Where do the other watts go? Need basic understanding please

[johnah5]

I see, so is Slew rate a measure of how an amp can ramp up power for the peaks in the music/movies?


Not sure how to determine the sensitivity of the pass labs or my current SPL S1200 amp but happy to post it if someone can tell me where it is?

 

[pseudoid]

And Yes, on a Class A amp, the extra watts do get wasted into heat.
On Class A/B, not so much.
On Class D, almost none.

Those were parts of the "fish", now he has no need for the "fishhook"!

 

[Ken Tajalli]

Those were parts of the "fish", now he has no need for the "fishhook"!

C`mon !
Let's answer his question.
He could always have researched here and there.
Lets make him feel lucky he tried here.

 

[Doodski]

I see, so is Slew rate a measure of how an amp can ramp up power for the peaks in the music/movies?

Slew rate can be used to explain the rate of change in voltage of a amplifier and it can be used to calculate the bandwidth of the amp too by taking the slope and using that to determine the maximum frequency the amp is capable of.

 

[Ken Tajalli]

Slew rate can be used to explain the rate of change in voltage of a amplifier and it can be used to calculate the bandwidth of the amp too by taking the slope and using that to determine the maximum frequency the amp is capable of.

True.
better still, consider it not important for as far as you are concerned.
Sales talk.

 

[MRC01]

... Music has a peak-to-average ratio of about 50:1 (17 dB) and movies can be 100:1 or more. Think a very quiet passage to a very loud passage, or whispers to gun shots, plus there are transient peaks from things like drums (and guns) that require peaks well above your average level. So at 1 W you may need 50 W or more to avoid ever clipping on dynamic music or movies. That is why people get bigger amps than they need for just the average listening level. Of course, some go way overboard...
HTH - Don

Yep. It's worth emphasizing something @DonH56 mentioned yet may slip past the casual reader: the increase in watts needed for increasing sound levels is not linear! That's the main reason why some people get such hefty amplifiers. A contributing factor is that some speakers aren't as efficient as yours.

What perceptually is twice as loud? In voltage, 6 dB is a doubling. In power, 3 dB is a doubling. But I'm talking about how we perceive the sound. Our perception is not linear. The question is subjective, but commonly agreed to be 10 dB. And 10 dB louder is 10 times the power. By this definition, perceptually speaking, a 400 W amp is only twice as loud as 40 W, which is only twice as loud as 4 W.

There's a common phrase that says, "The first watt is the most important." It's because you only need a watt or two for normal listening. But as soon as the music has wide dynamic range, or you play it louder, the power required can quickly go through the roof due to the exponential relationship between perceived loudness and power.

 

[johnah5]

@Ken Tajalli great quote and yes. It was a two part thought that I didn't articulate well. Your post answered the first thought but my main question was when you have a 300wpc amp and 93bd speakers how is that not overkill and unneeded watts? Like when you drive a 93db speaker at 8ohms why do you need an amp that has more than say 20wpc? I should have asked "why all the watts?"

Still not sure on why amps are made above say 25 watts
Thanks
jh

 

[MRC01]

... Still not sure on why amps are made above say 25 watts
...

Here's an example: Magnepan 3.6/R which produce 86 dB @ 500 Hz @ 2.83 V. At 500 Hz they have 4.2 ohm impedance so 1.9 W makes 86 dB. If you want to achieve the THX standard (85 dB plus 20 dB headroom = 105 dB) you need to go 19 dB louder, which requires 160 watts.

PS: at the opposite extreme, if you have very efficient speakers, especially in a small room, then 25 watts might be more than you need.

 

[Doodski]

@johnah5 this is a SPL calculator. If you look at it for a couple of minutes and plop in some test numbers you can see that amplifier power, distance from speakers and efficiency of speakers are all related and changing one changes the end outcome. I suggest make up some fantasy numbers and pop them into the calculator and see what happens.

 

[johnah5]

I certainly was NOT going to get into how some becomes heat, or other parts are kicked back to the utility, or power factor and not touching noise and emi...
@johnah5 sounds like he already knows the Ohm's Law and can jump around the logarithmic scale readily, so why should we bloviate?

Honestly have no clue and don't get off stirring a pot for the fun of it. I am new to the hobby and love learning about things.

 

[DonH56]

Yep. It's worth emphasizing something @DonH56 mentioned yet may slip past the casual reader: the increase in watts needed for increasing sound levels is not linear! That's the main reason why some people get such hefty amplifiers. A contributing factor is that some speakers aren't as efficient as yours.

What perceptually is twice as loud? In voltage, 6 dB is a doubling. In power, 3 dB is a doubling. But I'm talking about how we perceive the sound. Our perception is not linear. The question is subjective, but commonly agreed to be 10 dB. And 10 dB louder is 10 times the power. By this definition, perceptually speaking, a 400 W amp is only twice as loud as 40 W, which is only twice as loud as 4 W.

There's a common phrase that says, "The first watt is the most important." It's because you only need a watt or two for normal listening. But as soon as the music has wide dynamic range, or you play it louder, the power required can quickly go through the roof due to the exponential relationship between perceived loudness and power.

Thanks for the kind words and additional clarity.

