Amplifier Power: How Much is Enough?

Considering the amount of careful research, cautious theorizing and wild speculation that have been lavished on the amplifier power question, we should expect to be considerably closer to the answer in 1962 than we were five years ago. This does not seem to be the case.

We have instruments for measuring sound pressure levels in the air, for measuring electrical power, and for analyzing distortion content to the third decimal place, and the literature is full of learned dissertations on the structure of musical sounds, their behavior in concert halls and living rooms, and the relationships between ears and the sounds around them. Yet one audio expert still maintains that 0.5 watts of amplifier power is all you ever need, while another says 50 watts is barely enough. Who is right?

As is often the case in such a diagonal disputation, both are partly right. One source of the widespread disagreement stems from the lack of any standardized criteria for judging power requirements. Thus, one expert may be stating how much power we need to produce a certain volume of sound during crescendos, while the other may be telling us how powerful an amplifier we must have before any further increase in available power ceases to yield any perceptible improvement in sound. On the other hand, another expert—the field is thick with them—might be figuring power requirements on the basis of a high-efficiency speaker system like the Klipschorn, while yet another expert may have decided that the only speakers worth listening to are low-efficiency types like the AR-1, so he bases his estimate on its power requirements.

All are legitimate approaches, but it is obvious that no one of them can supply a universal answer. Hence the compounded confusion.

Let's get one thing straight at the outset: "Need" has no bearing on the matter. It is senseless to ask how much power we need, because the answer is "none." We don't need high fidelity, when it comes right down to that. Nobody would die, no governments would collapse, no panics would ensue if, all of a sudden, high fidelity had never been.

All right, then, how much power should we have? Simply stated, we should have enough power to reproduce the desired sound at the desired level without exceeding a certain limit of distortion. This reads like a masterpiece of evasion, but it is a step in the right direction, for no expert will disagree with it.

But what level is the "desired" level? Background music level, foreground listening level, or the kind of ear-shattering level that a conductor might hear from his podium?

The hi-fi system owner who does not plan to use his rig for anything except background music can just forget all about power requirements. At very low listening levels, the ear's powers of discrimination are poor, so any amplifier that is sold under the guise of high fidelity will do. A cheap 5-watter will be adequate, and it isn't too important if its distortion is fairly high, because nobody really listens attentively enough to background music to notice its sound.

The only time we benefit from high fidelity is when we concentrate on the program, because that is when we start to get finicky about the sound. Our ears are most responsive to upper frequencies when the sound is loud, and it is at high levels where a hi-fi rig's distortion is prone to be most severe. If the amplifier is clipping the tops off peaks at high listening volume, the resulting raggedness of sound is much more audible than it would be were the amplifier doing the same thing at a much lower volume level. This, of course, helps to befuddle the issue, because the higher the listening volume, the lower the amplifier's distortion must be in order to sound pleasant. And we all know that the harder we push an amplifier, the more distortion it generates.

So, for the purposes of this article, we are going to assume that you will, at least occasionally, play your system at foreground level.

What about orchestra-in-the-room level? Although a popular advertising gambit, this is an absurd notion. To be mundane about it, there simply isn't room for a symphony orchestra in the average home, so even if it were possible to re-create the original volume of the orchestra as heard from the conductor's podium—which it is, but it takes scads of power and a highly efficient speaker—the effect could not be realistic. It would also be very un-neighborly. A solo performer, or a chamber group, could be in your living room, and sounds very convincing when so reproduced. But recording engineers realized long ago that orchestra patrons listen from out in the hall rather than from the podium, so they do their microphoning to convey as well as possible the illusion of listening from a mythical "best seat in the house." Their recordings sound best when reproduced to scale; higher volume levels make them sound overblown and unnatural.

As sound waves travel away from their source, their total acoustical power remains essentially the same, but as each wave spreads out over a wider area, it thins itself out. Thus, the actual intensity of a sound some distance from its source will be considerably lower than its intensity right at the source. For this reason, we measure sound intensities in a concert hall in terms of variations in air pressure (or the sound pressure), rather than in terms of watts of acoustical power. The original power at the source can then be computed, if desired, by a simple formula based on the fact that sound pressure weakens by a square root function as its distance from the source is doubled.

