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Author Topic: Why do Class D amps seem to pull less power than they make?  (Read 20705 times)

Ivan Beaver

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #20 on: January 24, 2015, 10:22:54 am »

I completely agree about the loudspeaker analogy. I've actually done this before. EV claims to have measured the ETX-18SP to output 135db peak with broadband pink noise, whereas I measured around a 130db peak at 50hz.

Anyways, I didn't mean to sound like I had anything against Powersoft or their products, I'm just still confused how they can make a claim that their amps make more power than they consume. If they are indeed referring to peaks in the rating, it still doesn't make any sense that the amp could pull a very very brief peak of 1250 watts, and then somehow turn that brief peak of 1250 watts into a brief peak of 1500 watts and send it to the speaker. I am basing this all off of their specs, which dictate that at 1/8 max output power at 4 ohms (max output power at that resistance being 12000 watts, 1/8 being 1500 watts) the amp is only pulling 1250 watts. At 1/4 max output power at 4 ohms its rated to pull 2500 watts, with 1/4 max output power at that resistance being 3000 watts. Hopefully you can point out my error in this and I can learn from it and stop asking these stupid questions, lol!
The problem with SPL and power measurements is the integration time (response time) used for the measurement.

I bet if you had a SPL meter that could measure the instantaneous peak SPL, it would be higher.  That could be up to 10dB higher than the "fast" response time of the meter.

Regarding amps-I will say ONCE AGAIN, it is for a VERY BRIEF period of time-NOT continuously.

The power supply has a "voltage supply" that is constantly being charged up by the wall power.  When a short peak comes along-it can "dump" a bunch of that supply into the load (loudspeaker), but it cannot dump it for very long or it "runs out" and the power output drops (as much as 6dB or more)

An analogy is drag racing.  There are those people that "believe" those cars can go over 200MPH.  BULLSHIT!!!!!!!!!!!

Have you ever seen a drag race in which the cars even drove for a MINUTE-MUCH LESS and HOUR!!!!!!!!!!!!!!  The race is 1/4mile long-NOT for an hour.

No-the cars can't do that-but people "say they can".  Heck they can't even hold that much fuel------------.

Yes they can do that speed for a very short period of time-but not for very long.

So I dare you to go to a drag strip and try to bring up how they are lying about how fast the cars go-----------------

Music is the same way.  Just look at any musical waveform. The "average" level is FAR below what the peaks are.  And the peaks are what the large power is for.

It can be as much as 20dB (which is a factor of100 times) greater.

I don't know of any other way to explain it.

NOW-for what it is worth-times have changed.  I have an entire wall of my office that is full of old amplifiers that were designed to produce SINE WAVE outputs for long periods and that is how they were rated.

But music is not continuous sine waves.  So while the old methods were fine at the time-modern amps are more "realistic" in terms of power.

HOWEVER they are not rated the same.  This "peak power" that they can deliver varies quite a bit from model to model.  The real answer when it comes down to comparing amps is as follows: "How long can it produce that peak AND what level does it drop down to after that-if driven with a sine wave".

This matters more on some types of music than others.

It is also the "we don't want to talk about that" point that most amp manufacturers try to avoid.

HEY-LET'S JUST TALK ABOUT THE "POWER SPEC" and hope people believe what we are saying.

As I say all the time-you can't answer a complicated question with a simple answer.  But people try to all the time
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Ivan Beaver
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Keith Broughton

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #21 on: January 24, 2015, 10:26:25 am »

An analogy is drag racing.  There are those people that "believe" those cars can go over 200MPH.  BULLSHIT!!!!!!!!!!!
Your responses are quite entertaining Ivan :)
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Ivan Beaver

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #22 on: January 24, 2015, 10:31:34 am »

. The first amp needs 98V entered in the peak limiter to deliver 1200W, the second needs 138V according to manufacturer software. With these settings both amps deliver the same power. None of the manufacturers talk about Vpk or VRMS, just V. Maybe thats where my thoughts went south... :(
AGREED-this can be a very  deceiving point of argument.

It depends on what you call "peak".

From a purely engineering standpoint, the 138V is correct-but it is not what you would read on a standard voltmeter.

It IS IMPORTANT to understand exactly what the numbers mean-what the manufacturer was intending.

Sometimes being "technically correct" gets in the way of "general usage/knowledge".

