Hacking Schiit

Discussion in 'Modifications and Tweaks' started by Marvey, Jul 22, 2021.

  1. Grattle

    Grattle Friend

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    I'm thinking that I'm chasing windmills. I need to buy better gear and then evaluate. The Modi MB is great as is. It doesn't need any help from me or anyone else. I should appreciate it for what it is and look forward to hearing what others who know what they are doing have in store.
     
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  2. purr1n

    purr1n Super Friend

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    Oh, you will! Different sounding. OPA2156 is in the vein of the OG MB sound. It's less warmpoo and outright organic than OG - instead has an emphasis in the mids (lower mids to middle mids). If you want a bit of that 300B sound, this is it. OPA1656 is flatter or at least without any areas of emphasis other than having a slight darkish slope.
     
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    Last edited: Sep 7, 2021
  3. Grattle

    Grattle Friend

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    2156 transplant complete. Letting amp and DAC warm up.

    I’m not sure I have matching firmware in them though.
     
  4. Biodegraded

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  5. purr1n

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    ^ LOL. It's the same EPROM chip. Different programming. I will post mehsurements on differences.
     
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  6. Biodegraded

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    Last edited: Sep 8, 2021
  7. purr1n

    purr1n Super Friend

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    Not necessarily - a sticker may or may not say something - I would not count on it.

    Anyway - same exact unit with the OPA1656 - but this time around comparing initial release firmware and later firmware (released years ago - remember, the Modi MB is pretty ancient).

    Initial release firmware 0dbFS 24-bit input 48kHz
    [​IMG]

    Later firmware 0dbFS 24-bit input 48kHz
    upload_2021-9-8_15-13-38.png


    Initial release firmware -90dbFS 24-bit input 48kHz
    [​IMG]
    [​IMG]

    Later firmware -90dbFS 24-bit input 48kHz
    upload_2021-9-8_15-18-40.png
    upload_2021-9-8_15-21-8.png

    What sorcery is this?

    Bonus points to whoever wants to take a shot to explain (noting differences in the graphs above). Science is fun.
     
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    Last edited: Sep 8, 2021
  8. Grattle

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    Would the number of resistors used in a ladder DAC not determine the smoothness of the sine wave? More ladder steps (resistors), more smoothness? Dither mitigates this, does it not?

    Edit: assuming the above is true, this would seem to indicate that the better multibit chips have more resistors and possibly bettter resistor orecision as well.
     
    Last edited: Sep 8, 2021
  9. Hands

    Hands Overzealous Auto Flusher - Measurbator

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    See page 16: https://www.analog.com/media/en/technical-documentation/data-sheets/AD5547_5557.pdf

    You will probably want to ensure the Iout and Agnd pins from the DAC do not connect to the standard I/V opamp, only the Magni 3 (or I/V and output stage of choice).

    I believe you will still want to keep Rcom and Vref from the DAC connected to the second half of the dual opamp.

    I am not totally certain on what to do about the feedback, though. I use a Magni 3+ as the I/V and output for my DIY AD1862 DAC. I was too stupid to figure out how to use the AD1862's own feedback resistor with the Magni 3+, so I didn't connect it and bumped up the Magni 3's feedback resistor to 3K. (I mean, no matter what I tried, it just wouldn't work...Mostly just a monkey throwing shit at the wall to see what sticks, so I may need to revisit.)

    That said, let's assume you'll want to use the DAC's feedback resistor in combination with the Magni 3 to ensure proper accuracy.

    And only tried it as an external I/V, output stage for non-Schiit DACs at that!
     
  10. Hands

    Hands Overzealous Auto Flusher - Measurbator

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    Yes. If you have a multibit DAC that is truly accurate and precise enough to resolve more than 16-bits, run a measurement test where you play a 16-bit, -90dbFS, 1KHz sine vs a 24-bit sine. The former will show quantization steps, the latter will look more like a sine wave.

