Series III Input Topology
Posted: Sat Jan 19, 2019 9:26 am
Absent schematics, does anyone know what type of input topology is used in the Series III mic/line inputs? Is it a three-stage op-amp instrumentation input with output offset servo-control? Single-stage differential input op-amp?
The difference isn't trivial: 3-stage op-amp inputs (or high-quality transformer) offer best common-mode rejection ratio (CMRR). Cheaper product often use single-stage op-amps to achieve a differential input.
With a 3-stage input functioning as an instrumentation amp (not to be confused with an instrument input!) unbalanced sources can be connected directly to the balanced input without tying one side of circuit ground and otherwise affecting circuit CMRR.
Paul
The difference isn't trivial: 3-stage op-amp inputs (or high-quality transformer) offer best common-mode rejection ratio (CMRR). Cheaper product often use single-stage op-amps to achieve a differential input.
With a 3-stage input functioning as an instrumentation amp (not to be confused with an instrument input!) unbalanced sources can be connected directly to the balanced input without tying one side of circuit ground and otherwise affecting circuit CMRR.
Paul