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Complex Load for Power Amplifier torture testing

JohnYang1997

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That "industry standard testing protocol" was made to draw 0,00001% THD and 5-100000 khz. In other words that protocol was implemented to sell crap.



Yes, it is a way to compare different amps. But it's a compete ignorance to judge how those amp will work on real load.



Sure it can. Take any 2-3 way speaker with more or less complex crossover and such load can simulate every situation. You all forget also that any speaker is also a generator (press on your woofer by your hand while it is connected to multimeter and you'll see it generates something). And when such generator is powered by amp with deep negative feedback it all cause very high distortion (sorry, my english is limited. I don't know all terms).
And when we're measuring even on such load as topicstarter made we don't see that effect.
1, Can you hear 20000hz? If answer is no then nothing is wrong with measuring up to 10khz for THD.
2, Negative feedback only improves immunity of back EMF not the other way around. Again, need proof.
 

JohnYang1997

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Answer on all questions is yes. But I'm pretty sure that when I'll show you what is happening on same frequency on resistor and on speaker on a screen of my oscilloscope you'll have 200 more questions and will try to deny my measurements cause they're oppoisite to what religious sect named ASR is used here.
Where is your proof? I don't need a YES that's actually false. Also I have designed amplifiers with extreme low distortion. I don't think most things you said is accurate or remotely correct.
 
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pma

pma

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Thread Starter #204
I would like to show an effect of inadequate (underrated) power supply transformer to amplifier parameters in case of heavy complex load.

Power amp module with 4 pairs of MJL3281/1302 output transistors is supplied by the power supply with 100VA transformer. This power supply gives 2 x 45Vdc at power amp supply terminals at idle (no input signal). The amplifier is loaded with R//(R+C) load, 4ohm//(0.44ohm+6.8uF). This load would not be unrealistic if we speak about ESL panels.

Now, in case of 1kHz 14Vrms output voltage, into this load, power supply DC voltage at amp supply terminals drops from 2x45V idle to 2x35V loaded. This determines maximum output power at low frequencies where the load is almost resistive and close to 4ohm.
If we increase frequency to 10kHz and keep same input signal voltage as in the 1kHz test, the power supply voltage drops to 2x25V, because of the inadequate, underrated power transformer, which is not able to feed current demands of the amplifier operating to 1.9ohm/-50° at 10kHz.

We can see what the inadequate transformer makes with power frequency response and distortion, in the plots below. The top curve is always for 17.6Vrms output voltage, curves are stepped by -1dB.

R-RC_load.png


trafo100VA_14V_step1dB.jpg

Frequency responses into complex load, with inadequate 100VA PSU transformer. From 17.6Vrms output voltage step -1dB.

trafo100VA_inadequacy.png

Distortion vs. frequency with inadequate PSU transformer, due to DC voltage drop. Fs=48kHz, so there is a jump at 7.5kHz in higher Vo plots, as H3 measurement finishes there.
 
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eboleyn

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An interesting thread here, though I see it's gone quiet for a while. Plenty of, err, "spirited" debate. I'll preface this by saying I'm not up to speed on back-EMF from speaker drivers and reactive loads, but my interest in reactive load measurements are more oriented in low-wattage performance cases.

At one point a few years ago, I was comparing some amps at low levels, that is outputting less than 1 watt in my experiments for 50W+ rated amps to mostly 4 ohm but a few 8 ohm loads on a few different multi-way speakers (from 2 to 4 ways). I'd definitely noticed that "difficult" vs "simple" speaker loads can make a huge difference in the ability of some amplifiers to get said speaker to accurately reproduce the signal. The speaker models I won't comment on as I'm not sure it's relevant to the point here.

The one higher-end class D amp I tested is a NAD C390DD (goes directly from digital to amplified signal driving speakers, sometimes called a "PowerDAC"), and was comparing it to a class A (Pass Labs XA30.8) vs. a high-current A/B amp (Pass Labs X250.8). I tried a few others in passing but these were the ones I focused on.

The long and short of it is that the simpler the speaker load I used, the more the NAD unit shined. It really didn't seem to like the complex crossovers. The Pass XA30.8 was in the middle, where it always sounded pleasant, but had the well-reported result of being "weak" in the bass on the more difficult-to-drive speakers and in one case had some other artifacts. The standout was the Pass X250.8, which drove everything extremely consistently and pleasantly.

Having said the above, when recently I have been performing digital crossover bi-amp experiments with no passive crossover at all between the amps and the individual drivers, the NAD unit sounded simply stunning, possibly the best of all. For this setup the NAD is somewhat inconvenient to use since it's not well-suited to being just a power amp, so I'm acquiring some Hypex NC400 monoblocks to see if they behave similarly or not.

Anyway, all of this is just fodder for the commentary where both complex crossovers and the back-EMF from multiple drivers I'm sure gets messy, so love to hear about measurements that try to account for this even somewhat.
 

SIY

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Perhaps you could share measurements and methodology?
 

eboleyn

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Sadly the above results were using human ears (mine and some friends), so not nearly the level of what's being used in this forum. We tried to correct for levels to get matching around 0.5 db or less, but it wasn't super-rigorous.

