Axiom Home Page Forums General Discussion Technical Questions More on Amp power

This is a follow up to a discussion here on what a manufacturer is promising in their specifications on amp power. The following was posted by Mark Seaton on AVSforum

"The amp being referred to isn't the Speakerpower amp, but the one FunkyWaves mentioned, but I think you are implying a definition of "RMS" to mean long term continuous output. The two are not the same. RMS is a root-mean-square average which does imply that the power exists long enough to create and measure a sine wave. "RMS" makes no indication of how long that sine wave must be produced. Most of the modern, >kW Class D amps are designed a bit more practically with the capability of sustained power ranging from 5 seconds to a few minutes, depending on the design. Most common is about 15-45 seconds with the power then ramping down to anywhere from 1/6th to 1/2 of the maximum RMS rating. The new high power Speakerpower amps are in the 1/3rd-1/2 sustained realm, ICEpower modules are closer to 1/6th, with the PowerSoft designs having a rather short duration at very high power, and much less on the sustained power (I don't recall the ratio).

Lesson of the day? Don't read more into a spec than is specified."
So, my understanding is still that when a manufacturer promises 130 watts x 2 they are talking about RMS power. In the above, Mark is repeating what I was told by a senior engineering manager for Peavey in another AVSforum thread.

Johnk. Does this contradict what you have been writing here or are we talking different things?

Fred, it depends on what part of that quote that you're specifically referring to. If Mark was condoning the use of "RMS" with reference to power(I don't grasp the full context of the discussion from the quote), then that's imprecise use of language, which generally sets the teeth of EEs on edge, since there is no such thing. A root mean square average is used to calculate the voltage and current which results in an amount of power, but the power itself, i.e., the watt, is a fixed quantity which isn't subject to an average.

If you're referring instead to the manufacturer's rated power spec, as has been pointed out several times here, the FTC regs require that it be measured at the full rated power output for at least five continuous minutes.

Hmmm. OK. You have told me before that the FTC rules do not apply to car audio. Can I assume this is also true for Pro audio?

Here are some specs from a Peavey Class D amp I am looking at:

Rated Power (2 x 2 ohms) - 800 watts per channel @ 1 kHz at <0.1% T.H.D. both channels driven.
Rated Power (2 x 4 ohms) - 530 watts per channel @ 1 kHz at <0.1% T.H.D. both channels driven.

Current Draw @ 1/8 power - 550 watts @ 2 ohms, 390 watts @ 4 ohms, 250 watts @ 8 ohms
Current Draw @ 1/3 power -1,160 watts @ 2 ohms, 810 watts @ 4 ohms, 460 watts @ 8 ohms

Lets start with another question. Does the current draw @ 1/3 power of 1,160 watts translate to 560 watts x 2?

From what the pro amp guys, including the Peavey engineer, have written, their amps will deliver the 1/3 power draw all day long, but the full power (rated power) only for short periods of time (up to 45 seconds).

Another curious statement on the spec sheet is:

2 ohm power is time limited by circuit breaker.

I don't quite know how to interpret it or the other specs until I understand what rules apply.

Perhaps one of you engineer guys can comment on this, assuming the wattage ratings are accurate(according to manufacturers claims)....

Scenario 1) an amplifier rated at 125 watts per channel, playing into a pair of three-way(passive) tower speakers

...vs...

Scenario 2) 3 amplifiers rated at 75 watts per channel powering the same(in theory) three-way tower speaker but with an active crossover.


Questions...which would theoretically play louder and cleaner? would the separate (less powerfull) amps to the separate drivers put out similar db's overall, and would they also play with less distortion being separated into 3 amps. Would there be some type of correlation wattage-wise between these two scenarios(eg. single amp/100 wts per ch/passive...equals...3 amps/50 watts per c/with active crossover)

Fred, here again are the regs for reference. You'll note that the title relates to "home entertainment products", but so-called "pro" amplification equipment which is also sometimes sold for home use must also be in compliance.

The quoted Peavey power specs do in fact comply with the regs, stating power, impedance, frequency and distortion. As required by interpretation, this is with both channels driven.

If I follow what you mean by "translate", yes, the consumption figure(and of course it's consumption, not output)can be viewed as each channel being involved in the consumption of 580(not 560)watts. The indication from the numbers is that since the 1/3rd power figure is 533 watts({2x800}/3), the efficiency at that output is about 533/1160=45.9%.

Adrian, that would depend on how much efficiency in the use of available amplifier power was gained by removing or bypassing the internal speaker crossovers and instead applying external electronic crossovers before the amplifiers. It's possible that the 75 watt amplifiers would be able to supply the speakers with more maximum power than would the 125 watt amplifier, and if this additional power was actually needed at a particular instant, the peak would be played more cleanly.

