Winston OBoogie
Well-known member
john12ax7 said:I was approaching it from a line input perspective, it's possible that was incorrect.
Not really. I would say it's the same exercise, just as Abbey and John R. have explained above.
Of course, the exact source impedance that would feed the filter is unknown and out of your control, but you can look at worst case (higher source Z) and use that as your lowest possible dominant pole. These days most any source is low impedance - within the low to mid end of the range PRR said above - and the series R you use in the filter doesn't have to be so high as to degrade the noise performance to any great degree.
Edit, forgot: HPF - for better or worse, I'm still attracted to using low noise J-Fets as input devices which, besides offering good immunity against gremlins, allow the use of a nice film cap to block any incoming DC and define the dominant low end. John R's technique of using a later in the topology higher Z point sounds good for when the input is BJT with a lower input impedance.