Power amplifier tests with respect to FTC: 16 CFR Part 432 (July 5, 2024) requirements on output power claims

[pma]

Some of us feel the need of honest specifications of amplifier output power according to acknowledged documents like FTC regulations and IEC standards. The reason is to get comparable data, protect potential customers and draw attention to false claims of some manufacturers.

I have decided to make such tests myself ad so far I have tested several well known amplifier and also one of my DIY designs. The links to the tests and results can be found below:

AIYIMA A07 amplifier test review

1ET400A amplifier test review

Topping B100 RCA amplifier test review

A250W amplifier test review

NC252MP amplifier test review

 

[tmtomh]

I checked out your Purifi test results - you got 180 watts into 4 ohms at 1%THD, yes? @amirm's tests with an EVAL board-based unit (2019) and the Audiophonics amp (2022) each showed a little over 250 watts per channel into 4 ohms at the onset of clipping, which is at a point lower than 1% THD.

So I assume the test procedure or conditions are different - is that right?

 

[miero]

1. Test conditions
The amplifier (Purifi 1ET400A,EVAL2 mono board) is supplied from Hypex SMPS400A180 main power supply, that allows to get about 200W/4ohm/1kHz/THD1%at best.

Source: https://pmacura.cz/FCT/1et400a.htm

vs.

Hypex SMPS1200 was used in both amplifiers measured by Amir:
- https://www.audiosciencereview.com/...easurements-of-purifi-1et400a-amplifier.7984/
- https://www.audiosciencereview.com/...ics-hpa-s400et-review-stereo-amplifier.32014/

 

[pma]

I checked out your Purifi test results - you got 180 watts into 4 ohms at 1%THD, yes? @amirm's tests with an EVAL board-based unit (2019) and the Audiophonics amp (2022) each showed a little over 250 watts per channel into 4 ohms at the onset of clipping, which is at a point lower than 1% THD.

As @miero has mentioned and as I write in the test, I use SMPS400A180 power supply which gives 2x46Vdc. As 1ET400A EVAL2 is sold as an amplifier module, without a power supply, the test results are of course valid only with the power supply used. The same applies to the AIYIMA A07 test linked here above, that is also sold without a power supply. Based on my previous tests and experience with this small amplifier, I would not recommend higher supply voltage for the A07 than 36Vdc.

On the other hand, the NC252MP is a complete of amplifier and power supply on-board. So, it is tested if it meets manufacturer specs,

with the huge, 3U x 390mm heatsink used, under FTC conditions. It did not pass the 5 minutes maximum power test at 2 x 250W/4R/1kHz, but it was able to make it for 4 minutes. However, with dangerous signs like hot air and smell from the amplifier top vent slots. It would probably make something like 2 x 220W/4R/1kHz for 5 minutes. Another observation is THD slightly above 1% between 50Hz - 100Hz (the same or similar issue we can see in the 1ET400A test) and this continues even at lower power, so strictly put the power frequency response of NC252MP with THD+N<1% would be about 2x200W/4ohm/THD+N<1%. I find the weak point is the SMPS power capability at lower frequencies and also the heat transfer mechanism from SMPS to the module Al base plate. Though the switching MOSFETs heat seems to be dissipated well through the base plate to the heatsink, the rest of the module is only "air cooled" and this might be a reliability issue in the long term operation at high power. The beefy A250W class AB with linear supply produces more heat during operation at lower power, however, when pushed hard, it has lower both LF and HF distortion at high power and, the power dissipation at full power is lower than at half power for class AB. These may be interesting points.

 

[somebodyelse]

On the other hand, the NC252MP is a complete of amplifier and power supply on-board. So, it is tested if it meets manufacturer specs,

with the huge, 3U x 390mm heatsink used, under FTC conditions.

You really ought to include the notes 1-5 with this:

Note 1: The stimulus signal is a continuous 1 kHz sine wave. The true rms output voltage is measured across
a load resistor. Max output power is time limited due to thermal properties.
Note 2: Typically, this is 1/5 of the peak output power. Apply sufficient cooling.
Note 3: An Audio Precision AES17 20 kHz is used during this measurement.
Note 4: Current limited.
Note 5: Voltage limited

Given the much lower continuous output spec I was expecting the time limit for maximum to be a lot less than the 4 minutes you found.

 

[restorer-john]

You really ought to include the notes 1-5 with this:

Given the much lower continuous output spec I was expecting the time limit for maximum to be a lot less than the 4 minutes you found.