Thanks John. OK, it took me a couple of reads to understand the consumption part.

I am still confused as the Peavey guy told me that it would not do continuous output at full rated power. I think that is where the 2 ohm output being breaker limited comes from.

So, someone at UL or similar organization /facility put this thing on a bench and tested it continuously at its full rated power.?

You lost me on that last part. The 1160 watts is a single channel? My assumption was that since there was no specification of channels, the 1160 watts would be total consumption for both channels of output. Should it not be 533/580 which gives us 91% efficiency? That would fit wwith the stated efficiency of class D amps.

Adrian. I am no engineer so my answer is just an educated guess, but I would think its about the total watts available to drive the whole system. Three 75 watt two channel amps would give you 225 watts per channel vs 125 for the first scenario.

Cleaner would depend on the equipment in the signal chain, but most electronics are pretty good in this regard. If you are using pro amps as your outboards then the receiver would be cleaner on paper, but I doubt you or anyone could hear the difference.

I always thought that going to active crossovers is all about controlling the signal going to various channels. I guess in the pro world this would allow you to tune your system to each specific venue.

No; the 1160 watts is for both channels operating at 1/3rd power. Those 90% efficiency numbers for class D amplifiers are at full power output. All amplifiers drop in efficiency at lower output percentages. For example, class AB amplifiers at 1/8th output(the output used for UL testing and typically for the power consumption number)are about 20% efficient. It wouldn't be unusual for a class D to be around 50% efficient at 1/3rd power.

No, Fred; the amount of power available per channel would be that of one amplifier, not three.

New book idea:

Electronics From Dummies; All the wrong answers you won't get anywhere else.

I'm gonna start writing tonight...

Make sure it has lots of pictures in it Fred, with illustrations..."never cross the positive and negative terminals, like this... BSHHHZZZTT!!! "

Thanks guys for the input on the amps(mostly to John ).

I've read user reviews(with a grain of sodium) about comparisons of some 2 channel amps where users have claimed such things for instance, as, 2 channel Amp "A" rated at 75 watts per c. is as powerfull as 2 channel Amp "B" rated at 200 watts per c. Wishfull thinking? trying to justify a new purchase? insider?

So to simplify for us noobs,

-- In a medium sized room, say about 14x22x7

-- Using a pair of M80's

-- Attached to good receiver (AVR-3311ci - 125 watts/channel)

-- Playing at a comfortable listening level

Would the M80's sound better, worse or the same if one added a 2-Channel 200 watt/channel amp?

You're still going to get about 15 different answers to that question. Different people think different things about that question. I can just about guarantee that it wouldn't sound worse...

Physics. It works Bitches!

If I'm reading that right, you are saying that to pump out volume 'x' a speaker will require 'Y' Amps.

If it gets more than 'Y' amps it won't use them at volume 'x'.

Just like if I plug a 60watt light bulb into a 11,000 watt plug (110volt*100amp) It still only draws 60watts.

Am I on the right course here?

That's correct, but it's actually a bit more nuanced than certain people like to portray it as. Sound reproduction doesn't use a constant amount of power--it's peaking and dropping all the time. So the theory goes (not sure if I subscribe to it or not) that the more you have in reserve, the easier it is to achieve those peaks.

Yes, program material has dynamic peaks. That is why you add in some headroom when you are calculating the amount of power needed.
This Crown page talks about power requirements, dynamics in different program material and a few other things.

Is is worth while noting that good 'ol rock 'n roll requires very little head room, maybe 6 db.

To complicate things more, you have a little power 'in reserve' in your receiver in the form of charged capacitors that can handle very brief peaks in power demand. I have seen some pro amps described as 'more ballsy' because they have larger caps capable of sustaining peaks for longer periods of time.

The short answer is that if you give yourself 12 db of headroom and subtract 5 for room gain and then calculate the power you need in average sized rooms when listeneing at around 10-12 feet, you need a lot less power than you would think

So the theory is that a 1" garden hose would fill a 10 Gallon bucket faster from a 1,000 gallon tank than from a 100 gallon tank? Ok I guess you could make a case for higher water pressure generated from the 1,000 gallon tank. But thinking way back to basic physics, flow in an electrical circuit is governed by resistance not wattage.

Really? Let's try it!

I took my SPL meter to a National Symphony Orchestra concert last week. During the crescendos of "Also Sprach Zarathrustra" (aka the 2001 theme) the meter peaked around 90dB (we were in the cheap seats). So we'll use that.

