EMI susceptibility of audio and USB clocks

[Nick]

Hi,

I’m really quite excited about this post, I was going to add this post to the “hunting for noise” thread as a packaging addendum to the clock posts in that thread but what has happened here draws on thoughts from a number of other recent topics and discussions regarding grounding, EMI susceptibility and clocks, so here is a new post.

If you buy into the need to drive audio circuitry with low jitter / noise oscillators and think for a moment about the challenges of generating a “clean” clock wave form then clocks are debatably one of the most EMI susceptible elements of the replay chain. For about a year now my DAC has used a non-standard audio clock, then two upgraded USB clocks were added to the system. Two of the clocks ended up perched precariously on a plastic sheet resting on top of my NOS1 box. This is not because they have not been mounted with the NOS1 case, they have, but because with the resolution of the NOS1 and the rest of the system you can very easily here negative impacts on sound quality arising from placement of both the clock circuits and their associated power supplies. The PC USB Clock is equally sensitive, both Paul and I have posted about changes in sound as this unshielded clock is moved from location to location. Based on previous experiences using third party clocks and recent experiments reported in the “Hunting for Noise” thread it has always been the intension to individually fully (well almost fully there are a couple of holes in the enclosures) shield all three clock PCBs and their PSUs. Over the last month or two the first stage of this work to shield the clocks has been completed. The PSUs are still to be finished if the back order for the enclosures that have been ordered ever gets delivered !!

From a packaging point of view to get two of the clocks (audio and USB) to fit into my NOS1 case (Im desperate to get the top back on again !), it meant choosing quite compact metal enclosures for the clocks. Its been a challenge to make the clocks fit and in the end it was necessary to change components on the clock PCBs to make them fit into the small enclosures.

Very careful attention has been paid to the grounding of the clock PCB, PSU and in particular the grounding of the enclosures that the clocks are now inside.

The results.


What follows is with the Audio and USB clocks now installed within the NOS1 case and the PC USB clock fitted outside of the PC chassis.

First sound quality change in summary is very significant indeed, I would place the improvement in the region of 1/3 of the step that all of the modifications the my NOS and PCIe USB card have made so far, so very significant indeed. The clocks have indeed proved to be very sensitive to shielding. Just how much of difference it makes has come as a real surprise. It’s no secret that I’m a real believer in the merits of the upgrading USB clocks at both end of the USB Link bit this has moved things to a new level again.

DAC Audio Clock -see Update below


This is where things get interesting. The baseline position was that I personally preferred the Dexa clock that I have been using subjectively to the NOS1’s oscillator module but I was happy to accept that the difference was not that great and was likely to fall into the “personal preference “ type of change for folks listening to the two side by side. Certainly those that have heard these clocks back to back have noticed this difference but stopped short of investing. With the shielding applied however  there is a marked improvement in sound quality I would now have to say that there is a gap between the performance of the non standard clock (now a clear a favourite for me) and the standard clock in terms of subjective sound quality.  It will be very interesting for other folks to re-listen again as see what they make of this.


UPDATE 2/3/14

I have had the privilege of visiting Peter for a couple of days last week. Whilst at Peters we had the chance to listen the NOS standard 22.5mhz clock and the dexa 22.5mhz clock to get a feel for how both sounded. In addition to this Peter used his FFT measurement equipment to measure the the audio band psu noise of the NOS clock and the dexa clock.

This all made for some interesting results which I need to update here in order to keep information current.

After listening to both dexa and the NOS standard 22.5mhz clocks, the NOS standard clock provides the best sound quality.

The audio band FFT traces of the power supplies to the dexa and NOS1 clock showed the NOS1's clock supply to be around 45db quieter than the dexas psu. This is not a direct indication of the clocks performance but it does show how the lengths to which peter has gone to give the NOS1 clock the best environment to perform in.

In summary the NOS1 standard clock sounds better than the dexa used as an audio clock. Stick with the standard clock it works exceptionally well !
   


