Do I still need a DDC in between?

If you're not syncing dac to any outboard clock whether it be in ddc or streamer doesn't much matter. If you're syncing have to compare clocks in the two units. Also, evaluating quality of clocks involves how they are implemented, not only clock quality but also power supply to the clock. Quality of I2S cable also matters, no longer than .5m. By the way, I have Harmony with Denafrips Gaia DDC, sync'd to Gaia via Tubulus Ximius I2S cable. This superior to optimized usb vis Sonore OpticalRendu via Audioquest Diamond usb.

Good question.  Probably comes down to which clock is better and not sure if Volumio offers galvanic isolation, which is important (assume the U18 does?).  At the very least you’ll likely get a better quality streamer either way, so given the unknowns this seems like one of those situations where just getting the Volumio and trying it with the U18 is really the only way to know.  Sorry, I know that’s not terribly helpful.  Best of luck. 

The harmony (micro) Dac is the only Dac I know so far that allows one to choose which clock to go with, I2s vs local (dac) clock.  I try both and it seems local clock option yield slight better SQ but I won’t swear to it.  Both units (U18 & micro Dac) might have similar grade of clock.  I use Ifi Ipower X for the micro Dac and I use 0.5m DHL hdmi 2.1 cable as shown earlier.

With that said, @sns, what you have said below confuses me a bit and please provide some clarification.

By the way, I have Harmony with Denafrips Gaia DDC, sync’d to Gaia via Tubulus Ximius I2S cable. 

What clock option you have chosen in Harmony Dac to sync., local (dac) or i2s clock?

This superior to optimized usb vis Sonore OpticalRendu via Audioquest Diamond usb.

This makes sense.  The U18/micro DAC via I2s also outperforms the usb (galvanized isolated) input on micro Dac BUT that is because of the weaker source (Wiim Ultra; but still very good when combined with DDC).  That is why I am thinking about the upgrade.

Yes, @soix, the U18 has galvanic isolation on its USB input, and so does the Micro DAC. As I mentioned earlier, the Micro DAC also allows the user to select either the I²S or the local clock, and I slightly prefer using the local clock.

The reason I'm asking now is because the return deadline for the U18 will have passed by the time the Rivo+ arrives.

Anyway, thank you for your response.

i2s may or may not sound better 

with digital it all depends on implementation 

@audiotroy  I think Passion for Sound addresses your question.  The i2s sounds comparably, if not better, to usb which is perceived better than AES.  Did the homework.  But nobody as far as I know does the comparison with and without DDC.  Anyone? 

https://www.youtube.com/watch?v=D-fEVEdFBnE&t=317s

Ya return the ddc

Return the DDC. It’s garbage.
Save the money and get a better dac and better streamer.

@audphile1 word

I’m fine with returning the U18, but I’m fully satisfied with the Harmony Micro DAC and the Rivo Plus/LPS I’m considering. If I ever need a DDC again, I’ll just get a better unit like the SU-6 or the Micro DDC. I’m not sure the folks making that statement have ever actually auditioned these units. I know exactly where the point of diminishing returns is for my system.

You do not need a DDC. To improve your system further when the Volumio arrives and goes thru full break in, try a good Ethernet cable. 

Sync means using Gaia clock. By the way both DDC and dac use I2S clock, I2S is native pathway in every dac, you're bypassing dac I2S clock when syncing or using DDC or streamer with I2S interface. Perhaps some have used inferior DDC if found to be lacking vs other interface, I don't see much sense in using I2S if I2S clock implementation in dac superior or equal to DDC/streamer clock.

By the way both DDC and dac use I2S clock, I2S is native pathway in every dac...

No.  Don’t assume that.  Go back to read your Harmony manual again regarding I2s Clock (Sec. 4.2).  It clearly says "This feature enables you to select between the local DAC clock or the I2S clock when utilizing the I2S input."  The ’local DAC clock’ option is manifest by itself.  If you opt for ’I2S clock’, the DAC will be slaved to DDC’s clock and use the I2S clock unless the I2S clock becomes unstable or incompatible. 

This is an unique feature offered by Harmony DAC and µDAC. 

It is good idea to try a good ethernet cable.  Thx.  Do I also need a good switch box and / or ethernet-optical-ethernet converter there, @audphile1, you think?

