With the MK2, you will get great results taking in a USB input directly. This was not the case with the MK1, which most folks felt had a noisy USB port and opted to convert the USB source to I2S with the help of a DDC, LPS and I2S cable.
On the MK2, with a high quality USB cable (I’m using a Shunyata Omega), the sound of a USB source and I2S source are indistinguishable.
My own MK2 experience was that I preferred USB input more than I2S, which was not the case with MK1 for sure. I had a Euphony Summus 2 streamer that takes both USB and I2S inputs. So, the comparison was fair to some extent.
However, I used a topline Shunyata Omega USB cable to compare with a topline AQ Dragon48 HDMI. It could be that Omega is a superior cable, and it costs more than the Dragon. But a cable can only make a minor difference. The USB input sounded as quiet as I2S, and it sounded more musical and fuller in my system.
Many MK2 owners have the same opinion as I remember.
Thank you…
I agree with the cable. I have tried Stealth, Viablue and Carbon. Vastly different in price but the sonic difference is minimal. I hear more differences when I change XLR. I’m running an Aurender with only USB out. Sounds like direct to DAC is the ticket. No point adding boxes I don’t need.
My experience matches @dchang05’s. I even have the same cables now, Omega USB and AQ Dragon 48. Between PST as the I2S source and Holo Red for my USB, it’s pretty much the same to my ears. Even with other cables like Thunderbird48 and Phasure Lush 3e the differences between USB and I2S are subtle. Noticeable, but subtle.
The improvement to USB on MKII and all digital inputs/outputs able to have the grounds lifted was one of the reasons I made the switch from my DSJ. There’s also the fact with MK1 DSD 256 only worked through I2S and on Sunlight firmware.
Well Zombie, I’m not too worried about the tweeters really but the point about ultrasonic noise is certainly valid. It’s a “feature” basically of the DS DAC’s, and there is a lot of ultrasonic noise present in the output of the Mk2, as there was also with the Mk1. And the designer did say that the Mk2 relies even more on the leakage inductance for overall LPF than did the Mk1.
-So I think to-do and TBD, we’ll explore the possibilities of some additional LPF on the output, maybe a passive CM/Diff mode LC type of thing. We’ll do some investigating. It’s not the end of world though and I think we can improve that situation. Even with transformers present there’s going to be ultrasonic noise in the output.
I think the board even maybe provides some pads already for that sort of additional LPF thing (possibly), but I don’t know much about them yet. And I don’t have a lot of Mk2 schematic info to jump start progress either.
Sonically though it defintely does work; in that regard it’s as close to DC coupled as one gets basically without actually being DC coupled.
But truth is, my ears are older now and far from perfect, so we’ll see what the other member says when he updates his own unit.
Also keep in mind that many, many NOS DACs are playing into high quality systems that have a ton of 44K1 ultrasonic noise in the output. I’ve also fed the straight staircase output of a true NOS DAC or two into my own system(s) and it generally can sound quite good, but the ultrasonic noise present can have different secondary effects on different systems
As for interconnects, I finally also got around this week to trying out the “Schroeder method” of paralleling interconnects. Letting some cable splitters get some break in time for now, but so far it sounds like it could be verrrry interesting /Arte Johnson
If anyone’s interested:
I’m using an Audio Sensibility splitter set:
It’s easy to do currently as I have a direct XLR connect DAC → amp. And 2 pairs of a couple different very good quality XLR on hand.
-So far the sense is, the interconnects have almost disappeared from the experience. We’ll listen more later though. It’s too early to crow about it yet, but it seems quite profound actually.
And for the record, I’m also using Audio Sensibility Signature V2 speakers cables and liking them a lot. What attracted was, the construction/design approach is ~ the same as the path that I have come to by now. Ie, independently we have converged basically 
Steven is a great person to deal with as well IME and the pricing is very reasonable as well.
TK
" ..From this variety of setups an absolutely consistent result was seen; in every instance the Schroeder Method of doubling interconnects handily outperformed single interconnects by a wide margin in every parameter of sound quality. This was so regardless of the brand being compared singly, i.e., the doubled interconnects were always better regardless of the single pair being compared. The Schroeder Method was also superior regardless of of the type of speaker used; I compared using the PureAudioProject Trio 15 Horn 1, Kingsound King III electrostatic, and Legacy Audio Whisper DSW Clarity Edition." Indeed..
Did you disable power to the USB as described in my post above when comparing to the i2s input?
I thought the same thing until I followed the procedure above to disable power to the USB when comparing the two. It’s immediately noticeable especially when comparing USB power on vs off while playing music. Make sure you unplug the USB cable before disabling power as described above.
I have never blown a tweeter when using the stock or modded MK2 with APS transformers so it is very important to have that last part of the filter in place as designed.
@Zombie can you give any more thoughts on the APS transformers. I had mine installed before the latest firmware and I was away for a while so mostly remember the more extended upper end.
I’m quite disappointed there is no firmware updates going forward and trying to decide what to do. Wondering if the new Dac will sound better than a MKII/APS. A bit disillusioned at this point.
I have compared the Modded MK2 with many DAC’s costing 3x more and end up liking the MK2 best. I feel Ted’s designs sound the best though they do not measure as well as others. They do sound the most transparent and real with APS mods. I had the new APS R2 transformers installed as well as their new pure copper IEC AC inlet with silver wiring. This made a huge improvement over the stock unit and no fear of blowing tweeters. 
