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"[D]oes this circuit need a pre-amp or it can drive power amp directly?" The source Z of an op-amp's output is essentially 0. Note that there's a 220 ohm resistor on the last amp's output. So ithe output Z of the filter is 220 ohms. The input Z of a typical power amp is something like 5-10K, so the filter will have no problem driving an amp directly. "I can't find potentiometers stated in the diagram here in our place we have here 50k and 25k pots..." You'll note from the schematic that the pots are connected as rheostats. 25 and 50K will be fine. You could even use 100K pots, though adjusting them might be touchy.