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Proposed change for THD+n power measurement

mike7877

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Edit: if a mod sees this thread can you please move it to the amplifier review forum? And delete this line if it's not too much trouble. Sorry!



So this picture here:

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Is this picture above, + IMD + Harmonic Distortion from 32 tones across the spectrum at the same level are playing... no?



Since most music energy and dialogue in visual media has the majority of its acoustic power under 300Hz, and very little over 1000Hz, I don't think it's the most accurate to be saying THD+n is as high as it is at the higher power outputs for amplifiers that look like the example above (consistently falling THD+n until a point where THD+n decreases more slowly, usually between 2 and 10W).

A simple solution could be to analyze the sound from a collection of movies to find out how much sound energy there is across the spectrum, maybe ignoring under 50Hz because who cares if low booms have a little second and third harmonic energy? More impactful! Right? lol. I'm joking. Moving on..:

The same could be done for music using 50 songs from each genre, then weighting them for how popular each genre is.

Then, weight movies 70%, music 30%.


Then, adjust the level of the tones across the spectrum to represent the spectrum analysis.

Because a receiver is never asked to make 100W of 17067Hz!


If the THD+n isn't done the way I assumed, then I guess this thread is useless. Or maybe not - the concept could just need to be applied differently.
We'll see
 

jasonhanjk

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Normally I would measure THD+N vs Frequency with LPF 22kHz. Such measurements are very common in datasheet, eg TI.
 

amirm

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Is this picture above, + IMD + Harmonic Distortion from 32 tones across the spectrum at the same level are playing... no?
??? No. Multitone is at a fixed output. These are sweeps. They are also single tones, not IMD that is dual tones.

Note that the second set of graphs uses fewer steps/points as to not overstress the amplifier. So the clipping point is not always at the same wattage as the main power sweep at 1 KHz. Some amps also get upset at higher frequencies so I wind up not pushing the amplifier as much. The purpose of the second graph then is to see the relationship between frequencies and not as much with respect to power limit.
 
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mike7877

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??? No. Multitone is at a fixed output. These are sweeps. They are also single tones, not IMD that is dual tones.

Note that the second set of graphs uses fewer steps/points as to not overstress the amplifier. So the clipping point is not always at the same wattage as the main power sweep at 1 KHz. Some amps also get upset at higher frequencies so I wind up not pushing the amplifier as much. The purpose of the second graph then is to see the relationship between frequencies and not as much with respect to power limit.

So the first graph is at 1KHz?


What do you think about a new test kind of like how I described - for example something like a THD/IMD test done using multitone, where there are 3 pulsed low frequency tones at 15% duty cycle at 50% maximum amplifier power, 4 pulsed mid frequency tones at 40% duty cycle at 15% maximum power, and 5 high frequency tones at 100% duty at 1-5% maximum power (similar to music).

I think this could emulate what kind of performance to expect with music playing at a high-ish volume for the amplifier being reviewed. It also shouldn't cause clipping unless all of the waves crest simultaneously (should easily be able to be avoided by using choice frequencies and duty cycles).
 
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mike7877

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Somewhat unrelated- I've always wondered what those two THD peaks at around 50mW are from. This 4800 and the 3700 have it, the 3600 has it at 200mW. I'm betting the only difference between the amps (in the 3700/4800) is a few volts on the rails and maybe, maybe, a bigger output transistor.
My guess for the peak is the switch from Class A operation? 40mW and 8 ohms is just under 0.6V so it's plausible IMO, but I'm not sure
 
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