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Comparison of power amplifier distortions and FR into complex speaker dummy load

pma

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Comparison of AIYIMA A07 with DIY class AB amplifier into complex speaker dummy load (and more amps added soon)

This is a measurement comparison of AIYIMA A07 amplifier (powered by SMPS Mean Well RS-100-24 27V)

https://www.audiosciencereview.com/...-100-24-smps-power-supply-measurements.19507/

with my new 250W/4ohm class AB amplifier

https://www.audiosciencereview.com/...on-blameless-topology-and-measurements.21542/

with respect to operation into complex dummy load that simulates a 2-way real speaker.

I have built a dummy load to simulate speaker impedance years ago. It simulates a simple 2-way box and uses highly nonlinear ferrite 18mH coil to simulate woofer impedance nonlinearity.

The load looks like this
dummy_real_small.jpg


This is the circuit schematics of my dummy load
dummyload_PMA_cir.png


and this is a measurement of the dummy load impedance
PMA_dummyload_impedance.png


The plots were shown in the DIY amp thread but I re-post them for easier orientation.

First, let's see frequency response of both amplifiers into the dummy load. We can see quite considerable modulation of the AIYIMA frequency response due to its output LC filter interaction with the complex load impedance.

aiyima_vs_250W4Ramp_freqresp_dummy.png

Frequency response of two amps into dummy load


The complex load includes highly nonlinear ferrite core coil of 18mH, which simulates quite well the nonlinearity of the real woofer near resonance. So let's measure THD vs. output voltage at 70Hz first. At 70Hz, we have 8ohm magnitude and -45° phase.

aiyima_vs_250W4Ramp_thdampl_dummy_70Hz.png


We can see quite similar distortion of both amps at this frequency, with AIYIMA having slightly lower distortion between 2V and 8V and of course a big difference in output power. Please note the fast rise of ferrite coil nonlinearity effect above 10Vrms.

Now let's make measurements at 70, 130, 400 and 1300 Hz. Why these frequencies? Because there is a non-comfort combination of impedance magnitude and phase. 70 Hz was already mentioned, 130 Hz has 6 ohm minimum of magnitude with 0° phase, 400 Hz has 12 ohm magnitude with +45° phase and 1300 Hz has 15 ohm impedance with -40° phase.

Class AB distortion vs. amplitude into dummy load at 70, 130, 400 and 1300 Hz

250W4R_thdampl_dummy_multi.png



AIYIMA distortion vs. amplitude into dummy load at 70, 130, 400 and 1300 Hz
aiyima_thdampl_dummy_multi.png


Distortion at 70Hz is similar, at 130Hz and 400Hz AIYIMA has lower distortion but at 1300Hz AIYIMA has considerably higher distortion.

Now let's compare distortion vs. frequency, it was measured at 9Vrms which is still comfortable and almost optimum for AIYIMA.

aiyima_250W4R_thdfreq_dummy_dB.png


One can see fast rise of distortion below 80Hz, which reflects high nonlinearity of the ferrite core 18mH coil, this is reflected in nonlinear load current and this again in voltage distortion at amp terminals due to its finite output impedance.

AIYIMA has a bit lower distortion near woofer resonance (due to lower output impedance at lowest frequencies), slightly lower distortion between 80Hz and 1kHz, however much higher distortion above 1kHz.

Conclusion
So it goes, there are hints that the level matched AB listening test will bring differences. Mostly because of the differences in frequency response, but even the distortion differences might be interesting.
 
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pma

pma

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Thread Starter #2
I am starting to believe that this is the method that may explain why amplifiers sound different and why they should be tested into complex loads, not only to resistive load.

I have added one more amplifier into the comparison plot. The results quite precisely reflect what is heard during the level matched AB listening test.

A07_A250W4R_Z30_dummy.png
 

ebslo

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#3
Great stuff, I've often wondered if this kind of test would expose audible differences between amps that resistive loads do not show. I also appreciate how Soundstage! measures amps with an actual speaker in addition to resistive loads like here, second plot under heading "RMS level vs. frequency vs. load impedance (1W, left channel only)", although only measuring at 1W doesn't show what happens when the driver goes highly non-linear as you show here. A counter argument, however, could be that if the driver's electrical response has gone non-linear it's acoustic response has probalby also degraded and that likely dominates acoustic error.
 
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pma

pma

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A counter argument, however, could be that if the driver's electrical response has gone non-linear it's acoustic response has probalby also degraded and that likely dominates acoustic error.
That's absolutely correct, the acoustical distortion will always be greater than the electrical distortion. This is because the speaker impedance near resonance is highly nonlinear and even if the speaker was driven from an ideal voltage source with zero output impedance, the speaker current would be distorted due to Ohm's law (linear voltage divided by nonlinear impedance makes nonlinear current). However, it is still interesting to observe how speaker nonlinearity is reflected at the amplifier output.

BTW very good set of measurements at Soundstage.
 
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