    Do the same on a DAC that maxes out at 16-bits, and it will probably still show the same 16-bit results even with the 24-bit file.

    One possible exception to this, as is the case with the newer Modi MB firmware, would be if dither is utilized.
     
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  11. Hands

    Hands Overzealous Auto Flusher - Measurbator

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    I might agree with this. I've hesitated to offer any possible differences I heard between the two given they are much more similar than not. Could be placebo. But, if I were forced to compare them, I'd probably have made similar remarks as you did.

    I've heard the 1656 as perhaps slightly sharper and slightly less warm than the 2156. Higher contrast, in a sense, which may or may not be to one's tastes. I can see how this would match up, relatively, with your comparative findings.

    They do seem to have ever so slightly different measurements as well in this use case, but shouldn't be enough to matter. Could also be parts variation.

    For me, it was more a matter of the sound between the 1656 and 2156 being close enough for me to doubt discernible subjective differences, and so I went with the more precision-oriented approach. I mean, I've got a precision, non-audio DAC, so why not match it up with a precision part that's also not explicitly marketed as an audio part? :)

    Also not complaining about keeping some of that warmth to the sound. Not always a bad thing.
     
  12. purr1n

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    Yup. It's all dither. If we go back to the prior FFTs, we will see a slightly higher noise floor (8db higher) in the audio band for the revised firmware.

    Smart dither too. The dither noise looks like it's pushed into the ultrasonic region (broad noise floor bump peaking at sound 40kHz). In essence we are trading the extra bandwidth (x4 oversample) for moar bits. Note -75db spike (and harmonics) at 192kHz, confirming x4 OS for the 48kHz input signal.

    Modi MB 1M.png
     
  13. Grattle

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    [​IMG]

    What I find most interesting is in your chart above, how the spikes seem to alternate channels.
     
  14. purr1n

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    Huh, I'll check that again. Could be possible I may have had even pre-production firmware. The scope also suggested the channels are different - overshoot on one.
     
  15. spoony

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    OK, so I went ahead and replaced U10 with the OPA2156, which indeed gave me a different sound (smoother?), however, after a while the left channel started shutting off intermittently. I inspected everything and even re-soldered the part to no avail, then decided to replace it with a new part, which doesn't shut off but I still hear some rustling noises on the left channel now and then. I used 60/40 solder and made sure I had no dirty contacts or bridges.

    Before I reinstall the stock part, anyone has any idea of what could be the cause of this?, I think I like the sound of this part better.

    Setup is Modi 3+ -> Jot 2 -> Headphones

    Using a secondary USB input to power the Modi didn't make a difference.
     
  16. purr1n

    purr1n Super Friend

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    Got a microscope? Check adjoining parts. Could be bad part. Or maybe solder underneath. It happens.
     
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  17. spoony

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    Thanks, I'll re-check every contact. Now that I've got your attention, by any chance do you know the part number for the optical transceiver?, I fucked up the dust lid thinking it was removable, I may want to replace it in the future.

    Edit: Well, back goes the 1622, the second 2156 developed left channel issues too. Subjectively, the 2156 seemed smoother and a bit laid-back, highlight for me was less prickly highs, maybe less "contrasty"? I'll try the 1656 when it gets back in stock.
     
    Last edited: Sep 11, 2021
  18. Scott Kramer

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    Thanks @purr1n & @Hands for this, great stuff! Went OPA2156 in my ModiMB v2. It's richer, cleaner, a lil' bigger, and hits nicer too. Sweet upgrade for this "archaic" dac haha.
     

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    Last edited: Oct 4, 2021
  19. purr1n

    purr1n Super Friend

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    Basically OPA2156 to keep in the spirit of the original opamps without the warmpoo and OPA1656 for neutral. The difference isn't huge, it's still a Modi MB. It's only there if I listen hard. I actually have two in different spots where I move stuff around and I don't really notice a difference.
     
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  20. dBel84

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