How much amplifier power do you really need?

[RebelMan]

I found the video per the link below pretty much by accident when searching for something rather different.  I have to be honest, I found this to be utterly gobsmacking, I had always suspected that amps needed a little more power than simple maths based on required dB(A) and speaker efficiency would suggest, but nothing like this.  The video of the amp's display is a little blurry, but's lets just say that I has mentally parked the decimal point in the display a factor of 10 backwards until it dawned on me what was really going on.

I would say it is well worth 10 minutes of your time to watch.

https://www.youtube.com/watch?v=bRMR9JZ1m0s&app=desktop

I had the pleasure of attending a DAC shootout today.  The system included a beefy SAE 2HP-D power amplifier driving a pair of Focal Utopia Evo speakers.  Peak output power never breached 6 watts and RMS power was even lower.  The sound was much louder than I typically listen to at home.  I didn't sample the SPL but it was clearly louder than the mid 80s.  The Evo's have a sensitivity of 93dB and dip below 3 ohms, an average load to drive I would say.  So if we assume for a moment that the Evo's were less sensitive than they are, say around 84dB, it would have taken less than 50 watts of peak power to produce the same SPL in that room.  Given that the amplifier was a 600W behemoth but would only need 50 watts at most kinda goes against the grain that the video above is trying to make claim to.  I have said this once before and it bears repeating, EVERYONE needs to measure the power they are using before making any conclusions on how much power they actually need.  And once again more power does not sound better than less power all else being equal.

[thumb5]

As I pointed out before, for a sine wave the peak power is exactly twice the RMS power, because the (peak) amplitude is the square root of 2 times the RMS value of the signal and power depends on amplitude squared.

A lot of heavy weather seems to be being made of this...

This statement contradict's itself.  Peak power "by definition" is the √2 (which is 1.414) times the RMS power as you say but is NOT twice RMS power as you also say. Calculus proves this, what math are you using?

My statement does not contradict itself; you seem to be confusing power with amplitude. What calculus are you using to derive your (incorrect) conclusion?

[RebelMan]

"How much power do you need?" is a complicated question and I think there's a tendency to think that there's a simple answer based on speaker choice/sensitivity and the peak volume your speakers are capable of but it isn't as simple as that and in reality you can find 2 people with the same speakers, one of whom might be really happy and perfectly satisfied with a 140 Pro and another who needs a 1000 Pro and who can't wait for Devialet to produce a 2000 Pro.

Not so complicated.  People tend to mistakenly think they need more power because they believe more power will sound better... because it's more power.  The sound quality of an amplifier has absolutely nothing to do with the amount of power it generates.  Sound quality is dependent on factors such as the choice of architecture (Class A, Class A/B, Class D, etc.), choice and quality of components (solid state, valves, hybrid, etc.) circuit topology and design (multi-layer boards, discrete vs surface mount technology, etc.).  Power is simply a means to an end.  To produce electrical energy that can be converted into electromechanical energy. The only way to know for sure how much power you need is by measuring what you need.  Most people will be surprised by how little they really need.

[RebelMan]

                 Expert 140 Pro            Expert 220 Pro
8 Ohms      105 W                        165 W     
6 Ohms      140 W                        220 W
4 Ohms      210 W                        330 W
2 Ohms      420 W                       330 W

I have asked Devialet about the 2-ohm continuous power of 220 Pro in two different occasions before and got two different answers: 660W and 500W respectively. Not that I think the answers I got are more accurate (I shouldn't have got different answers in the first place), but I can't understand how a 220 Pro could do worse than a 140 Pro for a 2-ohm load, given that they have the same chassis and the power supply of 220 Pro should be no worse than 140 Pro (if not better). I am curious to know if the above power figures quoted for 140 and 220 were all provided by the same person? 

BTW, just for additional information, I found another review from the Stereo magazine ( https://stereo-magazine.com/flipview/epaper/stereo-magazine-20-2019-20/ ) which posted somewhat better measurements for the 220 Pro than what Hi Fi World did: 173W at 8ohms and 343W at 4 ohms. Not sure which review is more correct, but to me the differences are not night and day and could be just due to different test conditions.

