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Power measurements
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Old 26th October 2009, 01:05 PM   #1
Kurt von Kubik is offline Kurt von Kubik  Denmark
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Default Power measurements

This post was originally posted in the "Densen Amp" Thread, but I think this is of general interest, and I would very much like to see how other standards are done.
I can only explain the IEC, maybe some others know about CEA 490A.

When you look a stated data from any manufacturer you might choose, these data was measured or calculated in respect of certain different relations. To make these data comparable, or at least understandable, standards are developed and maintained by often federal organisations or sometimes even private ones.
In Europe the measurement standard is called IEC, and their website can be visited here: IEC - International Electrotechnical Commission > INTERNATIONAL STANDARDS AND CONFORMITY ASSESSMENT . The American equivalent is found here: CEA - Consumer Electronics Association

In the following I will try to take you through the most important matters regarding measurement of output power of amplifiers for sound reproduction, the standard is named IEC 60.268-3, and is often renamed in most countries, i.e. ** 60.268-3 is the British Standard, which is completely the same as IEC. This is done the same way througout most of Europe.

Rated condition
The manufacturer states his data, which are called "Rated condition" and are to inform about:
Rated power supply (230/240V)
Rated source impedance
Rated source e.m.f. (which is the input voltage, compensated for voltage drops as a result of low input impedance)
Rated load impedance
THD or rated distortion limited output voltage or power
Rated mechanical and climatic conditions

This might make the data look like 100W @ 8 Ohm @ 0,5% THD @ 25 dgr. C @ 230V @ 1V input @ 100 Ohms with free airconvection.

Every single point of these data are to be fullfilled, both the manufacturer and the customer knows exactly what it is all about.

This was the rated data. IEC 60.268-3 point 3.1.2.

From these you extract a special condition called "standard measuring condition" This is a kind of preconditioning of the amplifier, as well as it also is to be used between every measurement, if conducted as a series of measurements.
This condition is never used for any output power measurement, but as a state of preparation and rest. The "standard measuring condition" is defined as "rated condition - 10dB refered to the input e.m.f."
This can be read about in IEC 60.268-3 point 3.1.3

Special conditions as i.e. position of tonecontrols, are to be specified clearly.

IEC also defines classes of operation, i.e. class A is defined as:
"In which the current in each active devise supplying the load current is greater than zero throughout each cycle of the signal for all values of load current up to and including the value determined by the rated output power or voltage and the rated load impedance"
In short it could be: bias = peak current.

Output power:
There are several ways of measuring output power. First the manufacturer can stae almost anything, he just need to consider the matters mentioned above in "rated output power"

1 Distortion limited output voltage/power IEC 60.268-3 14.6.3.
2 Short term maximum output voltage/power
3 Long term maximum output voltage/power
4 Temperature limited output power
5 Sustaining time for rated distortion limited output voltage/power

The ones of interest to audiophiles and music lovers are the distortion limited output power and the temperature limited output power.
But ill try to explain them all.

Distortion limited output power: IEC 60.268-3 14.6.3
The voltage measured across the rated load impedance @ which the rated THD level is produced.
Distortion limited power P=U^2/R

Method of measurement:
The amp is preconditioned for 1 hour @ "standard measurement conditions"
The amp is brought under "rated conditions" with respect for all the conditions mentioned earlier.
The amp is operated under these conditions for 60 sec. then the source e.m.f. is readjusted to produce rated THD.
Output voltage is measured
Maybe repeated @ different frequencies if required.

Short term maximum output voltage/power IEC 60.268-3 14.7.2.
This is a test done with a tone burst signal @ 1KHz, and is done regardles of distortion. This is mostly to specify whether the output voltage is dangerous to humans or animals. The measurement is for only 1 sec.
So I will not dig any deeper into this.

Long term maximum output power IEC 60.268-3 14.7.3.
This is a measurement where the input e.m.f. is 10 times the rated input, creating massive overload, the signal is simulated program, the measurement is to be done 1 min. after the input signal is applicated.

Temperature limited output power IEC 60.268-3 14.7.4.
This is the interesting part.
To be specified is:
The output power which the amplifier is capable of supplying continuously, at a pecified ambient temperature, without exceeding the maximum permissible temperature in any component.