Considering @pma has the entire module bolted intimately to a huge 3U (132mm high) x 390mm long heat-sink, it was getting every possible chance it could. I reckon the heatsink would be 0.6-0.9C/W (guessing).

The 'good' NC-252MP I tested a while back, could not do 170W continuous, no matter what you did- even forced air cooling on a sizeable heatsink. Then it blew up, like several of the others I have here sent by members to investigate.

 

[pma]

Considering @pma has the entire module bolted intimately to a huge 3U (132mm high) x 390mm long heat-sink, it was getting every possible chance it could. I reckon the heatsink would be 0.6-0.9C/W (guessing).

It might be a good guess, I would say. The issue is definitely the power supply part. As long as the only one channel is loaded, the situation is much better and power BW is higher, well below THD+N<1% at 1x250W/4R. But, when both channels are loaded .... it gets stressed. Might be interesting if our dear OEM assemblers, @Buckeye Amps , @boXem , @Audiophonics , @marchaudio made the test as well, with their products. It is not only about datasheet specs.

 

[somebodyelse]

The 'good' NC-252MP I tested a while back, could not do 170W continuous, no matter what you did- even forced air cooling on a sizeable heatsink. Then it blew up, like several of the others I have here sent by members to investigate.

It's specified for 50W continuous. What made you think it would manage 170W continuous?

 

[Matias]

Here we go again with FTC tests in long term max power output...

This is as relevant as testing a car engine running 10.000 RPM for how long until it overheats and breaks: an useless test for an unpractical real use case. Yet the FTC specifies such test for some clueless reason, and even worst, some people insist on it...

 

[Davide]

Considering @pma has the entire module bolted intimately to a huge 3U (132mm high) x 390mm long heat-sink, it was getting every possible chance it could. I reckon the heatsink would be 0.6-0.9C/W (guessing).

The 'good' NC-252MP I tested a while back, could not do 170W continuous, no matter what you did- even forced air cooling on a sizeable heatsink. Then it blew up, like several of the others I have here sent by members to investigate.

The transmission of heat from the MOSFETs to the plate takes place with silicon thermal pads, silicon that tends to evaporate with high temperatures and aging (the famous oil bleeding), so performance decreases over time.
There is also the pressure applied on the pad that may be reduced due to PCB bending/plastic deformation.
Furthermore, the thermal resistance between the metal plate and the case heatsink is not so low if thermal paste is not used.
All this can cause the module to heat up a lot and the heatsink remains lukewarm, just as PMA finds.
I also have an NC252MP and I played it a lot with heat dissipation...
The 50W continuous per channel declared by the manufacturer make sense for integration with a proper dissipation system.
Anyway, it seems that the primary source of heat is the power supply.

 

[JustJones]

The FTC test is not 5 minutes at maximum output but what they call "rated power" which is continuous average power output not short term power output.

So according to the Hypex NC252MP data it would need to do 5 minutes from 250mW to the rated power of 50W , 20 Hz to 20 kHz without exceeding 1.0% THD+N to meet FTC rules.

The Purifi 1ET400A doesn't specify a continuous output power but simply states it's limited by thermal system.

(e) Any power level from 250 mW to the rated power shall be obtainable at all frequencies within the rated power band of 20 Hz to 20 kHz without exceeding 1.0% of total harmonic distortion plus noise (THD+N) at an impedance of 8 ohms after input signals at said frequencies have been continuously applied at full rated power for not less than five (5) minutes at the amplifier's auxiliary input, or if not provided, at the phono input.

(g) Rated power shall be minimum sine wave continuous average power output, in watts, per channel (if the equipment is designed to amplify two or more channels simultaneously), measured with all associated channels fully driven to rated per channel power.

Bold mine except at the end.

So yes the NC252MP could very well blow up , shut down, excessively complain, or give you the middle finger at 170 watts at 5 minutes, no surprise there.

 

[tmtomh]

So for the Purifi the test is really about the PSU, not the amp module itself. And for the NC252MP the 5 minute test is not actually in disagreement with the data sheet.

 

[Koeitje]

I think its impressive that it managed 4 minutes at 4 times the rated continuous output.

 

[Davide]

Here we go again with FTC tests in long term max power output...

This is as relevant as testing a car engine running 10.000 RPM for how long until it overheats and breaks: an useless test for an unpractical real use case. Yet the FTC specifies such test for some clueless reason, and even worst, some people insist on it...