PC USB Clock


Proper shielding makes a huge difference in performance particularly at the PC end. It’s worth pointing out that prior to putting the USB clock into its new enclosure and setting up its grounding the clock was not living inside of the PC (eg in harms way for EMI as it were). Rather it lived outside of the case in the best location I could find for sound quality. With the enclosure and grounding scheme in place there are more options for location of the clock now without impact on sound. And sound is just superb, it is one of those changes where everything improves from the lowest of lows to the highest of highs and everything between. Bass in particular is very improved, not overstated at all just much much better there is resolution way way down now and low “bass effects” mixed in during production that were not easily audible are playing along tunefully now.

DAC Interface USB clock


Again a significant difference in sound quality although not so large as that gained from shielding and grounding of the PC end USB clock. Similar sound quality gains characteristics but slightly less pronounced defiantly worth having though.

Thoughts.


Firstly as anticipated to sweat more performance out of the clocks it’s very well worth while taking precautions to prevent EMI.

Changes to the clock shield / PE electrical linkage for each individual clock changes sound characteristics. Get this “right” and sound beautifully clicks into place.

The behaviours from these experiments with the shielding strongly point to the PE grounding practice that Mani and Paul have implemented whereby each main component PC, DAC, AMP have their own PE spur running in star ground fashion to a dedicated PE spike. Following their PE star setups was on the plan after listening to Mani and Pauls systems but this has shot right up in priority after playing about with clock shielding. NICE ONE GUYS !

Finally it will be interesting applying enclosures to the three Clock PSUs which are currently still unshielded. I would not anticipate quite the same level of impact on sound quality but who knows.

As a final foot note.


The intension here is to highlight the susceptibility data and audio clocks to EMI and the effect on sound quality this can have. I guess to some extent or other it will be the same if the clock is a DIY module build, a standard XTAL or XCO fitted directly to  PCB or a 3rd party product such as the PPA card or Dexa. It’s just good to understand that whatever the clock is, the conditions it operated in might REALLY matter for sound quality.

As usual no measurements here just subjective assessment of sound quality changes, although differences in sound are so significant that conclusions about the effectiveness are easily formed.

Best Regards,

Nick.

[listening]

Wow, what a nice cookbook! There will be a lot of changes in electronics and my house 

Georg

[Scroobius]

On Sunday as the River Thames retreated from our doorstep (close Old Father Thames but not this time!!) my thoughts returned from the trivialities of sandbag humping and potential house flooding to the far more important subject of hi fi.

Previously I have posted about the improvement in sound quality when I moved the USB PCIe Dexa clock inside the PC case but well hidden from the rest of the PC (and its RFI) behind the hard disc case metal work. Wow what an improvement that provided and very surprising it was too at the time.

On Sunday I went one stage further and located the Dexa clock inside a dedicated metal box which was isolated from the PC earth and connected direct to a clean earth.

The result this time was subtle, I found it hard to note any significant improvement in sound quality. What this must mean in my system is that the location inside the PC was in fact well screened from RFI. Anyway the installation of the Dexa clock in a box looks much better and professional so the time was not wasted.

For sure though - good RFI screening is crucial as I found the first time I moved the clock.

I also tried a temporary lash up with the NOS1 USB Dexa in a box but again any improvements were not immediately noticeable.

Cheers

Paul

[Nick]

Hi,

I just want to update information above regards using a dexa clock to replace the 22.5 MHz audio clock of the NOS1. I have also copied the following text into the first post in this thread so that the comments are also in context of the original post.

I had the privilege of visiting Peter for a couple of days last week. Whilst at Peters we had the chance to listen the NOS standard 22.5mhz clock and the dexa 22.5mhz clock to get a feel for how they both sounded. In addition to this Peter used his FFT measurement equipment to measure the the audio band psu noise of the NOS clock and the dexa clock.

This all made for some interesting results which I need to update here in order to keep information current.

After listening to both dexa and the NOS standard 22.5mhz clocks, the NOS standard clock provides the best sound quality.

The audio band FFT traces of the power supplies to the dexa and NOS1 clock showed the NOS1's clock supply to be around 45db quieter than the dexas psu. This is not a direct indication of the clocks performance but it does show how the lengths to which Peter has gone to give the NOS1 clock the best environment to perform in.

In summary the NOS1 standard clock sounds better than the dexa used as an audio clock. Stick with the standard clock it works exceptionally well !

