(Suggestion on) Testing power amplifiers according to IEC 60268-3 standard

[pma]

(Suggestion on) Testing power amplifiers according to IEC 60268-3 standard

I would like to start the debate on ASR power amplifier testing and suggest some degree of conformance with current standards.
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The part 3 of IEC 60268 applies to analogue amplifiers, and the analogue parts of analogue/digital amplifiers, which form part of a sound system for professional or household applications. It specifies the characteristics that should be included in specifications of amplifiers and the corresponding methods of measurement.

1. Terms, definitions and rated values
1.1. Terms and definitions

1.1.1. Class of operation

1.1.1a. class A
class of operation in which the current in each active device 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.

1.1.1b. class B
class of operation in which the current in each active device supplying the load current is equal to zero for exactly one-half of each cycle of load current. In common usage, the term Class B is extended to the case where current flows for slightly more than one half-cycle.

1.1.1c. class AB
class of operation in which the current in at least one of the active devices supplying the load current is zero for some part of each cycle of load current for some range of values of load current not exceeding the value defined by the rated output power or voltage and the rated load impedance. At sufficiently low signal levels, a Class AB amplifier usually operates in Class A.

1.1.1d. class D
class of operation in which all active power devices are switched between fully on and fully off at a rate faster than the highest frequency of interest, and where the wanted signal is encoded in the switching pattern.

1.2. Rated values

The rated conditions for amplifiers are:
– rated power supply voltage;
– rated source impedance;
– rated source e.m.f.;
– rated load impedance;
– rated total harmonic distortion, or rated (distortion-limited) output voltage or power;
– rated mechanical and climatic conditions.
– rated voltage gain;
– rated distortion limited output voltage or power (when not adopted as a rated condition);
– rated signal-to-noise ratio;
– rated equivalent noise source e.m.f.

1.3. Standard measuring conditions
These are obtained by bringing the amplifier under rated conditions and then
reducing the source e.m.f. to a level of -10 dB referred to the rated source e.m.f.

1.4. Pre-conditioning for measurements
Before beginning measurements on an amplifier, it shall be operated under approximately
standard measuring conditions for a period of 1 h, or as specified by the manufacturer.
Before operating the amplifier the manufacturer's instructions concerning initial operation
should be studied.
The amplifier is then brought under standard measuring conditions (see ). Due to internal
heating, the output voltage may subsequently vary with time. Unless excessive, this effect is
ignored during the pre-conditioning period. When the pre-conditioning period is over, the
amplifier shall be brought under rated conditions or standard measuring conditions, as
required.

1.5. Series of measurements
If a series of measurements is made, the amplifier should preferably be maintained under
standard measuring conditions in the periods between measurements.
If the amplifier has to be put out of operation for an extended period between measurements,
then pre-conditioning shall be repeated before each set of measurements, unless this can be shown to be unnecessary.

2. Amplifier characteristics

2.1. Input characteristics
2.1.1. Rated source impedance
2.1.2. Input impedance

2.2. Output characteristics
2.2.1. Rated load impedance
2.2.2. Output source impedance
2.3. Output voltage and power (distortion-limited)
a) Distortion-limited output voltage.
b) Distortion-limited output power.
c) Distortion-limited output voltage with complex (that is, partially or wholly reactive) load
impedance.

2.4. Maximum effective output power (distortion-limited at 10 %)
a) The r.m.s. voltage, measured across the rated load-impedance, at which the total
harmonic distortion at 10 % is produced.
b) Maximum effective output power. The power produced in the rated load impedance by the
distortion-limited at 10 % output voltage.
c) Distortion-limited at 10 % output voltage with complex (that is, partially or wholly reactive)
load impedance.

2.5. Overload restoring time

2.6. Limiting characteristics
2.6.1. Overload source e.m.f.
2.6.2. Short-term maximum output voltage and power
2.6.3. Long-term maximum output voltage and power
2.6.4. Temperature-limited output power

2.7. Characteristics of protection circuits
Protection circuits in amplifiers may be classified as follows:
a) protection against excessive load current, or potentially damaging combinations of load
voltage and current;
b) protection against the presence of d.c. voltage between the load terminals (d.c. offset
protection);
c) protection against potentially damaging input signals (for example, excessive amplitude at
high frequencies).