A correction needs to be made.  The amount of power the 140 can feed into a 2 ohm load is 210 W RMS, not 420 as originally disclosed.  This is because of a limitation built into the software.  When the 140 is configured as dual-mono mode (A.K.A. a 210) the software unlocks the lmitation and the output is as follows...

Expert 140 Pro in Dual-Mono (A.K.A. 210)
8 Ohms      157.5 W  
6 Ohms      210.0 W
4 Ohms      315.0 W
2 Ohms      420.0 W

The information I received initially was misleading and provided by two different individals.

What I find interesting is when the Expert 220 Pro is configured in Dual-Mono the output power looks like this...

Expert 220 Pro in Dual-Mono (A.K.A. 440)
8 Ohms      330 W  
6 Ohms      440 W
4 Ohms      660 W
2 Ohms      660 W

More power is available in the latter for 4 and 2 ohm loads but runs flat at 660W whereas less power is available in the former for 4 to 2 ohm loads but runs more linear at 315W to 420W respectively.   An amplifier's output that follows the impedance curves of the speaker more closely than another will perform better, meaning less likely to distort/clip.

[RebelMan]

As I pointed out before, for a sine wave the peak power is exactly twice the RMS power, because the (peak) amplitude is the square root of 2 times the RMS value of the signal and power depends on amplitude squared.

A lot of heavy weather seems to be being made of this...

This statement contradict's itself.  Peak power "by definition" is the √2 (which is 1.414) times the RMS power as you say but is NOT twice RMS power as you also say. Calculus proves this, what math are you using?

My statement does not contradict itself; you seem to be confusing power with amplitude.  What calculus are you using to derive your (incorrect) conclusion?

Negative.  I managed to dig up a video tutorial for you...  RMS vs Peak

[thumb5]

This statement contradict's itself.  Peak power "by definition" is the √2 (which is 1.414) times the RMS power as you say but is NOT twice RMS power as you also say. Calculus proves this, what math are you using?

My statement does not contradict itself; you seem to be confusing power with amplitude.  What calculus are you using to derive your (incorrect) conclusion?

Negative.  I managed to dig up a video tutorial for you...  RMS vs Peak

Sorry to be blunt, but you are simply wrong about this.

As its title says, the video you linked to above explains why the relationship between RMS and peak value of a sine wave is a factor of root 2.  That means a sine wave of amplitude (value) 1 has an RMS value of ~0.707, or conversely if the RMS value is 1, the peak value (amplitude) is ~1.414.  Since power depends on the square of the value, the relationship between RMS and peak power is a factor of 2.

Since you are apparently fond of tutorial links, here's one for you: https://en.wikipedia.org/wiki/Root_mean_...rage_power.  Note that for a sine wave both Irms and Vrms are ~0.707 times the respective peak values so when multiplied together (P = IV) the RMS power is a factor of 0.707 x 0.707 = 0.5 times the peak.

[David A]

"How much power do you need?" is a complicated question and I think there's a tendency to think that there's a simple answer based on speaker choice/sensitivity and the peak volume your speakers are capable of but it isn't as simple as that and in reality you can find 2 people with the same speakers, one of whom might be really happy and perfectly satisfied with a 140 Pro and another who needs a 1000 Pro and who can't wait for Devialet to produce a 2000 Pro.

Not so complicated.  People tend to mistakenly think they need more power because they believe more power will sound better... because it's more power.  The sound quality of an amplifier has absolutely nothing to do with the amount of power it generates.  Sound quality is dependent on factors such as the choice of architecture (Class A, Class A/B, Class D, etc.), choice and quality of components (solid state, valves, hybrid, etc.) circuit topology and design (multi-layer boards, discrete vs surface mount technology, etc.).  Power is simply a means to an end.  To produce electrical energy that can be converted into electromechanical energy. The only way to know for sure how much power you need is by measuring what you need.  Most people will be surprised by how little they really need.