Cheers,

Nick.

[manisandher]

Hey Nick, thanks for the update. So the 'modded' NOS1 that Peter is currently using - is this in any way related to the mods you've implemented in yours?

Mani.

[boleary]

Hi,

I just want to update information above regards using a dexa clock to replace the 22.5 MHz audio clock of the NOS1. I have also copied the following text into the first post in this thread so that the comments are also in context of the original post.

I had the privilege of visiting Peter for a couple of days last week. Whilst at Peters we had the chance to listen the NOS standard 22.5mhz clock and the dexa 22.5mhz clock to get a feel for how they both sounded. In addition to this Peter used his FFT measurement equipment to measure the the audio band psu noise of the NOS clock and the dexa clock.

This all made for some interesting results which I need to update here in order to keep information current.

After listening to both dexa and the NOS standard 22.5mhz clocks, the NOS standard clock provides the best sound quality.

The audio band FFT traces of the power supplies to the dexa and NOS1 clock showed the NOS1's clock supply to be around 45db quieter than the dexas psu. This is not a direct indication of the clocks performance but it does show how the lengths to which Peter has gone to give the NOS1 clock the best environment to perform in.

In summary the NOS1 standard clock sounds better than the dexa used as an audio clock. Stick with the standard clock it works exceptionally well !

Cheers,

Nick.

Nick, I am so very happy that you were able to spend some time at Peter's with the"Dexa" mod, such commitment on everyone's part! Really an incredible community here.  Am also glad that I can stop wondering about how a non-DIY person could manage the "Dexa" upgrade. 

Keep tinkering!

[Nick]

Thanks Brian,

I just wanted to make sure that everyone is aware of the comparison. I would hate for anyone to invenst in the 22.5 or 24.5mhz dexas for the NOS after what we found.

Actually Paul has also helped reaching thisc onclusion as well. A couple of weeks before spending the time with Peter, Paul came over to mine and we had a chance to compare our DACs, mine with a dexa 22.5mhz clock and Pauls with the standard clock module. This also cast doubt on the dexas relative performance.

At Peters listened to under the frankly microscope performance of Peters system (WOW its good) it was clear that the standard module was just more musical.

I'm happy that this is the case, over the years I've spent too much on expensive aftermarket clocks for equipment so it's nice when a properly implemented module at a sensible cost sound better :-).

In the area of 24mhz usb clocks "the jury is out" just now. They defiantly did not help at all in Peters system but they are still a big upgrade here in my system and I think in a number of other folks, so ill be looking carefully again at them, but it just might be that their effectiveness is in some way system dependant. I guess we will see.

Best,

Nick.

[acg]

Hi Nick,

I am glad that yourself and Peter finally managed to get together and thank you for reporting this to us all.

Regarding the 24MHz dexa's do you have any theories as to why they did not help in Peters system?  This is perplexing for me because I have used and modified the PPA studio card which is basically a clock modification that is not that dissimilar to your mods, and for me there was a difference whether that was due to improved clock performance or some grounding issue or something else.

Have you tried Peters Silverstone card with the isolations in the PC and NOS1 for yourself? 

Regards,

Anthony

[PeterSt]

Hey,

My take :

No matter it may sound snobbish (say I just am) I sort of dare to predict how the outcoming of certain tweaks or mods will be. So, Nick will remember that I offered a bet on the Dexa USB mod of a 1000 which seemed to few for Nick, and after quickly changing that into a 10000 I withdrew that right away because I could loose and then it is a lot of money. And for Paul, a bet over a bottle of nice whiskey also was not suitable because Nick just brought a very nice bottle so that didn't seem fair. But really it went like that, including the outcome I predicted. But for me this is easy : the Silverstone carries virtues which in my view for sure nothing else has, and "Nick's case" would be lost on that alone. And so it did. But you must know what to listen for and as I told Nick this starts with a relatively very fast system to begin with, or otherwise no Silverstone virtues will come out anyway.
So think about that.