So 90dB + 12dB for headroom

At home, the primary seating is ~3-Meters from the speakers.
The M22's have 93bd Sensitivity. Plugging all that into the calculator:

Listener Distance: 3m
Desired Level at Distance: 90db
Sensitivity: 93
Headroom: 7db (12db -5bd? for room gain and I'll have to google what that is)

I need, wait for it.... 23 Watts.
Umm, err, 23? Wait, why in the hell were we talking about amps in the first place?

Exactly.

One point. If your meter peaked at 90 db, then that number includes the required headroom and you are using even less than that whopping 23 watts.

Edit: But then again, maybe you want to transport youself from the cheap seats to the front row when you are at home.

Wow. I thought all this "Applied Physics" stuff was just good for jokes on "The Big Bang Theory". Who knew it could actually save you real money!

Not sure but I think you might be double-dipping on the room gain.
The M22s are 89dB anechoic and 93dB in room.
Also I suspect that the calculator is looking for anechoic sensitivity as it is more of a standardized measurement while “in room” would be dependent on room size used.

All of these calculations are for 1 channel. What is the resulting decibel level of having, say, 3 channels all producing 90dB?

The same I would think. The calculator gives the power required to generate that volume whether you use one speaker or 5.

Johnk will be along sometime this evening to correct me if I am wrong.

Assuming you are right, let's re-run the numbers:
Distance: Still 3m (10Ft)
Desired volume: 90dB
Sensitivity: 93
Headroom: 12db

New Tally Please..... 71 Watts. That gives me up to 90db average sound with peaks up to 102db correct?

Scamp, as I've mentioned several times, I'm unenthusiastic about the Crown calculator. The main reason is that if it was used as shown on the calculator page the result would be far too high. Only in their discussion page do they point out that the result pertains only to anechoic or out-of-doors conditions and suggest using a 6dB correction factor to account for a typical room contribution. Of course, 6dB means that the result would then be 1/4th of the calculator result.

What my analysis would be is that the M22 has a sensitivity of 89dB(anechoic)and the studies of Dr. Toole and others indicate that sound level in home listening rooms(counting both direct sound and room reflections)drops off about 3dB per doubling of distance. At 3 meters(also about what my listening distance is)the level would be a bit over 4dB lower than at the 1 meter measurement distance. For about 85dB at 3 meters the M22 would use 1 watt. For the 90dB peak you measured during Also Sprach Zarathustra a bit less than 4 watts would be used.

Incidentally, I've also taken my SPL meter to concerts a couple times. As compared to what I could afford when I was a student, I now sit a lot more up front and the peak measurement on the Max setting was 106dB(during the Shostakovich 5th Symphony). I also had to explain what the meter was to some rather apprehensive patrons. They were satisfied that it wasn't a bomb and likely felt that although I was a nut, I wasn't a dangerous one. To duplicate that 106dB at home(I certainly almost never listen that loud)requires about 100 watts used by an M22.

You have it right now Scamp. I understand John's concern about the calculator not having something specific for room gain, but if you understand how the formula works, you can add it in where appropriate. Axiom just adds it to their sensitivity number.

I can't say I have ever listened to something at an average level of 90 db in my livingroom. I usually listen between 75 and 85 db depending on my mood. 90 just hurts (I did try it with spl meter in hand).

John. I'm trying to picture you with an spl meter in hand at a concert. Quite funny!

Damn. The kids really will be able to do some damage with these... Hopefully they won't figure out how to re-set the volume limiter.


The concert, Movies, outdoor events, me and my spl meter are inseparable. I may need a "weird but not dangerous" T-Shirt.

Are you suggesting that 2 speakers producing 90dB each aren't louder than one or that the amplifier power required, per the calculator, can be divided by the number of channels or both?

There is a little loss, running more channels. But the power required to deliver a specific sound pressure level at the listening position is the same no matter how many channels are involved. So closer to the later of what you're asking.

D, for example(disregarding losses Chris mentioned), if you have one speaker using 1 watt for a relatively loud 85dB at the listening position and then add a second speaker also using 1 watt for 85dB, 2 watts are now used, but the combined output is now 3dB higher, for a new 88dB sound level. If the overall sound level was instead to remain at the original 85dB, each speaker would use 1/2 watt for an 82dB output, with the result that 1 watt would be used for an 85dB sound level, as was the case using a single speaker.

What I wonder is how does one calculate adjustments for wind noise when waving the meter around like a baton?

Of course this also explains why they often have big foam bubbles over the microphone end. It's to prevent you from taking out your neighbors eye during a particularly lively crescendo.