Customizing the NFB 11.28 for a more liquid sound

Discussion in 'Headphone Amplifiers and Combo (DAC/Amp) Units' started by Junki, Aug 20, 2017.

  1. Junki

    Junki Facebook Friend

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    Hi, thought I'd share some experiments I did with my NFB 11.28 (Amanero interface with TXCO upgrade) in optimizing for my subjective audio preference.

    I used a pair of RBH HP-2 with very fast beryllium dynamic drivers for monitoring.

    When I first started using the NFB (ESS SABRE 9028), I was very impressed with the transparency it was able to reproduce. Switching back between the NFB and my Dragonfly Red (ESS SABRE 9018), which I've been evaluating against a Stoner Acoustics UD125 (AKM "Velvet Sound" AK4490) for office listening, I noticed I enjoyed music from the DFR more. Careful A/B listening made me realize that while both were more than adequate in detail retrieval, clarity, etc., the DFR produced what felt like warmer, more lush, more liquid sound. This was particularly evident when listening to the track Luminous by Alice and the Glass Lake. Sourced from the DFR, vocals were smooth and often times euphoric and the piano was liquid smooth, while the NFB produced what felt like relentless analytical superiority--there was a slight overall level of graininess and I could hear the hissy breath intakes and leaks even during the airy vocal parts heavy with reverb, which distracted from any feeling of euphoria.

    Hypothesis 1: Warmness

    [​IMG]


    My first hypothesis was that DFR produced a warmer sound. Fortunately, the NFB is configurable! I set the jumper in place for the warmer sound signature.... and it was disappointing. A/B testing it several times, I sounded like that it merely recessed the treble range rather than change or improve the timbre of vocals and instruments. While I could see how that would help with brighter headphones (as the feature is stated to be designed for), the HP-2 is definitely not a bright headphone by any measure, and the result was softer and quieter details in the music, which resulted in an overall feeling of relative muddiness compared to stock. I took the jumpers off.

    Hypothesis 2: Digital Filter

    [​IMG]
    (The above displays the PCB of the NFB-1, which is similar to but not exactly the same as the pin layout in NFB-11.28. Specifically, FIR_0 and FIR_1 are switched on NFB-11.28. You can tell by reading the pin labels.)

    Digital filters can have subtle and meaningful impacts to the sound, especially around frequency transitions. Looking inside my NFB, I realized that the NFB 11.28 stock filter setting is for "soft roll-off, minimum" with a jumper already on FIR_0 from the factory. According to this incredibly helpful article, minimum phase soft roll-off filters are one of the newer filtering methodologies that adds a bit of (not very audible) phase distortion for better time response. Better time response (and drivers fast enough to take advantage of it) makes sense for a more analytical sound, but I really wanted that liquid smooth sound in my music. Keeping the stock jumper in place, I pushed another jumper into FIR_1 for a soft roll-off, linear phase filter setting. (See Appendum section below for an in-depth explanation between minimum phase vs. linear phase filtering.)

    And wow. The sound my NFB produces now is very close to the DFR. A/B listening, I don't think they're exactly the same, but I really can't pinpoint where they differ. Both sound lush, liquid, and smooth and when listening to Luminous both send shivers down my spine. The graininess and articulation I complained about previously have softened up. I'm loving it!

    APPENDUM:
    Here is another great article on minimum phase filtering vs. linear/zero-phase filtering: https://www.dsprelated.com/freebooks/filters/Minimum_Phase_Filters.html. Specifically the section titled "Is Linear Phase Really Ideal for Audio?"

    TL;DR: More or less aligned to the article in the OP: Minimum phase removes pre-ringing by reproducing an impulse with minimum delay, whereas linear/zero-phase filters equally space out every impulse and evenly divides the ringing before and after the impulse. The result is the former results in no audible pre-impulse ringing while the later reproduces the wave form to the maximal extent.
     
    Last edited: Aug 21, 2017
  2. Scubadude

    Scubadude Almost "Made"

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    Very interesting, thank you! I have the exact same model, also with upgraded USB and upgraded PSU. I found leaving it on fleshes out the sound quite nicely.

    Where can I find more info on the jumper configuration of the NFB11.28? Does tinkering inside void the 10 year warranty?
     
