NC252MP (class D) vs. A250W4R (classAB) burst measurements into 4ohm//2.2uF load
As I have been doing, quite intensively, wide range of power amplifier measurements, I have decided to compare 2 different power amplifier design philosophies, class AB and class D. The amplifiers chosen have equal rated power, 2 x 250W at 4ohm load. The main difference, except for class of operation, is that NC252MP uses a common power supply for both channels, however A250W4R are in fact 2 independent amplifiers in one case, completely separated, with 2 independent linear power supplies (2 transformers and two rectifiers/filters). So each channel of A250W4R "does not care" if the other one is loaded or not.
The bursts were generated with DAC at 96kHz sampling rate. It can be seen in the class AB measurements, in the class D amplifier this fact is masked by its high ultrasound noise.
The amplifiers under test are described here and here.
1. Complex load used
The load used for the test is 4ohm resistor in parallel with 2.2uF polypropylene capacitor, KEMET automotive type with max. allowed dv/dt = 200V/us. Below please see measured impedance of this load:
At 20kHz, we have 2.762ohm impedance magnitude, -40° phase and 1.07ohm EPDR (equivalent peak dissipation resistor).
2. Measurements with 1kHz burst
(Maximum allowed distortion is 1%)
A250W4R maximum 1kHz burst power is 2 x 308W/4ohm, one or two channels driven (no difference)
NC252MP max. 1kHz burst power is 2 x 234W/4ohm (distortion 0.4%)
2. Measurements with 10kHz burst
A250W4R max. 10kHz burst power is 2 x 299W/4ohm (load 4ohm//2.2uF)
NC252MP max. 10kHz burst power is 2 x 227W/4ohm (load 4ohm//2.2uF)
3. Measurements with 20kHz burst
A250W4R max. 20kHz burst power is 2 x 298W/4ohm (load 4ohm//2.2uF). With respect to EPDR, max. burst power into this load is 2 x 1112W/1.07ohm.
NC252MP cannot handle the same input burst amplitude as at 1kHz or 10kHz. Distortion rises to 6.4% (H3). The shape is not sinusoidal.
Reduced amplitude for NC252MP 20kHz burst
With burst amplitude reduced to get 20.5Vrms output from NC252MP, the amplifier works "better", but still unacceptable.
4. Conclusion
The "beefy" class AB amplifier has a bit higher distortion with traditional measurements at 1kHz (with 20kHz BW), but it handles much better the complex load with capacitive component of -40°/20kHz. It is also more powerful because of two separated power supplies, though the rated power is equal.
4ohm//2.2uF load is not good for NC252MP. It activates oscillations near to 80kHz, as an interaction of NC252MP output filter with the 2.2uF capacitor. Care with complex load with such amplifiers.
There was a different windowing of 10kHz/20kHz measured spectra for A250W4R and NC252MP, for the reason that A250W measurements are older and taken with Picoscope 6 SW (older version). So the spectral resolution with Picoscope 6 was lower at lower frequencies, thus the different shape below 50kHz.
The longer I test test the Hypex NC252MP, the more my initial passion for this class D amplifier is fading away and the more my scepticism to class D is growing.
Next step would be capturing music files from the output of both amplifier loaded with 4ohm//2.2uF and the ABX test. Output power will be up to 2 x 200W/4ohm.
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Appendix:
NC252MP comparison of THD+N vs. power at 1kHz into 4ohm resistor load and 4ohm resistor in parallel with 2.2uF capacitor load
This is the proof that the NC252MP is unusable with 4ohm//2.2uF load (simulation of the worst-case elstat. speakers)
As I have been doing, quite intensively, wide range of power amplifier measurements, I have decided to compare 2 different power amplifier design philosophies, class AB and class D. The amplifiers chosen have equal rated power, 2 x 250W at 4ohm load. The main difference, except for class of operation, is that NC252MP uses a common power supply for both channels, however A250W4R are in fact 2 independent amplifiers in one case, completely separated, with 2 independent linear power supplies (2 transformers and two rectifiers/filters). So each channel of A250W4R "does not care" if the other one is loaded or not.
The bursts were generated with DAC at 96kHz sampling rate. It can be seen in the class AB measurements, in the class D amplifier this fact is masked by its high ultrasound noise.
The amplifiers under test are described here and here.
1. Complex load used
The load used for the test is 4ohm resistor in parallel with 2.2uF polypropylene capacitor, KEMET automotive type with max. allowed dv/dt = 200V/us. Below please see measured impedance of this load:
At 20kHz, we have 2.762ohm impedance magnitude, -40° phase and 1.07ohm EPDR (equivalent peak dissipation resistor).
2. Measurements with 1kHz burst
(Maximum allowed distortion is 1%)
A250W4R maximum 1kHz burst power is 2 x 308W/4ohm, one or two channels driven (no difference)
NC252MP max. 1kHz burst power is 2 x 234W/4ohm (distortion 0.4%)
2. Measurements with 10kHz burst
A250W4R max. 10kHz burst power is 2 x 299W/4ohm (load 4ohm//2.2uF)
NC252MP max. 10kHz burst power is 2 x 227W/4ohm (load 4ohm//2.2uF)
3. Measurements with 20kHz burst
A250W4R max. 20kHz burst power is 2 x 298W/4ohm (load 4ohm//2.2uF). With respect to EPDR, max. burst power into this load is 2 x 1112W/1.07ohm.
NC252MP cannot handle the same input burst amplitude as at 1kHz or 10kHz. Distortion rises to 6.4% (H3). The shape is not sinusoidal.
Reduced amplitude for NC252MP 20kHz burst
With burst amplitude reduced to get 20.5Vrms output from NC252MP, the amplifier works "better", but still unacceptable.
4. Conclusion
The "beefy" class AB amplifier has a bit higher distortion with traditional measurements at 1kHz (with 20kHz BW), but it handles much better the complex load with capacitive component of -40°/20kHz. It is also more powerful because of two separated power supplies, though the rated power is equal.
4ohm//2.2uF load is not good for NC252MP. It activates oscillations near to 80kHz, as an interaction of NC252MP output filter with the 2.2uF capacitor. Care with complex load with such amplifiers.
There was a different windowing of 10kHz/20kHz measured spectra for A250W4R and NC252MP, for the reason that A250W measurements are older and taken with Picoscope 6 SW (older version). So the spectral resolution with Picoscope 6 was lower at lower frequencies, thus the different shape below 50kHz.
The longer I test test the Hypex NC252MP, the more my initial passion for this class D amplifier is fading away and the more my scepticism to class D is growing.
Next step would be capturing music files from the output of both amplifier loaded with 4ohm//2.2uF and the ABX test. Output power will be up to 2 x 200W/4ohm.
------------------------------------------
Appendix:
NC252MP comparison of THD+N vs. power at 1kHz into 4ohm resistor load and 4ohm resistor in parallel with 2.2uF capacitor load
This is the proof that the NC252MP is unusable with 4ohm//2.2uF load (simulation of the worst-case elstat. speakers)
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