Sound pressure readings are made using a special microphone probe and a meter that resembles a tape recorder's VU meter but is calibrated in dynes/cm2 of pressure or in decibels above the threshold of human hearing. The sound meter shares the same shortcoming as a VU meter in that its indicator needle, having some inertia, does not respond fully to transients, but gives an average (or RMS) reading.

The RMS level of sound during an orchestral crescendo, as heard from a fairly close seat in the concert hall (row C, for instance), measures about 100dB on a sound level meter. The acoustical power (not electrical, please note) needed to create this sound level, at a distance of 15 feet from a loudspeaker in a 10' by 15' by 20' room, is on the order of 0.4 acoustic watts.

If we used a 100% efficient speaker (which is unlikely, because there's no such thing), we could recreate the RMS power of the original sound with 0.4 watts of electrical power. To find the amplifiet power required to get this acoustical power from a practical speaker, we simply multiply the reciprocal of the speaker's efficiency rating (in percent) by 40. Thus, for a 10% efficient speaker, we have: 40 x 1/10W, which works out to 4 watts. For a typical "low-efficiency" speaker of about 1% efficiency, we would need 40 watts of amplifier power to produce 0.4 acoustical watts.

The power figure derived by the above calculation represents the minimum amount of RMS power needed to reproduce an orchestral crescendo at its original measured sound pressure. The figure will apply as a total power requirement for both channels of a stereo system, but it will not apply for a monophonic system, because mono sound of a certain measured pressure level does not sound as loud as the same level when the reproduction is stereophonic. This means that, in order to reproduce monophonic material at the subjective level encountered in the concert hall, we need more power than would be indicated on the basis of sound level meter computations.

How much more is a moot point, because the disparity between stereo and mono power requirements varies with the program material, the way it was microphoned, and the acoustics of the listening room. It usually works out to about a 1–2dB difference, which seems negligible until we remember that it takes double the power to raise the listening level by a mere 3dB. To cope with a 2dB increase, we must up our original power estimate by a factor of about 1.6. Hence, if our original figure came out to 4 watts, we would have to multiply this by 1.6 to get our power requirement for monophonic listening, and this would come out to 6.4 watts for the 10%-efficient speaker.

The formula that we described for arriving at our minimum RMS power figure assumed that the loudspeaker radiated its sound in all directions away from the source. In truth, some speakers don't. The best loudspeakers for small-room listening are direct-radiator types, simply because they do radiate the sound over a broad area. But horns. which usually behave best in very large rooms, tend to direct most of their output forward, so a higher proportion of the radiated sound goes directly toward the listener. This would tend to reduce the power requirement even more for a horn-loaded system, but the high efficiency of the average horn puts its power requirement so low to begin with that it is pointless to quibble over an extra watt or two, even though this may represent a doubling or halving of the computed figure.

An orchestral crescendo, or a full choral passage, contains transients that are fully 10dB higher than the average volume of the sound, as measured by a sound level meter. A 10dB increase in level represents a 10-fold increase in power, so how can we possibly hope to cope with this sort of thing? Fortunately, we don't have to. Recording studios and broadcast stations use peak limiters to keep these huge transients out of the received signal, and tape recorders have their own built-in limiting action. Transients are high- frequency phenomena, and tape will saturate instantly if a strong treble impulse is fed to it. The result is a shearing-off of the peak, and if the overload doesn't last too long, this won't cause any more audible disturbance than a good peak limiter.

ARTICLE CONTENTS

COMMENTS
HammerSandwich's picture
Quote:

A 10dB increase in level represents a 100-fold increase in power

Actually, 10dB represents 10x power, so 100x is 20dB.

Odd that JGH missed this after correctly writing that doubling power is 3dB. I suppose even he could make a potential mistake.

John Atkinson's picture
Thanks
HammerSandwich's picture

Your keyboard seems to have moved while you were correcting this. :)

audiolab's picture

This was written the year I was born (53 years ago), and just like me things have changed alot. To many out of date misconceptions & assumptions to be of any real value today. However, it makes interesting reading to see where we have come from. I have recently finished reading two books by Briggs on loudspeakers and it was all somewhat a familiar experience.