Nope. Keep it simple. Just the highest quality cable you can get. 

Keep it simple!  I like that answer as long as it is effective.  Any suggestion?  DH Labs or other vendors that won't break my bank?

@lanx0003 You're misunderstanding me. I2S is the native digital pathway in every dac, this is the DEFAULT clock used regardless of input chosen. Section 4.2 simply states one can sync or use DDC I2S clock OR use dac internal clock using async. Important point is local clock is in FACT a I2S clock, it is the native pathway in every dac, this just a matter of semantics. 

And these are NOT unique features of udac and Harmony. Many dacs with I2S input and DDC offer sync or async options, as well as configurable pin. 

@sns  I am not trying to be argumentative but I respectfully disagree with the generalization you have made regarding:

I2S is the native digital pathway in every dac, this is the DEFAULT clock used regardless of input chosen.

"local clock is in FACT a I2S clock", "just a matter of semantics"

That directly conflicts with the statement in the manual: 'When users switch to other digital inputs, the DAC seamlessly transitions back to using the local clock.' When other inputs are selected, there is no such thing as an 'I²S clock' in that timing information is not pipelined separately from the signal data. Please be precise with the terminology. You may want to double-check with Harmony.

Your Gustard U18 mitigates jitter to as low as 38fs according to the specs. Have you seen anything in the other DAC or source that says they beat that? 

@lanx0003 when I went to the last audio show in Florida the DH Labs guys were there. I literally had the Mirage USB in hand to purchase & we started talking cables & equipment. He ended up telling me if I was only going to buy one cable he recommended his Ethernet cable. Saying it would give me the better result over switching my USB cable. 

Unfortunately for him I ended up not buying either cable because I wasn't exactly sure what exact length I needed in Ethernet form. I still haven't committed to buying the best I can afford as I know my current streaming set up will be moving. But I did go to LanShack & get custom length 23awg solid copper conductor Cat8 for now. Individual twisted pair shielding with outer braid shielding as well. An improvement over my basic cable & didn't cost much. Connectors are not as nice as the DH Labs Reunion cable. 

@moonwatcher The 38fs is the jitter rms level in the femto CO (crystal oscillator).  The jitter level at outputs can not be that low.  Here are some measurements taken from coaxial output of various DDCs by Goldensound for their RMS electrical noise and jitter noise at 48khz.

                                  Jitter(rms)   Electrical(rms)   Jitter(peak)  Electrical(peak)

Holo Red                     73ps           2.97mV                213ps           12.9mV

Singxer SU-6              75ps           3.3mV                  225ps            13.7mV

Singxer SU-2 KTE      85ps           3.2mV                247ps            13mV

Gustard U18             453ps           5.2mV                1133ps            22mV

You could see U18 jitter and electrical noise performance is lagging.  That is why I mentioned I could pick up better DDC like SU-6 or micro DDC later on if needed.

@mattw73  Thank you for the recommendation. I like that the cable is affordable, and Cat 6a is more than sufficient for audio applications. I’ll gladly take a $15 cable as long as it works. For the same length, a DH Labs Ethernet cable would cost ten times more (around $300). I’ll definitely give it a try.

@lanx0003 ah, that Singxer SU-6 looks a lot better...Thanks. Just wonder what numbers the OP would end up with going directly from his Volumio Rivo Plus to the LAIV Harmony DAC via I2S? Would they be helped by the Singxer SU-6? 

Thanks for the input. It is telling that often specs that are advertised aren't always necessarily the ones to be basing purchasing decisions upon. 

@moonwatcher You are welcome.  It is interesting that you are curious about the Rivo+ jitter performance.  There is no measurement yet since it is just released but I found the measurement for Motivo, which is basically Rivio+ plus the DAC and headphone amp.  Their streaming circuit and performance is neck to neck according to iiwi but the Rivo+ has a slight edge.  See below.

The average (Lt / Rt channel combined) RMS (or so-called 1-sigma) jitter is around 8.2ps (I am not sure which output this measurement is taken from though). See yellow highlighted.  Quite impressive.  

