The aim of four stage amplifiers was to offload the gain profile of the LTP to an assistant second gain stage before the main stage, the requirement of the assistant second gain stage was that it was to have virtually no distortion of its own and have a substantially lower gain than the LTP. With the A70 the second stage is coupled to the main gain stage almost as a current mirror, while SANSUI uses self cancelling. Here's my rendition of what SANSUI was aiming for https://www.pcbway.com/project/shar...audio_power_amplifier_a_rebirth_4a7881e9.html, while for reference is the A70. Are these better than the 3 stage Morpheus? https://www.diyaudio.com/community/threads/morpheus-ultra-low-thd.406485
These are instrumentation amplifier architectures that are best suited in opamps to lift low level signals from the wood work. Do they sound good when used to drive speakers in high power amplifiers? They probably do still sound good.
When it comes to bringing up a phono 100uV/500uV/5mV to 1volt, you need such architectures.
For 100uV/500uV you probably want to do two no feedback stages of 20dB for a total 40dB and then use an opamp with RIAA filter to get to line level. For 5mV you just need one 20dB no feedback stage before the opamp with RIAA filter to get to line level. The ghost of pearl II lingers on, its weakness is its discrete opamp, you need a beast like these fourstagers here
When it comes to bringing up a phono 100uV/500uV/5mV to 1volt, you need such architectures.
For 100uV/500uV you probably want to do two no feedback stages of 20dB for a total 40dB and then use an opamp with RIAA filter to get to line level. For 5mV you just need one 20dB no feedback stage before the opamp with RIAA filter to get to line level. The ghost of pearl II lingers on, its weakness is its discrete opamp, you need a beast like these fourstagers here