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Old 10th March 2009, 07:08 AM   #1
OMNIFEX is offline OMNIFEX  Jamaica
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Question Sinusoidal, Pink Noise & Duty Cycles.

Hi.

If one should measure an amplifier using pink noise, would the output wattage be higher than one measured using a sine wave?

Many professional audio amplifiers are not using sine waves to market their output power any longer. Instead, they choose pink noise.

As far as the manufacture states, it is a better representation of music than sine waves. I donít agree on such nonsense. Nevertheless, how would an amplifier stand measured with sine waves under pink noise conditions?

Pink noise seems far less demanding than sine waves, which would more than likely mean the power supply can deliver more power due to an easier duty cycle.

What are your thoughts on the matter?

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Old 10th March 2009, 01:23 PM   #2
wg_ski is offline wg_ski  United States
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You could claim higher power when testing with pink noise because the supply is only partially loaded and you get full peak voltage. The actual effective power with a PN signal is less - because of a crest factor of around 10-12 dB. Amps are *supposed* to be power tested with sine wave, all channels driven, but I'm sure not all of them are. A valid sine wave power test need only be run for a few seconds - long enough for the power supply to stabilize at its fully loaded voltage, but not long enough for the mains breaker to trip. A lot of big pro amps can draw 30, 50, or even 80 amps with a sine wave for some length of time, and you can't legally do that off a standard wall outlet and they needed some way of solving this dilemma. Pink noise, with its lower effective duty factor is supposed to be used for heat dissipation and mains draw testing as it is "more representative of real world use." But that can vary substantially depending on application and how much clipping the user is willing to tolerate.

For my own applications, I'll only run an amp down to the lowest Z that carries an FTC rating (which, nowadays has the burn-in done at 1/8 power, PN compared to 1/3 power sine in years past). With most amps that's 8 ohms. Some will take 4R, and a handful will go to 2R (RMX5050 being the only one I can afford). When I build them, I power test with 20 Hz sine, and design to tolerate 1/8 power PN at 2R and 1/3 power PN at 4R indefinitely without taking components beyond their ratings (except for the mains plug which tops out at 20A) That's a tall order for a store-bought amp.
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Old 10th March 2009, 01:58 PM   #3
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Cheers wg_ski!

I would imagine this is why some amplifiers claim more wattage using lower voltage rails than others when both offer the same power supplies.
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Old 11th March 2009, 01:26 AM   #4
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even with FTC guidelines in place, a lot of manufacturers still have some kind of "fudge factor" in place in their testing methods. i got a system in for repair recently that said "1000W RMS CONTINUOUS POWER" in big huge letters all over the shipping box. funny thing was, the line fuse was a 5 amp fuse. the other 400W must be sucked out of the ether somehow? even though this receiver has class D amps, there are absolutely NO amplifiers on the planet that run at 160% efficiency.... maybe the manufacturer has found a way to circumvent the laws of thermodynamics. this has to be the best kept "free energy" secret of all time!!!!! i'l hook up a couple of them to run each other's line voltage supply off of the speaker outputs, and never have to pay a dime for electricity ever again......
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Old 11th March 2009, 09:08 AM   #5
OMNIFEX is offline OMNIFEX  Jamaica
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Cheers unclejed613!

Iíve grown accustomed to evaluating the amperage drawn and disregard what the marketing specs state.

Possibly the manufacture followed the volt times volts divided by the load method and, use that as the marketing wattage figure?
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Old 11th March 2009, 11:37 AM   #6
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here in the US, these specs are supposed to follow some basic rules. one of them being that output power is to be measured as RMS power at clipping all channels driven into whatever load is specified in the specs. but what appears to happen with these "low-fi" and "mid-fi" systems is that they measure one channel at clipping into a load, and then multiply by the number of channels. the particular system in question required a new power supply, which is listed in the PARTS LIST as a 500W supply. being that it's a SMPS, not a linear supply, that pretty much assures that the 500W rating is pretty much a brick wall. any more than 500 watts and the power supply goes into shutdown mode, where a linear supply would just sag a bit. a SMPS also doesn't have any huge energy storage mechanism like a linear supply has. once you get to the current limit, that's it.
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Old 11th March 2009, 12:49 PM   #7
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I would suggest you download ( pdf ) manuals from the various amp manufacturers, especially from Crown.
You can compare the various techniques and "industry standards" that are used to generate wattage numbers.
Pay particular attention to the power consumption measurements and how they are Qualified.
Because Big wattage numbers sell amps, it is almost a standard practice to run tests to produce as large a number as possible, for the consumer's eye.
It most cases: the conditions under which those numbers were obtained are not stated as they do not reflect actual operating conditions.
There are a lot of ways to get exaggerated power figures.
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Old 11th March 2009, 03:17 PM   #8
wg_ski is offline wg_ski  United States
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Quote:
Originally posted by unclejed613
even with FTC guidelines in place, a lot of manufacturers still have some kind of "fudge factor" in place in their testing methods.
The FTC guidelines are more lax than they used to be. And they don't enforce it anymore - it's optional for a product to carry an FTC rating. Some do, most don't. The lower-tier Crowns and QSCs just claim an EIA power rating. And with the EIA standards, you can run the distortion pretty much as high as you want for the test, as long as it's specified. Most of the time, it's way past clipping.

Quote:

Pay particular attention to the power consumption measurements and how they are Qualified.
The power consumption numbers listed for sine, 1/8 power PN and 1/3 power PN give you an idea of how much power an amp can really put out. Many times this is way higher than the maximum 16A allowed from a standard 120V/20A circuit. It better be for an amp claiming to put out 2kW. The RMX5050 claims it draws 87 amps (!!!) at 2R sine wave.
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Old 12th March 2009, 01:29 AM   #9
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i recently had a sony system come through the shop that was a "component" system that looks like a very large boom box. it had "bi-amped" speakers' it had a power rating of "500W/ channel * ".

if you look below the picture on the box at the small print, you saw what the asterisk was indicating.... " * at 10%THD"

500W.... yeh right.... with the usual 22guage speaker wires..... funny

even funnier, it had 2 50W/ch stereo amp chips in it 1 for the woofers, 1 for the "twids".
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Old 12th March 2009, 02:26 AM   #10
OMNIFEX is offline OMNIFEX  Jamaica
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Thank you Gentlemen.

I downloaded a few schematics from Crest, Crown, & QSC. If I wanted to estimate the Sinusoidal output, how would I go about doing so?

Should I follow the volts times volts divided by the impedance and, subtract the figure by 58%?

Peavey suggest doing so with their old CS models.


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