CLOCKS, what should we look for in next generation

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iFi PRO looks to have some engineering problems, likely caused by trying to offer so many features at a low price:

 

https://www.stereophile.com/content/ifi-audio-pro-idsd-da-processorheadphone-amplifier-measurements

 

Evaluating jitter and/or clock performance by listening tests alone is fraught with difficulties.  Every system is different, and virtually every system has some existing "problems" which may obscure the results of listening tests.  I participated in some listening tests where we correlated various jitter levels and distributions (measured) with actual listening observations.  I came away from these tests realizing that in some systems, some listeners are definitely going to prefer the sound of the higher jitter option.  I suspect this effect is very likely responsible for "improved subjective results" some people report when applying certain external clock solutions.  As JS has mentioned, an external clock solution with extremely low phase noise, combined with a DACs which implements a "perfectly chosen" DDS device, and does perfect implementation, may be able to improve on the jitter performance of a good (say Crystek 575/957) internal clock.  But it is highly unlikely that the internal clock receiving/distribution is done perfectly.

 

I am getting great results in my DACs with both Crystek 575/957 and NDK SDA series XOs, with great deal of attention paid to power supply for the XO and good distribution techniques.  Could it be better, sure the same approach with a really, really good (local) OCXO could be better technically, would that difference be audible... that is the question.

 

On other trick one can do, is they can measure and hand select clocks for lower phase noise at low frequencies-the clocks referred to above all vary by as much as +/- 5 dBc/Hz.  If one has the ability to measure for phase noise, and can afford to discard (or use for less demanding circuits) a handful of XOs, you can achieve better performance.  Unfortunately, the tools for measuring phase noise are prohibitively expensive for most.  For a project awhile back, Sonore got a small batch of hand selected NDK oscillators-the few customers who received these got a very special upgrade indeed!  Unfortunately I do not believe the source who made those selections is offering the service anymore.

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6 minutes ago, Superdad said:

BTW, here's the plot for an eye-poppingly great OCXO we have been looking at.  Have no idea the cost yet--likely $600 a piece.  But check it out: -110dBc/Hz at 1Hz, -143 at 10Hz!  (this is 10MHz part, but even in the mid-twenties audio clock range these are spectacular).

 

Yup, pretty good even for 10 MHz!  Do they have plots for audio rate OCXOs?

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10 minutes ago, Miska said:

 

My question would be how good the clock performance is after feeding 10 MHz clock through DPLL to generate actual clocks needed for audio...

 

I'm hardly getting excited about any clocks anymore until I see measured proof of the final performance from DAC output...

 

While I tend to agree, I would get excited if i could purchase a DIP packaged XO @45.1584 with <115 dBc/Hz @ 10 Hz, or a 27.5792 with <120 dBc/Hz @ 10 Hz for $500.00 USD or less.  Just for bragging rights and subjective testing.  I would want to receive a phase noise plot for each individual clock as well.

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3 minutes ago, Miska said:

 

I'm quite happy with my CCHD-957's. Anyway the clock module is rarely the performance limiting factor. Especially on wider scale there are so many bigger things to work on in the D/A converter itself...

Like I said, I agree, mostly...  But I would like to try something better here anyway, just to satisfy my curiosity.  If I did then hear a difference I do have access (not really super easy access) to an AP for testing the DAC output...

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33 minutes ago, Miska said:

 

I'd be curious to see J-test24 results for various different clock modules if you can easily swap them!

 

Yeah, well if I could get the desired "better" clock it would be possible.  Unfortunately the one XO which used to be available (Pulsar Clock) is no longer, and I am not aware of other drop in clocks with this kind of performance/price at audio frequencies without having a custom frequency run done with MOQs I could not afford...

Anyone know of alternative XOs at 45.1584 MHz with < -115 dBc/HZ @ 10 Hz spec (confirmed performance) to the Pulsar Clock at anything close to reasonable prices?  NDK DuColon is too expensive...

The DAC for testing uses a socket, so clocks can drop in...

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5 minutes ago, PeterSt said:

 

Define the output noise level first. 

You mean the analog noise floor of the DAC?  I would not do it first, as I have to ship the DAC to where the AP is for testing, but then the full range of tests can be done.