I'm sorry but it is a complicated question. You assume that "people" believe more power will sound better so they're asking how much power they need to make things sound the best that they can sound but if people simply believed that more power sounded better everyone would be buying D1000s and no one would be buying D140s. While you may argue that the depth of our wallets is a limiting factor that too isn't universally the case. Lots of people buy smaller amps than they could afford to buy. You've got a straw man argument going on.

The reason the question is going on is because of the factors involved, factors such as preferred listening and peak listening levels, speaker sensitivity, listening distance, room size and setup of the speakers and listening position in the room, room construction and furnishings, openings to other spaces and the characteristics of the other spaces. All of those things affect the level of sound we hear in the room for a given amount of power and none of them have anything to do with a belief that more power sounds better.

In an earlier post above you said  "…more power does not sound better than less power all else being equal."  I agree, all else being equal an amp with more power will not sound better than an amp with less power when both are producing the same SPL at the listening position but how often is all else equal? If you have a situation where an amp with more power sounds better than an amp with less power when both are producing the same SPL at the listening position then obviously all else is not equal. If you want to claim that an amp with more power can't sound better than a smaller amp which is also capable of producing the same level then you need to show that all else is equal and you haven't attempted to do that. I would respectfully suggest to you that when you compare amps with different power outputs, even amps from within the same range such as the 140/220/250 you will find differences apart from the power output so all else is not equal. That being the case I have to say that, at the very least, a difference in sound quality is possible.

[Confused]

@David A - I pretty much agree with your comments above, but I think there is one additional point to add. The big take away I received from this thread is just how much difference the type of music makes to the demands placed on an amplifier. In an earlier post I linked to the Harbeth forum, which has a longer version of the video posted in the first post. In this slightly longer version the last two minutes the music is changed from the Electronic Pan Sonic track to a rather milder Corelli Sonata. The gain on the amp was left unchanged. The peak power used when playing Corelli was 28 watts, versus the 760 watts peak seen with the Pan Sonic track. So in this specific example, a simple change of music resulted in over 27 times the amount of power being required. From a more subjective perspective, it is interesting that nobody commented on the volume level when playing the Pan Sonic track, but the comment was made that "we were playing louder than we would normally play Corelli at" for the later section of music. So taking the exact conditions that the video was made, the room, the speakers, the volume, a 140Pro would probably sound very similar to a 1000Pro, whereas with the Pan Sonic track, the differences would likely be far more noticeable.

@RebelMan - the Archimego link is interesting, although I am not sure about the merit of testing with test tones at specific frequencies. For example, you might do the test with a 120Hz tone as suggested, and get a particular result. However, with a specific speaker the result at 60Hz or 200Hz might be very different due to variations in the impedance curve. So I think the proposed test might give you an indication of the power required, but only an approximation. I guess you could try the test using real music, but then the results would depend on the response time of the voltmeter used. I am not an expert with respect to voltmeters, but I suspect that the response time is typically somewhat less than would be ideal for this sort of test.

One other factor that interests me is the ability of a specific amplifier to respond to the required transient peaks in the music. In other words the ability to go from low volts / amps, to a very transient peak of high volts /amps. Subjectively, this is referred to as the "speed" of the amplifier. Technically this is known as "Transient Distortion", which is not something you often see in amplifier specifications, and is not something you could readily measure with a volt meter.

[David A]

…
One other factor that interests me is the ability of a specific amplifier to respond to the required transient peaks in the music.  In other words the ability to go from low volts / amps, to a very transient peak of high volts /amps.  Subjectively, this is referred to as the "speed" of the amplifier. Technically this is known as "Transient Distortion", which is not something you often see in amplifier specifications, and is not something you could readily measure with a volt meter.

I've got a background in an area where testing is important, it just happens to be health and safety which is a long way from electronics and the kind of testing we're interested in here in this sort of discussion. Having said that, there are some things that are relevant to testing and the interpretation of tests in every field, whether that be health, medicine, or audio. One of those things is a very simple and brutal fact: test results tell you a lot about what you tested and they tell you nothing about what you didn't test.

