It resolves to a simple two transistor gain stage, Q701 and Q703.
https://www.semiconductorforu.com/two-transistor-amplifier-working-and-its-applications/
The output stage is 'bolted' on to the output and the feedback now taken from the main output. Voltage gain is determined by the ratio of R713 and R715 which in the link above would be the 10k and 100R resistors.
All the complex looking stuff at the input is to try and get some sort of DC stability vs temperature because being a single ended input stage means the design has no pretensions of DC precision. Being able to set and maintain a low DC offset is a major challenge on a design like this.
Q704 simply makes a 'super high gain PNP transistor' in combination with Q703
https://www.semiconductorforu.com/two-transistor-amplifier-working-and-its-applications/
The output stage is 'bolted' on to the output and the feedback now taken from the main output. Voltage gain is determined by the ratio of R713 and R715 which in the link above would be the 10k and 100R resistors.
All the complex looking stuff at the input is to try and get some sort of DC stability vs temperature because being a single ended input stage means the design has no pretensions of DC precision. Being able to set and maintain a low DC offset is a major challenge on a design like this.
Q704 simply makes a 'super high gain PNP transistor' in combination with Q703
The emitter of Q701 is the negative input (feedback) of what lately we call a CFA, and Q703 is the VAS (Voltage Amplifier stage). Without the whole schematic, it's hard to say but it appears to have a bipolar power supply, ie no output coupling capacitor. Then pot R708 adjusts the bias on Q701 for the output DC offset. I'm curious if this is an old product with some before it's time features or a new product that uses a few older ideas. I'm not a fan of the schematic style that requires careful study to sort of the crammed drawing.
Thanks Mooly and stevenu!, i now understand the basics of it. The reason for asking is that whenever i repair an amplifier i try to understand the schematic but this was the first time i have seen this type. I think its from 1985 or close. The bridge function is also very different. Another strange thing is that it sounds very good.
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