Special conditions as i.e. rackmounting are to be regarded.

Measurement is done as follows.

Amplifier is brought under "standard measurement conditions", except for the power supply, which is brought to the upper limit of the rated data i.e. 240V.
The ambient temperature shall be measured.

The source e.m.f. (input signal) is adjusted in steps to increase output voltage, each time waiting for temp. to stabilise. This procedure is repeated until one or more components reaches their temp ratings. i.e. junction temperature @ 150 dgr. or electrolytic caps @ 85 dgr. or so. If the amplifiers protection circuits are triggered, then the output voltage shall be measured @ the highest level possible, without interruptions.

The temp limited output power is then U^2/R

The amplifier is to work in this condition for 4 hours at least.

Sustain time for rated output power.
This is a duration test, where the amplifier is brought to "rated conditions" and then you basically wait for the distortion to rise or for an hour to pass, which ever is less.

These are the IEC measurements relevant for amplifier output power.
Ive expained them from the Standard as faithfull as possible. I cannot give you any link to the standards, because they are copyrighted stuff, maintained by IEC. The standard is named IEC 60.268 Sound system equipment - part 3 Amplifiers.
To market amplifiers in Europe this standard is to be regarded, though the manufacturer must rate distortion limited output power, but can choose to rate the others if needed.
If nothing is rated or the rating for some parameter is missing, you can only do the temperature limited measurement, because output power without a corresponding THD percentage makes no sense.

I hope someone can enlighten us with the CEA490A standard for power measurement. That would be interesting.

Maybe we ought to start a new thread regarding measurements and standards.
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Old 27th October 2009, 01:13 PM   #2
Kurt von Kubik is offline Kurt von Kubik  Denmark
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Is there no one out there who can explain CEA490A?
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Old 27th October 2009, 03:40 PM   #3
qsa is offline qsa  Germany
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EIA/CEA 490A replace old IHF-A-201, 1966 and RMS Standard from 70s for professional PA and consumer Amplifier manufacturers

Power Amplifier
A device having separate input and output terminals, whose purpose is to provide a larger output power
than its required input power over the audio range (or a portion thereof), normally construed to mean 20
Hz to 20 kHz.

Since adoption of the IHF Amplifier Standard (IHF-A-201, 1966), many changes have occurred in
amplifier design. These changes have been prompted by technological advancement in circuit topology,
in component characteristics, in test equipment and methods of measurement, and in engineering
psycho-acoustics. Also, changing conditions in the marketplace driven by consumer demand for new
features like home theater and surround sound have prompted industry to consider and implement
additional design changes resulting in a proliferation of multi-channel amplifiers in receivers. However, the
purpose of an amplifier remains the same—to increase the power level of an electrical signal that
represents speech or music. Ultimately, that signal will be reproduced as sound.

Todays engery efficient SMPS analog or CLASS D amplifier output power is tested with EIA, test condition is 20ms burst or similar 20
Hz to 20 kHz

for example QSC, BGW, LABGRUPPEN, Powersoft

Last edited by qsa; 27th October 2009 at 03:48 PM.
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Old 27th October 2009, 04:04 PM   #4
Kurt von Kubik is offline Kurt von Kubik  Denmark
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Quote:
Originally Posted by qsa View Post
EIA/CEA 490A replace old IHF-A-201, 1966 and RMS Standard from 70s for professional PA and consumer Amplifier manufacturers

Power Amplifier
A device having separate input and output terminals, whose purpose is to provide a larger output power
than its required input power over the audio range (or a portion thereof), normally construed to mean 20
Hz to 20 kHz.

Since adoption of the IHF Amplifier Standard (IHF-A-201, 1966), many changes have occurred in
amplifier design. These changes have been prompted by technological advancement in circuit topology,
in component characteristics, in test equipment and methods of measurement, and in engineering
psycho-acoustics. Also, changing conditions in the marketplace driven by consumer demand for new
features like home theater and surround sound have prompted industry to consider and implement
additional design changes resulting in a proliferation of multi-channel amplifiers in receivers. However, the
purpose of an amplifier remains the sameto increase the power level of an electrical signal that
represents speech or music. Ultimately, that signal will be reproduced as sound.

