diyAudio

diyAudio (http://www.diyaudio.com/forums/)
-   Class D (http://www.diyaudio.com/forums/class-d/)
-   -   Impossible efficiency? (http://www.diyaudio.com/forums/class-d/123568-impossible-efficiency.html)

Philco 22nd May 2008 10:39 PM

Impossible efficiency?
 
Today I noticed that in the Crest audio CD3000 class-D amplifier specs, it lists power output as 3000W bridged into 8 ohms. However, it also lists maximum input prime power as 20Amps from 120VAC, which is a total input power of 20A x 120V = 2400 Watts; 600 Watts LESS than the power coming out of the amp.
I knew class D was efficient, but wow.

http://www.crestaudio.com/media/pdf/CDseries_specs.pdf

Can anyone explain this? Is this marketing going haywire, or has Crest solved the world's energy crisis?

I was thinking of investing a million dollars in CD3000s, then I'd feed one of them with a 60 Hz sine wave audio input, then put its (155V,19.3A) 8 ohm output into a transformer, and use it to power another CD3000, etc.etc. Then, I'll take all the extra power and sell it back to the grid. And make a killing.

kevinkr 22nd May 2008 10:41 PM

Might be short term for a couple of cycles.. :D Laws of entropy, perpetual motion machines, etc.. :devilr:

Nelson Pass 23rd May 2008 02:07 AM

Quantum theory allows you to borrow large amounts of energy,
but the more you borrow, the faster you have to put it back.

Same thing as if you embezzle at a bank.

TheMG 23rd May 2008 02:21 AM

There's an explanation for this, it's rather simple.

You only get an amplifier's RMS output rating if you feed it a continous sine wave. Is the music you will be playing just one constant sine? Thought not.

In real-world usage, the amplifier will NEVER deliver it's full RMS rating. This is because music and sounds are very complex, with peaks, pauses, soft parts, and a whole mix of frequencies.

This is why amplifiers designed for audio use, while they are rated in RMS, use smaller fuses, transformers, and cooling, less than what would be sufficient to obtain the full RMS power rating. Yep, if you try to put sines through your amplifier and crank it you'll likely end up with a blown fuse or the amplifier going into protection.

Same goes for speakers. If a speaker is rated for say 100W RMS, it doesn't mean you can put 100W RMS sinewave into it and expect it to survive, it won't, it will fry in no time at all. The speaker RMS rating is saying that if you match the speaker with a 100W RMS amplifier, it will survive when playing audio.

theAnonymous1 23rd May 2008 02:55 AM

It doesn't say anywhere that it's 3000W RMS. It does say it has a max output voltage of 85V RMS, which is 1,806W RMS into an 8R BTL.

A 20A breaker can handle 1.8kW just fine.;)

dweekie 23rd May 2008 02:56 AM

Quote:

Originally posted by Nelson Pass
Same thing as if you embezzle at a bank.
Can you explain this in greater detail with some schematics and workarounds to problems? :devilr:

DigitalJunkie 23rd May 2008 03:25 AM

Quote:

Originally posted by dweekie


Can you explain this in greater detail with some schematics and workarounds to problems? :devilr:


Yes,and perhaps how one might go about getting a job at said bank. :smash:

fredos 23rd May 2008 02:16 PM

Power consumption of commercial amplifier is rated at 1\3 maximum RMS power at nominal load. This the 0065 UL\CSA norm. So a 3000watts 100% effiency audio amplifier should be rated at 1000 watts power consumption. This not apply to chiness amplifier rated at 50 000 watts.. :)

Fredos

kevinkr 23rd May 2008 04:19 PM

Quote:

Originally posted by fredos
Power consumption of commercial amplifier is rated at 1\3 maximum RMS power at nominal load. This the 0065 UL\CSA norm. So a 3000watts 100% effiency audio amplifier should be rated at 1000 watts power consumption. This not apply to chiness amplifier rated at 50 000 watts.. :)

Fredos


IIRC I am pretty sure this is not quite correct having been involved in the certification of amplifiers for UL/CSA/VDE in the past. Generally the stated power is the maximum power consumption at the rated power or whatever power level results in the highest overall power consumption into the lowest specified load impedance. The current stated on the ratings label should be the highest continuous current the amplifier will ever draw in normal use over the specified range of line voltage. (You must determine what "normal" condition results in the most power consumption overall and use that for ratings.) This does not include inrush current or currents under fault conditions obviously.

The 1/3 rating condition is actually close to the operating point of greatest thermal duress in class A/B amplifiers and is why the FTC original chose this point for power testing when they decided to regulate the HIFI industry's rampant and unrealistic claims for output power. This point is not usually that close to the overall maximum power consumption. (It is close to the point of minimum output stage efficiency in a typical SS class AB amplifier.)

Incidentally the existing preconditioning requirement is probably meaningless with a class D amplifier design.

It is quite possible for an amplifier to produce 3KW on peaks for 10mS or so without exceeding the average stated power consumption, provided there is sufficient capacitance in the supply to support the momentary pk load current - obviously this is not the continuous power consumption.

I no longer have access to the standards so take my comments with a grain of salt and bear in mind my last exposure to the standard was in 2004.

Philco 27th May 2008 10:37 PM

Quote:

Originally posted by theAnonymous1
It doesn't say anywhere that it's 3000W RMS. It does say it has a max output voltage of 85V RMS, which is 1,806W RMS into an 8R BTL.

A 20A breaker can handle 1.8kW just fine.;)

It says
power specifications 8 stereo 4 / 21 stereo 8 / 41 bridged
CD3000 800W 1500W 3000W

There it is, "3000W".

There is a semi-serious side to my question, which is, what does a 3000W power specification really mean? Is it continuous RMS power? Is it short-term RMS power (i.e. for less than the milliseconds duration that the hold-up caps can keep the rails up)? Is it (even worse deception) the instantaneous peak power for the microsecond spent at the peak voltage of the sine wave?
If I were to build and market some amplifier which I measure to have some performance, what can I claim as its power specification?
In this data sheet, Crest claims the test conditions for the power rating are: 1kHz, 0.4% THD+N for 2 ohm stereo and 4 ohm bridged operation.
This implies they used a sine wave, but they must have gated it to only last a few tens of milliseconds? Otherwise, if 3000W output were CW, then it would have to blow the 20A circuit breaker, right?


All times are GMT. The time now is 02:52 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio


Content Relevant URLs by vBSEO 3.3.2