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No, it's not on the audio circuit, it's in the compressor detector, it's not allowing your very low frequencies to trigger very hard the compressor and squishing all the signal. The compressor is working always on all the spectrum, but the compression is not triggered by the very low notes (on my 5's is very effective).So it sets the frequency that you want to bypass compression? Allowing you to have a compressed signal while not squishing your lows?