DaveinSM's picture

Actually, What struck me is how much of this article is still relevant today. It reads like it was written last week. Too many people underestimate the impact of listening habits in terms of volume level, room size, etc., on power requirements.

Bkhuna's picture

You can never have too much overhead.

dalethorn's picture

Seems like, a company that makes premium loudspeakers, should be able to tell you more than "Use an amp with at least 50 watts of power", or, "We like the ABC amp best, but the XYZ amp is good also". Seems like they should have a booklet or set of papers that describe their experiences more fully.

Allen Fant's picture

JGH was the master! We audiophiles could never have enough high-power nor high-current, IMO.

Ronald Koh - SG's picture

Just in hope that we it'll be clearer in simple terms for those who are not familiar enough with technical terms too. As, we need to know in simple terms that:

1. Sound levels in decibels and light in lumen too follow the inverse square law with change in distance. Meaning, twice the distance from sound or light source, 4x power is need. Inverse that distance to 1/2 and the power requirement becomes 1/4.

So SPL, simply put is 1 divided by the change in D sqd. Thus:

2x the distance is 1/2 sqd = 1/4 of the original SPL, means 4x SPL is required to maintain the same SPL at the original distance from the sound source. Thus, the original SPL at 1x is only 1/4 of the the new 2x distance from 1x.

2. In the earlier JGH article on "Too Good to Be True' of 1 Jan, 1963 at:

http://www.stereophile.com/comment/554772#comment-554772#oAVcTQJDvBC7DbM...

I pointed out about source to load matching impedance-modulus to the technically knowledgeable about this. So it will suffice to note that the loudspeaker impedance curves and voltage to current lead-lag phase angles comments by JA in his reviews of speakers are additionally also helpful in determining/choosing power amplifiers' RMS & dynamic/transient power capabilities.

And important is also to note JA points of speaker sensitivities are usually measured at the standard distance (or other by him accordingly) at 1kHz etc per his expertise qualified methods in typical listening rooms. So they are not Anechoic Chamber measurements which are also without typical room gain in the bass regions.

And since we are about bass, in the JGH era, it was told that even 20% speakers distortion is not audible? Would JA like to give us a brief on this if you can find time?

3. Just sharing for how anyone may view it from any perspective per their experience.

I use Infinity IRS dipole ribbons with integral servo unit drive between preamp & power amp. Twin 12" bass units wired in reverse in each bass bin, are directly driven. Their foam surrounds are modified & coated for long term durability to suit our very humid tropical climate for longevity. So the electronic servo unit was modified to offset the modified surrounds tension & motion sensors' sensing behind the dust cap in one bass unit in each speaker. Also, channel tuning are individualised for fine tuning to integrate L & R performance as seamlessly as possible acoustically.

They were made in 1987, and I had them for as modified for some 15 years without needing to change surrounds every 10 months. Yes! Their bass rendition change that fast due to foam deterioration! And both don't deteriorate equally too! As replacement costs increased with foam surround quality & replacements skills become questionable to now a dying trade. Was why I did it as a do or die effort attempt.
Positioning them quite an effort too for best results. I read Stereophile's 3 reviews long ago of a higher 4-units infinity model with separate bass towers. Can't remember the model now.

They are so transparent & musical that they are literally almost like computers - software rubbish in rubbish music out and vice-versa! They can be Heaven or Hell literally, depending on recordings. No back wall too with some ~40ft in front & ~10 ft from the front wall with windows I can open.

Just wonder what designer Arnie Nudell will say if he gets to hear about this "idiotic" effort of mine. He has retired from Genesis too years ago. Am 70+ with still unbelievably good balance ears able to hear tinkling highs beyond 13kHz. Can't complain at my age!

Thanks for your Merkin Hall CD in 2008 dear JA. Cheers! :)

Roger Paul's picture

Mr. Holt,

I am the designer of H-CAT and recently had a major breakthrough. This has a direct impact on your assessment of how much power you need. The significant difference is in the distortion present at various SPL. As you know most if not all amplifiers will begin to strain or breakdown in purity as you drive them harder. You were correct in assuming that a higher powered amp will at least take you farther. What happens if there is no distortion at low levels or high levels? What do you get?

