Power . How much do we need...

[Orb]

Forgot to mention one other critical aspect and that is the speaker loading/impedance.
Not going to rehash what most of us have read, but it is worth considering that a speaker's impedance usually fluctuates between low and higher frequencies, which can impact an amplifier if its performance-spec (including power reserve that I have raised earlier) are not up it.
It is interesting to see charts showing an amp's performance to clipping for different impedances.
And again this is compounded that real music-recordings do not behave like a single static sinewave at a set frequency.

Cheers
Orb

[j_n]

But that is of the audio track when the power supply is NOT clipping! This is why a cheap, low powered amp can destroy the tweeters of an expensive, high power speaker: the waveform of a clipping signal has a huge boost in the level of high frequency energies, courtesy of the sharp corners of the clipping, which get fed straight through to the tweeter. The kids have a loud party: bye, bye, little speaker! Frank

I have had two different scenario's for blown tweeters.

1. Total power: vc burnout as a result of beating the living daylights outta the system. Autopsy of the tweeter shows a dark brown mess, burned epoxy and such. I have one such example I use as part of my tours...a combination of "what happens when you don't use superconductors, how a charged particle moving in a magnetic field causes a force orthogonal to both, how lorentz forces build up in magnets very quickly, and of course, what happens when you loan your system to your son to use at a beach party where he points the speakers away from the turntables..

2. Ultrasonics. This is the most interesting of the two intellectually. Clipping of the musical content causes ultrasonic energy. There is one location within a standard voice coil tweeter which is susceptible to overband content, this is the location where the voice coil end wires exits the vc form and heads out of the flux field to the terminals of the driver. I've had several different manufacturers product fail the exact same way.

Specifically, the unsupported tweeter lead wire is still within the magnetic flux of the gap for a small length, and this unsupported region will flex in response to current. For stimulus below the resonance frequency of this unsupported wire length, this section is better cooled than the vc, so there is no problem. When US content causes resonance in the flying wire, this unsupported section will flex until slip plane dislocation compromises the physical integrity of the wire..failure will occur. The failure location will look necked down at the break, consistent with the look of a tensile stress failure. Microscopic examination of the ends will show the slip plane texture on the wire surfaces. repeated flexure failure.

I see this failure mode confused with overheating many many times. Both occur, and it is dependent on spectral content as well as tweeter VC design. The explanations of tweeter burnout and power/clipping is many times muddied...because nobody considers the resonance failure.

Cheers, jn

[Robh3606]

I look at if from a system standpoint. I look at the speaker sensitivity and listening distance and know up front what my SPL tarket is at the listening position. Then look at the power required and if the speakers can safely handle it. Then I add some headroom and there you have it.

When I did my HT I wanted to be able to reach the 105db reference level with 10db of headroom above that. I don't tyically listen at reference level but even if I do the set-up sounds effortless with the added headroom.

The point is there is no magic number. It is system, room and SPL dependant.

Rob

[Grainger49]

Of course, there is no way for me to wade through all 11 pages of responses. So I will give my standard answer.

You need enough to drive your speakers, in your room (the larger the more power needed to reach a physical volume level) to the volume you like to listen to.

No one else should tell you how to listen.

[amirm]

There are however clear cases where it is more our bias expectations that anything else at play .... and the power required to drive a given speaker is one of them ... Most speakers are grossly inefficient. The sensitivity numbers do not do justice to the vast inefficiency of our loudspeakers.

The wiki on this is pretty good and goes even further: http://en.wikipedia.org/wiki/Loudspe...s._sensitivity

"Loudspeaker efficiency is defined as the sound power output divided by the electrical power input. Most loudspeakers are actually very inefficient transducers; only about 1% of the electrical energy sent by an amplifier to a typical home loudspeaker is converted to acoustic energy. The remainder is converted to heat, mostly in the voice coil and magnet assembly. The main reason for this is the difficulty of achieving proper impedance matching between the acoustic impedance of the drive unit and that of the air into which it is radiating (at low frequencies improving this match is the main purpose of speaker enclosure designs). The efficiency of loudspeaker drivers varies with frequency as well. For instance, the output of a woofer driver decreases as the input frequency decreases because of the increasingly poor match between air and the driver.

Driver ratings based on the SPL for a given input are called sensitivity ratings and are notionally similar to efficiency. Sensitivity is usually defined as so many decibels at 1 W electrical input, measured at 1 meter (except for headphones), often at a single frequency."

Notice the bolded section from me. Efficiency is a marketing number for speakers so one has to believe that some stretching of the truth is going on in some or many cases.

My experience certainly mirrors that of Frantz in that the math of efficiency does not agree with the amount of power I see necessary to drive the speakers.

[Orb]

I have had two different scenario's for blown tweeters.

1. Total power: vc burnout as a result of beating the living daylights outta the system. Autopsy of the tweeter shows a dark brown mess, burned epoxy and such. I have one such example I use as part of my tours...a combination of "what happens when you don't use superconductors, how a charged particle moving in a magnetic field causes a force orthogonal to both, how lorentz forces build up in magnets very quickly, and of course, what happens when you loan your system to your son to use at a beach party where he points the speakers away from the turntables..

2. Ultrasonics. This is the most interesting of the two intellectually. Clipping of the musical content causes ultrasonic energy. There is one location within a standard voice coil tweeter which is susceptible to overband content, this is the location where the voice coil end wires exits the vc form and heads out of the flux field to the terminals of the driver. I've had several different manufacturers product fail the exact same way.

