me leary of underpowering.


Maybe I'm being naive here, but the thing that kills the speaker is the total power absorbed, regardless of frequency, right?  As in if you put X amount of power in for Y seconds, it burns up or blows out or whatever?

If so, couldn't you do a frequency sweep at constant voltage, measure current and have a very good idea of the safe limits at each frequency?


No, that wouldn't tell you much because impedance varies highly with frequency. 



Yes and by keeping a constant voltage, sweeping frequency and measuring current you would have the impedance at all possible frequencies


Quote from: DigitalMan on
The power limits of the typical cone/dome speaker are usually the thermal dissipation in the voice coil or the physical excursion extremes of the voice coil in the magnet structure.  Speaker maximum power will be highly frequency dependent because of the different properties of drivers and the absorption of power in the crossover.  A speaker might absorb 100W all day long at 100Hz and only 15W at 15kHz for a few seconds.

For example, I've seen burnt out voice coils (wire melted) in tweeters and bent voice coils on woofers due to the voice coil hitting the magnet during extreme signals.


So you're saying the safe power limit is itself a function of frequency, independant of the impedance at a given frequency?  That certainly complicates things.

WATT theory?

Reply #32 –
As I understand it, one of the main reasons why speakers can die when fed clipped signals is because of a lack of cooling for the voice coil, resulting from insufficient movement of the cone. This site explains it way better than I can, though.

WATT theory?

Reply #33 –
Quote from: Mike Giacomelli on
So you're saying the safe power limit is itself a function of frequency, independant of the impedance at a given frequency?  That certainly complicates things.
It does complicate things a whole lot. Speakers are designed this way because most music has fairly little energy in the higher frequency bands. For example, the maximum safe power with white noise will likely be less than that with a real musical signal. Measuring the safe maximum power of a speaker is very complex. It's not surprising that manufacturers do it in a way which makes them look good.

WATT theory?

Reply #34 –
Quote from: Cartman_Sr on
As I understand it, one of the main reasons why speakers can die when fed clipped signals is because of a lack of cooling for the voice coil, resulting from insufficient movement of the cone. This site explains it way better than I can, though.


An interesting read, which I can reccommend.

I do wonder how much of it (especially the 'cooling' part) applies to electrostatic speakers though.

Also, I'm slightly puzzled by the claims that there is no air flow if the speaker coil is not moving (in the flat area of a clip). While this sounds like common sense in general, the reality (I think!) is that you can view the speaker as a piston. If it moves 'slowly' (non-clipping), then the air will be pulled in nice and neat. If however it clips, it will draw vacuum (or much closer to that state), drawing in air while the vacuum equalizes itself even though the actual speaker 'piston' doesnt currently move.

I guess the 'smooth' way of things is preferrable though.

WATT theory?

Reply #35 –
I think maybe what he means is that for the periods of time when the signal is clipped, the cone moves out to its maximum excursion and just sits there and doesn't move because for those fractions of a second there's no change in the audio signal. Also, all of the electrical power from the amplifier is going through the speaker, heating it up and damaging it. But I could be wrong, I'm no expert.

WATT theory?

Reply #36 –
Quote from: Cartman_Sr on
I think maybe what he means is that for the periods of time when the signal is clipped, the cone moves out to its maximum excursion and just sits there and doesn't move because for those fractions of a second there's no change in the audio signal. Also, all of the electrical power from the amplifier is going through the speaker, heating it up and damaging it. But I could be wrong, I'm no expert.


Expert or not, you've got it right..

BUT

My point is that the cooling should continue. Imagine pulling all air out of a room, but leaving a crevice open. The air will not 'teleport' in, but flow in. Violently at first, lightly later, until it's equal again. And we're speaking about fractions of seconds with clipped audio. I could imagine that a tightly designed speaker will still have air flowing, even in clipped mode.. perhaps not enough though.

Anyway, I've actually gave the techs at Bel Canto a call, very nice people. Same with the Quad people, mailed them, got an answer.

Both seem to agree on a few key points: 400Watts@8ohm is not necessarily overpowering an 140W rated speaker. The speaker ratings audio manufacturers list sometimes even include some buffer for people with bad amps  probably less common with high-end speakers.

The guy at Bel Canto even said he wouldn't be surprised if even at 'full blast' the amp would not break/kill/overload the speaker.

The same story came from a reviewer (on which I partially based my purchase choice): http://www.stereotimes.com/amp100702.shtml

Anyway, I've bought the amp, and we shall see(hear) what happens soon enough.

WATT theory?

Reply #37 –
Quote from: puntloos on
Anyway, I've bought the amp, and we shall see(hear) what happens soon enough.


