What does moving from a 150 watt amp to a 400 watt amp get you?

From Musical Fidelity Website around 2009

Technical background to our dynamic range/power claims.

This section is slightly technical and may take you a few minutes to read. We would like to take you through to the basic technical elements that make up a hi-fi system and explain how they go together.

Loudspeaker sensitivity

Loudspeaker sensitivity is a measure of how much sound a loudspeaker will give for 1 watt at 1 metre. It is critical to note that the basic measure of sensitivity is at 1 metre and not at a typical listening distance of about 10 feet or 3½ metres.

Sound attenuates (reduces) over distance at the rate of 6dB with each doubling of the distance. At 2 metres distance from the loudspeaker its perceived sensitivity is reduced by 6dB. At a normal listening distance of about 10 or 11 feet from the loudspeaker its perceived sensitivity will be reduced by approximately 10dB.

This is the factual basis for our claims about how much power a system would need for a decent hi-fi dynamic range. We reiterate that this is not made up or marketing hype, it is scientific fact.

Loudspeaker sensitivity – is it real?

Regrettably, most loudspeaker sensitivity ratings are not particularly accurate and are regularly overstated by 2 or 3dB. We have seen several examples of respected manufacturers’ products specifications overstating their sensitivity by 5dB or more.

This does not sound like a big deal, but it has tremendous implications for the power required by the loudspeaker to deliver proper dynamic range.

Amplifier power – confusion reigns.

This is the source of much misunderstanding. Amplifier power is specified in watts, which are a measure of heating power. They have no apparent relationship to what we hear, as they are a linear measure. Loudspeakers (and our ears) perceive things in dB (decibel) steps. These are based on a logarithmic relationship.

1

This is the fundamental mismatch between what your ears perceive and how amplifiers are specified. The solution to the problem is to recalibrate watts into dB steps. The results are below, in a chart of watts converted to dB steps. For convenience we have started our chart at 50 watts. These figures are not made up they are fact.

dBW Watts         dBW Watts

17      50             24      251

18     63             25     316

19     79             26     400

20     100           27     500

21      126           28     630

22      156           29     795

23      200           30     1000

As you can see, as soon as you calibrate amplifier power in dB watts, you get a dramatically different view of what amplifier power really means.

First off, you can see that what looks like a large increase in amplifier power, for example from 50 watts to 100 watts, only gives an increase of 3dB.

Things get really interesting as when you get to higher powers. You start needing vast amounts of power for each dB step. For example, only 1dB (remember 1dB is the smallest change in sound pressure level that the human ear can perceive UNDER IDEAL LISTENING CONDITIONS) is the difference between 400 watts and 500 watts. If you really wanted to hear a difference above 400 watts you’d probably need to go to 800 watts (3dB) which should be audible.

You can see why amplifier manufacturers want to sweep these figures under the nearest carpet; they make most of their claims look ridiculous as they predict that most loudspeaker/amplifier combinations will have only limited dynamic range.

2

How much dynamic range do I need?

Some years ago John Atkinson (current editor of the Stereophile) made some measurements of live music using accurate equipment. He recorded 109dB peaks (brass and percussion) and the quietest was 63dB (solo violin) a variation of 46dB from the quietest to the loudest moments – a huge dynamic range.

The question is what sort of dynamic range a really good hi-fi system should have.

In our opinion, an ability to produce unclipped peaks of 105dB is the minimum starting point for a really good hi-fi system. You are welcome to debate different figures, but that is our basic position.

If you listen to small scale chamber music or usually listen at quiet levels, you will not need the peak capacity we deem necessary. But if you are trying to recreate the listening experience at reasonable levels, 105dB peak is not overly generous.

How do you put all this together.

Assuming that you have accepted the scientific facts this is how you determine what your system can produce.

1. Take your loudspeaker sensitivity (better yet check back to a technical review to find out what its sensitivity really is). Deduct around 10dB for the SPL (sound pressure level) attenuation over distance. And then add back 3dB because there are two loudspeakers in the room.

Now you have arrived at the practical, real world, in-room sensitivity of your loudspeaker system.

2. Decide what peak level you want to achieve. We think 105dB is about right. Some people think 110dB is more appropriate. It’s up to you.

3. Deduct the result of 1 above from your decision about 2. This is how much amplifier power you require in dB watts.

4. Use the chart above to translate your dB watts result in to ordinary watts.

General overview.

None of this is intended as criticism of amplifier or loudspeaker manufacturers.

You could regard it as a criticism of magazines and shops for not bringing it to your attention. We have been banging on about this for about ten years and many people have reacted adversely because they believed it was just marketing hype.

This is not marketing hype, this is scientific fact. Many people do not like the result of a scientific analysis of their equipment but that does not alter the scientific facts.