So instead of calling internal clock in dac I2S clock, it is most commonly or rightly called the master clock, this clock positioned adjacent to digital to analog conversion circuitry. Only a ddc or streamer with I2S clock can replace the internal master clock in dac. Using usb or any other input cannot defeat or bypass the internal master clock. The only reason external I2S clock can replace this dac master clock is it's use of I2S protocol. 

Wait I’m confused….

I thought there isn’t a scenario where the DAC master clock is bypassed. Doesn’t matter what input you use the DAC clock has to provide timing reference for the Digital to analog conversion. Where is it stated that the internal clock is bypassed if i2s input is used? 
Is there a clock sync on the LAiV DAC?

Once again, if the user opts for the "I2S clock," the DAC or micro DAC will be slaved to the DDC’s clock and will use the I2S clock unless it becomes unstable or incompatible. This is a unique design feature of the Harmony. Still not convinced? Take a look at Harmony’s email response:

When you select “I2S Clock” and it is stable and compatible, the µDAC will indeed lock to and use the incoming I2S clock signal instead of its internal local DAC clock.

In this case, the local DAC clock is effectively bypassed, meaning it is not used during playback. The µDAC will rely fully on the external I2S clock.

It is amazing that some AGs here presume they know everything...

@lanx0003 I don't know what you're problem is, yes, I'm agreeing with you, the ddc clock bypasses the dac local clock, I've only said the same thing time after time! Perhaps I've called it internal I2S clock, local clock, master clock, its all the same thing. You really need to gain some reading comprehension skills, I'm done with this silliness.

@sns I was simply helping to clarify the confusion @audphile1 has with the confirmation from the manufacturer.  All right?  Cool.

You’re too jittery mah man. Chillax. 

@audphile1  Thanks for suggesting I change out my Ethernet cable — it made a significant improvement.

@mattw73  While waiting for the LanShack cable (S/FTP) to arrive, I also grabbed a UGreen Cat 8 (F/FTP) cable from Amazon for just six bucks. Upgrading my existing Cat 5e (U/UTP) to UGreen Cat 8 resulted in a significant improvement in sound clarity and a darker background. It also helped eliminate some digital glare. I believe this is largely due to the double shielding feature of Cat 8.  The degree of improvement is comparable to what I experienced with HDMI cable upgrade from AG to DH Labs.

The UGreen cable is made of 26 AWG stranded copper, while the LanShack uses 23 AWG solid copper, which I hope will yield additional sonic benefits. I believe in larger conductor and solid copper over stranded.  Thank you for the recommendation.

I like my U18 despite the negative opinions.  Especially in combination with my LHY ock-2 clock.  But, I'm using it to convert USB to I2s. 

I don't know if it would help much if you could go I2s to I2s. Probably depends on the gear that you're using.  It's too bad that you can't keep it long enough to try it. 

@sls883  I like the WiiM Ultra + U18 combo more and more each time I switch from going USB straight into the microDAC. The soundstage, clarity, reverb, and other aspects are all improved. Coincidentally, another listening section tonight has made me rethink the whole idea of replacing this combo of wonder with a supposedly better streamer.

Feeding an external 10MHz clock to synchronize the streamer/DDC and DAC is still an exciting and promising concept that's on my mind. Whether it would actually outperform—or even match—the results of a higher-end streamer or vice versa remains uncertain. I need to sleep on it.  I still have some time to think this through.

@lanx0003 I was using a Ugreen before the LanShack. Hopefully you will perceive the improvement I did. Ugreen actually makes quite a few extremely cheap cables that are properly shielded & good in my opinion. If you go into their Amazon home page/store & go into their categories you will see a wide assortment of video/audio products. Dirt cheap. A couple years ago they use to carry these little usb-c to 3.5mm dongle dacs. Get a 2-pack for like $16 bucks & I swear they were so good lol. 

@mattw73  Same here. I had the UGreen USB-A to Lightning dongle for my EarPods after comparing 2–3 products side by side. That’s why I picked one up this time without hesitation, even though other brands have gold-plated RJ45 connectors—which LanShack doesn’t have either. I hope I ordered the right one. Is this it?

@mattw73 Good call on the UGREEN cable! I have lots of high-end cables throughout my system but hadn’t yet bought a decent LAN cable - for whatever reason I wasn’t thinking it would make much difference. I subbed out my cheap cable for your bargain cable and immediately enjoyed better sound. 

@lanx0003 you got it!