The new Blue Sky firmware is the best sounding and I do not think firmware upgrades are necessary with the mods that were done.
The new DAC also will not be offering firmware upgrades. Guess we will find out how good it is when it’s released however, I have no desire to upgrade. I’m very happy with the MK2 and the APS LESS+ MK1 I had done a while back. I like them both.
I like what APS transformer did to my MK2. The Mod took a few hundred hours to break in. Interestingly when they were first installed the unit ran hot for a couple of months. Gradually it turned cooler, and now MK2 is just warm like the stock one. This is how I can tell when it completed the break-in (in addition to SQ improvement).
Blue Sky firmware was such a major improvement that I tend to think MK2 has reached most of its potential already. It is hard to imagine another major improvement like Blue Sky could be accomplished. I am very happy with PST/MK2 going through MU2’s pre, and I even lost my desire for rapid upgrades lately. 
Agree with you on Blue Sky. But then again I felt the same about Massive when that came out.
I had an APS transformer modded and Massive installed Mk2. A great DAC.
So it turns out, I think we can get over -50dB attenuation @ 7MHz on the output with a few pretty easy LPF cap additions. And I think it should be sonically a non-issue, only starting to roll in above 20KHz. I’ll see about implementing it on mine soon.
Currently I see about a few hundred mV noise at about that freq on the output, triangular or sawtooth looking pulses; after the LPF additions it should reduce that substantially (at least a factor of .003x). As far as I’m concerned that would be fine.
TK
Ok so I have a decent model using the AD8139, left over from the Mk1 development efforts. In the Mk2 case now, a few relatively simple changes/additions and we go from about -42dB down on the output @ 7MHz to -98dB down, an improvement of some 56 dB attenuation. Yay! Nearly 60dB attenuation (60 dB is 3 orders of magnitude(!)), that is nice and meaty and should be fine I think. And just what we need. Overall response looks fine; step response looks fine; 20KHz rolloff is acceptable. I’ll implement it asap. Need to take stock of parts on hand first before we open 'er up again.
There is an R701/R702 shown in the design after the transformer, they’re in parallel with the output of the transformer and a relatively low value, about 1K16. I think it’s in place to tune the response of the transformer output. I don’t think we need it with the caps in place though, and it also knocks down some the overall output stage gain (about -1dB or so) (and needlessly loads the diff amps output now) so I’ll probably remove that as well.
I also have a small stock of some 68R SMD Zfoils, I think maybe enough to do the whole OPS (output stage) here; these are left over from the Mk1 efforts. If I do have enough I’ll likely install those as well into the Rout locations of the high speed diff amps as it does alleviate a little bit of the 20KHz overall rolloff that results from the final RC passive output filter (uses 87R currently). It’s rather aggressive there and IME that location is responsible for a large part of the 20K rolloff. We can back off on that a bit now as the new additions up front alleviate having most of that LPF burden onto the final passive LPF. So, a little more of our cake to eat too basically..
I didn’t plan to dive too deep into the deep end here with the Mk2 but ok, here we go, into the 5’ adult section at least 
Stay tuned Sonically it’s worth it.
TK
Any Aussie members has their DSD Mk2 dacs modded?
No, not this time. Probably not going down this path with this DAC, I am not sure of my longer term play with this DAC at this point so mods are of the table.
That’s my sentiment too. MU2 still seems end game, combining DAC & Pre against PMG separates. Either way, I’ll have to wait
Ok, I have implemented the LPF additions and she’s playing happily here now. As the simulations suggested, the ultrasonic noise is ~ gone now basically. I think it’s maybe a world’s first for a DS DAC lol
Baseline noise envelope:
Sorry for the poor image quality, the dust and the bad light; these were taken quickly on the workbench, not a photog studio.
After the LPF updates:
I zoomed way in (500mV/div → 10mV/Div) to measure the updated ultrasonic noise envelope, but honestly we’re below the ability of my scope and measurement setup to resolve reliably. My measurements, about 380mV → less than 5mV suggested about a -38dB reduction in the noise envelope. In reality it’s probably putting out a mV or so of ultrasonic noise now.
Issue is solved and we’re listening now. It sounds amazing.
On another note, the caps I have in there are not film and thus they do have a small amount of leakage at power on and reduces from there for a few minutes after. I added some 20K bleeder resistors on the output as well. My project amp is DC coupled and has a real pit bull of a DC offset monitor; sometimes even a turn on thump or similar will trip it. Thus the bleeders added now to make the amp happier at DAC power on.
Also note to self, need to remember to remove the 1K16 differential load from the output as well. R701//R702 I mean.
Next, think I’ll try to see if my THD+N meter can see any difference/improvement with high quality upsampled vs not upsampled. The meter might be happier as well now with the well LPF’d DAC output. TBD.
That reminds me, I do also have an old AP System 1 here, donated from a retired engineering colleague but have never tried to connect it up. The stumbling block has been that older system needs an old ISA bus PC for its add in card, and that was donated as well but he couldn’t remember if the ol’ Win95 machine has a pw or not lol (or what it might be). Maybe we’ll find out soon.
TK
We had the newly improved Lone Star Audio fest here in the Austin area this weekend and it was real nice IMO. Had a great time on Fri and I really hope it will grow even more next year.
Of note there was at least one DS Mk1 being used for demo, and at least one DS Mk2. Yes, I did talk to those gentlemen quite a bit
TK
