change a square wave into a sine wave (3 pole R-C filter)

How to change a square wave into a sine wave (or triangle wave) with the help of a 3 pole R-C filter. The showed circuit works good on 31 KHz. The signal weakens when it passes the filter, so amplificaton (1 transistor at the output) could be necessary.

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Видео change a square wave into a sine wave (3 pole R-C filter) канала radiofun232