2.7.1. Sustaining-time for rated (distortion-limited) output voltage or power
The time for which the amplifier can produce rated distortion-limited output voltage or power.
The rated value shall be stated by the manufacturer in the specification.

2.8. Gain

2.8.1.Voltage gain and e.m.f. gain
The manufacturer may use one of the following two methods for stating the rated value of gain
in the specification, either as a direct ratio or in decibels.
a) The voltage gain, being the ratio of the output voltage U2 to the input voltage U1 under
standard measuring conditions.
b) The e.m.f. gain, being the ratio of the output voltage U2 to the source e.m.f. Es under standard measuring conditions

2.8.2. Maximum e.m.f. gain
The e.m.f. gain measured with the volume control set at the position for maximum gain and
the source e.m.f. being set so as to restore the output voltage for standard measuring
conditions.
The manufacturer may optionally state the rated value in the specification.

2.8.3. Attenuation characteristic of the volume control

2.8.4. Attenuation characteristic of balance controls for multi-channel equipment

2.9. Response

2.9.1. Gain-frequency response

2.9.2. Gain-limited effective frequency range

2.9.3. Distortion-limited effective frequency range

2.9.4. Phase-frequency response

2.10. Amplitude non-linearity

2.10.1. Rated total harmonic distortion
The value of total harmonic distortion stated by the manufacturer in the specification or
specified in an IEC standard, above which the performance of the amplifier is considered
unacceptable for the intended purpose. Published research on distortion perception by the human ear suggests that the limiting value of distortion for most purposes is in the order of 1 %. The overload restoring time is also significant.

2.10.2. Total harmonic distortion under standard measuring conditions
Method of measurement
The amplifier is brought under standard measuring conditions and the output voltage U2 is
measured.
The level of total harmonic distortion of the source of signals shall be at least 10 dB below
the lowest level of distortion to be measured.

2.10.3. Total harmonic distortion as a function of amplitude and frequency
The total harmonic distortion, determined for different frequencies and output voltages.
Care is necessary to ensure that the frequencies of significant distortion components do not fall above the upper frequency limit of the analyser.
For example, if the upper limit of the gain-limited effective frequency range is 30 kHz, and the highest significant harmonic is the fifth, the highest fundamental frequency for which a value of total harmonic distortion is valid is (30/5) kHz, that is, 6 kHz. If the highest significant harmonic were the third, however, values of total harmonic distortion could be stated for frequencies up to (30/3) kHz, that is, 10 kHz.
The manufacturer may optionally present these data in the specification.
Some amplifiers produce a spectrum of harmonics including small but measurable harmonics of high orders. The highest frequency component of this spectrum, the amplitude of which is significant, may in general be taken as the highest harmonic whose r.m.s. value exceeds one-third of the total harmonic distortion at the same fundamental frequency. In some cases a different criterion may be necessary, in which case it shall be stated.

2.10.4. Harmonic distortion of the nth order under standard measuring conditions
The harmonic distortion under standard measuring conditions due to the component of the
output signal of harmonic order n.
It is recommended that the manufacturer state rated values for this characteristic, at least for
values of n from 2 to 5, in the specification.

2.10.5. Harmonic distortion of the nth order as a function of amplitude and frequency
The total harmonic distortion of the nth order under standard measuring conditions, determined for different frequencies and output voltages.
The manufacturer may optionally present these data in the specification.

2.10.6. Modulation distortion of the nth order (where n = 2 or n = 3)
The following characteristics should be specified:
a) Modulation distortion of the second order
When f1 and f2 are the frequencies of two sinusoidal input signals of specified amplitude
ratio, the second-order modulation distortion is given by the ratio of the arithmetic sum of
the output voltages at frequencies f2 + f1 and f2 - f1 to the output voltage at frequency f2.
b) Modulation distortion of the third order
When f1 and f2 are the frequencies of two sinusoidal input signals of specified amplitude
ratio, the third-order modulation distortion is given by the ratio of the arithmetic sum of the
output voltages at frequencies f2 + 2f1 and f2 - 2f1 to the output voltage at frequency f2.

