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Old 1st June 2014, 02:47 PM   #1
kinku is offline kinku  United States
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Default low pass filter for DAC output

Hi,
I am wondering if someone can help me with a Second order low pass filer using transistor for PCM1704 DAC output. Will a passive filter work better than an active one?
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Old 1st June 2014, 03:47 PM   #2
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Yes a passive filter will generally sound better than an active one as it won't be prone to IMD. Why do you want only 2nd order and what function is the transistor playing?
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Old 1st June 2014, 05:00 PM   #3
kinku is offline kinku  United States
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I am not sure if it need to be one with transistor ( or active) but no OPAMP based on is being considered.
There is no particular reason for selecting second order. That is what being used in other DAC models, active though.
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Old 2nd June 2014, 02:40 AM   #4
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I'd suggest going for a higher order filter if you're going to use the PCM1704 in NOS. I use 11th order currently (5 inductors, 6 caps per channel). If you want to run OS then you can use fewer components in the filter as it doesn't need to be as steep. However I reckon NOS and a steep filter sounds best.
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Old 2nd June 2014, 02:59 AM   #5
kinku is offline kinku  United States
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I am using it as NOS configuration, I would appreciate if you please share the filter schematics/ details. Also curious to know how you made those inductors.I guess the filter is a Bessel-Butterworth?

Last edited by kinku; 2nd June 2014 at 03:18 AM.
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Old 2nd June 2014, 03:39 AM   #6
sreten is offline sreten  United Kingdom
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Hi,

You will certainly need golden ears to hear the effect of an 11th order
supersonic filter over one that simply does the the job required on a DAC
output, which is typically 2nd order LP op-amp based I/V conversion.

Assuming NOS means no output stage and OS means output stage,
how on earth do you come up with a 11th order filter as sensible ?

rgds, sreten.
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Old 2nd June 2014, 03:45 AM   #7
kinku is offline kinku  United States
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This is my DAC design, I do not want to make it too complicated. So any idea is welcome as long as it is practical and does not involve extreme measures. Sreten I have the I/V conversion taken care of from PCM datasheet. Not sure if I need a low pass filter or not after IV. A transistor based Low pass filter is the one I am looking for. Second or third order perhaps. But not sure which way to go
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Last edited by kinku; 2nd June 2014 at 03:49 AM.
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Old 2nd June 2014, 04:05 AM   #8
kinku is offline kinku  United States
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Quote:
Originally Posted by sreten View Post
Hi,

Assuming NOS means no output stage and OS means output stage,
how on earth do you come up with a 11th order filter as sensible ?

rgds, sreten.
Non oversampling (NOS) and oversampling prefilter sorry
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Old 2nd June 2014, 04:17 AM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by kinku View Post
Non oversampling (NOS) and oversampling prefilter sorry
Hi,

Sorry on my part, that makes a lot more sense.

rgds, sreten.
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Old 2nd June 2014, 04:56 AM   #10
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Quote:
Originally Posted by kinku View Post
I am using it as NOS configuration, I would appreciate if you please share the filter schematics/ details.
Its on my blog, here - http://www.diyaudio.com/forums/blogs...ev-filter.html

Quote:
Also curious to know how you made those inductors.I guess the filter is a Bessel-Butterworth?
This particular filter uses bought inductors (TDK) but in previous designs I've used pot cores to fashion inductors from. No, the filter's Chebyshev to get a steep slope before the imaging components kick in. In going NOS there's not much room (frequency-wise) between the top of the audio specturm and the start of the imaging products (24.1kHz). Which is why so many DACs have historically used oversampling so the steep filter's digital, rather than analog.
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