I have the same problem. Quiet clicks when switching between tracks. DSD. And also at the beginning and end of the album. DAC Holo Cyan2. I adjust the volume with HQPlayer Desktop. Amplifier Benchmark AHB2.
They should properly mute the DAC output with muting relays around the format switch. DSD spec requires 50 ms mute period around start/stop.
Please check that you have Idle Time set to something like 30 seconds. Quick pause is not enabled, and that you donāt have adaptive rate enabled (fixed output format, for example always 44.1x256).
Yes, these are my current settings. I have updated since my last post to idle time of 30 secs, quick pause off and all my profiles have a defined output format, no auto and auto rate family.
Iāve noticed an edge case that I canāt systematically reproduce where when playback is paused (or stopped, Iām not sure) then started again, playback will be plagued with static clicking like at least once per second. I have to kill hqplayer or power off the DAC, it is unbearable and I fear for my amp and speakers (probably OK, itās not that loud )
I notice that Gustard are using NAA 4.4.0 on the AH90, probably on a old stable Debian. I havenāt investigated trying to update the Linux system on the sdcard, or at least the binaries it runs.
They are on kernel v6.x
They definitely can update to v5 NAA - ask them. I did but the more that ask, the quicker it may happen 
It is similar. I like the sound of the transformers very much so I wonāt change. Also the low end sounds fantastic, but the sound using the transformers is especially sweet for the higher frequencies. They beat any solid state implementation.
I buy them here: https://nl.aliexpress.com/item/4001146194450.html?spm=a2g0o.order_list.order_list_main.56.9fc479d29LhaCF&gatewayAdapt=glo2nld
They have a more solid sound and improved bandwidth with respect to the yellow ones in your DSC2.
I wonder if the thing actually has output muting relays/switches. Sometimes manufacturers cut costs by omitting those. And result is snap-crackle-and-pop in the output when the DAC chips go through reset and reconfiguration.
No, unfortunately I donāt have one to make such.
When the computer stops the data stream to the DAC, it likely switches back to PCM mode or something. For USB it depends on what the interface controller is programmed to do, and also on the OS drivers at the source side. On such built-in NAA it depends on what itās drivers are programmed to do in such cases and how the DAC side reacts to such.
Typically DAC chip has also itās own mute logic output signal, and in this case the AK4191 has MUTEN output pin. It has also AMUTE input to create an aggregate mute output through MUTEN. MUTES register setting controls the level of āsensitivityā for this mute output logic.
With AH90, PCM To DSDD there is a definite relay click (the satisfying type of click). I imagine like your A26?
I donāt hear the low level clicks reported though - maybe amplifier dependant?
I donāt really remember, been a while since I last used itā¦ Iāve noticed though that the chassis anodizing is fading, it has turned from black to grayish-brownā¦ 
Could be a DC offset in the output too, which may be unit specific depending on manufacturing tolerances. And depending on possible amplifier input DC blocks. If thereās a lot of gain, then even tiny DC jump across mute relay switching will cause a pop.
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Yup Iām thinking it could be this
Thanks for the response, this led me on the right path to understanding the issue. I have been trying this new class D amp with a āvolume bypassā switch that keeps the amp at max gain, acting like a power amp. I was compensating by lowering the source (HQp) and limiting max volume there. But then once the DAC introduces its sound signature, there is only the amp to bring that forward. DSDD mode implies that AH90 is set to max volume, and there is no preamp in the chain. Setting the source back to fixed volume and lowering the gain on the amp, I barely noticed it once between tracks. Iāll try with a preamp in that setup and see how that goes. Cheers!
Actually they are O-type transformers. They are the same 2 used in the May KTE. So now there are 4 O-typte transformers on the shelf, the May KTE arrived. Both the have the same low hum. I may try a cmx2+ and see if it helps.
On another noteā¦ The Mayā¦ wow! 
I feel like a kid in a candy store again! I can now hear big differences in Modulators. I am able to run DAC correction at DSD256 with select modulators using a M1 studio max.
Not sure what is changing but there is a favorable difference with DAC correction on.
Thank you @jussi_laako!
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And I guess the RME ADI-2 DAC fs is nothing to correct still?
At the moment Iām not considering making corrections for ADI-2, since there are three different DAC chips used in this product. And I would need to make three distinct correction sets for it. And I would need to have all three models to do so. So Iāve given up on idea to do it.
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Quick question for those with the AH90 DAC. In DSD direct mode, can I only use volume in Fixed mode?
So I need to use a preamp, and canāt go direct into power amps?
well you COULD go direct into power amps and just use HQPlayer digital volume control
It can be dangerous though
The Holo Spring DACs come with optional analogue vol control (extra cost)
The T+A DACs also have analogue vol control.
Reduces box count but extra $$$$
I have a couple of different preamps, but wanted to have a listen directly into power amps.
The T&A 200 does look good and I did see a newish one pop up on Stereonet for $8500. But since Iād need to win the lottoā¦
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For example Schiit has some decently priced devices that offer full balanced analog volume control.
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Iāll second what Jussi saysā¦ the Freya+ or Kara (I prefer having the tube option, but the Kara is a wonderful SS device) can take balanced XLR in from a DAC like the Holo Cyan 2 (or anything else) and provide balanced outs with a very solid pot on the analog volume control to either balanced monoblocks via XLR or traditional stereo power amp via RCAs. The magic thing these pre-amps do also is that you can have unbalanced in and balanced out, or balanced in and unbalanced out if you prefer. To be perfectly honest, I listen 99% of the time with the analog volume turned all the way up so itās at unity gain - I could easily forego the pre-amp at this point, because Iām using HQP for volume control. I just really like having the tubes for some recordings that I donāt fully tame / enrich via HQP.
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I also use a Freya+ preamp, often in tube mode, between a Dac200 balanced output and balanced Atma-sphere class D GaNfet mono blocks. It seems out of itās league on paper but itās a very versatile device and with quality tubes - I use a quad set of Linlai Elite 6SN7ās - it adds just the right amount of musicality and warmth to the digital chain that I miss if removed. Finding a truly balanced 6SN7/small tube preamp with remote control otherwise requires big $$. And it even measures pretty well in tube mode.
Interestingly, I also use a Freya + with a pair of LINLAI Elite 6SN7, once in a while.
I have it connected to my benchmark LA4 by XLR so once in a while if I like some tube magic I would use that.
I run a Nuprime Evolution STA class d amp.