# Input-power listing vs Output-power rating

ataylor
Posts:

**3**Just out of curiosity, I've noticed the power-input label on the XLS-1002 reads 175-watts while it's rated output is 700-watts into 4-ohms (both sides driven).

Now, given that efficiency even in class D rarely exceeds 90%, how do we get more out than what goes in?

The same is true for the XLi-800, the XLS-1002's class AB predecessor. It reads 430-watts input, but is rated for 600-watts output into 4-ohms (both sides driven).

Is this a function of a Harman-proprietary rating method? Is it not RMS?

Don't take this wrong, it's an honest question and I have both amps (just bought another one last month), but I'd like to know why there's an apparent discrepancy between the input power listing and the output power rating.

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## Comments

205If I am reading this right, you're comparing the 120VAC line voltage to the amplified speaker outputs (which are DC)? They're not directly comparable.

3No, the question relates to the fact that a power-amplifier simply acts as a 'controller' for the associated power supply, therefore the rated output is limited by the capacity of the power-supply.

In this case, it controls the amount of 'power' delivered to the load (speakers) at an audio-rate.

The XLS-1002 back panel says it needs 175W @ 120VAC for power input.

The XLi-800 back panel says it needs 430W @ 120VAC for power input.

Power-amplifiers usually have a 'gain' of around 26-28db or so (this would be the voltage component), but since power-amplifiers are 'current' devices, current is the variable that determines the 'power' output.

So no, we're not talking Vcc here (not DC), we're talking amplified signal: P=I/E, which by definition is alternating (since it's audio).

Normally, the E at the power-input (120VAC) is not stated in PP values, but rather RMS, so is .707 of peak, therefore the resulting power input requirement of the amplifier is in RMS which in the case of the XLS-1002 is stated as 175W on the back panel.

If the power input on the back panel is accurate, it would seem the total output power of the XLS-1002 could not exceed 175W RMS (given 100% efficiency), but it's rated at 700W output total (bridged or both sides driven).

For 700W advertised output, the XLS-1002 efficiency would be: (700W/175W)x100 = 400%

For 600W advertised output, the XLi-800 efficiency would be: (600W/430W)x100 = 139%

Regardless of the 'class' of the amplifier (A,AB,D,H,etc.) or whether the power supply is 'Linear' or 'Switching', the output power cannot under any circumstances exceed the power coming into it (can never exceed 100%).

So, let's try a little experiment: Using the XLi-800's efficiency shown above (since it's a lot closer to 100% than the XLS-1002), let's see if we can determine the 'True RMS' output.

We take 139% and multiply it by .707 (which is how we convert from Peak to RMS or 'average' power).

So: 139x0.707 = .98 = 98%

Now that looks a lot closer to believable since it's less than 100%, so let's apply that to the advertised output: 600Wx0.707 = 424W-RMS total output.

Or about 212W-RMS/ch into 4 ohms and (200x0.707) 141W-RMS/ch into 8ohms.

As for the XLS1002 and other 'DriveCore' amps, I haven't a clue as to what 'mojo' was used to rate that output, maybe you can ask around and clarify, but for the XLi-800 and other class AB amps, it looks like the output is rated in Peak rather than RMS, which in my experience is somewhat misleading and something I wouldn't have expected from Harmon/Crown - input power is RMS, output power should also be RMS, but that's just me.

Thanks,

Tony

4My guess is that the input power label is assuming normal use. If a person is playing music and keeping the levels such that the amplifier is not clipping it will be drawing much less power than it will with a continuous sine wave input at max power. Crown's web page says they test the amps with continuous sine waves at rated distortion so the power rating is real. It wil draw more wall current doing that but there is no reason for them to rate the AC input wattage at a number that reflects testing, not usage in the real world. Audio professionals will be sizing their power distribution networks using this number. Quadrupling it to reflect continuous sine wave testing just costs the professional user money for no reason.

78Rated power is usually expressed and measured using a continuous sine wave signal. This is a very robust signal! And yes, if you do the calculations, powers don't add up.

Most equipment manufactures have 'discovered' that users don't use a sine wave to listen to music but program material. Program material is not so different from peak material and it is real world situation.

I have mentioned previously about under powered systems. Basically, recommendation is to have an amplifier with twice the rated power of the speaker network. Half of the power is for continuous use (rated power of the network) and the rest for headroom purposes (program peaks). So, amps are rated higher to reflect this headroom objective.

Furthermore, power output efficiency has to do with the SMPS design, when applicable (PFC management), and the way the output device switches modulates/demodulates the output signal. This also has to do how the output system accumulates energy in caps and coils and then release it. A bit like F1 cars!

Alain