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.