The breakthrough at H-CAT is that you can now amplify without distortion. (Zero) As incredible as it sounds - its true. (It only took me 20 years) So what does it sound like? It sounds un-amplified, 100% effortless and live. Since there is no indicator that alerts you that you are running out of power, you have no idea if you are using a 50 watt amp or a 500 watt amp. In addition to having no distortion this new amplifier method includes the "wave" as captured at the original venue and its velocity is synced to the real world at Mach One. Your central nervous system accepts the sound event as happening live. No other amplifier can perform this function.

I am not trying to "pitch" my products - I am simply making you aware that sound reproduction as we know it has just had a complete game changer. The movie industry especially the 3-D film companies are the first to jump on this. I have been in touch with 4 of the top 10 film makers already regarding things like compatibility with Dolby Atmos and THX certification.

The amplifier method I'm using is 100% pure analog. You simply replace the analog amps in the theaters with "Live amplifiers" and the audience gets a bonus experience they never had. They get to hear actual live sound from the block buster movies. You can see my press release via my website.

Roger
www.h-cat.com

corrective_unconscious's picture

While claiming zero distortion.... And you are not pitching, you are spamming.

Roger Paul's picture

I do apologize for addressing Mr Holt on this site as I was not aware of his passing. Perhaps it is a testimony to my absolute disconnection with the audio industry in general. As you can see from my web site I have currently one review and one press release. I did not want to make an amplifier that sits on a shelf with every other amp. I did not want to spend money by sprucing up the aesthetics of a product with solid gold handles and force customers to hire piano movers to bring them their new mono blocks. I would rather keep my day job.

Instead I have been laser focused on one single issue that has grabbed my attention since I built my first amplifier in 1969 - Distortion. My curse, if you will, has been the puzzling nature of electronic amplification and why it does not just work.

I think outside the box and it is the only reason that I have been able to conquer distortion. I would have to include my faith as well because I do believe I have been given a gift of insight into the inner workings of electronic circuitry. Many people who know me privately will tell you it is true. At the risk of ruining my own reputation, I will gladly tell you why I have been able to accomplish a nearly impossible feat. I can look at a schematic diagram of a complete circuit a see it operate with all of the devices appearing as simultaneous moving parts. Some engineers would say that's nothing and they can also envision a working circuit having been schooled in the field of electronics. The difference is that they can't see or failed to notice something that has been hidden in analog circuitry since the grid was added to a rectifier to make a triode.

Almost like the glass "half full or half empty" it is all in the viewpoint. Take two approaches to making a statue of Abraham Lincoln.
One artist may start by continuously adding clay from the bottom up and shaping the arms,legs and head etc. Another artist may start with a solid block of granite knowing that Abraham Lincoln is already in the center and all you need to do is chip away everything that is not him.

This is how I built the first true 3-D playback system. I already knew it was possible to project sound into mid air. The trick was finding out why it wasn't working. This took decades. No other designer or giant multi million dollar corporation bothered to pursue this goal because they thought that amplifiers have come a long way and distortion has already been reduced to "low enough" levels to where it "doesn't matter". Wrong.

There are many audiophiles reading this posting that I'm sure are yelling "what about the distortion in loudspeakers being higher than the amplifier so you would not be able to hear a perfect amp if there was one?". The answer is simple. Whatever the impurities are in the transducer, one thing is still certain - however they alter the sound "image" it is constant and more important it is stable. Analog amplifiers are unstable and do most of the damage to the sound of a system. By locking the delivery speed to the speed of sound, an amplifier can appear as a parallel path to sound waves. In other words, the amplifier output can produce a "wave" of sound that would be in sync with a real sound wave traveling outside the amplifier. Conventional amplifiers have a tendency to accelerate or de-accelerate the flow of information headed to your speakers. Even though this instability in speed is tiny, your brain knows instantly it is fake and any hopes of hearing live sound are lost.
Real live sound is naturally stable because the medium of air is stable.

Don't blame the speakers - blame the electronics. I have had customers that were baffled by their own speakers saying "I had no idea my speakers could do this". What they thought were speaker problems were in fact amplifier problems.

Roger

corrective_unconscious's picture

Accordingly, I am unable to gather anything from your website as I have not visited it.

How are the efforts at perpetual motion devices coming along?

