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Cybershaft rubidium clock...low priced option?


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A rubidium standard offers NO advantage over the very good OCXO. The rubidium standard has two systems, a rubidium oscillator, which has high jitter but very good long term stability and an OCXO with very low jitter but not as good long term stability. Some complex circuitry in side there reads both and every so often slowly tweaks the OCXO to match the long term averaged frequency of the rubidium oscillator.

 

Because the OCXO is an adjustable oscillator it actually has slightly higher jitter than the fixed OCXO. Audio could care less about absolute frequency accuracy over tens of years time frame so the rubidium version has higher jitter and costs more, not particularly a good combination for audio use. (unless it is all about bragging rights, but that is something else!)

 

The phase noise specs for those cybershaft OCXOs are actually very good for the price. The big issue with any such external frequency reference is how it gets into the DAC and what happens to it in there.

 

First off, many frequency standards are sine wave output, a lot of DACs that have external inputs want a square wave not a sine wave. Make SURE the reference and the DAC will work together before spending any money.

 

Almost no DAC or audio device uses 10MHz directly. In order to use it the frequency has to be converted. What that conversion does to the phase noise of the input can vary wildly. The absolute best systems out there are at about on par with the phase noise from the premium. So with a premium you would be getting about twice the jitter inside the DAC. With the limited the internal jitter is going to be several times higher than the reference, hence there is not going to be much of an actual difference in the DAC, for a much bigger cost.

 

The above assumes your DAC has a state of the art frequency conversion circuit, these are pretty rare and expensive but COULD exist in one or two audio devices. The problem is that any device with such a conversion circuit probably already has a REALLY good local oscillator, so using one of these OCXOs going through the conversion is not necessarily going to give you lower jitter in the DAC. It may, but it may not.

 

John S.

 

Hi John

 

Any thoughts on swapping out the XO directly on the PCB?

 

These days, some DACs seem to be encouraging this with 14/4 DIP compatible PCB lands. And in most (all?) cases there are only 3 pins to solder.

 

The top-end XO's and OCXO's seem to be around the $400 mark, which is a fair bit cheaper than these offboard clocks. (Which are admittedly much better looking than my soldering!)

Front End: Neet Airstream

Digital Processing: Chord Hugo M-Scaler

DAC: Chord Dave

Amplification: Cyrus Mono x300 Signatures

Speakers: Kudos Titan T88

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  • 3 weeks later...
Here is the revised clock graph from the first page. I have also added the Sforzato PMC-01 BVA clock, which is the only clock in the world based on the Oscilloquartz BVA-8607 OCXO module:

 

M5te0H.gif

 

The numbers used in this table, are based on minimum manufacturer guaranteed values. The actual samples may exceed those numbers.

 

The perofrmance of the Sforzato PMC-01 BVA clock is so good - it should be considered the curent king of the hill in clock world. No wonder there were reports people in Asia dumping the Esoteric clocks for those.

 

Antelope on the other hand, looks like a complete garbage.

 

I plan shortly to install onboard the following OCXO:

 

http://www.pulsarclock.com/ds/Pulsar_Clock.pdf

 

The RMS jitter has been calculated at 20.174fs.

 

The part is about $400.

 

I'm actually very happy with the Crystek CCHD-950 which I recently installed, and is much cheaper.

Front End: Neet Airstream

Digital Processing: Chord Hugo M-Scaler

DAC: Chord Dave

Amplification: Cyrus Mono x300 Signatures

Speakers: Kudos Titan T88

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I'm not sure how you have calculated those jitter values.

 

Using the same calculator and range I used to calculate the jitter of the above clocks, I calculated rms phase jitter of this Pulsar XO at 446fs, not 20.174fs. 20x difference.

 

See post #253 here:

 

LKS Audio MH-DA003 - Page 17

 

The tool focusses on jitter in the 10Hz-10Khz range.

 

Some calculated values appear in post #274.

LKS Audio MH-DA003 - Page 19

 

As stated, I currently use the Crystek CCHD-950, replacing the stock CCHD-575 in my machine. The 950 has a worse RMS jitter spec, but sounds better in my DAC. However, I have the 575 in another of my DACs and it sounds just fine. This suggests that experimentation is the best way to find the best XO for a particular DAC, rather than raw specs.

