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Akai AA-1150 Low Filter

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The Akai AA-1150 stereo receiver has a 'Low Filter' button. This activates a high-pass filter, which is specified to be -6 dB at 50 Hz. Does anyone know what is the roll-off rate of this filter? Is it a first-order or second-order filter or higher? It's an old piece of audio equipment and I haven't been able to track down any information on this.

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If it specifies the -6dB point (not the -3dB point) ... then it is an even-order HP filter (most probably Linkwitz-Riley) - so either 12dB or 24dB.

 

Andy

 

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From schematic of AA-1175 (also -6dB @50Hz)

image.png.e5be627ba0ae3b7773a707c38ee6fa5b.png

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On 02/08/2020 at 6:05 PM, andyr said:

If it specifies the -6dB point (not the -3dB point) ... then it is an even-order HP filter (most probably Linkwitz-Riley) - so either 12dB or 24dB.

Thanks for responding. My guess would be that it isn't a Linkwitz-Riley filter, even though a -6 dB point is specified. That's because Linkwitz's journal paper was published in 1976, which probably predates the AA-1150 amplifier by just two or three years.

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On 02/08/2020 at 7:34 PM, mbz said:

From schematic of AA-1175 (also -6dB @50Hz)

image.png.e5be627ba0ae3b7773a707c38ee6fa5b.png

Many thanks for that information. I managed to find the schematic for the AA-1150, which has very similar component values. It looks like the low filter is a first-order RC high-pass network. Although the -6 dB point specification is a little unusual, it does seem to tie in with the circuit's component values. I guess that the aim of the low filter was to try and prevent lots of low-frequency energy getting to small vented bookshelf loudspeakers, thus greatly reducing cone excursion, particularly when playing warped LPs.

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Yep, simple first order filter. Just about mandatory for turntable rumble on any size spkrs.

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