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Author Topic: Why do Class D amps seem to pull less power than they make?  (Read 24709 times)

John Roberts {JR}

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #50 on: January 30, 2015, 09:46:21 am »

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.
Load phase angle mattered most for old school class AB amps. Modern class D amps do not experience the same dissipation or secondary breakdown behavior in power devices from high current with high voltage at the same time. In class D the power devices are either off or switched hard on so no voltage and current at the same time.

JR
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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 #51 on: January 30, 2015, 09:53:55 am »

Load phase angle mattered most for old school class AB amps. Modern class D amps do not experience the same dissipation or secondary breakdown behavior in power devices from high current with high voltage at the same time. In class D the power devices are either off or switched hard on so no voltage and current at the same time.

JR
See-you learn something every day.  :)

My experiences with the "hard loads" was back in the "AB" days.

I can't say I have experienced those sorts of problems with the class D amps-but then again I am not driving them with those types of loads either.
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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 #52 on: January 30, 2015, 10:44:08 am »

See-you learn something every day.  :)

My experiences with the "hard loads" was back in the "AB" days.

I can't say I have experienced those sorts of problems with the class D amps-but then again I am not driving them with those types of loads either.

I have heard musings by someone smarter than me that a reactive load (like a capacitive electrostatic loudspeaker) might be more efficient to operate since you don't have all that resistance wasting power as heat. The reactive load would just charge and discharge not really consuming as much power.

This is hypothetical and I am not aware of any practical examples.

JR
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Art Welter

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #53 on: January 30, 2015, 11:31:08 am »


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.
Just before the holidays I conducted a battery of tests on some old amplifiers and a new Behringer NU4-6000 four channel amplifier. To my surprise, the  performed as well on low frequencies as on mid/high frequencies, and is capable of full power sine wave output with all four channels driven to rated output at two ohms, or two bridged mono pairs driving four ohm loads each. The  $350 NU4-6000 is within 3 dB of the K10 on sustained (more than 1 second) output.

The NU4-6000 with  two bridged mono pairs driving four ohm loads just below the illumination of the clip/limit light each put out 85.5 volts at 60 Hz (1828 watts), 84.6 volts at 30 Hz (1789 watts), dropping the mains voltage on a 100' 10AWG 120v line from 118.1 volts down to 107.2 volts, drawing 31 amperes.

Using just one bridged mono pair, the amp ran for 40+ seconds before I terminated the test, as the amp was drawing 19.8 amperes, and the "tired" 20 amp mains breaker had popped several times in various tests already. The amp would have put out more power given a full 120 volts, but the test represents "real world" situation, we don't generally plug our amplifiers in to an outlet two feet from the mains transformer.

I also tested my old "heavy iron"  bass favorite, a Crest CA9, bridged into a 4 ohm load it dropped the mains to 99.6 volts, drew 37.8 amps but only put out 80 volts (1600 watts). The NU4-6000 put out more power, and drew only 50% of the power from the mains compared to the CA9 !

My back (and bank account) are very pleased with the NU4-6000.

Art
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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 #54 on: January 30, 2015, 11:53:57 am »

Yup, in case I haven't said this lately the modern cheap class D power amps are pretty remarkable. Peavey has been messing with class D variants for decades, but back when I was involved we just didn't have the high frequency, high voltage switching devices to make serious power. Back then it was a stretch to make 1kW. Nowadays I'm sure these guys are using a dedicated chip set for the front end and nice off the shelf power devices (speculation I've been out of the trenches for over a decade).

Regarding mains power, to accurately test a big dog amp you need a serious power drop. I recall having to re-wire dedicated mains power feeds for the amp designer's test bench. I always wince when consumers "test" power amps, because as often as not, they are are seeing the limitations of their power source. Of course that is real world, but can lead to some ugly conclusions if not factored in. More good news is that the modern designs are more power friendly too.

The budget amps are probably not taken very seriously, but that sounds like pretty serious power. i remember when UL would make us put camper plugs on smaller amps than that... Times have changed for the better.

JR
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Richard Turner

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Re: Why do Class D amps seem to pull less power than they make?
« Reply #55 on: January 30, 2015, 03:46:16 pm »

Just before the holidays I conducted a battery of tests on some old amplifiers and a new Behringer NU4-6000 four channel amplifier. To my surprise, the  performed as well on low frequencies as on mid/high frequencies, and is capable of full power sine wave output with all four channels driven to rated output at two ohms, or two bridged mono pairs driving four ohm loads each. The  $350 NU4-6000 is within 3 dB of the K10 on sustained (more than 1 second) output.

The NU4-6000 with  two bridged mono pairs driving four ohm loads just below the illumination of the clip/limit light each put out 85.5 volts at 60 Hz (1828 watts), 84.6 volts at 30 Hz (1789 watts), dropping the mains voltage on a 100' 10AWG 120v line from 118.1 volts down to 107.2 volts, drawing 31 amperes.