Soon enough = now  - the amp arrived, hooked it up, and well, the short version is:

Selecting a volume of '80%' on my preamp, hooked up to the 400W power amp will not overload the speakers. My conclusion is that they can indeed handle at least 300Watt. I don't dare (plus I don't like to, my ears start to hurt) to turn it up higher. Im a happy camper

WATT theory?

Reply #38 –
Quote from: cabbagerat on
Quote from: MedO on

Maybe I missed something, but isn't 200 Watts quite a lot for home use? A friend once showed me the volume of a pair of speakers running with 1 Watt (measured at the speaker cable), and it was already too loud for normal listening imo.
The efficiency of the speakers comes into play quite heavily here. This can range over about 100x (20dB), so judging one set by another is not really that educational.

I have linked to Rod Elliot's article on why small amps kill tweeters many times. It's really worth a read for those of you who haven't read it yet.

Boojum: What is the real power of those 1200W amps? If they can really put 1200W of sin wave into an 8ohm load then I am seriously impressed. I have a 6kW test load that I use as a footrest at work (don't use it much) which could do with some warming

So is there a difference between blowing your speakers with an over-powered amp not clipping or blowing them with an underpowered amp overdriven into clipping? Either has the same end result. Surely the better option is an underpowered amp where the distortion can be heard and interpreted as a warning to reduce the volume?

I too have questions for Boojum: what do you power these amps from? 1200W output implies an input in excess of 1800W (at best, for an efficient Class B amp anyway), times two for a stereo pair is approaching the limits for a (UK) power outlet! How far do you retreat at these levels? ;-)

WATT theory?

Reply #39 –
Quote from: antz on
So is there a difference between blowing your speakers with an over-powered amp not clipping or blowing them with an underpowered amp overdriven into clipping? Either has the same end result. Surely the better option is an underpowered amp where the distortion can be heard and interpreted as a warning to reduce the volume?


Amp clipping can occur without smooth onset... suddenly you end up in square wave territory and your tweeter is gone. I think distortion caused by over-driving your speakers has a smoother onset.

WATT theory?

Reply #40 –
I think the assumption that you'll hear the clipping from an under-powered amp before the damage is done isn't correct.

-brendan

WATT theory?

Reply #41 –
Quote from: bhoar on
I think the assumption that you'll hear the clipping from an under-powered amp before the damage is done isn't correct.
Absolutely, especially after a couple of beers, or during a party. The only time i've ever killed a tweeter (luckily the speakers were old and damaged anyway) was while I was mowing the lawn and cranked up the volume. You aren't always aware of distortion before it's too late 

WATT theory?

Reply #42 –
Would it help if instead of mere HPF on crossover we put another LPF just before the tweeter, effectively making a BPF into the tweeter?

Or even build your customized crossover with LPF into the woofer, BPF into the midrange, and another BPF into the tweeter?

WATT theory?

Reply #43 –
Quote from: cabbagerat on
Absolutely, especially after a couple of beers, or during a party. The only time i've ever killed a tweeter (luckily the speakers were old and damaged anyway) was while I was mowing the lawn and cranked up the volume. You aren't always aware of distortion before it's too late 

Am I the only one who pictured you being drunk and running over a small bird with your lawnmower? *TWEET*

WATT theory?

Reply #44 –
Quote from: puntloos on
Am I the only one who pictured you being drunk and running over a small bird with your lawnmower? *TWEET*


Just the thought of trying to listen to music while lawnmowing had me rofl-ing already

WATT theory?

Reply #45 –
Quote from: Patsoe on
Quote from: antz on

So is there a difference between blowing your speakers with an over-powered amp not clipping or blowing them with an underpowered amp overdriven into clipping? Either has the same end result. Surely the better option is an underpowered amp where the distortion can be heard and interpreted as a warning to reduce the volume?


Amp clipping can occur without smooth onset... suddenly you end up in square wave territory and your tweeter is gone. I think distortion caused by over-driving your speakers has a smoother onset.

ANY clipping is the equivalent to some square-wave content, so by definition you're correct. However, the power levels will be small and the frequency highest at the onset of clipping. I've never tried detecting clipping audibly to see at what level it becomes noticeable, so I can't claim to know if it's a valid method. You have intrigued me enough to do some testing though, if I can figure a way to do it sensibly.

As an aside, I did do some square-wave testing on my own amp (Technics SU-V3) a few years ago, loaded by my speakers, not resistors. A 'scope connected to the output couldn't discern any rounding or ringing until over 10kHz, something I was seriously impressed with. I don't suppose I had the volume too high though, but it sure never blew the tweeters! The amp is rated at 40+40W and the speakers are, nominally, 75+75W so I'd be in dangerous territory according to the theories. Rough calculations suggest that the amp has just over 4dB headroom before clipping (allowing for some "sag" in the power supply).