  3. Junki

    Junki Facebook Friend

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    Tinkering inside is part of Audio-gd's user manual, which includes the jumper configurations. However, there is a sticker seal on the case that when opened says it voids the warranty. So, maybe it does?
     
  4. Junki

    Junki Facebook Friend

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    Here is another great article on minimum phase filtering vs. linear/zero-phase filtering: https://www.dsprelated.com/freebooks/filters/Minimum_Phase_Filters.html. Specifically the section titled "Is Linear Phase Really Ideal for Audio?"

    TL;DR: More or less aligned to the article in the OP: Minimum phase removes pre-ringing by reproducing an impulse with minimum delay, whereas linear/zero-phase filters equally space out every impulse and evenly divides the ringing before and after the impulse. The result is the former results in no audible pre-impulse ringing while the later reproduces the wave form to the maximal extent.
     
  5. dropadred

    dropadred New

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    @Junki, so would you say the 11.28 in the default is just a tad harsh?
     
  6. purr1n

    purr1n Desire for betterer is endless.

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    Minimum phase filters sound like shit. With ESS Sabre, double that shit. Less ringing is less ringing. The belief that no pre-ringing is ideal or more according to nature is bullshit, especially when we are trading no pre-ringing with more post-ringing. The laws of the universe dictate that nothing is for free and everything has a cost. Squeeze things one way, shit comes out the other.

    My preference is linear with a sharp knee, but I understand why you would prefer a soft knee, especially with ESS Sabre.
     
    Last edited: Sep 13, 2017
  7. monacelli

    monacelli Friend

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    The trade-offs of going from linear phase to minimum-phase digital filters are severe. Any nonlinearity in the phase response effectively induces a frequency dependent time delay. So, roughly speaking, the delay imparted to cymbals would be different from the delay imparted to a double bass. You can't do this (by even a small amount) without f'ing with imaging and soundstage in really undesirable ways. If what you want is a softer roll-off, then I agree with Marv. It would be greatly preferable to implement a lower order filter with a linear phase response. The ability to implement linear phase FIR filters is probably the greatest advantage of digital signal processing, so why squander it?
     
  8. TomHP

    TomHP Facebook Friend

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    Based on this, I assume you're completely opposed to the whole MQA "ideal A/D D/A" chain idea? (can of worms... hugely off topic... sorry)
     
  9. capetownwatches

    capetownwatches New

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    Since I now own the very amp to which you refer, I can confirm that it is a NFB-11and not a NFB-11.28. So you would have had to look VERY hard to find the relevant jumpers since they do not exist on this model! Which is a good thing for me since I would have been going nuts trying out every possible configuration had they been. Luckily I took it off your hands before you had the chance to open it up...:D

    So far as I'm concerned it sounds just great as is - the 9018 implementation is excellent, with none of the infamous glare evident in lesser devices.
     
  10. Greg121986

    Greg121986 Almost "Made"

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    I'm going to necro this thread instead of posting a new one.

    I got an NFB11.38 recently for use as a DAC/Pre for a bedroom stereo. I have been using it in my office since I sold the DAC that was previously dedicated to this headphone rig. I must say, I really like the sound of this NFB11.38. It is really excellent with Focal Clears. I am very annoyed by this because I like having an unnecessarily large setup with separate DAC/Amps at my office. In reality I can be very satisfied with just the NFB11.38. So far I think I prefer the internal headphone amp as opposed to feeding the NFB11.38 to an Auralic Aries MKII. This all seems quite peculiar.

    I was messing around with filters this morning before I stumbled on this post. I found that the Slow Roll Off - Minimum phase filter is a stark difference from the stock config, and it may very well be my preferred flavor. I've not tried any other options but I am so far very happy with this signature so far.
     
  11. jsmiller58

    jsmiller58 New

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    I too recently picked up the 11.38.

    I have so far been playing with slow roll off - linear... much less harsh sound than the stock setting (which, the idiot that I am, I did not note before I pulled one of the jumpers out so don’t know what the default is I am comparing to!)... what setting was default on yours?

    What IIR settings are you using? I have left it with no jumpers (50K).
     
    Last edited: Jul 17, 2018

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