Todays engery efficient SMPS analog or CLASS D amplifier output power is tested with EIA, test condition is 20ms burst or similar 20
Hz to 20 kHz

for example QSC, BGW, LABGRUPPEN, Powersoft
Could more be said about precise conditions?
Load, Temp, time and so on?
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Old 30th October 2009, 10:32 AM   #5
Kurt von Kubik is offline Kurt von Kubik  Denmark
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Are there really nobody out there with knowledge of CEA490A and eventually other standards?
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Old 30th October 2009, 11:21 AM   #6
SY is offline SY  United States
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Power measurements
Quote:
Originally Posted by Kurt von Kubik View Post
Are there really nobody out there with knowledge of CEA490A and eventually other standards?
Conformity to bureaucratic standards is generally not high on the list of diyers' priorities, so don't be surprised that most of us haven't paid a pile of money to secure copies, nor spent a lot of time testing our equipment to those standards.
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Old 30th October 2009, 12:53 PM   #7
Kurt von Kubik is offline Kurt von Kubik  Denmark
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Quote:
Originally Posted by SY View Post
Conformity to bureaucratic standards is generally not high on the list of diyers' priorities, so don't be surprised that most of us haven't paid a pile of money to secure copies, nor spent a lot of time testing our equipment to those standards.
Id say this could actually save you a pile of money, by safeguarding you arround the bad designs

But it also is a kind of common language, wich make measurements and also manufacturers statements understandable to anyone.
I.e. IEC it also is precisely described how to measure output impedance, which is in fact completely different to what many nerds might believe.

I think these objectives are to be considered a lot more than they are at present. That would definately rule out some designs.
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Old 30th October 2009, 12:55 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Kurt von Kubik View Post
I.e. IEC it also is precisely described how to measure output impedance, which is in fact completely different to what many nerds might believe.
tell us how IEC recommend it be done.
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Old 30th October 2009, 02:01 PM   #9
Kurt von Kubik is offline Kurt von Kubik  Denmark
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Quote:
Originally Posted by AndrewT View Post
tell us how IEC recommend it be done.
Ill give it a try then!

Characteristics to specify:
The internal impedance masured between the output terminals under specified conditions. Everything aabout this matter must be stated by the manufacturer.

Measurement:

The amplifier is brought to "standard measurement conditions" which is rated conditions -10 dB.

Then source e.m.f. is reduced to 0,00, and the rated load impedance is removed.

Then you have another amplifier with at least 10 times the expected output impedance value of the amp subject to be measured, in series with an AMMETER connected to the output terminals of the amplifier, playing sinusoidal current.
A voltmeter is also connected to the output terminals, and then the current is adjusted to the value, which would flow through the output terminals under "standard measurement conditions"

So you see - you have another amp playing into the output of the amp beeing measured.

Then you calculate the current so that it produces the voltage level of -10 dB referred to rated distortion limited output, with regard to the rated load.

You then measure the voltage across the output terminals.
Then the output impedance is calculated as : Z=U/I.
May be repeated by different frequencies.

This is how it is done according to IEC 60268-3 point 14.6.2.
And thus you can calculate your damping factor as: Zload/Zsource = DF @ any specified frequency.
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Old 3rd November 2009, 10:06 AM   #10
Kurt von Kubik is offline Kurt von Kubik  Denmark
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A freindly soul mailed me the complete CEA490A standard, so here we go.

Power measurement by this standard is somewhat different from the IEC.

Preconditioning:
30 min. of operation into rated load @ 1/8 of rated power all channels on, with a 1KHz sinusoidal tone.

Measurement of power:
5 min of operation @ 1%THD with a sinusoidal 1KHz tone into 8 Ohms.
Power is calculated as Square RMS voltage devided with load.
V^2/R

Damping factor:
This is measured the same way as directed by IEC, which means a current source is to send its signal into the output terminals of the subject to be measured.
Then a sweep is done, so you will see the output impedance versus frequency.
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