I usually list the volume steps for folk, was busy today (and at 8 pm just finishing work after a 5:30 am start, blah). Here is my usual post, which just repeats yours, but mine has bullets!

In the midrange (the curves compress for deep bass and very high frequencies):

• 1 dB is barely noticeable if you change the volume by that much and requires about 26% (1.26x) more power.
• 3 dB is what most people will do when asked to "make it a little louder" and takes twice (2x) the power.
• 6 dB is much louder to most people and takes four times (4x) the power.
• 10 dB sounds twice as loud and takes ten times (10x) the power.
• 20 dB, four times as loud, takes 100 times the power!

IME most people overestimate their average power level and underestimate the power needed for peaks.

 

[DonH56]

I see, so is Slew rate a measure of how an amp can ramp up power for the peaks in the music/movies?

Slew rate is a measure of how fast the amp can ramp up power for peaks (or anything else). The slew rate of modern amplifiers far exceeds what we as listeners need worry about.

 

[MaxwellsEq]

There are all sorts of special cases.

But for an ordinary amplifier or AVR that you can buy in your high street shop, those extra watts don't go anywhere. At normal listening levels, your 300 Watt amp is only drawing, say, 10W from the wall socket. It isn't drawing 300W and having to throw away 290W somewhere.

 

[Ken Tajalli]

@Ken Tajalli great quote and yes. It was a two part thought that I didn't articulate well. Your post answered the first thought but my main question was when you have a 300wpc amp and 93bd speakers how is that not overkill and unneeded watts? Like when you drive a 93db speaker at 8ohms why do you need an amp that has more than say 20wpc? I should have asked "why all the watts?"

Still not sure on why amps are made above say 25 watts
Thanks
jh

Because things are not as simple as that.
- An amp that quotes 100W continuous output Capability, may only be Clean for the first 5W! you'd be surprised!
But most people, driving 90+dB speaker efficiency, wouldn't need more than 20W in an average room. But those with less efficient speakers, in larger rooms, listening to Bass heavy music at disco levels, may need more than 300W. You see, horses for courses.
- speaker efficiency figures are given at 1kHz at one meter for one watt, that's a very limited criterion. Most of the music's energy is at lower frequencies, well below 1kHz. I would say 75 percent of the energy is at 200Hz and below, about 20 percent from 200Hz to 2kHz and the rest for higher frequencies (not accurate figures, just enough to give you an idea). We don't know the efficiency of most speakers at these lower frequencies, nor the impedance (Unless we look deep into measurements). So a simple calculation, based on efficiency figures given to calculate needed power, may not do.
- Others have already explained about SPL calculations and powers needed, the difference in loudness of a 20W amp and a 200W one is not ten to one! But fortunately, on Class A/B amplifiers (very very common type), the amp can actually deliver more than the continuous power rating, but for a short time! Meaning a 100W amp can deliver 150W or so, but for a short period. This has got something to do with the power supply section and recovery.
A class-A or class-D perhaps not.

Going back to "where is the extra wattage", you should know by now that:
- On Class A amps, the extra wattage, if not used by the speakers, gets wasted as heat! A class A amp burns a steady amount of power (electricity from mains), whether playing music or not, regardless!
- A Class A/B amp, is like a Car! When you start your car, it ticks over gently, idling, burns a minimal amount of fuel and does have minimal power output, enough to roll the car over (This is when it's operating in class A mode). As you put your foot on the gas pedal, it burns more fuel and generates more power! (Class B mode), the idling of the engine (class A) ensures smooth power delivery as your demands go up and down - imagine if the engine turned off and turned on for stop and go! The movement would otherwise be jerky.
So in a Class A/B amp, if you don't turn your volume up, the amp will not produce more power (nor burn more electricity). The wattage quoted is a mere Capability.
- In a class D amp, for the lack of a better example, it is akin to an EV! Almost all the electrical power goes into driving the wheels! At traffic lights, it almost uses nothing.
This is the most efficient type of amplifier.

 

[pseudoid]

This is the most efficient type of amplifier.

..which has brought with itself a 'slew' of challenges and complexities which may impact some of qualities of audio... but have been creatively solved thru additional designs.
The yin-and-yang of audio for the sake of musical enjoyment.

 

[voodooless]

Why worry about amps.. the real burners are your speakers:

96dB is just above 2% efficient. The rest is converted to heat.

 

[Ken Tajalli]

Why worry about amps.. the real burners are your speakers:

96dB is just above 2% efficient. The rest is converted to heat.

Good one!
But since our ears (like microphones ) require minuscule amounts of air pressure power, so far "This Point" has been accepted as OK.
But you are correct, speakers are that!

 

[pseudoid]

Why worry about amps.. the real burners are your speakers:

What about the sound absorption of the couch that I am sitting in? That eats up much of the power too?

 

[pseudoid]

What about the sound absorption of the couch that I am sitting in? That eats up much of the power too?

I finally figured out [all by myself] the reasons why peeps prefer IEMs. They all must be big fans of *simplicity and of *energy conservation, to the max!

 

[voodooless]

What about the sound absorption of the couch that I am sitting in? That eats up much of the power too?

Nope, far less than 1%.