This is a matter of phase distortion. Simply put, every transformer (and any other signal processor, unless DC-coupled) behaves as a 6 dB/octave high-pass filter. This filter creates phase distortion that’s significant at about 10 times its ‘cutoff’ (−3 dB) frequency and gets worse as frequency decreases. Therefore, to avoid significant phase distortion at 20 Hz, the cutoff frequency must be 2 Hz or lower. In many Jensen designs, this frequency is well under 1 Hz. Again, this is a matter of time alignment of musical overtones. For example, a kick drum, when heard through a Jensen transformer will actually sound like a live kick drum because the fundamental note and all its overtones have arrived at your ears at the same time!
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