2.10.7. Difference-frequency distortion of the nth order (where n = 2 or n = 3)
The following characteristics should be specified:
a) Difference frequency distortion of the second order
When f1 and f2 are the frequencies of two equal amplitude sinusoidal signals, separated
by a specified frequency difference, the difference-frequency distortion of the second
order is given by the ratio of the output voltage U2,f2 - f1' at frequency f2 - f1 to the
reference voltage U2,ref, which is equal to twice the output voltage U2,f2.
b) Difference frequency distortion of the third order
With signals as under item a), the difference-frequency distortion of the third-order is
given by the ratio of the arithmetic sum of the output voltages at frequencies 2f2 - f1 and
2f1 - f2 to the reference voltage U2,ref which is equal to twice the output voltage U2,f2.
It is recommended that the manufacturer present these data in the specification.

2.10.8. Dynamic intermodulation distortion (DIM)
The modulation distortion arising when a particular input signal is used. The input signal is the
sum of a sinusoidal signal of frequency fs and a low-pass filtered square wave of fundamental
frequency fq, where fq is less than both fs and the filter cut-off frequency fc. The peak-to-peak
amplitude ratio of the square wave signal to the sinusoidal signal is 4:1 and the dynamic
intermodulation distortion is then determined by the ratio of the r.m.s. sum of the output
voltages at the frequencies specified in Table 2 to the amplitude of the output voltage at the
frequency fs.
The manufacturer may optionally present these data in the specification.

2.10.9. Total difference frequency distortion
When f1 and f2 are frequencies of two sinusoidal input signals, where f1 = 2f0, f2 = 3f0 – δ,
δ being the frequency offset, the distortion is given by the ratio of the r.m.s. sum of the output
voltages U2,f2-f1' and U2,f1-f2' of the in-band second-order and third-order intermodulation
products at the frequencies f0 – δ and f0 + δ to a reference output voltage, equal to the
arithmetical sum of the output voltages U2,f1 and U2,f2 at the frequencies f1 and f2.
The manufacturer may optionally state the rated value in the specification.

2.10.10. Weighted total harmonic distortion
The characteristics given in 2.10.2, 2.10.3, 2.10.4 and 2.10.5 may also be measured and
presented as weighted values by including a weighting network between the amplifier output terminals and the distortion measuring instrument. Allowance shall be made for the insertion loss of the weighting network at the input signal frequency.
Because of the shape of the response of the weighting network, the measurements are valid
only for input signal frequencies between 31,5 Hz and 400 Hz.
Rated total harmonic distortion may also be presented as a weighted value.

2.11. Noise
The following characteristics are suitable for inclusion in specifications:
a) Signal-to-noise ratio
The ratio, expressed in decibels, of the rated output voltage to the wide-band, or a
weighted sum of the output voltages or the octave/third-octave band output voltages
produced by the different noise components, the amplifier being brought under rated
conditions and the source e.m.f. being then reduced to zero. The weighting curve and the
characteristics of the measuring instruments shall be as specified in IEC 60268-1.
Information about noise, excluding hum and the other spurious signal components, may be
given, where relevant. When this is done, it shall be explicitly stated.
If it is justified to use a reference output voltage other than the rated output voltage, the
level of decibels of this reference voltage with respect to the rated voltage (0 dB) shall be
stated when reporting the results.
b) Noise output voltage
The output voltage of an amplifier which is due to noise generated both within the amplifier
and within its rated source resistance. This voltage is measured at the output of the
appropriate filter or weighting network according to IEC 60268-1.
For many purposes, it is the value of this noise output voltage which is significant rather than its
ratio to the rated distortion-limited output voltage.
Also, the specification of noise output voltage (instead of signal-to-noise ratio) avoids conceptual difficulties which arise when the noise performance is to be specified under measuring conditions where rated distortion limited output voltage cannot be obtained.
c) Residual noise output voltage
The noise output voltage as defined in b) above but with the volume control set to its
minimum position.
d) Equivalent noise source e.m.f.
The e.m.f. of a source giving a sinusoidal signal of a specified frequency which will
produce an output voltage equal to the noise output voltage produced by the amplifier.
The frequency of the equivalent source is preferably the standard reference frequency of 1 000 Hz.
The manufacturer shall state rated values for one or more of these characteristics in the
specification.

