Note how the rising edge of the square wave has an indentation when crossing zero - this is indicative of low current capability of the VAS, but probably more of low bias.
Keep in mind that when you change the rail voltage, the VAS current changes in proportion, and also the bias voltage and hence standing current changes - in your case both get lower. You need to re-adjust bias when you change rail voltages on this design.
Adjust the bias current so this dissapears, just be careful to monitor what it is as on the MF12 design turning the bias pot all the way to the end will result in infinite standing current!
The reason why you get rising edge distortion, but not falling edge is because the VAS current pulling down has a FAR larger limit. In essence, the lower MOSFET can be driven on with a very large current (limited by R19). The upper transistor can be driven up only by the current of the bootstrap VAS (Vrail / (R3_R4)).
If you get oscillation when zero crossing, you may also experiment with the gate damper resistors on the MOSFETs, especially the upper one. This will not make the notch dissapear but may make it less jagged. Then, adjust as best possible with bias current.
Keep in mind that when you change the rail voltage, the VAS current changes in proportion, and also the bias voltage and hence standing current changes - in your case both get lower. You need to re-adjust bias when you change rail voltages on this design.
Adjust the bias current so this dissapears, just be careful to monitor what it is as on the MF12 design turning the bias pot all the way to the end will result in infinite standing current!
The reason why you get rising edge distortion, but not falling edge is because the VAS current pulling down has a FAR larger limit. In essence, the lower MOSFET can be driven on with a very large current (limited by R19). The upper transistor can be driven up only by the current of the bootstrap VAS (Vrail / (R3_R4)).
If you get oscillation when zero crossing, you may also experiment with the gate damper resistors on the MOSFETs, especially the upper one. This will not make the notch dissapear but may make it less jagged. Then, adjust as best possible with bias current.
I just noticed in your diagram you were running the amp into clipping, in which case most of what I said in my last post may not apply. It is difficult to see anything of the natural response of the amp when driven by a square wave into clipping... the notch on the rising ende may be due to that alone.
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