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Thoughts on a bunch of DACs (and why delta-sigma kinda sucks, just to get you to think about stuff)

Discussion in 'Dedicated Source Components' started by purrin, Dec 5, 2013.
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  1. Currawong Contributor

    Their latest versions have galvanic isolation, which reputedly improves that.  It will be interesting to see if the V2 of the Geek Out does as well, as it was also vastly improved with a better PSU. The trouble they have, like any regular manufacturer, is that a regular dealer/distributor/marketing sales model can't compete with direct sales that have none of those three things.  Hopefully though I'll get a TT in for review, because irrespective of that, there are people out there who are simply willing to shell out even serious money for something that sits neatly on a desk that does the whole headamp/DAC/preamp thing and sounds great.
  2. zerodeefex
    Word has it that your trash option costs hundreds of dollars for me. I'm done with the infinite upgrades. I'll enjoy the gear I've paid for (except for the studio, take my damn $7k already).
    Yggy > EC Studio + speaker amp > HD800 + philharmonic speakers at home
    TransDAC > Liquid Carbon > Slants at work
    Geek Wave SE (whatever the hell the name is)/Geek Out V2+ SE > UERM on the go
  3. zerodeefex
    Galvanic isolation is a small part of the puzzle. The important thing to note is how different operating systems and machines implement USB. It's disturbingly variable. You actually can't rely on the fact that your user will have a machine that supplies enough current, that supplies the correct voltage, that packets are sent reliably, or that any other aspect of USB is implemented to spec. 
  4. jexby
    just sold a kidney in hopes of inching towards such a trio of sweetness.
  5. Currawong Contributor

    Indeed, but it seemed the primary issue was with noise in the USB lines. What you're saying makes it seem like making a USB-powered device is extremely problematic, which makes a lot of sense.  Maybe because of these issues, USB audio receiver manufacturers have improved a lot over the years. 
  6. purrin
    Hahahaha. Amos, screw that! While I'm willing to shell out $700 for tube amp transformers, I'm still too poor for a TT!
    For something that sits neatly on a desk, I'm getting myself a multi-bit Bifrost. Heck for TT money, I'm getting multibit Bifrost + Upscale Audio Regen. Then I'm putting it in custom chassis from FPX. Then I'm laser etching the faceplate with the visage of Idris Elba, since he's like the dude who stands before Bifrost and Schiit. Then I'm spending the money I have leftover on a Fuji X-T1 with the Fujinon 23mm/1.4 prime. And then I'm upgrading my daughter's laptop with a SSD. And then I'm taking my family to Din Tai Fung (hopefully on a good day when they make stuff right.) P.S. I should get my wife something too, like tickets to see Masters of Percussion or the Palast Orchestra at UCLA.
    recarcar likes this.
  7. Currawong Contributor