I'm quite interested in better dynamic measurements of whole chain response (from digital source out through a speaker then measured via microphone) and have been tinkering in that area due to some personal subjective experiences of "better quality" audio from a RAAL 70-20xr ribbon tweeter which objectively by THD standards measured no better than another tweeter, but don't have anything worthwhile to say on it so far.
 

SIY

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Sadly the above results were using human ears (mine and some friends), so not nearly the level of what's being used in this forum. We tried to correct for levels to get matching around 0.5 db or less, but it wasn't super-rigorous.
Clearly. :cool:

The two things that matter are source impedance low enough to not have audible frequency response errors with the target load, and the ability to stably drive current into that load. Both incredibly well-understood for decades and both routinely done except for the very cheapest and very most expensive amps.
 

eboleyn

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Clearly. :cool:

The two things that matter are source impedance low enough to not have audible frequency response errors with the target load, and the ability to stably drive current into that load. Both incredibly well-understood for decades and both routinely done except for the very cheapest and very most expensive amps.
Yes I'm aware of this issue. The amps I listed all are sub-0.1 ohm across the frequency spectrum (modulo the "impedance setting" on the NAD unit which is kind of strange) and even tested output level at several frequency points to determine if the response was flat. But that was just calibration. The actual results were using human ears and so would still fall well into the "subjective" category.

I'm indeed interested in performing these kind of measurements more rigorously using equipment, but don't yet know the right criteria I'm looking for.
 

SIY

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The actual results were using human ears and so would still fall well into the "subjective" category.
Unfortunately, no, this was not by ear if you didn't put in basic ears-only controls. So the differences you think you heard may or may not be real. Start there to confirm that there actually ARE audible differences before trying to figure out what you need to measure.
 

amirm

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I'd definitely noticed that "difficult" vs "simple" speaker loads can make a huge difference in the ability of some amplifiers to get said speaker to accurately reproduce the signal.
I built a complex load emulating a 2-way speaker and started to run it on a few class-D amp. Except for very cheap amps (less than $100), it didn't show anything revealing other than a fraction of a dB variation around 20 kHz so I stopped running it.
 

March Audio

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An interesting thread here, though I see it's gone quiet for a while. Plenty of, err, "spirited" debate. I'll preface this by saying I'm not up to speed on back-EMF from speaker drivers and reactive loads, but my interest in reactive load measurements are more oriented in low-wattage performance cases.

At one point a few years ago, I was comparing some amps at low levels, that is outputting less than 1 watt in my experiments for 50W+ rated amps to mostly 4 ohm but a few 8 ohm loads on a few different multi-way speakers (from 2 to 4 ways). I'd definitely noticed that "difficult" vs "simple" speaker loads can make a huge difference in the ability of some amplifiers to get said speaker to accurately reproduce the signal. The speaker models I won't comment on as I'm not sure it's relevant to the point here.

The one higher-end class D amp I tested is a NAD C390DD (goes directly from digital to amplified signal driving speakers, sometimes called a "PowerDAC"), and was comparing it to a class A (Pass Labs XA30.8) vs. a high-current A/B amp (Pass Labs X250.8). I tried a few others in passing but these were the ones I focused on.

The long and short of it is that the simpler the speaker load I used, the more the NAD unit shined. It really didn't seem to like the complex crossovers. The Pass XA30.8 was in the middle, where it always sounded pleasant, but had the well-reported result of being "weak" in the bass on the more difficult-to-drive speakers and in one case had some other artifacts. The standout was the Pass X250.8, which drove everything extremely consistently and pleasantly.

Having said the above, when recently I have been performing digital crossover bi-amp experiments with no passive crossover at all between the amps and the individual drivers, the NAD unit sounded simply stunning, possibly the best of all. For this setup the NAD is somewhat inconvenient to use since it's not well-suited to being just a power amp, so I'm acquiring some Hypex NC400 monoblocks to see if they behave similarly or not.

Anyway, all of this is just fodder for the commentary where both complex crossovers and the back-EMF from multiple drivers I'm sure gets messy, so love to hear about measurements that try to account for this even somewhat.
Well you need to cautious of some of the information posted in the thread. Creating pathalogical situations and loads that are totally unrepresentative of the real world is not useful and in fact potentially misleading.

PMAs post above showing a load impedance falling to 1 ohm at 20kHz is encountered with some speakers, but not many, and is erring toward the more extreme. However the big mistake he has made there is applying a 14 volt signal across the entire frequency range. Music simply just does not behave like this so its an unrealistic test providing no useful information. Music will typically have signal levels at 20kHz that are 60dB or more lower than at 1kHz. This means a 14 volt signal at 1khz will fall to just 0.014 volts ! So the amplifier (or power supply) is simply not taxed, and will never be, as is indicated in that post.

So what he considers an "inadequate" power supply is in fact [probably what i would consider an "intelligent" design. It hasnt been over engineered to account for situations that just dont exist in the real word. A larger transformer that is not needed would just add significant cost for no benefit.

You can see my point perfectly demonstrated in one of PMAs graphs.

When the signal level falls by just 6dB (not 60dB) the output remains consistent to 20kHz.

1593395477503.png


Second thing to note is that music is transient in nature. Its not a consistent load. Unregulated supplies, as is typical with amplifiers, will cope better with transients and maintain their output voltage better than with constant loads.
 
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