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Hi ClieOS,
Thanks for explaining what causes the roll off.
But why are such audiophile focused players (Hifiman Cowon, Colorfly) designed to have roll off when other players like the Clip+ measure flat?
You'd think being mid / high end focused audiophile player would purposely be built to measure flat or have flat frequency response. but instead they seem to have roll off there for a reason?
The answer is multi-dimensional.
First, almost none of the audiophile DAP maker has access to either a software or hardware team that is capable of making anything remotely half decent of a SoC solution. Beside Apple (which is the only one with enough money to ask others to do the engineering for them), everyone else is buying their solution from off-the-shelf solution provided by another company. Sandisk has bag of money, but not nearly as much as Apple, so they get their solution (at least on Fuze and Clip) from AMS. Try and go to AMS to order a few hundreds to a few thousands of their SoC unit and they will laugh at you. We are talking about ten of thousands of unit here (if not 100x more), and that's the level the big boys are playing. Companies like HifiMan or ColorFly, which only expect to sell a few hundreds unit, to a thousand if they are extremely lucky (and I do mean
EXTREMELY), will never get entertained by an decent SoC provider. Instead, they need to rely on the small SoC provider that ain't known for great engineering nor great software support - but they are the only people around that will take your 'tiny' order, so what can you do? You just have to suck it up and deal with them. These SoC provider not just here to give you a chip, they are the solution provider - that means you tell them what kind of hardware you want to add on top of their existing SoC design, and they modify it just enough to make it work, then try to modify the stock firmware so it will play nice with everything, or at least not crash in the first few seconds. At 99.95% of the time, the audiophile brand themselves has little to no control over firmware, as software engineer is way to expensive to hire by a small firm, and the SoC provider will NEVER release their source code to you. Remember, most of these SoC provider is only doing your business because large company usually don't want to deal with them, yet your hundreds unit order isn't important enough for them to dedicate too much resource for you - so here you are, stuck in a loop that is audiophile DAP manufacturing with all the difficulty in the world. If you know how much investment you need to put into an audiophile DAP to turn it into real product, you will likely not going to do it in the first place. That is one of the reason why the few audiophile DAP that do come to live want to charge you for big money - because that's the only way they will even make any money out of a few hundreds unit of DAP they are going to sell. It is the power of mass production that makes thing cheap and good, Apple is the best example. For HifiMan or ColorFly, they are like the medieval of an electronic manufacturing, so their cost always skyrocketed and the finishing will never be the same level to Apple or Sandisk.
With the first handicap in mind, now comes the part of trying to piece together a design that works - I am not too familiar with others so I'll stick to HM801 in this case - first, the choice of Butterworth filter is intentional. It is how Fang like it and it is a well regarded design in the past (you can find the same filter in WOO's DAC, plus quite a few others). Even top ESS chip has a Butterworth filter built in, they just also have a few others option for you choose which to use. Fostex HP-P1 has a slow roll-off filter too, yet you rarely see people complain it being not HiFi enough - the true is, it is all about turning to the desired sound, it isn't like Butterworth filter has no benifit either - it has no group delay or rippe when compared to the sharp Chebyshev Type 1, but how many times do you see anyone complaining about not having those? It is really a silly mentality among audiophile that choose to only look for a certain fact while totally ignore the others - but as human, that's what many of us do. On the case of Clip - which I have no idea what filter it used (could very well be a high order Butterworth) but it has the benifit of big manufacturing and miniturization, so even very complex filter design (like a fifth order Butterworth) that won't fit on HM801 PCB can be shrinked down to a tiny part of the SoC. Again, we go back to the first point where audiophile DAP maker is limited by what that is availble to them.
You might think audiophile DAP maker should have all that they need to build the perfect DAP - yet the real world is quite the opposite. It is the big companies that has every resource available, yet they are also the least interested in making a Hi-end DAP with the best sound parts and best measured spec, becuase those are not something they can sell by truck load. Yet, the few companies that do want to make audiophile DAP is lacking all the resource to compete. As audiophile, we are stuck between them.