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The survey above is not complete, but covers (IMO) the most important amplifier characteristics to be specified and measured. Detailed description of individual items is not provided, as it would result in >50 pages. Individual items may be a subject of further discussion and explanation.

 

[Triliza]

Adhering to standards is a good thing in my book, but lacking any relevant knowledge to add something meaningful in the discussion, I'll list some possible issues that came to my mind and may be related to the discussion (or not, I'm just trying to understand why this is not already happening).

1. Will the majority of the people be able to understand and make use of the proposed metrics? Making myself the common denominator will set the bar way too low, but would someone need a degree in electronics to understand them? But then again, live and learn, not a bad principle to follow.
2. Will it need a lot of time and effort (by Amir and others) to do the testing? May affect the willingness of them to join the cause.
3. Will the reviews be much larger that the current ones? Many things can be attachment, so this may not be a real issue.
4. Is there any risk involved to the amp tested, I bet the owners wouldn't like for their amps to be stress tested to their limits.

 

[pma]

Adhering to standards is a good thing in my book, but lacking any relevant knowledge to add something meaningful in the discussion, I'll list some possible issues that came to my mind and may be related to the discussion (or not, I'm just trying to understand why this is not already happening).

1. Will the majority of the people be able to understand and make use of the proposed metrics? Making myself the common denominator will set the bar way too low, but would someone need a degree in electronics to understand them? But then again, live and learn, not a bad principle to follow.
2. Will it need a lot of time and effort (by Amir and others) to do the testing? May affect the willingness of them to join the cause.
3. Will the reviews be much larger that the current ones? Many things can be attachment, so this may not be a real issue.
4. Is there any risk involved to the amp tested, I bet the owners wouldn't like for their amps to be stress tested to their limits.

Let me answer according to my knowledge and experience -
1. Not the majority, definitely yes in case of some members, let me name @restorer-john , and he is not the only one,
2. Yes, it woul need time, efforts and technical skills like preparing test accessories. On the other hand, it is not about most expensive instrumentation. IEC 60268-3 suggest quite simple test circuits. However, you need time. But, there are general suggestions that would be easy to follow, like pre-conditioning, distortion limited power, harmonic distortion measurement bandwidth etc.
3. Yes and different.
4. Yes.

The suggestion is not about cherry-picking of certain parameters. It is not about ASR mainstream opinion, it is about professional approach to the tests and methodology.

 

[MaxwellsEq]

Thanks @pma this is very interesting. I would find it very interesting to see measurements count this way. It would also potentially clarify whether different amplifier architectures have the sonic characteristics sometimes ascribed to them.

The following also caught my eye: "For many purposes, it is the value of this noise output voltage which is significant rather than its ratio to the rated distortion-limited output voltage."

 

[Triliza]

Let me answer according to my knowledge and experience -
1. Not the majority, definitely yes in case of some members, let me name @restorer-john , and he is not the only one,
2. Yes, it woul need time, efforts and technical skills like preparing test accessories. On the other hand, it is not about most expensive instrumentation. IEC 60268-3 suggest quite simple test circuits. However, you need time. But, there are general suggestions that would be easy to follow, like pre-conditioning, distortion limited power, harmonic distortion measurement bandwidth etc.
3. Yes and different.
4. Yes.

The suggestion is not about cherry-picking of certain parameters. It is not about ASR mainstream opinion, it is about professional approach to the tests and methodology.

Oh man, couldn't you just lie about some of them, especially point 4, I for one would have believed you

Jokes and difficulties aside, I hope something fruitful comes out of this, more facts and data never hurt anyone.

 

[restorer-john]

The following also caught my eye: "For many purposes, it is the value of this noise output voltage which is significant rather than its ratio to the rated distortion-limited output voltage."

The noise output voltage, tested with shorted input, volume at minimum, maximum and a worst case position is what really sorts out the suitability of amplifiers for desktop/nearfield use and with high sensitivity speakers. If the amplifier has a volume control/gain control, those tests are vital. Some are covered above, but not all.

