Hi Ted. You’ve made references to this a few times now but seemingly without any explanation. Am I just being dense or are you being coy about something you’d rather keep under wraps?
Sort of both 
When I first build the DS prototype I used differential opamps differentially (i.e. when the “plus” of one of the opamps went up another opamp’s “-” went up.) This cancels 2nd order distortion from the power supplies. I then used a pair of these to drive one input winding of the output transformer and another pair to drive another in parallel. With the original prototype I also doubled everything above to lower the uncorrelated noise by another 3dB.
With the DS we couldn’t afford 8 opamps per channel (tho the TSS uses 8 per channel), but there were still two pairs of opamps being used differentially.
What I didn’t mention above was that each pair of opamps was driven by a separate differential digital DSD signal. That’s the way it was in the releases up to Snowmass.
Then I realized that I could take advantage of sometimes driving them non-differentially to add more low level detail. That’s what the new releases do. Those exceptions I put in amount to noise if you only look at one side of an opamp pair. But with both sides it’s more detail.
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Thanks. That’s along the lines of what I thought you might be implying but it’s a big assumption to make. It sorta-almost-kinda sounds like a multi-bit SDM…
Ted, is there a way to generate a step or square wave test sequence?
Turbo
There aren’t any built in test signals.
I use Adobe Audition, If you have a path from your computer to the DS, you can generate and play any standard test tone at any level pretty simply with it. Audacity (audacityteam.org) is a similar, but free, program.
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Yes, have used Audacity alot actually. But dont’ know how to generate a DSD step test signal 
Turbo
You can’t very easily. The best you can do with things like Audacity is to generate a high sample rate test signal and then use something like HQPlayer (Signalyst) to convert it to DSD. With HQPlayer you have a lot of choices for conversions and (at least the last time I looked) the differences aren’t all explained in detail.
JRiver MC also has a driver that can be used by Audacity and can convert to DSD.
Still different DSD upsamplers all will sound different and there’s no one that’s mathematically more correct than all of the others.
Also, the DS will upsample any incoming DSD to it’s own quad rate DSD internal format in any case.
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Thanks Ted. So I guess that, since there are no test signals built in I’ll have to inject an analog step or square wave to see the filter step response in the actual circuit. D*rn.
I do recall that the DAC would pass a 50KHz sine wave if fed a high sample rate
Todd
The DS’s output bandwidth is limited only by the output filter (well, up to 5 MHz). The ultrasonic noise rises, but it too is limited by the output filter. Send in a 352.8k and you’ll see a 176.4k response.
[Edit: - ] Actually I might have misspoke, Depending on the release the DS has shared the same filter for multiple of the higher sample rates. The Mk II and TSS have a complete set of filters for all PCM sample rates, but the DS might have lower cutoffs for some of the higher sample rates.
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I have a new LP filter to try, a CLC that offers a lot more rejection of the DSD bits frequencies, and maybe better analog band performance. I mean in the analog band basically will be just a piece of Cu wire from OA output (68R) to the transformer. As opposed to 3 individual SMD resistor joints and resistances in series now. It’s teasing me… We’ll see…
Also found a nice VHF inductor that we’ll try. It has a simple construction and a high self resonance freq well above the DSD fundamental here (>35MHz). Though I’m not sure regular old PSU inductor would have been any worse here (self resonance ~ 3-5MHz).
Trick will be, getting the filter to behave as expected in the actual circuit. Hopefully it will, if it matches the simulation close enough at least.
Great eclipse view here!
Turbo
CLC filter performance:
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Per Ted’s suggestion we may have what we need via Audacity to test the step response.
As example, below is a Pontus 2 DAC rendering an 8x48K 440Hz square wave perfectly IMO. Fed over I2S.
We’ll soon see how it goes through the DS fw.
Turbo
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384000 isn’t a valid input sample rate for the DS, I’d suggest either 176.4k or 352.8k and, say 441Hz (to keep from having a tick when the source signal wraps.
Here’s a 441Hz signal with a 705.6k sample rate on the Mk II:
Here’s a 441Hz signal with a 352.8k sample rate on the Mk II:
Here’s a 441Hz signal with a 176.4k sample rate on the Mk II:
But, I be careful using the phase perfect rendition:
A perfect square wave requires an infinite series of frequencies to represent digitally.
The shape of the edge when zoomed in should reflect the sample rate and be bouncy (705.6kHz digital edge in Adobe Audition):
If it doesn’t bounce the right amount then it’s not representing the input faithfully. Here are zoomed in versions. They all have the same scale, but I captured the screen by hand so they aren’t quite aligned.
Here’s a 441Hz signal at a 176.4k sample rate on the Mk II,
Here’s a 441Hz signal at a 352.8k sample rate on the Mk II:
Here’s a 441Hz signal at a 705.6k sample rate on the Mk II:
The 176.4k and 352.8k are what you’d expect from proper upsampling to 705.6kHz.
The 705.6kHz is upsampled to 11.2896MHz so it’s bounces are very short and can’t get thru the analog low pass filter.
These are all 3dB higher than full scale outputs (the 16V voltage range is too high for the 4VRMS / +/-5.65V Peak to Peak standard), I’m experimenting with SDM stability and had forgotten that when I recorded these.
[Edit: - ] I accidentally had the scope on AC coupling instead of DC so I reposted the images, not what you want when looking at edges.
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Thanks Ted. Understand about the square wave and Gibbs phenom (if I recall the name right).
We’ll try it out soon. But may have to fire analog step pulses into the filter to see what it does. I do have a good 80MHz Agilent function generator at least.
Unfortunately I managed to partial tear my achilles so am moving slow.
Turbo
Yep, the Gibbs phenomenon.
Take care of you achilles tendon. I once simply bruised one of mine and it took years to heal because I never stopped using it all day, every day. Heal well.
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Yikes T. Take care of yourself. That’s a very painful injury. Your work on this is amazing. I need to ask though; what DAC do you keep discussing as your reference? Im guessing its the Denafrips?
Wishing you full recovery. Take care.
Update: pops came back, and stopped only after I removed Bridge II. So, it’s not the transformers or the analogue board!
Guys thank all of you so much for the kind well wishes Yes, good advice, I’ll wear the boot as much as I can Right foot though so cant drive with it.
DG, the DAC I referred to is an Abbas 2.3SE. My sister now owns it while I wait for a 2.4SE.
Turbo
Serhan are the pops loud enough to hear on a cell recording? Could it be you have a cold solder joint? I’m not sure what has been done to your DS so far but you might consider blasting any areas that have been reworked with freeze spray to see what happens.
Turbo
Thank you for caring 
The solder was done at 350 - 400 celsius. When desoldering, I used suction, flax and cleaned up with a wick. I put the XS4400s, pops and crackles started, first sporadically, then frequently! I removed them and put the originals back. Things went fine for a while, then pops and crackles came back when using I2S and Bridge. I tested DS DAC without Bridge for a few days now, pops were gone. A reinstall of Bridge brings them back! I may get greedy and reinstall the XS4400s again.