Using just one bridged mono pair, the amp ran for 40+ seconds before I terminated the test, as the amp was drawing 19.8 amperes, and the "tired" 20 amp mains breaker had popped several times in various tests already. The amp would have put out more power given a full 120 volts, but the test represents "real world" situation, we don't generally plug our amplifiers in to an outlet two feet from the mains transformer.

I also tested my old "heavy iron"  bass favorite, a Crest CA9, bridged into a 4 ohm load it dropped the mains to 99.6 volts, drew 37.8 amps but only put out 80 volts (1600 watts). The NU4-6000 put out more power, and drew only 50% of the power from the mains compared to the CA9 !

My back (and bank account) are very pleased with the NU4-6000.

Art

I've been eyeballing that exact amp, Does it sound as musical as compared to old crest iron?  putting 3 units in a rack box and coming out under 50 lbs and having up to 12 channels in a flexible package for under $1200, Would it be too good to be true?

Is it still working as advertised after initial break in? No funny smells no melted bits after running fill tilt for 50 hours? Definelty attractive for a monitor rack and utility amp rack.

Heck 3 year warranty, if they don't take 90 days to effect repairs that's a solid selling point right there. Or is it in some fine print that actually using it commercially cripples the warranty term?
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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 #56 on: January 31, 2015, 09:32:23 am »

Just before the holidays I conducted a battery of tests on some old amplifiers and a new Behringer NU4-6000 four channel amplifier. To my surprise, the  performed as well on low frequencies as on mid/high frequencies, and is capable of full power sine wave output with all four channels driven to rated output at two ohms, or two bridged mono pairs driving four ohm loads each. The  $350 NU4-6000 is within 3 dB of the K10 on sustained (more than 1 second) output.


Interesting results.
i have been toying with the idea of racking up 3 or 4  iNuke  6000DSP amps and putting them into real world use.
AT $385 CDN(dealer) you cant' beat the price!
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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 #57 on: January 31, 2015, 10:18:56 am »

The NU4-6000 with  two bridged mono pairs driving four ohm loads just below the illumination of the clip/limit light each put out 85.5 volts at 60 Hz (1828 watts), 84.6 volts at 30 Hz (1789 watts), dropping the mains voltage on a 100' 10AWG 120v line from 118.1 volts down to 107.2 volts, drawing 31 amperes.

I looked up the specs on the NU4-6000 and Behringer states that the amp will produce 2 x 3,000 watts into 4 ohms.  They do not give any parameters for how those ratings were achieved.  Art measured ~1,800 watts which is well short of their advertised output, although still impressive for a 12lb, $350 piece of audio gear.

Trying to understand the gap even though JR has warned me about such foolishness, I found a bench test of the NU-6000 which states that, in the owner's manual, Behringer lists RMS figures of 1.1kW @ 8 ohms and 2.2kW @ 4 ohms for the 2 channel iNuke 6k.  Interestingly, the writer of this review reaches the same conclusion as Art did for the amp; stating that it should be good for 1.8kW long term into 4 ohms.  The reviewer also points out that the Behringer amp lacks Power Factor Correction and so may present a more challenging load on the mains circuit at your local pub. 

http://forum.speakerplans.com/behringer-inuke-nu6000-vs-kam-kxd7200-bench-tested_topic69202.html
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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 #58 on: January 31, 2015, 11:46:25 am »

I looked up the specs on the NU4-6000 and Behringer states that the amp will produce 2 x 3,000 watts into 4 ohms.  They do not give any parameters for how those ratings were achieved.  Art measured ~1,800 watts which is well short of their advertised output, although still impressive for a 12lb, $350 piece of audio gear.

Trying to understand the gap even though JR has warned me about such foolishness, I found a bench test of the NU-6000 which states that, in the owner's manual, Behringer lists RMS figures of 1.1kW @ 8 ohms and 2.2kW @ 4 ohms for the 2 channel iNuke 6k.  Interestingly, the writer of this review reaches the same conclusion as Art did for the amp; stating that it should be good for 1.8kW long term into 4 ohms.  The reviewer also points out that the Behringer amp lacks Power Factor Correction and so may present a more challenging load on the mains circuit at your local pub. 

http://forum.speakerplans.com/behringer-inuke-nu6000-vs-kam-kxd7200-bench-tested_topic69202.html
I too would be interested to see how they arrive at
"Delivers 2 x 3100 Watts into 4 Ohms, 2 x 1600 Watts into 8 Ohms "
as stated on their product page when the amp actually does 2200/4.
The "real" numbers are good to know and ,yes, it is still quite good for the price.
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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 #59 on: January 31, 2015, 11:57:59 am »

The crux of the biscuit is how do you come up with a single number for what is at least a two dimensional (power vs. time) envelope of power output capability. Any single power number is only describing one instant of that total power envelope. Further there is a short term and longer term power envelope limited by different parameters.

Note: Loudspeakers share a similar multi-dimensional power handling envelope.

Buy a powered speaker and let the engineers worry about this.

JR   
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Re: Why do Class D amps seem to pull less power than they make?
« Reply #59 on: January 31, 2015, 11:57:59 am »


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