Back in the 60's/70's half a century ago the big objectivist/subjectivist debate was about whether 2 amps with the same measured distortion could sound different. The objectivists said they couldn't, the subjectivists said they could. The measure of distortion being used in the debate was the simple THD spec still used today. The debate died when different testing regimes revealed that 2 amps could have the same % THD spec but very different distortion spectrums with different overtones making the greatest contribution to the % figure and, as we now know, that difference can easily be audible even though the amps have the same % THD. Measuring the % of THD tells you something about what's going on but it doesn't tell you everything and what it doesn't tell you is something that can and does make an audible difference to sound quality but as long as you're not measuring the distortion spectrum you're never going to know that there is a measurable difference there to be measured.

Transients require the amp to deliver brief bursts of higher power very quickly, usually for short period. An amp may be able to deliver the volts and amps but that doesn't necessarily mean that the amp can deliver them fast enough to deliver the transient with the kind of impact the music requires. Measures of output power don't measure how quickly the amp can deliver transient demands of 10 or more dB. Some sorts of music have more transients, including more frequent transients and repeated transients within short periods, than other sorts of music and I think it's pretty obvious that some amps are more capable of delivering that sort of music effectively than other amps, even though the other amps are capable of delivering the level of power required. It's a bit like 2 cars, each of which is capable of achieving 100 kph but one of which can achieve 0-100 kph in a shorter time than the other.

Take a look at the normal set of  performance specs routinely supplied by manufacturers and compare that with the test results you see in reviews. Most test reports limit themselbes to attempting to verify the manufacturer spescs but some reports, Stereophile for example, give much more detailed results based on a wider range of measurements and attempt to relate those results to the subjective listening report, something which is not an exact science. Testing takes time and money and manufacturers tend to report the minimum range of measurements required by industry standards but there are more tests which can be done and probably more than Stereophile does. It was a mistake 50 years ago for people to think that the THD % spec gave an complete and accurate representation of THD performance and it's a mistake today to think that amplifier specs completely capture every aspect of amplifier performance, especially when you look at the very different demands that different speakers can make of an amplifier and also the different demands that different sorts of music can make on the amplifier.

In the end I doubt we'll ever see a set of specs that will ever give a fully reliable guide to the performance of any component. In the end we're faced with the task of using our ears and being guided to some degree by our own subjective impressions and, yes, we sometimes make mistakes but relying on the numbers isn't any more reliable. 2 components with identical performance measurements can sound different because the measurements never show everything that could be measured and even if they did there are people out there coming up with new things to measure and new ways to measure things we already measure and provide more information than current measurements do.

I'm not discounting measurements. We wouldn't have the gear we have today if it weren't for measurements and the measurements we are provided with can tell us a lot about how a component will perform but when it comes down to deciding whether component A or component B better suits ouir personal needs and tastes  the only way to make that decision remains to listen to both and see what we think, and different listeners can come to different decisions about which is best.

[RebelMan]

Sorry to be blunt, but you are simply wrong about this.

As its title says, the video you linked to above explains why the relationship between RMS and peak value of a sine wave is a factor of root 2.  That means a sine wave of amplitude (value) 1 has an RMS value of ~0.707, or conversely if the RMS value is 1, the peak value (amplitude) is ~1.414.  Since power depends on the square of the value, the relationship between RMS and peak power is a factor of 2.

Since you are apparently fond of tutorial links, here's one for you: https://en.wikipedia.org/wiki/Root_mean_...rage_power.  Note that for a sine wave both Irms and Vrms are ~0.707 times the respective peak values so when multiplied together (P = IV) the RMS power is a factor of 0.707 x 0.707 = 0.5 times the peak.

I see where the problem lies.  To be equally blunt, there is NO SUCH THING as RMS power yet that was the terminology that was being tossed around here which made no sense to me.  There is (really) ONLY peak power and average power.  Instantaneous and continuous power also exist but we are not interested in the former and we (I am anyway) are assuming that continuous power is the same as average power for this discussion.  People are confusing RMS power to equal average power which it absolutely does not, because there is no such thing as RMS power.  I interpreted peoples use of term RMS power to be more than average power and therefore did not agree that peak power was twice RMS power which again it does not!  Peak power is twice average power, but that's NOT what was being said.  Let's be careful with our wording, DO NOT USE RMS power, there is NO SUCH THING.