@PENG,
So, on any AVR without disconnection of internal amps, when an internal amplifier starts clipping, since it would also affect the performance of the preamp section driving that power amp, doesn't that impairment of the preamp circuit performance add to the power amp's THD and stability? But I've never heard of a reference to that preamp effect. I guess that it would be almost impossible to measure it?
I may be wrong but I would guess the answer is Yes and no.. By the time the preamp's distortion would increase to that level, any more distortions due to the power amp clipping should be insignificant relative to the power amp's distortions resulted from severe clipping. This, imo can be fixed easily and we already know the SR8015 preamp is not affected by the power amp clipping effects any more. Also, keep in mind whatever the power amp's distortions is, only a small portion would be fed back to the amp's input, that is obviously connected to the preamp output.
I am not an amplifier designer but the feedback principle should be easy to understand. Take a look of the linked article below that I just Googled:
Note: I am not saying that this is exactly how the pre outs of the D+M AVRs distortions happened due to the power amp clipping, but you can see that it could happen this way.
Amplifiers Feedback (tutorialspoint.com)
Principle of Feedback Amplifier
A feedback amplifier generally consists of two parts. They are the
amplifier and the
feedback circuit. The feedback circuit usually consists of resistors. The concept of feedback amplifier can be understood from the following figure.
From the above figure, the gain of the amplifier is represented as A. the gain of the amplifier is the ratio of output voltage Vo to the input voltage Vi. the feedback network extracts a voltage Vf = β Vo from the output Vo of the amplifier.
This voltage is added for positive feedback and subtracted for negative feedback, from the signal voltage Vs. Now,
Vi=Vs+Vf=Vs+βVoVi=Vs+Vf=Vs+βVo
Vi=Vs−Vf=Vs−βVoVi=Vs−Vf=Vs−βVo
The quantity β = Vf/Vo is called as feedback ratio or feedback fraction.