Later Nick and me talked through my "predictive behaviour" like on this forum, and that a few forum words would not be able to come across. This is so I guess. So, how to make such "statements" more science if you want (these are now my words) which is not always possible;
In this case you know my statements about how noise will influence and no matter it can be hard to see or believe, for people with some more insight it is not difficult to see how a device will be noisy agains another device. So in the realm of the Dexa it was almost literally put in that corner by me in advance and per my own "theories" (or more insight knowlegde). So, fine ...

Not so fine is that while I had the chance to make screenshots of the noise emitted by the Dexa solution, I forgot to do that. So, missed chance. But for Anthony (who accidentally received some screenshots of the normal solution) - where you there see two small harmonics in the audio band plus 2 for the USB noise itself, the Dexa shows one big pile of uncountable very discrete harmonics. Thus, not a higher noise level as such, but harmonics of (now on estimate) 30dB sticking out the general noise level.
Not to make it complicated : this was measured at the DAC side with no reason to believe it would be different on the PC side (dedicated supply and all exactly the same). The complication : this was NOT measured from the Silverstone which could have told something when it had been done.

So, noise noise noise, and might someone think that this does not influence then he should come up with a better idea. And notice (my thinking) : Listening is NOT that better idea when first measurably all is for the worst. And oh, we can listen with our subjective ears and might have the idea that Griffith starts to sound reasonable at 95dB or so (hi Nick), but it really won't work out for the better if you realize how it can happen that a Griffith voice starts to sound for the better (which will be smeared and which is clearly audible through other music, plus it is totally predictable (ehm, by me)). Costello not much different (hi to Paul now).

Btw what Nick doesn't tell much is that we tried all combinations possible and no Dexa sounds for the better anywhere as soon as it is in the chain (somewhere). So think like Nick's NOS1 with/without Dexa USB, with/without Dexa Audio, my NOS1 without Dexa's with/without Dexa-PC and of course the full mony of Dexa's everywhere. It is just a no-go but it also can't be (too much noise and too lousy passive crystals - just saying that too).
But we sure tried ! (with actually 3 NOS1's all together).

So really, this is not a matter of "why does it not work in my system" and it merely will be about "maybe my system lets hear through some more". And also don't underestimate the power of, say, fiilters (which is what noise will be) against something which is not right to begin with. And we can really count in the Griffiths.
Regarding this latter : I have been attending some concert of some woman I don't know the name of, but her not amplified voice hurt my ears more than my audio system ever did as far as I reall. So voices *can* be mad, and there's nothing much to do about it except some kind of filtering which will demolish the remainder. Let me add that I think that Nick possibly heard the highest speed of on/off sounds/transients in his life which is a nice gag because it can be done, but don't let the Griffiths loose on that because it will show for the worse; Just logic.

That it is the Silverstone which highly contributes to this all is something else and accidental and *why* this happens I honestly still don't know. So far and after all this time I still don't recognize it as giving a nature to the sound, except for all becoming super fast. But what I could do (try to do) is measuring its (supply) noise against such another "normal" PCIe card. I am not saying it will be that what will show different, but I will say it is not the crystal being better and that it is about jitter or something, which just can not exist in that (USB) domain.
(although it sure has to meet jitter specs to let USB work at all).

Still not scientific, but I try.
Peter

[acg]

Hi Peter,

Based on all of my conversations with you, I rate your post just now as the instigator of one of my 'keystone moments' or "when it all seems to fall into place".  Of course I have a little more insight than most with our recent emails and our discussion of USB noise:  in those power supply noise plots that I have been privy to the USB fundamental at about 8MHz and harmonics further out really do stand starkly above the general noise profile, but they are really quite alone up there.  If the dexa's produce lots of this noise that sits well above the "normal" noise profile then I begin to see how they would alter the sound.

Since I have settled on the Silverstone and the isolations of the PC and NOS1 (along with the XXHE settings to suit) I have experimented by putting other usb cards back into my PC from time to time.  There is no doubt in my mind that the Silverstone in this arrangement is quite unique and I really struggle to persevere with the other cards, including the PPAstudio card.  The obvious question is "why", but of course no-one here really knows.  It would be nice to find out though, and I suspect that it has something to do with the all-SMD layout of that card.


Cheers,

Anthony

[AlainGr]

Peter,

Did you fall on the Silverstone by accident or by studying the specifications ? You could have chosen another card that would have been as good (no) ?