I believe it should be seen in reverse. FTC regulates that so that manufacturers declare the maximum power according (also) to that test, not only in accordance with distortion or electrical potential of the system, which may notoriously not be sustainable in the long term.
In fact it's referred as rated power, not maximum.
It is a fair regulation to protect consumers, and it can't be expected to apply retroactively.
Despite this, Hypex already distinguished Maximum from Continuous. I think it was honest in the absence of precise regulation. And it doesn't even make sense to me to declare only rated (continuous) power without peak power, it's misleading sometimes.
Of course, not immediate to understand for any kind of consumer (leaving aside that it is an OEM specification that is rarely transported to the final product).

 

[DonH56]

Here we go again with FTC tests in long term max power output...

The thread title is pretty clear; not reading the thread is a valid choice.

This is as relevant as testing a car engine running 10.000 RPM for how long until it overheats and breaks: an useless test for an unpractical real use case. Yet the FTC specifies such test for some clueless reason, and even worst, some people insist on it...

Here we go with the car analogies again... No, it is the same as running the engine at specified maximum load to verify that the times you need it going down the highway, it actually meets the manufacturer's specifications and defined government standards of performance and quality.

There was a lot of input to the FTC from apparently clueless people who prefer a defined standard power specification without unrealistic peak numbers. I and many others sent in comments for them to evaluate, and of course plenty of manufacturers did as well. The latter were among those pushing for a 1/8-power preheat and most all comments I read agreed with that based on typical musical signals and the wide variety of modern amplifier operating classes. It was not done in a vacuum, but rather an attempt to reign in the unrealistic specs that were being produced.

Running at full rated power for five minutes is not horribly stringent; in what other world is "continuous" reduced to just five minutes? If the amp cannot handle that, it is more likely to fail after hours of use at much lower power levels in a consumer installation. Heat is the primary cause of failure in electronics so assessing the thermal capacity of the amplifier is credible, at least to me and other engineers and scientists. I am used to having to perform life tests that are much harder (and longer!) than this test for components used in cars and other consumer products, and much more stringent tests for space and military applications.

There are several solutions manufacturers can use and all have been discussed. The simplest is to derate the continuous power rating appropriately, adding a CEA (formerly IHF) peak specification for short-term power. Marketing will naturally emphasize the peak numbers, but at least everybody will be following a defined standard. Improving thermal management, via larger heat sinks, additional venting, fans, and so forth is another approach more likely to be taken buy high-end manufacturers.

Clueless I may be, but I would prefer to know the actual continuous power ability of my amplifiers, as it provides insight into the manufacturer's design practice and reliability.

 

[pma]

Running at full rated power for five minutes is not horribly stringent; in what other world is "continuous" reduced to just five minutes?

That’s it. Thank you, Don!

 

[Sokel]

I really can't get the denial here.Rule does not force anyone to change their designs or make people spend more.

It only asks certain figures under certain conditions,nothing more.Amps will still sound the same,they won't go magically weaker.Selling points might be but that's for those that are not in the hobby,the ones already in (hopefully) know what they buy (even if it's written in the fine print,or by others)

Yes,there are people in the hobby saying "I push 100 W" out of a passively cooled matchbox but those are usually carried away by various fanbois,etc.That's a great opportunity for those too to embrace mother nature and its rules and move the magic at other territories.

(Yes,there are egos at play too,but that's the same old story as everywhere else)

 

[pma]

Forgive me to go slightly off topic. I started to build amplifiers in 1970, when I was 15, and to design my own circuits about 10 years later. Even then I had not realized how important the thermal design was. My modular amp from 1980 looked like this:



and it had similar issues like nowadays B100. The idle current had to be held very low to prevent thermal runaway. And I used similar reasoning as I can read here, 44 year after: "enough with music", "sine wave test is not needed". So it goes. The reliability of such "thermal design" was not great, believe me.

 

[Geert]

An amp from a renowed brand I bought new a few years back, spec'd 2×120 Wrms / 8Ω and equiped with 2 chunky 340 VA transformers. During summer, the thing shut down twice due to overheating while playing background music in my living room. The small (enclosed) heatsinks couldn't keep-up. The thermal protection sensor (thermistor) on the PCB wasn't even connected to the heatsinks.

So a bit of stress testing can't hurt I believe.

 

[Chr1]

Doh.

Small internal heatsinks.