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RE: Class D , Class-a , Zmin and phase is Very important

If what you claim to measure is true then I would suggest that the amp you measured is not purely Class B.

From Elliott sound products website:
"Crossover Distortion *# : A form of distortion caused by the power output devices in a push-pull amplifier operating in Class-AB. This occurs in valve and solid state designs, and is caused by one device switching off as the other takes over for its half of the waveform. There are some designs that claim to eliminate this distortion by never turning off the power devices, but in reality, only Class-A amplifiers have zero crossover distortion. This is generally measured as a part of the THD of an amplifier, and becomes worse as power is reduced from the maximum."

Note the last part, "and becomes worse as power is reduced from the maximum"

"Solid state amps tend to create high order odd harmonics, so there will be the 3rd harmonic, only a tiny bit less of the 5th harmonic, and the harmonics will extend across the full audio bandwidth. Transistor and MOSFET amps have very high open loop gains, and use feedback to reduce distortion. In all cases, the crossover distortion is caused because the power output devices are non-linear. At the low currents at which the changeover occurs, these non-linearities are worse, as well, the devices usually have a lower gain at these currents.

This has two effects. The open loop gain of the amplifier is reduced because of the lower output device gain, so there is less negative feedback where it is most needed. Secondly, the feedback tries to compensate for the lower gain (and tries to eliminate the crossover distortion), but is limited by the overall speed of the internal circuitry of the amplifier. This results in sharp transitions in the crossover region, and any sharp transition means high order harmonics are produced (however small they might be)."

"I do not believe that we can simply ignore crossover distortion on the basis the "everyone knows how to fix it, and it is not a problem any more". I would suggest that it is still a real problem, only the magnitude has been reduced - the problem is still alive and well."

In a Class B amp it is certainly very alive and very well.

Now, let' look at some amps that have visible crossover distortion in their traces and how they behave with THD+noise vs. power.

"Fig.9 indicates that the distortion is predominantly the subjectively innocuous second harmonic, though there is a suspicious-looking spike in the residual waveform almost coincident with each zero-crossing point. This spike is the result of a picket fence of higher-order harmonics (fig.10), though it's fair to note that these are all relatively low in level.
Read more at http://www.stereophile.com/content/dan-dagostino-momentum-monoblock-power-amplifier-measurements#oPtGg3HVVI7zzb8F.99"

"Perhaps more important, the distortion content (fig.8) has some higher-harmonic content present. This is coincident with the waveform's zero-crossing points, which suggests that the Paladin's output stage is a little under-biased. This harmonic content can also be seen in the spectrum of the amplifier's output while it drove a low-frequency tone at high power into 8 ohms (fig.9). The highest-level harmonic is the relatively benign third, at -64dB (0.06%), but the more problematic fifth, seventh, and ninth harmonics also make appearances, between -80dB and -90dB.
Read more at http://www.stereophile.com/content/portal-audio-paladin-monoblock-power-amplifier-measurements#SzLhG5FuAMhddzFW.99"

"However, when I examined the distortion content, I was surprised to see—superimposed over what appears to be basically a third harmonic (fig.4)—what looks like crossover distortion. (The spikes in this 'scope trace occur at exactly the zero-crossing points of the sinewave.) Yes, the measured level in this graph is very low (0.003%), but crossover distortion tends to be more audible than its level would imply, due to its high-order content and the fact that it tends to dominate at low signal levels. At high powers (fig.5), it tends to drop into insignificance. Intermodulation distortion was very low, the 1kHz difference component with the very demanding 19+20kHz test signal remaining at -90dB (0.003%), even just below the onset of visible clipping (fig.6).
Read more at http://www.stereophile.com/content/herron-audio-m150-monoblock-power-amplifier-measurements-part-2#YIt1qwqub8Fx6QM7.99"


Have a look at the McCormack:
http://www.stereophile.com/content/mccormack-dna-225-power-amplifier-measurements#6KjWBSgkwAzYIBZR.97

Look at the distortion waveform...you can see a slight bump at every zero crossing of the main signal...

All of these are modern AB amps that still have a visible crossover notch. Most of the rest have suppressed it further but the distortion from it is still evident in the FFT spectra.

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