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Author Topic: Why do Class D amps seem to pull less power than they make?  (Read 21813 times)

Brian Jojade

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #40 on: January 28, 2015, 12:17:06 pm »

If you are measuring joules, that is a measure of work. A small amp at a long gig is likely to pass more joules in total than a larger amp at a short gig and thus do more work.

Power is the rate of doing work. It is equivalent to an amount of energy consumed per unit time. The unit of power (MKS) is the joule per second (J/s); more commonly known as the Watt.

The question that seems to be in dispute is what time frame you use to measure the number of joules coming in and out of the amplifier to define the power rating.

Is publishing the specifications showing the instantaneous power over a millisecond lying or not? It all depends on whose version of the truth you believe.


This is the measurement that answers the OP's question. 'Why do Class D amps seem to pull less power than they make?'  Stating rated maximum watts with a variable duty cycle makes it seem that the amp is making more power than it is drawing. But when you measure true total power going into the amp and total power coming out of the amp, you'll find that it's not making power at all.
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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 #41 on: January 28, 2015, 12:24:40 pm »


 But when you measure true total power going into the amp and total power coming out of the amp, you'll find that it's not making power at all.
EXACTLY the reason they should be called VOLTAGE AMPLIFIERS and not Power amplifiers.

They do amplify the VOLTAGE of the input, but not the POWER of the input (signal and AC power combined)

Just being picky-but truthful :)
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Don Boomer

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #42 on: January 28, 2015, 07:29:14 pm »


 But when you measure true total power going into the amp and total power coming out of the amp, you'll find that it's not making power at all.

Exactly ... amps just redirect the power they take in ... well except the perpetual motion variety  ;D

I just wanted to add that the current draw you see listed on the chassis usually reflects operation at 1/8 of the rated continuous output for agency compliance.
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Peter Morris

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #43 on: January 28, 2015, 08:43:12 pm »

The computer ate my first answer so I'll keep this brief.

AFAIK the 5 sec duration for power testing is arbitrary so not an industry standard.

How the amps work for you in your application is the real test.

Don't just take my word for it, but have some trust in the market to identify and punish any serial liar who over-promises and under-delivers on amp power. Back in the '70s it was so bad that the government got involved. Any simple government imposed answer to a complex issue like amp duty cycle will cause unintended consequences (like the old FTC specs did).

Research actual end user reports about specific products on forums like this. Trying to understand dynamic power specifications is complicated (please trust me about that at least). 

JR

Hi JR,

As I understand with Powersoft amplifiers there is peak power mostly determined by the PSUs maximum rail voltage, and a short continuous power rating determined by the amounts the rail voltage sags under continuous load.

There is also a thermal limit; I assume determined by the thermal capacity of the heat sinks and switch-mode HF transformer. After a short time the amp turns itself down.  The gain is reduced and there is no distortion form what I can remember.

In practice you can occasionally notice the amp turning itself down. When this has happened to me I have been grateful; all my speakers survived despite the efforts of some stupid operator who was determined to destroy something.
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Steve Bradbury

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #44 on: January 29, 2015, 06:09:32 am »

Quote
EXACTLY the reason they should be called VOLTAGE AMPLIFIERS and not Power amplifiers.

They do amplify the VOLTAGE of the input, but not the POWER of the input (signal and AC power combined)

Just being picky-but truthful :)

Whilst most power amplifiers are voltage followers, in that the output voltage is supposed to be a fixed multiple of the input voltage irrespective of the load impedance, for them not to be power amplifiers the output current capacity would have to go down such that the input and output Volts x Amps product remained constant.

As both the output voltage and current are increased when compared to the input signal then I think that it is not too untruthful to call them power amplifiers.

Quote
Exactly ... amps just redirect the power they take in ... well except the perpetual motion variety  ;D

I just wanted to add that the current draw you see listed on the chassis usually reflects operation at 1/8 of the rated continuous output for agency compliance.

The problem is that the terms power and energy are frequently confused. Whilst the amount of energy coming out cannot exceed that going in it is possible for the energy to be transferred to the load at a greater rate than it is coming in, albeit for limited periods.

For example say you have a battery powered amplifier. The charger may be limited to 10W output, but once the battery is charged the amplifier may be capable of producing 100W. The trade-off is that the battery takes 10 times as long to charge than it does to go flat.
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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 #45 on: January 29, 2015, 11:49:40 am »

Hi JR,

As I understand with Powersoft amplifiers there is peak power mostly determined by the PSUs maximum rail voltage, and a short continuous power rating determined by the amounts the rail voltage sags under continuous load.

There is also a thermal limit; I assume determined by the thermal capacity of the heat sinks and switch-mode HF transformer. After a short time the amp turns itself down.  The gain is reduced and there is no distortion form what I can remember.

In practice you can occasionally notice the amp turning itself down. When this has happened to me I have been grateful; all my speakers survived despite the efforts of some stupid operator who was determined to destroy something.
Thanx.. yes that chart reveals the difficulty in coming up with an arbitrary peak power test time duration. Back in the 24x7 day that would be a 1000W amp, but I guarantee that those short term peaks play a lot louder than a 1000W amp in real world music use.

Powersoft designed their amp in response to their own bench work, hardware trade-offs, and years of customer feedback.  An arbitrary industry peak power test can make winners and losers depending on where they land wrt that metric. 5 seconds for 2 ohms would clearly understate the actual short term peak power available. In real music there are lots of transients that are shorter than 5 seconds, and very few that aren't (which is what their apparent 10 second threshold supports) .