Alain

[PeterSt]

Pure accident Alain.
Well, almost; I saw this relative expensive one and thought to give it a try for that reason.

Peter

[Scroobius]

What an interesting couple of days for Nick and great that we are gaining more information and understanding (well maybe we are but not Peter ha ha).

So the standard Audio clock in the USB card sounds better than the Dexa clock. I have to say (as Nick has pointed out) that does not surprise me at all. With the many listens to Nicks NOS1 I have had to date (in both his system and mine) with Dexa audio clock versus my NOS1 with standard clock I have never been convinced by the Dexa in place of the audio clock. So no surprise for me there.

As for Dexa's at both ends of the USB link I cannot see anything in the above posts that says the 24Mhz Dexa at the NOS1 end made the sound WORSE in Peter's system (please correct me if I am wrong) but it seems to be the case that it did not sound better (and maybe it should sound worse if the noise profile has the Dexa PS harmonics in it). I understand that Dexa in the PCIe card could not be made to work in Peter's system (again am I correct?).

I fully accept Peter's view that this is all about noise and the inherent speed of a system to fully hear the impact of noise.

Just now I am not tempted to remove the Dexa's from my USB link just yet as in my system they sound great! (even if that is for the wrong technical reasons). But now I know there is more work to be done (Arrgghh!!!).

Cheers

Paul

[PeterSt]

Hey Paul,

As for Dexa's at both ends of the USB link I cannot see anything in the above posts that says the 24Mhz Dexa at the NOS1 end made the sound WORSE in Peter's system (please correct me if I am wrong)

Correct, but it depends on how careful I write and how careful you read. So indeed, nothing shows (showed) that the Dexa at the NOS1 end made it worse, but this is because this combination was not tested, which in itself was because it could not be tested. But also : there is nothing in my mind which focused on that being a possible source for better or worse; you could almost say I forgot it (or we both did). On my part I don't see it anyway, and on Nick's part it will have been about "the two at both ends make the deal anyway" (which did not quite happen - but careful - with the Silverstone as opponent and nothing else).

The Dexa on its own at the NOS1's end was not easy to test because it would have needed a normal audio clock next to a Dexa USB clock which was the farthest of what at least Nick would have wanted to test (for obvious reasons), while once we would have tought of it it would have needed the Dexa for audio clock to be removed (again) and let's say that in 24 hours of time Nick was able to catch his train just by a minute left. Plane the same story.

What I could have added in my previous post - and I will do it now - is that in my view all what's left is the "tuning" facility/possibility of the Dexa's at both ends, but that will loose from the Silverstone's virtues and which was what my virtual bet was about. This can be subjective up to some point plus it can be subjective to the system as a whole, although I don't think the latter will be the case easily (and this in itself is because everybody I heard so far (and this is quite many !) perceives the same from the Silverstone - for the better).

But you are right - nothing tells that the Dexa-USB at the NOS1's is for the worse. But not for the better either.
Quite expensive toy ?

Let's not forget that you guys started out with this (against all odds ?) because it costs 600 euros or dollars or even Asutralian dollars, but the worth of it is beyond me. So that tuning against eachother perhaps; well, if you like me to provide something like that for maybe 100 euros for both ends then this seems a better option - it could have low jitter oscillators and possibly I earn some money at the same time. Haha.
But really ...

Regards,
Peter

[Scroobius]

Agree with all of that. I do have a Silverstone PCIe card (currently with Dexa clock) but I can easily convert it back to its original clock. So when my new XXPC arrives maybe I can try a few things.

[AlainGr]

Peter,

OT: am I ok to assume that between 2 USB ports, there should be a "master" and a "slave" ? If so, then to what level does the 2 clocks need to be tuned together (since I figure that this already  happens between them) ?

Alain

[Nick]

Alain, All,

After returning from my visit to Peter's I was left with a big question - Why did the USB clocks not perform as expected in Peters system ?