    Yeah, it's over my budget too, way over. It's for people who have sufficiently deep pockets that they don't have to care about that though. And while we're at it, should you really be breaking private confidences?
  8. purrin
    I'm simply surmising. I have not actually heard any mult-bit beef-roast nor do I know much about it really other than what little Jason / Moffat have said. However, it would appear that you may know more than me about this multi-bit beef-roast! Please tell me more! Many inquiring minds want to know! Maybe we can ask Darko?
    Yeah, I know. TT is more for my spoiled rich cousins and nephews in Taiwan who don't have to do anything.
  9. frenchbat
    Let me guess. He's a huge fan of Chord right ?
  10. zachchen1996
  11. KeithEmo
    All of the technical facts you stated seem to be true and accurate, but I do question your interpretation and evaluation of some of them. There is a common tendency (especially by fans of R2R DACs) to describe Delta-Sigma DACs as "using low precision internal DACs and tricks to get good measurements" and making it sound as if this is somehow a way to "foist" inferior products on audiophiles by the use of clever tricks. The first part of the sentence is entirely correct - the whole idea of modern Delta-Sigma DACs is to use some clever tricks to allow an internal function block with five or six bit precision to deliver an analog output with 24 bit precision. (And that is a pretty neat trick.) However, it could also be restated as: "A Delta-Sigma DAC that has circuitry with internal conversion precision of only five or six bits can deliver performance equal to or better than an R2R DAC with 24 bit precision - and do so for a lot lower cost. (When you state it that way it sounds more like a cool idea that lets you get better performance using cheaper parts - which sounds more like a good thing.)
    Also, when you talk about "converting distortion into noise", you need to be very careful of the context to avoid becoming mislead... The whole subject of how a Delta-Sigma DAC works is quite a bit more complicated than many people think... In general, you can ALWAYS "trade" bit depth against sample rate. This is what DSD does - when compared to PCM. Instead of a 16 bit (or 24 bit) signal at a certain sample rate, you instead have a one bit signal at a much higher sample rate. However, the process of "trading one for the other" isn't some sort of shady business deal, conducted in a back alley somewhere. Rather, it is perfectly legitimate math, and the "trade" really is fair and equal. You really CAN use less bits at a higher sample rate and get the same performance (within the limitations of what you're doing). A modern Delta-Sigma DAC isn't "doing something sneaky" either - it's simply using some clever math to balance sample rate against bit depth because it so happens that it's a lot easier to get DAC function blocks that can convert with five or six bits of precision, but do so at very high sample rates, than it is to get ones with 24 bit precision, that can do so at lower sample rates. (You can think of it as "dividing" that 24 bit sample into several smaller pieces, converting each piece very precisely and quickly using a DAC with less bits, then carefully putting all the pieces of output back together afterwards.) So, given that the precision of the results will be equal among those choices, it sort of makes obvious sense to choose the (equal) option that costs the least - right? Other than bragging rights, there's no technical benefit to doing something the more difficult and more expensive way unless it actually works better - right? (And arguing that a "real 24 bit R2R" DAC could do a better job than a 24 bit Delta-Sigma one is not only not necessarily true, but it's sort of moot - even the high-end R2R DAC used by Yggdrasil "only" has 20 or 21 bits of precision - not the full - and arguably unnecessary - 24 bits of precision.)
    What the fellow from Sabre was referring to was that, because of the way the process works, you sometimes end up with a noise floor that varies depending on the content of the signal you're converting (the noise floor is modulated by the content). Since most people agree that a smooth consistent noise floor is in general less annoying than one that is correlated with the signal in some way, this is something worth avoiding (by careful attention to the details of how that mathematical trick is actually accomplished). We can leave the question of whether you can hear the difference between a smooth noise floor and a bad one, and whether different types of noise modulation sound audibly different - when the noise floor in question is better than -120 dB down - for another discussion. (This is only relevant and meaningful if you actually DO notice that the noise between low level passages really is audible - and sounds audibly different between different DAcs.)
    Another thing that seems to need clarification is the subject of digital filters. ALL oversampling DACs require digital filters - and this includes BOTH Delta-Sigma DACs and other types of oversampling DACs as well. Since the oversampling is tied in intimately with the Delta-Sigma process, most Delta-Sigma DACs have an internal oversampling filter. Yggdrasil uses an R2R type DAC CHIP, yet it still oversamples, and still uses a digital filter to do so. (Schiit has developed their own digital filter, which functions somewhat differently than the one included in most oversampling DAC chips, and which they claim is audibly superior. Their oversampling is implemented outside the DAC chip.) 
    In this context, perhaps I should also clarify what is meant by "digital filter". The process of "oversampling" consists of converting a digital audio stream recorded at a certain sample rate to an equivalent digital audio stream at a higher sample rate. The way this is done is to create more samples. (The process may create all new samples, or keep the original samples and create new ones to "drop" between them at the appropriate times. Note that the process CANNOT create new information. The ideal goal is to create new samples that contain the exact same information as the original audio stream, without changing it in any way (except to express it at a higher sample rate). You can think of it conceptually as taking the original samples as points on a graph, drawing a line through them in the precisely correct place, then picking NEW points on that same line (but more of them spaced more closely in time). If you get this all just right, then your new points will define the same exact line as your original points. In practice, this can be done by calculating approximately where the new points should be, then applying a filter. By "filtering out the errors" the filter "forces the new samples into their proper values". Basically, if you make a guess, then eliminate the errors from your guess, the result will be the correct answer. And, yes, that's a horrible oversimplification. However, it can also theoretically be done in other ways.
    The purpose of all this is that, by raising the sample rate, it raises the frequency of the errors introduced by the "steps" in the conversion process which, in turn, makes them easier to filter out without altering the desired audio signal. My basic point, however, is that the term "oversampling filter" may be somewhat misleading to some people... and thinking of it as an "oversampling process" (which is usually done using a special sort of digital filter) may make the concept easier to grasp.  