I would add in terms of the J-Test, I am generally more interested in the spread of the skirt than the floor and sidebands (unless they are rather extreme)...

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22 minutes ago, Miska said:

 

Of course there must be no side bands at all, otherwise you have correlated jitter which comes from clock line interferences (usually PSU or PCB design).

 

So far, one of the best figures I've got is from Holo Spring 2 running at DSD256. And no COTS DAC chips, but just a discrete implementation.

Yes, wish Holo would put a good volume control in their DACs I might just pick up one of those then.  I am scared to rely just on HQP VC.  My DSC-2 build with analog VC from Pavel's board is coming along well...

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53 minutes ago, jabbr said:

 

What is the frequency resolution of the AP? At the levels of phase error under discussion you want to have sub-Hz resolution.

 

Some pointers: http://www.ke5fx.com/ke5fx_mud2010.pdf

(note the HP 8662/3 signal generator is awesome! also see 8561/2 spectrum analyzers -- ha ha but finicky  

As we still see, even in J-Test amongst some very good DACs (Jussi posted examples above) clearly there are differences.  My suspicions are also that the ancient J-Test approach (designed originally to test jitter on SPDIF interfaces, which are generally at higher levels, especially back then, think the nearly ubiquitous CS SPDIF receiver chip, pre-Wolfson, which  has >200 pS of added jitter) is inadequate to really measure where we are at now with the best DACs.

I certainly have heard differences from solely changing clocks from something good, to something just a little better, and no not with some weird flying lead contraption!  Just dropping in another clock to a socket (and before you suggest something here, I am not sure a socket will result in the best performance either, but it will degrade both clock signals equally).

 

As I regard subjective testing as being important if done carefully, i would not be so quick to dismiss the possibility of better clocks actually improving performance of a DAC.  I am not going to just agree with saying that the Crystal 575/957 parts are as good as it gets for DAC performance without further investigation, as too many times things which appear to not matter at first, have been proven to matter later.

 

For the more technically minded folks, say if one were to suggest that the J-Test is antiquated, is there, perhaps, a better testing protocol to look at the effects of clock performance on DAC performance, at the analog output?

 

Got to back to work now, out with soldering iron...

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@Miska, I will defer to your expertise then!

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4 hours ago, Miska said:

With HQPlayer Embedded I'm much less worried since the OS doesn't run or have any other audio applications or sounds. But on Windows or macOS I would be more nervous.

How much less worried?  In my set up I have to run pretty high gain in order to accommodate some low average level files (like Channel Classics, for example), and I have to attenuate the signal by at least -5 dB to accommodate oversampling to DSD and avoid clipping there.  This is definitely a case where I could clip the amp.  I wish I could just run the HQP volume control, as I would prefer that to analog, but the analog volume does give me safety.  I do not want to be replacing (somewhat expensive) drivers...

 

I am using a Muses chip (laser trimmed resistor ladder on a chip VC) with my DSC board right now, I did listen without it (under controlled circumstances) and with it, and it it seems to be really good from a subjective POV (transparent or very close subjectively, i could not A/B it quickly), but still, I am strong believer in digital VC when reliable and possible, and do rely on it with my ESS based DACs, even with high gain.

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28 minutes ago, fas42 said:

Digital volume control is the way to go - IME, analogue gain adjustment and trimming is always a weakness, unless the most extreme quality devices can be used.

Agreed, but with DSD there are few options for such, HQP is one, but controlling the volume via computer based software may be risky.  In my case I am talking about running the DAC(s) always at DSD 256 or 512.

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10 minutes ago, mansr said:

From Stereophile:

JVS: You don't believe in using an external word clock. Why?

EM: Because I think this is the most stupid thing I've ever heard in the audio business. That means you have a precision clock that you have to connect to a wire to connect to a DAC, when the clock should be straight away where it belongs, inside the DAC, beside the DAC chip, if there is such a thing—not through a cable in a different box. This is so idiotic, it's not even funny. It's a money grab.

I suppose that would be Ed Meitner?

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It is always nice to hear when an experienced digital engineer make no nonsense statements like that.