THIS IS COMPLICATED but not rocket science. IMO an industry standard could create winners and losers not necessarily a bad thing but it could be for the losers.

Power vs time plots like this are good and I think a few of the big dog amps have published such plots.   

JR

PS: It must be a slow week... lots of pedantry about what words mean... I thought that was only me making such petulant complaints.  8)
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John Schalk

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #46 on: January 29, 2015, 04:14:54 pm »

Powersoft designed their amp in response to their own bench work, hardware trade-offs, and years of customer feedback.  An arbitrary industry peak power test can make winners and losers depending on where they land wrt that metric. 5 seconds for 2 ohms would clearly understate the actual short term peak power available. In real music there are lots of transients that are shorter than 5 seconds, and very few that aren't (which is what their apparent 10 second threshold supports) .

Here is a link to a PDF that contains the graph posted above for the K10. The 23 page file compares an fP-6400, an i-T8000 and both a K6 and K10.  The commentary that explains each test is pro-Powersoft, but that doesn't take away from the interesting results.  It's worth noting that the K10's rated output into 2 ohms stereo is 6,000 watts, so the fact that it can make 7,500 watts for a tenth of a second is pretty cool, I think.

http://www.diy.poweraudio.ro/albums/userpics/10001/BENCH_COMPARISON_TEST.pdf

Here is a link to a very technical review of a K3, K10, and M50q, also with plenty of graphs, written by Langston Holland.  Some of it is a bit over my head, but interesting none the less.

https://soundforums.net/threads/3723-Powersoft-K3-K20-M50Q-(Part-II)

My take away from this discussion is that I will continue to run my amps at 8 or 4 ohms and avoid setting up systems that require 2 ohm stereo or 4 ohm bridged operation.  As to how long is long enough, I have not had any issues with my current system so 10 seconds would seem to be the answer for me. 
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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 #47 on: January 29, 2015, 06:30:26 pm »

Here is a link to a PDF that contains the graph posted above for the K10. The 23 page file compares an fP-6400, an i-T8000 and both a K6 and K10.  The commentary that explains each test is pro-Powersoft, but that doesn't take away from the interesting results.  It's worth noting that the K10's rated output into 2 ohms stereo is 6,000 watts, so the fact that it can make 7,500 watts for a tenth of a second is pretty cool, I think.
7500W/6000W is maybe 1 dBW no big deal and a reasonable spec headroom margin.
Quote
http://www.diy.poweraudio.ro/albums/userpics/10001/BENCH_COMPARISON_TEST.pdf

Here is a link to a very technical review of a K3, K10, and M50q, also with plenty of graphs, written by Langston Holland.  Some of it is a bit over my head, but interesting none the less.
yup Langston is (was) a regular here and a solid citizen.
Quote
https://soundforums.net/threads/3723-Powersoft-K3-K20-M50Q-(Part-II)

My take away from this discussion is that I will continue to run my amps at 8 or 4 ohms and avoid setting up systems that require 2 ohm stereo or 4 ohm bridged operation.  As to how long is long enough, I have not had any issues with my current system so 10 seconds would seem to be the answer for me.
+1 conservative system operators do not use 2 ohm operation for normal use. Handy for emergencies if you lose an amp channel or two and need to doble up a couple boxes.

Another aspect of system operation is that 8 ohm nominal loudspeakers can have regions of less than 8 ohm impedance, so some drive headroom is always a good thing. Amps also last longer if you don't flog them hard every moment.

JR
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Peter Morris

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #48 on: January 29, 2015, 10:30:49 pm »


Another aspect of system operation is that 8 ohm nominal loudspeakers can have regions of less than 8 ohm impedance, so some drive headroom is always a good thing. Amps also last longer if you don't flog them hard every moment.

JR

Absolutely, and in general mids are the worst where the speaker is operating in the frequency range where the impedance is the lowest. Subs however, especially reflex enclosures often have relatively high average impedances.  My Powersoft K10 tells me that my 4 ohm double 18s have an average impedance of 7 ohms, so I dont find any issues driving 2 x double 18s on one channel of a K10.

Its probably also worth mentioning that the maximum output current from a K10 is huge 125 amps peak.  In pratical performs terms the K10 is a very interetsing design / set of compromises. 
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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 #49 on: January 30, 2015, 08:09:23 am »

Absolutely, and in general mids are the worst where the speaker is operating in the frequency range where the impedance is the lowest. Subs however, especially reflex enclosures often have relatively high average impedances. 
I would not agree with that.

All loudspeakers (passive or multiamped) have peaks and dips in the impedance-depending on the freq you are talking aboutt.

HOWEVER-just to make it a bit more "interesting", the ohmage of the impedance is just ONE of the factors that makes it difficult for an amp to drive.

Something that is never talked about is the PHASE of the impedance.

This can put a VERY DIFFICULT load on amplifiers, causing them to get VERY hot and shut down, it it is to far "out of bounds".  Generaly +/- 45 is considered OK, beyond that it can start to be hard on an amp

Some examples:  THe old Servodrive speakers were HELL on an amp.  This was due to the almost purely inductive load.  Back when I was running them I had to pressurize my racks with every amp I tried or else they would easily shut down at no where near full output.

Piezo tweeters are another example on the opposite end.  They are almost purely capacitive-which some amps can't drive.

SO ONCE AGAIN- a simple number cannot possibly describe a complex situation.
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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 #49 on: January 30, 2015, 08:09:23 am »


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