Just like the original TEF audio measurement system.

It used BNC connectors for all put the mic inputs of the system.

The reason was that it was a piece of "test equipment" and "test equipment" of the day all used BNC connectors.  This meant you had to make your own cables-because nobody made a BNC to XLR cable.

THe new version is all XLR, but sometimes "engineering types" get in the way of practicality.

HENCE the reason FOR THE USER to understand what is really going on and not just rely on the "simple numbers" presented to them.

HOWEVER I could also argue that if you were to set the peak limiter to the RMS voltage (NOT POWER!!!!!! as some people here might think) fo the peak of the waveform, it would actually provide greater control and lower the possibility of damage-because it would be limiting early.

If it was the other way around-damage to the loudspeaker could happen easier.

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Ivan Beaver
Danley Sound Labs

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Alex Berry

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #23 on: January 24, 2015, 10:32:30 am »

The problem with SPL and power measurements is the integration time (response time) used for the measurement.

I bet if you had a SPL meter that could measure the instantaneous peak SPL, it would be higher.  That could be up to 10dB higher than the "fast" response time of the meter.

Regarding amps-I will say ONCE AGAIN, it is for a VERY BRIEF period of time-NOT continuously.

The power supply has a "voltage supply" that is constantly being charged up by the wall power.  When a short peak comes along-it can "dump" a bunch of that supply into the load (loudspeaker), but it cannot dump it for very long or it "runs out" and the power output drops (as much as 6dB or more)

An analogy is drag racing.  There are those people that "believe" those cars can go over 200MPH.  BULLSHIT!!!!!!!!!!!

Have you ever seen a drag race in which the cars even drove for a MINUTE-MUCH LESS and HOUR!!!!!!!!!!!!!!  The race is 1/4mile long-NOT for an hour.

No-the cars can't do that-but people "say they can".  Heck they can't even hold that much fuel------------.

Yes they can do that speed for a very short period of time-but not for very long.

So I dare you to go to a drag strip and try to bring up how they are lying about how fast the cars go-----------------

Music is the same way.  Just look at any musical waveform. The "average" level is FAR below what the peaks are.  And the peaks are what the large power is for.

It can be as much as 20dB (which is a factor of100 times) greater.

I don't know of any other way to explain it.

NOW-for what it is worth-times have changed.  I have an entire wall of my office that is full of old amplifiers that were designed to produce SINE WAVE outputs for long periods and that is how they were rated.

But music is not continuous sine waves.  So while the old methods were fine at the time-modern amps are more "realistic" in terms of power.

HOWEVER they are not rated the same.  This "peak power" that they can deliver varies quite a bit from model to model.  The real answer when it comes down to comparing amps is as follows: "How long can it produce that peak AND what level does it drop down to after that-if driven with a sine wave".

This matters more on some types of music than others.

It is also the "we don't want to talk about that" point that most amp manufacturers try to avoid.

HEY-LET'S JUST TALK ABOUT THE "POWER SPEC" and hope people believe what we are saying.

As I say all the time-you can't answer a complicated question with a simple answer.  But people try to all the time

Perfect! This is exactly the answer I was looking for! I completely forgot about the entire aspect of capacitors, which totally explains the ability to dump more than it pulls, but only for tiny amounts of time, equating like you said to peaks in music. Unfortunately for me, the type of music I play is extremely compressed and there aren't really any noticeable peaks. An unfortunate effect of the loudness war.

Many thanks for your help! Sometimes I get so caught up in something like "this amp claims to be 120% efficient" that I completely forget about electronic basics like the capacitor. And I agree with Keith, your responses are quite entertaining. :D
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Ivan Beaver

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #24 on: January 24, 2015, 10:33:24 am »

Your responses are quite entertaining Ivan :)
I just try to put things in term people can relate to.

OK- OK SOME people-----------------------

Others don't get my responses at all ;(
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Ivan Beaver
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Ivan Beaver

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #25 on: January 24, 2015, 10:37:23 am »

Perfect! This is exactly the answer I was looking for! I completely forgot about the entire aspect of capacitors, which totally explains the ability to dump more than it pulls, but only for tiny amounts of time, equating like you said to peaks in music. Unfortunately for me, the type of music I play is extremely compressed and there aren't really any noticeable peaks. An unfortunate effect of the loudness war.

