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The passive filters defined here are single order, -6dB per octave filters, with 45° phase shift at the turn-over frequency and a total of 90° phase shift at the extreme range outside of the bandpass. Single order RC networks also occur where ever there is resistance being fed by capacitance or looking into capacitance, e.g., the proceeding stage output impedance working into the Miller capacitance of a triode grid in an amplifier.
The active filters defined here are second order, -12dB per octave filters, with 90° phase shift at the turn-over frequency and a total of 180° phase shift at the extreme range outside of the bandpass. Audio Gadgets covers the two common filter types: Bessell and Butterworth. Choose the Bessell filter alignment for better delay response; Butterworth for better amplitude accuracy.
Active High Pass
Active High Pass filters pass the high frequencies and attenuate the low frequencies. Every coupling capacitor in the signal path is a High Pass filter that not only blocks DC, but attenuates and phase shifts the lower frequencies. The -12 dB slope is a moderately steep one, which helps to protect the device following it from too low and too strong a frequency. Active high pass filter are usually used for crossovers for bi-amped loudspeaker designs and rumble filters for phono playback.
Active Low Pass
Active Low Pass filters pass the low frequencies and attenuate the higher frequencies. The -12 dB slope is a moderately steep one, which helps to protect the device following it from too high and too strong a frequency. Active low pass filter are usually used for crossovers for bi-amped loudspeaker designs and for subwoofer implementation, along with CD player post I to V filtering.
Passive High Pass
High Pass filters pass the high frequencies and attenuate the low frequencies. Every coupling capacitor in the signal path is a high pass filter that not only blocks DC, but attenuates and phase shifts the lower frequencies. The -6 dB slope is a very gentle one, which helps preserve the signal's integrity, but may not protect the device following it from too low and too strong a frequency.
Passive Low Pass
Low Pass filters pass the low frequencies and attenuate the higher frequencies. Most CD players use a low pass filter at the output to filter out any high frequency artifacts from the decoding process. The -6 dB slope is a very gentle one, which helps preserve the signal's integrity, but may not protect the device following it from too high and too strong a frequency. The input stage of a high power amplifier, let us say.
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