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Thread: CS42448 balanced output filter HELP!

  1. #1
    Junior Member
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    CS42448 balanced output filter HELP!

    Hello

    I'm wondering if someone could point me to some proper documentation on doing an active balanced output filter for the CS42448 chip.
    The datasheet only gives me circuits for a single-ended output, passive and active.
    There is a circuit that takes the balanced output of the chip and gives a single ended output but that's not what i need.

    The output impedance of the DAC is 100 ohms according to the datasheet.
    The datasheet also says that there is an inbuilt 0.5*Fs lowpass filter built into the chip but they also reccomend that a filter is to be used to further reduce noise above the audio passband.

    I've found balanced filters from other DAC chips but they normally have waaay lower output impedances, so the filter cutoff would probably change.

    Here's the two circuits that the CS42448 datasheet gives me.
    Click image for larger version. 

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  2. #2
    Junior Member
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    Dec 2016
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    Might I suggest what you are looking for is called a "fully differential amplifier". Here is a good article on the subject https://sound-au.com/articles/balanced-io.htm. Your old pro audio gear used to be entirely fully differential, but as you can see you have to double your part count. Recently more high end outboard gear has been going back to fully differential signal paths.

  3. #3
    I second the link from Sin_Phi. ESP has a wealth of example circuits with good explanations of the design decisions/trade-offs alongside.

    If you want to experiment with different input and output circuits for this CODEC on a breadboard, I made this breakout:

    https://easyeda.com/recursinging/kxmx_kaugummi_3.5-6

    The BOM is small and with the exception of the CS42448 itself, you can get everything from JLCPCB/LCSC for about 10 bucks shipped. I was able to hand solder it without any issues. If you use stackable headers, you can breadboard the Teensy GPIO to test things like mute circuits and so on. I have a Teensy 3.2 version as well, but I haven't built one yet. Teensy 4.0 shouldn't be hard to adapt to either.

  4. #4
    Figure 1 in the link Sin-Phi gave is fine, only use NE5532s and the correct value of output resistors (300 Ohm?) for your application.

  5. #5
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    Quote Originally Posted by MikeDB View Post
    Figure 1 in the link Sin-Phi gave is fine, only use NE5532s and the correct value of output resistors (300 Ohm?) for your application.
    I think i'd need to filter the output though, since there is still some high frequency noise in the signal.
    Granted it's above the hearing spectrum but still, Cirrus reccomends filtering it..

    I think you're right about it working as a driver. Maybe it'll work out just fine?
    Has anyone tried *not* filtering the outputs of the CS42448?

  6. #6
    Quote Originally Posted by hallmar View Post
    I think i'd need to filter the output though, since there is still some high frequency noise in the signal.
    Granted it's above the hearing spectrum but still, Cirrus reccomends filtering it..

    I think you're right about it working as a driver. Maybe it'll work out just fine?
    Has anyone tried *not* filtering the outputs of the CS42448?
    Yes I was assuming filtering before you get to the NE5532s. They are there as output drivers.

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