lowpass filter question
The first case ends up as a lowpass filter, the second case is either a lowpass (R=0), or a shelf lowpass (R&gt;0). Both attenuate the DC to some extent, due to the input pots:

For the 1st case, you can't isolate with only RC elements, you'll need some active element (opamp would be a good choice) for that. For the 2nd case, I have no idea what this means to you

Acoustically, this is closer to what I was looking for

but the corner frequency is lower than the 1st case, and due to the &quot;shelf&quot; response, high frequencies are preserved, but attenuated. I guess that might sound to you as more &quot;rich&quot;, more clear. I also don't know what range means to you here

The filter in fig.1 doesn't give the range I was looking for

but it seems to me that the corner frequency in the 1st case is ~50 Hz, while for the 2nd one it's ~6 Hz. You should first establish some parameters before continuing, for example what range of frequencies do you want, or is it a simple or shelf lowpass that you need. When I say frequency range I don't mean &quot;somewhere between 30-300 Hz&quot;. Establish your goals first, then build.