Peter provided an account and his view above in this post of what may have been happening regards USB clock noise etc. I know that the dexa clocks (read improved clocks on standard xtals in USB interfaces, forget Dexa as a make here) have made a significant positive impact in 5 of the 6 systems that they have fitted. Before posting my comments and thoughts on how the USB clocks performed at Peter’s and why, I wanted to spend time investigating what might have happened to impact their performance. I have been doing this for a couple of weeks now and am reasonably sure that I understand now what happened. I will post more on this shortly, but for now…

To the point that you make above Alain, the relative speed of the two clocks at the ends of the USB link is very important to sound quality. As part of the work to diagnose the problems at Peter’s I now have a way to show the relative speed of the two USB clocks in my system. This is done is such a way that the relative clock speed can be looked at in real time across the room from my listening position whilst listening to music. The display will resolve fractions of one cycle of phase shift between the clocks (eg a few degrees of one 360 degree “beat” between the clocks). Viewing the relative clock speed in real time whilst listening its clear that speeds do significantly impact sound quality. In fact it changes sound differently if the PC PCIe USB clock is running faster or slower then the NOS1 USB interface clock. Same speed, or as close as possible to this, is best subjectively for SQ.

The USB clocks in my system are tuned now to a frequency which is better than 1 part per million of each other (this 1ppm is the max and for some time they run at the same speed exactly). When we tried the clocks at Peters they were matched very badly (worse than 100 parts per million). Sound now is entirely different to what Peter and I were listening to.

If these results are valid and relative USB clock speed does impact sound quality, then without the capacity to tune the USB clocks it may very well just be luck that determines how good a particular PCIe USB3 card sounds with a NOS1. The luck being that all the many variables that impact the Xtal speed and stability of on the USB card and NOS1 USB interface just happen to “line up” such that the two crystals run at close speeds to one another and sound good. That is a worrying thought as even different instances of the same PCIe USB card model may sound different…..

Paul is bringing his Silverstone USB card, and DAC (fitted with USB Dexa clocks) to mine this weekend to look at this and we hope to get more supporting evidence. With respect to the SIlversone USB card, unfortunately the act of measuring the oscillation speed of a raw crystal’s output will change the speed (due to capacitance of the probe that is being used to do the measuring) so tuning the card to the Dexa in my NOS1 is unlikely to be possible but we will see what we can do. I will update when we have more information.

Regards,

Nick.

[CoenP]

Hi Nick,

Interesting progress!

We have a saying here: "when you have two clocks you never really know what the hour is...". My take on this phenomenon would still be a 'master USB clock' solution. This will always imply a fixed and identical base frequency yet the clock distribution is likely to introduce different phase noise at the different ends. I'm too distracted to look deeper into this subject, yet it may hold even more potential. At least it lacks the synching of the clocks and the clock source may be decoupled from the source and destination noise sources.

I would not be surprised at all that you will find absolute and relative wandering of the clock frequencies audible.

keep us posted,

regards, Coen

[Nick]

My take on this phenomenon would still be a 'master USB clock' solution.

Coen hi,

I agree this needs to be tried. I have had all the parts for a while now to give a single clock driving both ends of the USB link a go, just need to find the time  .

Regards,

Nick.

[listening]

Hi Nick,

very interesting point. I remember to a good friend of mine who has been an enthusiastic shortwave amateur who build his tube receivers by himself. And one very critical aspect was the frequency normal which was actively heated to a specific temperature for stability. The systems needs to heat up for over an hour for good stability. Do you think that temperature will influence the synchronization? Could it be possible that here is a reason for different SQ at various days?

Georg

[Nick]

Hi Nick,

very interesting point. I remember to a good friend of mine who has been an enthusiastic shortwave amateur who build his tube receivers by himself. And one very critical aspect was the frequency normal which was actively heated to a specific temperature for stability. The systems needs to heat up for over an hour for good stability. Do you think that temperature will influence the synchronization? Could it be possible that here is a reason for different SQ at various days?

Georg

Georg,

I think you could be bang on with what im seeing here.

The dexas have a simple resitor heater and thermostat control of the temperature of the 24mhz xtal can on the clock pcb. When the clocks are turned from on cold there is a very large difference in relative speed of the clocks. This reduces quickly (as the temps stabilise I think) at first quickly then further stabilises happens over about 10m. In the end the two clocks do not stay completely stable with respect to each other. I've got them trimmed now with a slow progression of 0 beats per second to around 10 beats per second (possibly less). The cycle from 0 beats to 10 beats and back takes about 30 seconds. I cannot prove it right now but I have a feeling this might be due to the heating cycle of the thermostat on both clocks slight over heating then under heating the crystals.