Specifically, the unsupported tweeter lead wire is still within the magnetic flux of the gap for a small length, and this unsupported region will flex in response to current. For stimulus below the resonance frequency of this unsupported wire length, this section is better cooled than the vc, so there is no problem. When US content causes resonance in the flying wire, this unsupported section will flex until slip plane dislocation compromises the physical integrity of the wire..failure will occur. The failure location will look necked down at the break, consistent with the look of a tensile stress failure. Microscopic examination of the ends will show the slip plane texture on the wire surfaces. repeated flexure failure.

I see this failure mode confused with overheating many many times. Both occur, and it is dependent on spectral content as well as tweeter VC design. The explanations of tweeter burnout and power/clipping is many times muddied...because nobody considers the resonance failure.

Cheers, jn

Ah yeah great point on the 2nd one, forgot about that and is a real interesting one especially when considering-expanding other factors such as a wide bandwidth amp and interraction with ultrasonics generated elsewhere (crude example being a poorly designed nos dac ).

Cheers
Orb

[Phelonious Ponk]

I'm no engineer and there's a lot I not only don't know and wouldn't even understand if you explained it to me. But among the things I've heard, over and over again, until they've finally sunk in is that loudspeakers are inefficient and very inconsistently inefficient at that. And so manufacturer's ratings are not reliable. I've also learned, as has been discussed here, that an amplifier can have lower watts than another, yet have higher headroom. It's pretty difficult to know what to do with such inconsistency if you're not an engineer. But then there's what my ears taught me: More powerful amps sound better. They sound better at low volume. They sound better at high volume. They sound better into efficient and inefficient speakers. Many years ago, in my passive midfi days, I had a pair of Altec Lansings that were very efficient. I was driving them with a Kenwood integrated amp rated at 60 watts. I sold the Kenwood and bought a Harman Kardon that was basically monoblocks in a single case - separate paths, separate transformers, etc. It weighed a ton. The dealer I bought it from, a guy I knew, told me that Harman's watts were louder. I didn't know what he meant by that, I still don't really understand it, but I know that Harman, at 45 watts per channel, just seemed to make those Altecs breathe easier.

Probably expectation bias . But whatever it was, I have, ever since then, been a believer in excess headroom. That belief was only reinforced when I got into actives. The really powerful active monitors with bigger amps for the woofer than for the tweeters, and more wattage (whatever it may mean...) than even seems to make sense for the size of the drivers, always seem to sound the best.

Or at least that's what I hear. Oh, and by the way, really good, heavily amped actives deliver, to my ears, the purest, cleanest, most transparent midrange in the world. Low power SET, runining at anything close to realistic volume, sounds like a special effect by comparison. MHO. YMMV. That doesn't mean it's not attractive, by the way.

Tim

[Ron Party]

Efficiency is a marketing number for speakers so one has to believe that some stretching of the truth is going on in some or many cases.

My experience certainly mirrors that of Frantz in that the math of efficiency does not agree with the amount of power I see necessary to drive the speakers.

As does mine. Amir, have you played around with the Salon2s? My local high end dealer who carries these speakers told me they really begin to sing when feeding them 1000 W.

[DonH50]

The issue of efficiency vs. frequency has long been understated... Not documenting this means the fairly severe impedance excursions most speakers exhibit are "hidden" factors driving how much power is really needed. Another factor is how we hear, i.e. the old loudness contours. That means a speaker with lower frequency response almost inevitably requires more power than one with more limited FR because more power is required to make the LF's louder to match the (perceived) loudness of the highs.

I consider loudspeaker efficiency numbers very useful, but only as general guidance on where to start.

As an aside, I would love to know what peak power a given speaker can take, based upon its drivers' maximum excursions. Of course, that also probably varies over frequency...

To throw out some numbers, in the midrange 1 dB is almost imperceptible to most people if you change the volume level (I am NOT talking about volume matching for AB tests) and takes about 30% more power. Around 3 dB is a noticeable change and requires twice (2x) the power. Doubling the volume is 10 dB and takes 10x the power. If we start looking at headroom over the average level, then 20 dB means 100x the average power, and 30 dB is 1000x the average power. So, even with just 1 W average, if we never want to clip (a debatable though worthwhile goal), it's easy to see why people often prefer amps capable of delivering 100 W or more. And, why a few dB increase in speaker efficiency means so much.

[Gregadd]

Interesting comment on SS vs. tube power... I would have guessed the opposite, with about 2x the power in a tube amp to match the bass tightness of a SS amp. In the midrange and up, I expect power to be a wash, with the tube amp sounding a little "sweeter". I have always felt bass to be one of the weaknesses of tube amps, primarily due to their relatively high output impedance (and low damping factor). BTW, assuming a good amp with stiff rails, the SS amp rated at 600 W should put out 800 W into 6 ohms; the tube will put out 300 W no matter the load as long as the tap is there and it is matched to the speaker's impedance.

88 dB/1 W/1 m, say we want it to reach 120 dB peaks at our listening spot, maybe 3 m away. That means we need an extra (120 - 88) = 32 dB, and another 6 - 12 dB for distance depending upon the room gain (size etc.) Let's say an even 40 dB, or 10,000 W (!) Maybe 110 dB is more reasonable, yielding 1000 W -- closer to your target goal. In fact, 600 W is about 28 dB above 1 W, so the speaker should output 116 dB at 1 m and maybe 110 dB at the 3 m listening spot driven by 600 W with a little help from the room; close enough! Good "guess", Gregadd! Leaves the tube amp lagging a bit, though, as it will be 3 dB into clipping at that power. We probably won't notice as it will only happen on the loudest peaks.

I thought this was about amps; it's a "guess the speaker" game? Alas, I cannot play, as I am unfamiliar with most of the newer speaker brands. 6 ohms rules out Magnepan, however.

Onwards - Don

p.s. I am an "objectivist" who has owned tubes until very recently, and sold out to SS due to cost and other considerations. I always did like falling outside the box...

Of course the rooms must be considered