People have objected to our position claiming that their system sounds great. It might. However, you can’t escape the fact that, if they have a low powered amplifier and relatively insensitive loudspeakers, the system must be clipping, distorting and limiting regularly, which must dominate the listening experience.

Maybe the listeners like distortion clipping. Well, each to his own and good luck to them. If you want your hi-fi system to produce as close an approximation to the real live performance as you can get, then you must ensure, for a start, that your amplifier is not clipping.

1)The reference to SPL (sound pressure level) falling off at 6 db per doubling of distance applies to most non-planar (box type) speakers. The rate of fall-off for planar speakers such as electrostatics is significantly less.

2)More often than not sensitivity is specified as the SPL produced at 1 meter in response to an input of 2.83 volts, rather than in response to 1 watt. For an 8 ohm impedance those results will be identical, since 2.83 volts into 8 ohms corresponds to 1 watt, but for a 4 ohm impedance the SPL produced in response to an input of 2.83 volts will be 3 db less than if the spec had been based on 1 watt, since 2.83 volts into 4 ohms corresponds to 2 watts.

In any event, Stereophile measured the OP’s speakers as providing an SPL at 1 meter of 89 db in response to an input of 2.83 volts, which corresponds to 1.6 watts into what I would consider to be its 5 ohm impedance (see my earlier post regarding the 5 ohm figure). 1.6 watts is about 2 db more than 1 watt, so the speaker will produce 87 db in response to an input of 1 watt. The 240 watt capability of the amp into 5 ohms (again, see my earlier post) corresponds to about 24 db more than 1 watt. Two such speakers listened to at a distance of about 10 feet when driven with 240 watts will produce an SPL of approximately 87 + 24 + 3 -10 = 104 db, very close to the 105 db minimum recommendation provided in the paper.

And as I mentioned earlier, an amplifier capable of providing 400 watts into 5 ohms would add very little to that figure, 2.2 db to be precise. Unfortunately, IMO, it appears that very few of the responses by the others have focused on the OP’s situation in a comparably specific manner.

All of this assumes, btw, that the speakers are capable of handling these high power levels without significant thermal compression or other non-linear effects becoming significant. I have no knowledge of the maximum power handling capability of the Revel Ultima Studio 2, or of how well it would perform as its power handling limit is approached.

I would add that while I personally listen to a lot of classical symphonic recordings that have been engineered with minimal or no dynamic compression, and consequently I have stated in some past threads here that 105 db at the listening position is the minimum peak volume capability of any amp/speaker combination I would consider (which happens to coincide exactly with the paper’s minimum recommendation), my perception has been that the majority of audiophiles do not listen to recordings that when played at their preferred volume levels would come anywhere close to such high peak levels. With peak levels not exceeding even 90 db in many cases that have been mentioned in past threads here. I believe the main reason for that is the high degree of dynamic compression that most recordings in various genres, especially pop and rock, tend to be engineered with.

Thanks again. Regards,

--Al

I'm sorry to say that most of the responses you've received are not useful, and some are nonsense.  (There are some good ones too, so don't everybody feel insulted.)

I'll try to help...  In general, more watts are not needed for more volume.  They are needed for more 'headroom'.  In a musical recording, the transients (leading edges of any sound) are MUCH higher amplitude than the rest of the signal.  Having plenty of headroom allows your amp to produce those (fraction-of-a-second) transients at full volume without attenuation or 'clipping'.  That results in a much richer, more robust and more REALISTIC presentation.

It's also true that less efficient speakers will perform better with plenty of wattage & current, because they simply need more power to 'sing'.  I tend to like the sound of low-efficiency speakers mated to powerful amps, but there are many exceptions (& that's my personal taste).

That being said, your Revels are rated at about 88 dB sensitivity w/ nominal 6 Ohm impedance.  That is a moderate, rather than difficult, load, and should not require 400 W/ch.

What's more important is the QUALITY of the amp!  The Ayre V-5xe is a nice-sounding amp. You could easily find a mega-watt amp that is not so well-designed... and take a step BACK in sound.  I would try to audition the Revels with a more powerful amp before spending any $$. You may find that there's no real advantage.

BTW... what pre-amp are you using?  Have you tried a tubed pre-amp?  I've had great success matching solid-state power amps with tubed pre-amps (incl. my current system). That gives you the tight, well-controlled bass associated with solid-state as well as the beautiful mids & highs that only tubes seem to provide.

Anyhow... I hope that helps. 

...for a 4 ohm impedance the SPL produced in response to an input of 2.83 volts will be 3 db less than if the spec had been based on 1 watt, since 2.83 volts into 4 ohms corresponds to 2 watts.

...for a 4 ohm impedance the SPL produced in response to an input of 1 watt by a speaker whose sensitivity spec is based on 2.83 volts will be 3 db less than if the spec had been based on 1 watt, since 2.83 volts into 4 ohms corresponds to 2 watts.