Filter Analysis using Audio Analyzer
If you have 100 or 600 ohms or whatever source resistance, you have to add that to the R in your RC. If you had a simple RC filter like 500 ohms and 100n you would get a corner frequency of 3.2 kHz. But if the source impedance of the circuit driving the filter is 600 ohms, you would get 1.4 kHz because the R in the RC is actually 600+500 or 1.1k. Big difference!