I am always looking for cheap amps in the local classified ads, but this one is not quite cheap enough. The reason I am posting is that the circuit is not typical. It would be interesting to give it a listen if the layout and construction wasn't so poor and the main capacitors so small. (I would replace the electrolytic coupling caps and inject the signal directly to the power amp of course.)
It has an additional LTP after the input. BUT, I wouldn't call it a dual-differential; because this one also has your typical single ended VAS like 90% of amps. One additional stage, therefore. It must have high loop gain at 100Hz and 10Hz, and according to Putzeys this is the way to go (regarless of loop gain at higher frequencies; it can be good, but it's not as important as the low frequency feedback).
It has an additional LTP after the input. BUT, I wouldn't call it a dual-differential; because this one also has your typical single ended VAS like 90% of amps. One additional stage, therefore. It must have high loop gain at 100Hz and 10Hz, and according to Putzeys this is the way to go (regarless of loop gain at higher frequencies; it can be good, but it's not as important as the low frequency feedback).
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I quote Bruno Putzeys, for clarity, because I mentioned his article:
As explained in the article, having a high number for the low frequency open loop gain is something that bears relation to the final error, which in control system theory means how close the output of the system gets to the setpoint (the input). When a voltage step is applied, the high OLG system "adjusts a smidgen further" compared to the system with lower OLG (I'm quoting from the article, it means the final error is smaller for the high OLG system).
Of course, having plenty of loop gain for high frequencies is desirable as well.
https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf
As explained in the article, having a high number for the low frequency open loop gain is something that bears relation to the final error, which in control system theory means how close the output of the system gets to the setpoint (the input). When a voltage step is applied, the high OLG system "adjusts a smidgen further" compared to the system with lower OLG (I'm quoting from the article, it means the final error is smaller for the high OLG system).
Of course, having plenty of loop gain for high frequencies is desirable as well.
https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf
The additional differential did not help the specs - 0.7% THD. https://www.hifiengine.com/manual_library/sherwood/rv-5050rds.shtml
It looks like a cheapie. 😉
Ed
It looks like a cheapie. 😉
Ed