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Old 4th December 2008, 11:37 AM   #1
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Default line level high pass filter

I am looking for some suggestions regarding a line level high pass filter for main speakers. I don't mind building a circuit as long as it is a good or better than something I can buy reasonably.

I would like to be able to filter out <~60hz signals from reaching the mains. If possible I'd like to be able to adjust this freq. I believe a 2nd order filter is what I'm looking for.

I am using a digital source -> DAC -> Amp -> Main Speakers

1) Should this be passive or active?
2) If there is consensus on #1, is there a recommendation for a high quality filter?

Thanks for any feedback.
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Old 4th December 2008, 11:59 AM   #2
SY is offline SY  United States
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At frequencies that low, I think that active is the way to go. You can do it pretty simply (Sallen-Key filter made from opamps) or you can get a bit more sophisticated (for example, the Acheron crossover shown on my website). Either approach will well outperform a large, clunky inductor and equally large, clunky capacitors.
“Instead of Rational Law, objective truths perceptible to any who will undergo the necessary intellectual discipline, Knowledge will degenerate into a riot of subjective visions. . ." - Auden
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Old 13th December 2008, 04:21 AM   #3
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How much attenuation is needed at 60Hz? Here is a circuit which will give -12.4dB at 60Hz. It was generated by Microchip's FilterLab software.
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Old 13th December 2008, 06:21 AM   #4
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The high pass of a PLLXO coule be used. 2nd order gets kind of droppy and if your amp has a low input impedance could be a no go... (i'd suggest 50k or higher input impedance, and if that low make R2' infinite and the cable short).

If the input impedance is an issue and you still want to approach it this way, you can sandwich the PLLXO between 2 buffers. I have a dual 9V battery powered one for one of my speakers, but it could be done with jfet beffers (like in Nelson's B1 or cathode followers (don't forget the CCS)

SY's XO is also very interesting -- i'm no big fan of SK... I'm happy he did the groundwork for this so the rest of us don't have to. Thanx SY.

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