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low pass filter for DSD
low pass filter for DSD
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Old 1st May 2013, 06:58 PM   #11
Julf is offline Julf  Europe
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Originally Posted by merlin el mago View Post
I have no idea that's the reason I'm asking help.
OK, but it sounds like you need help with much more than just a few component values. Doesn't your DSD DAC already have an output filter?
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Old 1st May 2013, 08:20 PM   #12
merlin el mago is offline merlin el mago  Europe
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I don't want to use my DSD DAC, only the filter + hi-gain tube stage.
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Old 2nd May 2013, 12:57 AM   #13
abraxalito is offline abraxalito  United Kingdom
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I suggest a 4th order LC filter for DSD. After all you'll want to aim for something approaching 120dB rejection at the switching frequency, which for DSD64 is 1.4MHz.
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Old 2nd May 2013, 05:58 AM   #14
Julf is offline Julf  Europe
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Originally Posted by abraxalito View Post
I suggest a 4th order LC filter for DSD. After all you'll want to aim for something approaching 120dB rejection at the switching frequency, which for DSD64 is 1.4MHz.
Agree, you need something much more complicated than just a first-degree filter - but a lot also depends on the HF behaviour of your tube stage.
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Old 2nd May 2013, 06:01 AM   #15
Julf is offline Julf  Europe
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Originally Posted by merlin el mago View Post
I don't want to use my DSD DAC, only the filter + hi-gain tube stage.
Ah, I see. What kind of circuit drives your filter? What is the HF behaviour of your tube stage?
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Old 2nd May 2013, 07:53 AM   #16
merlin el mago is offline merlin el mago  Europe
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Originally Posted by Julf View Post
Ah, I see. What kind of circuit drives your filter? What is the HF behaviour of your tube stage?
I don't know the HF behaviour of the tube stage, the tube stage to be used in attached pic.
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Old 2nd May 2013, 08:00 AM   #17
merlin el mago is offline merlin el mago  Europe
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Originally Posted by abraxalito View Post
I suggest a 4th order LC filter for DSD. After all you'll want to aim for something approaching 120dB rejection at the switching frequency, which for DSD64 is 1.4MHz.
Sorry my ignorance:

What Low pass filter: Butterworth or Chebyshev?
What is the corner frecuency necessary?
What kind of filter: series or shunt?
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Old 2nd May 2013, 08:35 AM   #18
abraxalito is offline abraxalito  United Kingdom
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Butterworth or Chebyshev would be fine - I suggest Butterworth as its simpler to design. The corner frequency depends on you - perhaps 50kHz -3dB would be fine as a first attempt.

A 4th order LC would have two series inductors and two shunt capacitors, followed by a resistive termination

<edit> Attached a schematic based on a design I did for an ESS DAC. Its Butterworth but 30kHz -3dB.
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File Type: png merlinLC.png (4.2 KB, 911 views)
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Last edited by abraxalito; 2nd May 2013 at 08:51 AM.
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Old 2nd May 2013, 08:36 AM   #19
Julf is offline Julf  Europe
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Originally Posted by merlin el mago View Post
What Low pass filter: Butterworth or Chebyshev?
What is more important for you - steepness of the filter, or minimizing ripples?

Quote:
What is the corner frecuency necessary?
As low as possible - I would put it at 22-25 kHz.

Quote:
What kind of filter: series or shunt?
When you go beyond a simple first-order RC filter, it doesn't really make sense to talk about a series or shunt filter - if you implement the filter as a passive filter, it will have both shunt elements and series elements, and be a fairly complex filter with both inductors and capacitors - and making a 4th order passive filter is not easy! Usually some active buffer elements are used.

If you want to go ahead anyway, all the required formulas are here and here.
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Old 2nd May 2013, 08:39 AM   #20
Julf is offline Julf  Europe
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Originally Posted by merlin el mago View Post
I don't know the HF behaviour of the tube stage
I am all for experimenting, but I have to ask what it is that you are hoping to accomplish? You are not likely to produce anything that has superior sound quality compared to systems designed by professional designers, and there is a real risk of burning out your tweeter or causing some other damage to your speakers.
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