The overload characteristics/recovery time are another area where amplifiers have wildly different results. 10% THD for maximum power is a strange one, as many modern amplifiers simply won't produce 10% before activating protection circuits. And, speaking of protection circuits, DC, overpower, overcurrent and excess input voltage are covered, but not the big one, which is over temperature. Modern gear often has overly sensitive temperature trips from various sensors feeding A/D lines where the micro decides to shut off the amp. (transformer/heatsinks etc). With preconditioning, you will see plenty of pieces of low cost Class D gear in sealed casework failing these tests.

Obviously, a shortened bank of IEC compliant tests that either supplements, modifies, or replaces some of the testing Amir has previously done would be the best outcome.

 

[amirm]

From my quick searching, there is no adoption of this standard either by manufacturers or reviewers. As is typical of IEC specs, they want a ton of money for it ($325 for Part 3). I have done some reading and will investigate more what makes sense to pull in.

 

[pma]

From my quick searching, there is no adoption of this standard either by manufacturers or reviewers. As is typical of IEC specs, they want a ton of money for it ($325 for Part 3). I have done some reading and will investigate more what makes sense to pull in.

Hopefully, from some Institutes of Technical Normalization in Eastern EU countries it is possible to buy the same valid standard at fractional price, like 32 EUR, which was my case as well. It is an International standard valid in countries of CENELEC members, which covers EU, Switzerland, Norway, Serbia, Turkey, GB.

 

[pma]

The noise output voltage, tested with shorted input, volume at minimum, maximum and a worst case position is what really sorts out the suitability of amplifiers for desktop/nearfield use and with high sensitivity speakers. If the amplifier has a volume control/gain control, those tests are vital. Some are covered above, but not all.

The overload characteristics/recovery time are another area where amplifiers have wildly different results. 10% THD for maximum power is a strange one, as many modern amplifiers simply won't produce 10% before activating protection circuits. And, speaking of protection circuits, DC, overpower, overcurrent and excess input voltage are covered, but not the big one, which is over temperature. Modern gear often has overly sensitive temperature trips from various sensors feeding A/D lines where the micro decides to shut off the amp. (transformer/heatsinks etc). With preconditioning, you will see plenty of pieces of low cost Class D gear in sealed casework failing these tests.

Obviously, a shortened bank of IEC compliant tests that either supplements, modifies, or replaces some of the testing Amir has previously done would be the best outcome.

Much more tests, like hum and noise tested separately, tests on thermal behaviour, tests of protection circuits etc. are covered by the standard, but for the obvious legal reasons I cannot copy/paste the whole thing here. Please note that 1% distortion is mentioned in the distortion tests. 10% is for maximum power, distortion limited. Many manufacturers use it - try to get 300W from A07 at any conditions …..

 

[pma]

This is my attempt to issue a sample test report of my PM-A4 power amplifier, with limited number of measured parameters, according to the subject of this thread
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1.1. Class of operation
class A

2. Amplifier characteristics

2.1. Input characteristics
2.1.1. Rated source impedance – 100 ohm
2.1.2. Input impedance - 47kohm
47kohm/100Hz, 45kohm/1kHz, 15kohm/10kHz

2.2. Output characteristics
2.2.1. Rated load impedance – 6.8 ohm for class A full range operation, or any higher. 4ohm in class A up to 18W and in class AB up to 30W.


2.2.2. Output source impedance – 0.002 ohm, 0.007ohm/10kHz

2.3. Output voltage and power (distortion-limited at 1%)
a) Distortion-limited output voltage – 12.23 Vrms
b) Distortion-limited output power – 22W / 6.8 ohm

2.4. Maximum effective output power (distortion-limited at 10 %)
The power produced in the rated load impedance by the distortion-limited at 10 % output voltage - 30W

2.7.1. Sustaining-time for rated (distortion-limited) output voltage or power
The time for which the amplifier can produce rated distortion-limited output voltage or power – 60 min

2.8. Gain
a) The voltage gain, being the ratio of the output voltage U2 to the input voltage U1 under
standard measuring conditions – 27 dB

2.9. Response
2.9.1. Gain-frequency response

2.9.4. Phase-frequency response

2.10.1. Rated total harmonic distortion – 0.01%

2.10.2. Total harmonic distortion under standard measuring conditions

2.10.3. Total harmonic distortion as a function of amplitude and frequency

2.10.6. Modulation distortion of the nth order (where n = 2 or n = 3)

2.10.7. Difference frequency distortion (at 6W/6.8ohm)

2.11. Noise

a) Signal-to-noise ratio – 111 dB(A)

b) Noise output voltage – 34.2 uV(A)

 

[pma]

Input and output impedance modulus plots and modulation distortion added in the post above.