Roger Paul's picture

This is why I have enjoyed being disconnected from the audio circus.
Since you don't use your real name its fun to poke fun at something that you don't understand.

BTW Its 2016 Isn't it about time somebody got rid of distortion?

I don't blame most audiophiles for being skeptical because it is quite a profound claim. But that is not my problem. The open minded ones will pursue an opportunity to listen for themselves. Everyone who has listened to this technology needs only about 10 seconds to determine that this really works and are totally baffled about how it is possible. After hearing the process they are spoiled and every other system now sounds distorted to them. You won't have to go to my web site to learn about it - just keep watching the evening news. You will still become familiar with the H-CAT logo. It will be in the credits at the end of your favorite movies. (This process has already started)

If you happen to be a designer of amplifiers yourself - well I know how you must feel. That's not my problem either. I did my homework.

This will be my last post to you. I will not engage in a discussion with someone who has predetermined that they know better even though they did not give this the benefit of the doubt until you hear it for yourself.

I wish you luck and hope you find your ultimate sound system someday.

Roger

corrective_unconscious's picture

While ruminating (spamming) about zero error....

On the plus side, there can be big money in communicating with those who have passed over to the other side. And by "the other side" I do not mean "digital."

Ronald Koh - SG's picture

Hi Roger Paul,

Thank you for sharing of your new technological breakthrough in audio analogue power amplification.

Please don't get me wrong. Your H-Cat analogue design at your do indeed sound amazing. And so at this juncture, more technological information at point must indeed be proprietary.

So, without prejudice, I'll must you the benefit of the doubt from our understanding of present non-H-Cat technology. It is only sensible as subjectivity is an intensely complex thing that can lead to fierce controversies versus objective technicalities too. This understanding is why I only read Stereophile with JA's technical measurement with his methods clearly explained. And also co-relating them with his own and/or his reviewers' review comments.

1. So I shall begin with other than power amplifiers and distortions in loudspeakers and our ears perception of distortions without factoring ear deficiencies & ageing defects to not complicate and culminate into long drawn technicalities & subjective controversies.

2. Being electro-mechanical, the inherent distortions in loudspeakers are invariably much higher and also different from electronic power amplifiers. But that is not to say that their lower distortions can't or won't be heard through higher distortions loudspeakers for obvious reasons. Truth is, their type of distortions that our log scale loudness sensing ears can still differentiate them to electronic distortions. Such as in power amplifiers of the push-pull varieties from high-efficiency & distortion Class B to > Classes AB1 > AB2 > Class A. Each have their pros & cons like push-pull crossover, phase, intermodulation and harmonic even & odd distortions. And like Class A that most ardent tube and vinyl analogue followers favour, my experience it that Class A lacks that dynamic peak of rise to fall speed "slam' that has been mentioned by Stereophile reviewers. Thus I use MosFET output power amps for my ribbons for their tube-like warmth and speed of transistors. And normal silicon Class AB1 with higher dynamic & damping factor power amps for the servo driven bass drivers. Just what I consider as a "best" of both worlds compromise that's all.

And thus, last but not least is also how our ears perceive them all distortion types as irritations or not. Along with its ageing & personal understanding of musicality by not focusing on sound-bites instead! Many audiophiles do per my earlier comments in the earlier JGH article on "Too Good to Be True" of 1 Jan, 1963.

3. Thus I'll end here in the hope of someone else with good technical & subjective respectable knowledge to join in and enlighten us all on an even keel, so that we shall all not end up none the wiser.

Thank you Mr. Roger Paul. I shall be looking forward to know more of your new H-Cat design for consideration.

Roger Paul's picture

Thank you Mr Koh for sharing your thoughts. Yes it is controversial and I have had my share of skeptics. If you look at the above posting - I do touch on the reasons that speaker distortion has less effect on the overall system distortion. The secret to an amplifier used for audio frequencies is that it must include a speed governor. Amplifying sound waves have to include the wave itself. The flow of sound as a wave is the phenomenon that has to be passed through the amplifier along with the electrical signal that represents the exact changes in air pressure. Transferring changes in air pressure without maintaining a constant speed will cause the holographic nature of the presentation to collapse.

Roger

corrective_unconscious's picture

The article is from about half a century ago?

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