Either a low pass or a high pass filter can change the amplitude of the signal. That depends on the frequency range that is contributing to the amplitude. For example, in the last plot we can see that keeping all frequencies up to 5 kHz results in a lower amplitude compared to keeping all frequencies up to 100 kHz. If most of the amplitude were coming from a 3 kHz feature, however, then the amplitude would not change much between these two different filters.
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