 

Installing the following DIP adapter on the board allows for fairly easy XO rolling:

1107741 Aries Electronics | Crystals, Oscillators, Resonators | DigiKey

 

This makes comparisons of sound a lot easier; easier, certainly than having to re-solder an XO every time.

 

I can't comment on external clocks as my DAC's don't support them (and I don't have the $$$ to play that ball game anyway).

Front End: Neet Airstream

Digital Processing: Chord Hugo M-Scaler

DAC: Chord Dave

Amplification: Cyrus Mono x300 Signatures

Speakers: Kudos Titan T88

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Hahaha! Made me laugh. Trying to achieve low phase noise and then using a plug in XO... Also, does not the 575 achieve lower phase noise than the 950 at 100 MHz... At least that is what I see in published Crystek specs...

 

My 950 is actually soldered in, the board is there for those who want to roll. It's actually good to hear the difference that different XO's can make to SQ. And it doesn't always follow specs. I expect not many people have had the chance to hear how a different oscillator can impact the sound of the DAC.

 

Out of interest, how does the plug-in aspect impact the phase noise? If XO's are that sensitive, then surely having a clock offboard is also impactful? Not trying to be funny, just trying to learn more.

Front End: Neet Airstream

Digital Processing: Chord Hugo M-Scaler

DAC: Chord Dave

Amplification: Cyrus Mono x300 Signatures

Speakers: Kudos Titan T88

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Even more interesting is the variation from one sample XO to the next. A few years ago we had some XO samples measured for close in phase noise and hand selected the best ones for a project, if one has the equipment, time, and price margin available to do this one can achieve even higher performance.

Plugging any component, whether an IC OPA, XO, or even a voltage regulator into a socket is always going to result in lower performance, as the contact resistance will be higher than a well soldered connection to a well designed PCB. Indeed, using external oscillators subject to multiple plug in connections and a cable as well is not going to be ideal. Best approach is always going to be a dedicated, low phase noise fixed frequency XO right at the DAC.

Best to leave external clocks for the pro world, where the may be needed to sync together multiple components to one clock source. In the playback world, lets put a really good XO as close to the DAC as possible, and then send it out to the source for syncing.

 

Very interesting. With this in mind, I'm tempted to solder in the Ticha 994 op amps I use in the I/V section as these are also socketed (by the manufacturer).

 

When I got the DAC, the XO was sat on a separate 'clock' board with a custom voltage regulation circuit.

 

2016-08-31 21.34.00.jpg

 

The XO's output ran a few cm along some coax - probably sub-optimal. I've soldered the new XO to the motherboard between the DAC's two ES9018 chips. It's as close as it can be, now. I still power the XO with the aftermarket board though, it just required some simple re-wiring.

 

I do wonder if there is any impact from an XO wired into two DACs rather than just one. I've never seen any other approach, but would be interested nevertheless in any views on this subject.

 

Great tip, by the way, on the XO batch variations. Maybe the 575 I had originally in my LKS was one of the runts of the bunch. :)

Front End: Neet Airstream

Digital Processing: Chord Hugo M-Scaler

DAC: Chord Dave

Amplification: Cyrus Mono x300 Signatures

Speakers: Kudos Titan T88

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But those measurements are for diffrent ranges ! One was in the 1Hz-100Hz range and the other one - 10Hz-10Khz range.

 

You cannot compare XO jitter figures in different ranges.

 

Jitter is not unlike car acceleration. The measured car acceleration (measured in seconds) only gets any meaning after you apply a speed delta to it.

 

What you just did here, was comparing car A acceleration in 0-60 range, with car B acceleration in 60-120 range. Makes absolutely no sense at all !

 

Fair point on the comparison.

 

There is nevertheless a valid debate to be had on whether one XO type is 'better' than another for DAC usage based simply on a comparison of the published specs. The jitter ranges I have provided links to show (or at least attempt to show) a better context for audio DAC usage. My own listening tests have led me to conclude that following the published specs will not always give you better sound - there are other factors at play.

 

To conclude, I would not have believed that an XO (and possibly its location) could have such an influence on sound these days. My hearing is not the best, and I am usually pretty cynical when it comes to real vs perceived differences, but I have found that swapping the XO influences a DAC's sound perhaps more than any other component change I have tried.

Front End: Neet Airstream

Digital Processing: Chord Hugo M-Scaler

DAC: Chord Dave

Amplification: Cyrus Mono x300 Signatures

Speakers: Kudos Titan T88

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