A subsonic filter is mostly used in low frequency technology. Most often, the very low frequencies create distorsions in music recording or reproduction. This happens independent of the applied recording medium. To remove this unwanted low frequencies, subsonic filters like the circuit featured here is used.
The subsonic filter is actually an active Tschebyscheff high pass filter of the fifth order. It has a cutoff frequency of 18 Hz. This filter has a very sharp cutoff character. Frequencies blow 10 Hz are attenuated by more than 35 dB. The typical Tschebyscheff resonance of 0.1 dB does not cause any distortion. The phase shift is neglegible.
If the circuit is applied to a stereo unit, build an exact duplicate for the other channel. Any difference could cause noticeable phase shift in the output signal. It is very important that the capacitors C1, C2, C4, C5 and C7 are of exactly the same values in both channels.
The current consumption is aound 5 mA. The maximum useful frequency is 3 MHz. In connecting the subsonic fiter, remove the decoupling capacitor from the end module to avoid distorting the signal.