    gevorg, zilch0md, Argo Duck and 3 others like this.
  12. frenchbat

    Happy to please :wink:
  13. KeithEmo
    Making a USB powered device IS somewhat problematic... and it also depends a lot on what you're connecting them to.
    I've owned several little "dongle DACs" and a lot of desktop units, and this is my experience.....
    Basically, the power supplies in most computers are very noisy (they're not intended to power high quality audio equipment - and they're plenty good for digital circuitry). However, If you're using a little DAC (like a DragonFly) for headphones, with the headphones plugged directly into it, then the DAC is basically floating - and so the noise shouldn't matter (or be audible). I've never had a noise problem with any little DAC when used this way.
    I've also never had this problem with an AC powered "desktop DAC". I've got a whole bunch of them, some with galvanic isolation, others not, but I haven't noticed this problem to a significant degree with any of them. (Note that most desktop DACs - even the ones with USB inputs - are NOT "USB powered" - they have their own AC power supply. The USB receiver may or may not be partially powered by the USB port of the source device.)
    From my experience, however, the problem is common when you want to connect a little dongle DAC to a stereo system or AC powered headphone amp - which has its own ground reference. Once you do that, the noise on the computer's power and ground are now referenced to the ground on your other equipment, through the connections to the DAC, and the noise tends to creep into the analog circuitry in various ways. (Usually the ground and power on a computer are noisy enough that the obvious answer - simply grounding the computer chassis to your stereo - may not work.) Whether you have trouble in this situation depends on your computer, the DAC itself, your stereo, and how your home is wired - and about the only way to find out is to try it.
    When it does occur, sometimes adding an external "USB isolator" will fix it - other times not. There are also two things you need to be aware of here. First, not all external USB isolators work at the higher sample rates, and some simply don't work with certain DACs at all for various reasons (presumably due to their architecture somehow). Second, a lot of the various "USB cleanup" devices currently being sold do NOT provide galvanic isolation. Some provide galvanic isolation on data and power; others filter the power but don't isolate the data lines, and others simply regenerate the data signal, but don't isolate it - and don't do anything to the power. Whether a given one of these will help with a problem you have will depend on what it does and the specifics of the problem in your particular setup.

    Whether a USB port can supply sufficient power to run the device at all is another concern with USB powered DACs. Many older computers had very limited power available at their USB ports, and on some modern computers the different ports are actually able to supply different amounts of power - and some designate one "high power USB port" for charging USB devices like phones. You must also keep in mind that each USB port has a limit on the TOTAL amount of current it can deliver. If you plug a passive hub into a USB port, that single port must supply the power to run that hub AND everything you plug into it. Note there that some USB memory sticks use a significant amount of current, as do many other USB devices. If, instead, you use a USB hub with its own power supply, then the hub draws little to no power, AND the hub's power supply powers everything you plug into that hub.

    However, to address something someone else mentioned in another post... the actual "quality" of the USB port itself is largely meaningless here. I've never seen a computer whose USB ports put out power that is "clean enough for audio applications". Therefore, it is up to the DAC to work with what's normally available and ignore that noise.
    Another issue with USB audio outputs from computers is that of what USB mode they use - and the quality of the digital audio signal they deliver (which is a separate concern entirely from the power). Most older DACs used Isochronous mode, in which the computer is largely in charge of clocking the data. In general, when using this mode, the audio quality you get will depend on the computer itself, and will often be relatively poor. However, most modern USB DACs use Asynchronous mode. In this mode, the DAC controls the clocking of the audio data, and the computer has very little effect on it. (Most people consider Asynchronous mode to be clearly superior although, as usual, a few disagree.)   
    zilch0md and Currawong like this.
  14. blasjw

    With all due respect, let's face it.  DS sucks.  It's a cheap-***, flawed, no-brainer solution.  Don't get me wrong, I love Emo.  In fact I'm running an Emo spinner (ERC-3) and amp (XPA-200).  But, tell your boys at Emo (for their own sake) to thow those POS XDA-2 and DC-1 DACs in the garbage, go back to the drawing board, and create a real DAC with a multi-bit chip in it.  There's a reason why companies like Schiit and MSB are using R2R with custom filters and SHARC DSPs.  Hell even Behringer is using a SHARC in it's ~$300 DEQ2496.  Peace out.
    Maxx134 likes this.
  15. Sonic Defender Contributor

    Wow, calling people's products garbage and saying with all due respect don't work. You essentially said his opinions are garbage and his explanations are nothing but spin. Pretty low-blow there. For the record my D-S DAC sounds quite good, but I'm sure I don't have good hearing and I just can't tell that it is crap.
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