Many thanks for your help! Sometimes I get so caught up in something like "this amp claims to be 120% efficient" that I completely forget about electronic basics like the capacitor. And I agree with Keith, your responses are quite entertaining. :D
And that is EXACTLY WHY you need to be looking at the DURATION TIMES that were used to come up with the output power numbers.

The longer the duration time-the better chance the amp has to deliver more power longer

Again-these vary WILDLY.  I have seen them as short as 0.02 seconds and up to 4 seconds.  0.08 seems to be some sort of a "standard"-but you need to look to be sure.
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Ivan Beaver
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Keith Broughton

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #26 on: January 24, 2015, 10:46:12 am »


Again-these vary WILDLY.  I have seen them as short as 0.02 seconds
And that's how Behringer makes a 6000 watt amp for $300!

I bought a couple iNuke 3000DSPs to do low level front fill for corporate. Works fine.
Then the boss said why not get some more for the main rig?
(sigh)
I just can't connect a Behringer amp to a Danley sub....I just cant! ;D
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Alex Berry

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #27 on: January 24, 2015, 10:48:18 am »

And that is EXACTLY WHY you need to be looking at the DURATION TIMES that were used to come up with the output power numbers.

The longer the duration time-the better chance the amp has to deliver more power longer

Again-these vary WILDLY.  I have seen them as short as 0.02 seconds and up to 4 seconds.  0.08 seems to be some sort of a "standard"-but you need to look to be sure.

As far as I can tell, the K10 doesn't specify the duration times. Like you said, probably not something Powersoft wants to publicly disclose.

Regardless, many thanks for all the help! Will definitely keep this in mind in the future.
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John Roberts {JR}

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #28 on: January 24, 2015, 11:05:28 am »

This is a very old topic so a search "should" reveal previous discussion threads.

As Ivan said this is a complex topic but consumers like to make purchase decisions based on simple data comparisons so it is not unheard of for power claims from overly enthusiastic marketers that defy physics.

I can't educate you all about how to make simple comparisons. That is why posts from forum member with experience using the subject amps with similar speaker loads and similar musical genres is so valuable. If you think you can understand this subject better than the equipment designers have at it. The marketplace is a reality check for new designs where amps that do not deliver adequate power get identified and shunned by customers.

Also why I continue to be a fan of powered speakers, let some design engineer with a pocket protector worry about these questions. 8)

JR

PS: @Ivan, only a fraction of the size and weight in your wall of old legacy power amps was to support 24x7 duty cycles. For example when the old school 24x7 CS800 was revised to allow higher short term output, the power changed from 800W to 1,200W. Significant but this alone does not explain the so much smaller size and weight of modern amp technology. Improved efficiency reduces the waste energy allowing for smaller heat sinks. An even larger reduction is due to power supply technology. Replacing the huge 50-60Hz power transformer with a small HF switching supply transformer scales down the transformer size proportionately with the increased power transfer frequency. This is a win-win situation only limited by cost of high frequency power devices and magnetics.     

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Brian Oppegaard

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #29 on: January 24, 2015, 11:11:09 am »


I wish the term "peak power" would go away. It is misleading and wrong. Power must be calculated with RMS voltage over at least one complete cycle to have any relevance to audio. For 20Hz, that's 50 ms. I prefer "tone burst power" Tone means audio, at least one complete cycle. Burst means time, how many cycles at what frequency. Power is the area under the curve which relates to the heating effect of the waveform.

Amplifier power can't be expressed with just one number these days. They are designed to have a short term power to get thru a Thump and a long term power to get thru EDM. A curve of power versus time will show both values and if the amp can transition smoothly between them without overshoot or oscillation.

In my bench testing I measured the K10 to do 3600W at 2 ohms for 1.5 seconds, then smoothly dropping to 1260W long term. Test conditions were 50Hz, both channels driven, 230VAC mains. Behavior was good but unfortunately Powersoft consistently overstates their power. I am sure their marketing people say "but everybody does it", and quite a few do, but I don't in my amps. http://www.speakerpower.net/comparative-performance.html

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Brian Oppegaard
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ProSoundWeb Community

Re: Why do Class D amps seem to pull less power than they make?
« Reply #29 on: January 24, 2015, 11:11:09 am »


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