Thinking about it the beat frequency cycle could be due to other effects as well. But hysteresis in the heating / temp sensing circuit seems like it might be the first place to investigate. It's fun this stuff, whats the betting that Paul's clocks behave completely differently when we look at the haha :-).

Best,

Nick.

[Nick]

Since my trip to Peters I have been working to understand why the dexa usb clocks did not perform as expected. The first step in understanding what happened is in this post.

http://www.phasure.com/index.php?topic=2888.msg30710#msg30710

In summary this post flags the sound quality performance difference that tuning the PC and DAC usb clocks makes to sound. Being a compulsive tweaker, when Peter and I listened to the clocks I had the damed clocks significantly out of sync (of the order of 1000s of hz against a USB chipset spec typically of 24mhz +- 100ppm). It was only after returning from Peters that I worked out how to tune clocks accurately to each other.

I the post above I had to stopped short of saying that the poor performance we encountered was completely explained. This was because although synchronising the USB clocks makes a massive difference to sound I still felt there was something else about the set up that had impacted sound. So more thinking and now what i beleive to be the final issue solved making the dexas perform as i had expected.

What had happened is that whilst tweaking, I had set the speed of the NOS interface USB clock fast by 1-2 kHz and lost track that I had done this ! I was stumped as to how I could get the NOS dexa close to dead on 24mhz again (my scope frequency counter is only accurate to 3 decimal places), then I remembered the DIY 24mhz clock I'd built to test the USB clocks  . Using the diy clocks as a reference the dexa at the NOS end was retuned again to very close to 24mhz. Then PC end dexa usb was re-tuned to match the DAC end dexa with both running much closer to 24mhz.

So now the system behaves exactly as expected (totally different performance) without question these two tuning factors are the root cause of the poor performance that Peter and I experienced.

I'm really glad to have found the cause of the problem, diagnosing what happened has helped to pin down a better understanding clock related variables and how usb clocks need to be set up. It would be fun now to retry the upgraded clock config in Peters system but not that practical unfortunately 

Anyway absolutely top, top draw performance again from the modified USB link.

Regards,
Nick

[manisandher]

Nice work Nick, and thanks for sharing.

Using the diy clocks as a reference the dexa at the NOS end was retuned again to very close to 24mhz. Then PC end dexa usb was re-tuned to match the DAC end dexa with both running much closer to 24mhz.

How close is 'close enough'? And how much do you expect the Dexas to drift away over time from their current settings?

Mani.

[CoenP]

Hi Nick,

I don't really understand what you have been doing to get it right. Are you saying that the absolute value of 24Mhz is of importance?

Regards, Coen

[Nick]

I don't really understand what you have been doing to get it right. Are you saying that the absolute value of 24Mhz is of importance?

Coen hi,

The posts are none too clear  There are two factors.

1) matching the speeds of the PCIe USB card clock and the DAC USB interface clock.
2) both clocks need to run at close to their nominal frequency of 24mhz.

When I went to Peters I had unknowingly tweeked clock speeds to the point that, the DAC end clock was running > 1000hz above the 24mhz nominal speed for the USB decoder. In addition to this the matching of the two clock speeds also was way out.

Fixing both these points has things very much back on track sound quality wise  .

Regards,

Nick.

[Nick]

How close is 'close enough'? And how much do you expect the Dexas to drift away over time from their current settings?

Mani hi,

Good question  I'm not sure about how close is close enough. The  sample 24mhz "DIY" clock i used based on a module would should run at 24mhz. I might investigate more but I need to spend more money on measuring equipment to do this.

Regards the dexas drift rate. As mentioned in other posts, there is a cycle of drift between two tuned dexa usb clocks, but this peaks at about 3 or so beats per second so not that large (your clocks were tuned to this tollerance). So far once tuned to each other the frequencies of the clocks appear to be stable over time. I'll keep checking on this but I don't think they are going to need retuning regularly.

Regards,

Nick.