 

[solderdude]

The part 3 of IEC 60268 applies to analogue amplifiers, and the analogue parts of analogue/digital amplifiers, which form part of a sound system for professional or household applications. It specifies the characteristics that should be included in specifications of amplifiers and the corresponding methods of measurement.

How many manufacturers do test their 'household' amplifiers and by that extension all audio gear, according to this norm and offer the results or found specifications to customers ?
Personally I think manufacturers should be doing tests like this before they market it and test for EMC/ESD and safety according to some directives as well.
Then the next problem is location. There are different requirements for different countries/regions, at least for safety/EMC.

This last aspect is an issue for the company I work for which sells (railroad) measurement systems globally. In these cases certain testing has to be done externally by notified bodies.

Understanding the test results from all such tests takes expert knowledge and is only valid for lab conditions under 'ideal' installation conditions. Even changing a cable or slightly different setup IRL can make a device not adhere to directives any more.

The fact that one often reads issues with music systems related to ground loops, compatibility or otherwise incorrect usage is testimony.
DAC's that perfectly pass audio tests but are plagued by ground loops IRL etc.

When say testing for the input parameters I am all for it for instance... but.... when reporting this aspect it only really matters when source resistances are known which I have bit seen in any review. With that info one could only say something like... This amplifier might not be perfectly suited for certain sources and explain what they are.

Producing a full report (and more than just this norm) would consume lots and lots of time and why stop at power amplifiers ?
Who else (reviewers) tests consumer audio gear this way ?

So while I am certainly sympathetic to this kind of testing and think it is valuable to a handful of people that would really like to see these results (and EMC, safety) as well as listening tests. And maybe even some loads that differ from the ones outlined in the IEC 60268-3 directive I suppose, in the end, as Amir is doing all the testing and reporting, can evaluate what tests he might want to add and report that could be useful to 'mortals' that want to buy an amplifier and see if it is recommended or not (by Amir in this case).

 

[abdo123]

I would expect Audio precision to have a script for all of this, but I guess members here are interested in more than just "PASS" ?

 

[pma]

This attached report is a good example of a professional measurement report.

 

[amirm]

I would expect Audio precision to have a script for all of this, but I guess members here are interested in more than just "PASS"

I looked and couldn't find any. They do have some component tests complying with this standard but no all up script.

 

[amirm]

This attached report is a good example of a professional measurement report.

They list all this gear:

Equipment used: Audio Precision System One Dual Domain audio analyzer
Rohde & Schwartz UPD audio analyser
Tektronix TDS410A digitizing oscilloscope
4 & 8 ohms resistive loads NRK
Baxendall test equipment

Lots of work and gear to setup and use. R&S has a sample report by the way but it is very limited in what I saw in there.

 

[Sokel]

Are there any specific safety tests in this standard?
Measurements about performance are nice but if we can also get indicators about (our physical) safety and the safety of the device according to an established norm will be far more useful and can debunk the ones that are not fit for purpose.

 

[restorer-john]

It's such a pity R&S pulled the plug on audio analyzers. Their gear was always way better than AP, but oh, so expensive. I used a UPL for a while and it was incredible, but died prematurely. And I had to give it back.

SR have killed their analyzer range too.

Sad.

 

[pma]

Are there any specific safety tests in this standard?
Measurements about performance are nice but if we can also get indicators about (our physical) safety and the safety of the device according to an established norm will be far more useful and can debunk the ones that are not fit for purpose.

They are mentioned, of course, however Safety as a main topic is covered by other standards.

 

[BR52]

Are there any specific safety tests in this standard?
Measurements about performance are nice but if we can also get indicators about (our physical) safety and the safety of the device according to an established norm will be far more useful and can debunk the ones that are not fit for purpose.

For the safety reason, I buy from a European in market bringer because he is responsible for the fulfillment of all the European standards. And is responsible not the Manufacturer, same where else.
Violating the rules can be a legal pain for them.
Hopefully they know it.