I'm on my 2nd C399 now and the issues I've had occurred on both.
The thump sound people are reporting looks like AC mains to me.
After running DL setup with 2.2 config.
1. Whenever a track starts streaming from Qobuz. Nevermind gapless playback, I'd settle for a lack of thump/pop to let you know when the next track starts streaming.
2. Depending on how the DL is run as to how bad the problem seems to be. 2x REL T5i subs. Set crossover on subs to max 120Hz and volume to 75%. The problem is very bad. Re-running DL setup with crossover set to 70Hz and volume to 50% the problem is much less apparent.
With DL on and sub X set to 70Hz in the BluOS app.
Streaming via Qobuz, Amazon, Spotify, Bluetooth - everything sounds very good.
Any of the other inputs (mostly I play my TV) : HDMI, Optical or Line in. Gives a thumping sound. It's as loud as dialog and would seem to me to be mains noise as it lasts 2/100s. Makes TV unwatchable. Turn DL off, subs still on and the problem goes away.
Problem does not occur with subs turned off and DL on (separate setup run just for 2-channel).
Here is from a 32s clip recorded from TV via HDMI in. Link to the sound clip hosted on Google Drive. I chose snooker as it has a nice quiet background with overlaid commentary. The only problem is that you can hear the clack of the balls when a shot is played, but it's clearly a different thing to this thump/pop noise.
I opened the recording in Composer. Dots placed where the thumps occur.
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The knowledgeable members here would know better than I do, but 2/100s sine wave looks like 50Hz AC to me (I'm in the UK).
When it occurs at the same time as voice you can see the effect.
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Quiet background.
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Do I have very noisy mains? Not that I've ever noticed before. Certainly the Lyngdorf TDAI-1120 that I had on 2 week trial didn't seem to think so, nor does my DDRC-24. No problems there. Nor indeed does the C399 seem to think so as long as I'm not using DL in 2.2 mode with external inputs to the amplifier.
With 3 other amplifiers (non-DSP) I've also never heard anything like it.
As I say, this has been evident with 2 units now.
Now I'm running with crossover in BluOS set to 50Hz and the thumping is barely or not noticeable on HDMI/Optical I chose 50Hz because the DL helps the speakers handle down to 50 very nicely in my room, and was hoping that with the crossover set to where the problem is it might mask it. And that seems to be the case.
Note that the actual sound quality from the DAC and Hypex amp really suits my speakers/subs/room combination incredibly well. It sounds really really good - significantly better than the Lyngdorf I trialled. The soundstage is quite spectacular, wide and deep and tall, it really seems like my whole front wall is a speaker or stage or concert hall. I've not owned anything higher end or more expensive but I recently demo'd some McIntosh, Linn and Naim amps at a dealer with much more expensive speakers and they did not sound as good as my current home system - though I expect that was mostly the demo room.
I'm thinking of doing one final speaker upgrade to something closer to full-range and getting rid of the subs, so I'm on the fence whether I should keep the C399 or not, in 2.0 with DL it sounds great.
I guess the main question is whether they will be able to fix this in software or not. If yes, then I can live with it until then. If not then I should probably get a refund and try something else.
Your thoughts appreciated.