I was comparing a few versions of the Seventh Circle Audio C84 (based on the Cohen topology) and noticed a difference between the DC servos.
Just to start with the first stage:
In the attached schematic (v1.6), a single inverting first-stage servo takes input from R27/28 and spits out the correction in front of the matched input transistors, through R5/6.
In the schematic linked here (v2.0), the first stage has two servos: an inverting servo that takes input through R29/30 but feeds a second pair of matched transistors, and a second, differential servo that only feeds the "-" input terminal through a 100K resistor, while the "+" input terminal goes to ground through another 100K resistor.
What's the difference? What are the different servos doing?
(Am I correct in guessing that, in v1.6, the single servo correcting for common mode DC, while in v2.0, the differential servo is handling common mode DC and the inverting servo/transistor array is correcting DC across the gain switch?)
Just to start with the first stage:
In the attached schematic (v1.6), a single inverting first-stage servo takes input from R27/28 and spits out the correction in front of the matched input transistors, through R5/6.
In the schematic linked here (v2.0), the first stage has two servos: an inverting servo that takes input through R29/30 but feeds a second pair of matched transistors, and a second, differential servo that only feeds the "-" input terminal through a 100K resistor, while the "+" input terminal goes to ground through another 100K resistor.
What's the difference? What are the different servos doing?
(Am I correct in guessing that, in v1.6, the single servo correcting for common mode DC, while in v2.0, the differential servo is handling common mode DC and the inverting servo/transistor array is correcting DC across the gain switch?)
